arXiv:0704.0192v1  [astro-ph]  2 Apr 2007
Submitted to The Astrophysical Journal
Star Formation in Galaxies with Large Lower Surface Brightness
Disks
K. O’Neil
NRAO, PO Box 2, Green Bank, WV 24944
koneil@nrao.edu
M. S. Oey
University of Michigan, Astronomy Department, 830 Dennison Building Ann Arbor, MI
48109-1042
msoey@umich.edu
G. Bothun
University of Oregon, Physics Department, 1371 E 13th Avenue, Eugene, OR 97403
nuts@bigmoo.uoregon.edu
ABSTRACT
We present B, R, and Hα imaging data of 19 large disk galaxies whose prop-
erties are intermediate between classical low surface brightness galaxies and or-
dinary high surface brightness galaxies. We use data taken from the Lowell 1.8m
Perkins telescope to determine the galaxies’ overall morphology, color, and star
formation properties. Morphologically, the galaxies range from Sb through Irr
and include galaxies with and without nuclear bars. The colors of the galaxies
vary from B−R = 0.3 – 1.9, and most show at least a slight bluing of the colors
with increasing radius. The Hα images of these galaxies show an average star for-
mation rate lower than is found for similar samples with higher surface brightness
disks. Additionally, the galaxies studied have both higher gas mass-to-luminosity
and diffuse Hα emission than is found in higher surface brightness samples.
Subject headings: galaxies: evolution; galaxies: colors; galaxies: luminosities;
galaxies: ISM; galaxies: photometry; galaxies: spiral
http://arxiv.org/abs/0704.0192v1
– 2 –
1. Introduction
Large low surface brightness galaxies are galaxies with disk central surface brightnesses
statistically far from the Freeman (1970) value of µB(0) = 21.65 ± 0.3 mag arcsec
−2, and
whose properties are significantly removed from the dwarf galaxy category (e.g. MB < −18,
MHI > 10
9M⊙). Studies of large LSB galaxies have discovered a number of intriguing facts:
large LSB galaxies, in contrast to dwarf LSB galaxies, can exhibit molecular gas (Das,
et al. 2006; O’Neil & Schinnerer 2004; O’Neil, Schinnerer, & Hofner 2003; O’Neil, Hofner,
& Schinnerer 2000); the gas mass-to-luminosity ratios of large LSB galaxies are typically
higher than for similar high surface brightness counterparts by a factor of 2 or more (O’Neil,
et al. 2004); and, like dwarf LSB galaxies, large LSB systems are typically dark-matter
dominated (Pickering, et al. 1997; McGaugh, Rubin, & de Blok 2001). These properties,
added to their typically low metallicities (de Naray, McGaugh, & de Blok 2004; Gerritsen
& de Blok 1999), lead to the inference that even large LSB galaxies are under-evolved
compared to their high surface brightness (HSB) counterparts. Once their typically low
gas surface densities (MHI ≤ 10
21 cm−2) (Pickering, et al. 1997) and low baryonic-to-dark
matter ratios (Gurovich, et al. 2004; McGaugh, et al. 2000) are taken into account, the
question becomes less why LSB galaxies are under-evolved than how they can form stars at
all (O’Neil, Bothun, & Schombert 2000, and references therein). Yet large LSB galaxies have
the same total luminosity within them as ordinary Hubble sequence spirals (O’Neil, et al.
2004; Impey & Bothun 1997; Pickering, et al. 1997; Sprayberry, et al. 1995). On average
then, star formation cannot be too inefficient in these large LSB galaxies in spite of their
unevolved characteristics, else their integrated light would be significantly less then in their
HSB counterparts.
In an effort to better understand this enigmatic group of galaxies and their evolutionary
status, we recently conducted a 21-cm survey to discover a larger nearby sample of such
objects (O’Neil, van Driel, & Schneider 2006; O’Neil, et al. 2004). We succeeded in identi-
fying about 25 candidates within the redshift range 0.04 < z < 0.08, whose combined HI
and optical properties suggest them to be large LSB galaxies. We obtained B, R, and Hα
imaging of 19 of these galaxies at Lowell Observatory to confirm whether these candidates
are indeed LSB galaxies, and to obtain a dataset of their fundamental parameters. These
observations are presented here, and interestingly, none of the galaxies ultimately turned out
to be LSB galaxies by the strict conventional definition; we discuss this result below in § 4.
However, these galaxies still represent a sample whose surface brightnesses are below average,
and whose properties are intermediate between those of the bona fide massive LSB galaxies,
and ordinary HSB galaxies. In this work, we quantify and parameterize the fundamental
properties of this sample of large, “lower surface brightness” galaxies.
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2. Galaxy Sample
There are three ways that disk galaxy surface brightness can be measured or quantified –
using a surface brightness profile and fitting an exponential disk to derive the central surface
brightness; measuring an average surface brightness within a given isophotal diameter; and
measuring the surface brightness of the isophote at the 1/2 light radius point (the effective
surface brightness). The latter two definitions suffer from the fact that the bulge light is
included in the surface brightness estimates, resulting in their prediction of the disk surface
brightness to be less accurate. As a result, the typical operational definition of an LSB
galaxy uses the first definition, and defines an LSB galaxy as one whose whose observed disk
central surface brightness is µB(0) ≥23.0 mag arcsec
−2. For reference, the Freeman value of
µB(0)=21.65 +/- 0.30 mag arcsec
−2 defines the distribution of central surface brightness, in
the blue band, for Hubble sequence spirals.
Regardless of the definition, without pre-existing high quality optical imaging of galaxies,
it is difficult to unambiguously identify a sample of disk galaxies that will turn out to be
LSB. With only catalog data available, one is driven to use the average surface brightness
and identify potential LSB galaxies as those whose average surface brightness is below some
threshold level.
All of the galaxies in this sample were identified as LSB by Bothun, et al. (1985) using
the magnitude and diameter values found in the Uppsala General Catalog (Nilson 1973), and
employing the general equation 〈µB〉 = mpg +5log(D)+8.63. Here, mpg is the photographic
magnitude of the galaxies, D is the diameter in arcminutes, and the constant, 8.63, is derived
from the conversion from arcminutes to arcseconds (8.89) and the conversion from mpg to mB
(-0.26, as used by Bothun, et al.) Bothun, et al. (1985) then made a cut-off to the galaxies
in their sample, requiring 〈µB〉 >24.0 mag arcsec
−2 to look for galaxies with lower surface
brightness disks, with the majority of the galaxies chosen having 〈µB〉 >25.0 mag arcsec
(The inclusion of a number of galaxies with 〈µB〉=24-25 mag arcsec
−2 was due to the 0.5mag
errors given in the UGC.)
The Bothun, et al. (1985) sample was further pared down by our desire to image large
LSB galaxies. That is, we wished to avoid the dwarf galaxy category entirely. To do this, we
required the galaxies to have MHI > 10
9 M⊙, W20 > 200 km s
−1, and/or MB < −19. These
criteria are sufficiently removed from the dwarf galaxy category to guarantee no overlap
between our sample and that category exists.
– 4 –
3. Observations & Data Reduction
Galaxies’ integrated broad-band colors represent a convolution of the mean age of the
stellar population, metallicity, and recent star formation rate; while measurements of Hα
luminosity provide a direct measure of the current star formation rate (SFR). With these
combined observations, is is possible to parameterize the current SFR relative to the overall
star formation history. As a result, these observations are widely used in many surveys
that target fundamental galaxy parameters, for example, SINGG (Meurer, et al. 2006) and
11HUGS (Kennicutt, et al. 2004), and others (e.g. Gavazzi, et al. 2006; Koopman & Kenney
2006; Helmboldt, et al. 2005).
19 galaxies were observed on 7-10 June, 2002 and 5-8 October, 2003 using the Lowell
1.8m Perkins telescope. The filter set used included Johnson B and R as well as three
Hα filters from a private set (R. Walterbos) with center frequency/bandwidths of 6650/75,
6720/35, 6760/75 Å. A 1065x1024 pixel Loral SN1259 CCD camera was used, giving a 3.3′
field of view and resolution of 0.196′′/pixel. Seeing in June, 2002, ranged from 1.8′′ - 2.4′′ and
from 1.4′′ - 2.2′′ for the October, 2003, observations. At least 3 frames, each shifted slightly
in position, were obtained for each object through each filter and were median filtered to
reduce the effect from cosmic rays, bad pixels, etc. All initial data reduction (bias and flat
field removal, image alignment, etc) was done within IRAF. The R band images were scaled
and used as the continuum images for data reduction purposes.
Corrections to the measured fluxes were made in the following way. Atmospheric extinc-
tion was obtained using the observational airmass and the atmospheric extinction coefficients
for Kitt Peak which are distributed with IRAF. Galactic extinction was corrected using the
values for E(B−V) obtained from NED, the reddening law of Seaton (1979) as parameter-
ized by Howarth (1983) (A(λ) = X(λ)E(B−V )) and assuming the case B recombination of
Osterbrock (1989) with RV=3.1 (O’Donnell 1994) (X(6563Å)=2.468). Contamination from
[NII] emission in the Hα images was corrected using the relationship derived by Jansen,
et al. (2000) and re-confirmed by Helmboldt, et al. (2004):
[NII]
= [−0.13± 0.035]MR + [−3.2 ± 0.90] ,
where MR is the absolute magnitude in the R band. Hα extinction was determined using
the equation found in Helmboldt, et al. (2004):
log (Hα)int = [−0.12± 0.048]MR + [−2.5± 0.96]
which was found through a linear least squares fitting to the A(Hα)int determined using
all galaxies in his sample with a measured Hβ flux. For this calculation, Helmboldt, et al.
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(2004) used the Hα to Hβ ratio measured by Jansen, et al. (2000), an assumed intrinsic ratio
of Hα
=2.85 (Case B recombination and T=104 K (Osterbrock 1989)), the extinction curve
of O’Donnell (1994), and RV=3.1 No correction for internal extinction due to inclination
was made for the B and R bands. It should be noted, though, that in a number of plots
inclination corrections were made to the B and R colors and central surface brightnesses, as
noted in the Figure captions. The corrections used in these cases are:
µ(0)λcorr = µ(0)
λ − 2.5Cλlog(b/a) (1)
mλcorr = m
λ −Aλ (2)
Aλ = −2.5log
1 + e−τ
λsec(i)
+ (1− 2f)
1− e−τ
λsec(i)
τλsec(i)
Here, CR,B=1 (Verheijen 1997); (b/a) is the ratio of the minor to major axis; f = 0.1 and
τR,B=0.40, 0.81 (Tully, et al. 1998; Verheijen 1997). Finally, a correction was applied to
account for the effect of stellar absorption in the Balmer line of
Fcor = Fobs
, (4)
where Fcor is the corrected and Fobs is the observed Hα flux, We is the measured equivalent
width and Wa is the equivalent width of the Balmer absorption lines. As we do not have
measurements for Wa, we estimated Wa to be 3±1 Å, based off the values found in Oey &
Kennicutt (1993); Roennback & Bergvall (1995); McCall, Rybski, & Shields (1985). Note
that this effect is potentially stronger in the diffuse gas than in the H II regions due to
the older stellar population likely lying in the diffuse gas. As a result we may still be
underestimating the total Hα flux in the diffuse gas within the galaxies. However, as the
diffuse gas fractions for these galaxies are extremely high (see Section 6, below), it is unlikely
that this effect is high.
Global parameters and radial profiles for the galaxies were determined primarily using
the routines available in IRAF (notably ellipse) and the results are given in Tables 1 and 2.
Galaxy images, surface brightness profiles, and color profiles are given in Figures 1 – 3. In
all cases the inclination and position angle for the galaxies were determined from the best fit
values from the B & R frames. These best fit values were then used for the ellipse fitting in
all four images (B, R, Hα and continuum with Hα subtracted), a practice which insures the
color profiles are obtained accurately and are not affected by, e.g. misaligned ellipses. The
same apertures were also used for all four images, with the apertures found through allowing
ellipse to range from 1 pixel (0.196′′) until the mean value in the ellipse reaches the sky value,
– 6 –
increasing geometrically by a factor of 1.2. Sky values were found through determining the
mean value in more than 100 5×5 sq. pixel boxes in each frame. The error found for the sky
was incorporated into all magnitude and surface brightness errors, which also include errors
from the determination of the zeropoint and the errors from the N II contribution to the Hα
(in the case of the Hα magnitudes).
The B and R surface brightness profiles of all galaxies were fit using two methods. First,
the inner regions of the galaxies’ surface brightness profiles was fit using the de Vaucouleurs
r1/4 profile
Σ(r) = Σeffexp
−7.669[(r/reff )1/4−1] → µ(r) = µeff + 8.327
, (5)
and the outer regions were fit by the exponential disk profiles
Σ(r) = Σ0exp
( rα) → µ(r) = µ0 + 1.086
. (6)
Additionally, we attempted to fit a disk profile (6) to both the inner and outer regions of the
galaxies’, to determine if a two-disk fit would better match the data (Broeils & Courteau
1997; de Jong 1996). Roughly one-fourth of the galaxies (5/19) were best fit (in the χ2-sense)
by the standard bulge+disk model. Another 47% of the galaxies were best fit by the two-disk
model. Of the remaining galaxies, 21% (4 galaxies) were best fit by a single disk, and one
galaxy (UGC 11840) could not be fit by any profile. The results from the fits are shown in
Table 3 and Figure 2, and an asterisk (*) is placed next to the best fit model. Note that
in a few cases (e.g. UGC 00189) only one model is listed in the Table. This is due to the
fact that in these cases the fitting using the other model proved to be completely unrealistic.
Finally, it should be noted that in all cases the same best-fit model was used for both the B
and R data.
The color profiles were similarly fit (using an an inverse error weighting) with a line to
both the inner and outer galaxy regions (Figure 3). Here, though, the “boundary radius”
was simply taken from the surface brightness profile fits, with the “boundary radius” being
defined as the radius where the inner and outer surface brightness fits crossed. If only one
(or no) fit was made to the surface brightness profile, then only one color profile was fit. In
a number of cases the difference in slope between the inner and outer galaxy regions was
less than the least-squares error for the fit. In these cases again only one line was fit for the
color profiles.
The HIIphot program (Thilker, Braun, & Walterbos 2000) was used both to determine
the shape and number of H II regions for each galaxy and also to determine the Hα flux for
each of these regions. The fluxes from the Hα, Hα-subtracted continuum, B, and R images
– 7 –
were measured in identical corresponding apertures, which are the H II region boundaries
defined by HIIphot. While HIIphot applies an interpolation algorithm across these apertures
to estimate the diffuse background in the Hα frames, we determined the background in other
bands from the median flux in an annulus around each H II region aperture. Results from
the analysis of the H II regions are given in Table 4, and sample H II regions are shown in
Figure 4. Errors for the Hα, SFR, and EW measurements are derived from the error values
reported with HIIphot. Errors for the B and R magnitudes, and colors, are derived from the
total sky and zeropoint errors, as well as the error in positioning of the HII regions. The
diffuse fraction errors are derived both from the total Hα flux errors and also include errors
in determining the total flux within the HII regions and for the entire galaxy. Finally, it
should be mentioned that the equivalent width (EW) was calculated simply as the ratio of
the Hα flux to Hα-subtracted continuum flux for a given region (or the whole galaxy).
The large distances to the observed galaxies (40 - 100 Mpc) results in many of the H II
region being blended together. As a result, any luminosity function derived for these objects
would be necessarily skewed towards larger HII regions (see Oey, et al. 2006). This can be
seen in the analysis done by Thilker, Braun, & Walterbos (2000) wherein the dependence of
the luminosity function found for M51 was examined. There one can clearly see the increase
in the number of high luminosity regions and subsequent reduction in the number of low
luminosity regions as the galaxy is ’moved’ to increasing distances. Examining their results
also shows that while the distribution of H II region luminosities changes with distance, the
total luminosity of the H II regions, as found by HIIphot, does not change significantly as the
galaxy moves from 10 Mpc to 45 Mpc. As a result, while determining luminosity functions
for the galaxies in this paper is not feasible due to the distances involved, derivations such
as the diffuse fraction are unaffected by distance. This fact is also supported by the SINGG
survey results (Oey, et al. 2006).
4. Surface Brightness
The distribution of central surface brightnesses found for the galaxies observed is shown
in Figure 5. As is plain from that Figure, the mean measured central surface brightness for
this sample, falls short of the definitions discussed in Section 2. Indeed only 4 galaxies in our
sample meet the operational definition of LSB galaxies as having µB(0) ≥ 23 mag arcsec
If we return to the Freeman value, however, we see that the operational definition of LSB
galaxies is 4.5σ from the value for Hubble sequence spirals, making it statistically extreme.
For the sample defined here, half have central surface brightnesses at least two sigma above
the Freeman value, a definition only 2.5% of the Freeman sample meets. As a result, while
– 8 –
the sample does not meet the operation criteria for LSB galaxies, we clearly do have a sample
with lower central surface brightnesses that would be found in the average Hubble sequence
galaxies.
It should be pointed out here that the main scientific focus of Bothun, et al. (1985)
was not oriented toward producing a representative sample of LSB galaxies as detected on
photographic surveys (that focus did not occur until Schombert & Bothun 1988), but rather
toward identifying cataloged galaxies for 21-cm based redshift determinations. The galaxies
were chosen to have surface brightnesses that were too low for reliable optical spectroscopy
(assuming emission lines were not present). This was done as a test of the potentially large
problem of bias in on going optical redshift surveys in the time (see Bothun, et al. 1986).
In fact, the operational criteria for selecting the galaxies that were observed at Arecibo 20
years ago, lay in the knowledge that these cataloged galaxies were never going to be even
attempted in the optical redshift surveys of the time and this raised the very real possibility
of biased redshift distributions and an erroneous mapping of large scale structure.
In the original redshift measurements of Bothun, et al. (1985) a significant number of
candidate LSB galaxies were not detected at 21-cm within the observational redshift window
(approximately 0-12,000 km/s). Many of those non-detections would later turn out to be
intrinsically large galaxies located at redshifts beyond 12,000 km/s (see O’Neil, et al. 2004).
As we are interested here in the Hα properties of galaxies with large, relatively LSB disks,
these initial non-detections comprise the bulk of our sample.
Surface photometry of this sample not only provides detailed information regarding
the galaxies’ surface brightness and color distributions, but it also probes the efficacy of
the Bothun, et al. (1985) average surface brightness criteria for selecting LSB disks. Here,
we used the magnitudes and diameters obtained in this study (Table 1) with two different
equations for determining a galaxy’s average surface brightness within the D25 radius. The
first equation used is that of Bothun, et al. (1985)
〈µ25〉 = m25 + 5log(D25) + 3.63 (7)
and the second is a modified version of the above equation from Bottinelli, et al. (1995)
which takes the galaxies’ inclination into account:
〈µ25〉 = m25 + 5log(D25) + 3.63− 2.5log
kR−2C + (1− k)R(0.4C/K)−1
. (8)
In both equations, m25 and D25 are the magnitude and diameter (in units of 0.1
′) at the
µ=25.0 mag arcsec−2 isophote, R is the axis ratio (a/b), and C is defined as (logD/logR)
and is fixed at 0.04 (Bottinelli, et al. 1995). Finally, k (the ratio of the bulge-to-disk lumi-
nosity) and K (a measure of how the apparent diameter changes with surface brightness at
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a given axis ratio) are dependent on the revised de Vaucouleurs morphological type (T) as
follows (Simien & de Vaucouleurs 1986; Fouqué & Paturel 1985):
T=1 → k=0.41; T=2 → k=0.32; T=3 → k=0.24; T=4 → k=0.16; T=5 → k=0.09; T=6 →
k=0.05; T=7 → k=0.02; T≥8 → k=0.0;
K = 0.12− 0.007T if T < 0; K = 0.094 if T ≥ 0.
The values for k at T≥8 are extrapolated from fitting the Simien & de Vaucouleurs (1986)
values.
The results of equations 7 and 8, plotted against the galaxies’ central surface brightness
both uncorrected and corrected for inclination, are shown in Figures 6 and 7, respectively.
The difference between the two plots is small, with neither equation doing an excellent job in
predicting when a disk’s central surface brightness will be low. The two equations (Bothun,
et al. (1985) and Bottinelli, et al. (1995)) have roughly the same fit (in the χ2 sense), which
at first appears surprising. It is likely that uncertainties in the inclination measurements and
morphological classification of the galaxies have increased the scatter in the Bottinelli, et al.
(1995) equation, increasing the scatter in an otherwise more accurate equation. As a result,
while the Bottinelli, et al. (1995) may indeed be the most accurate, the simpler equation is
equally as good to use in most circumstances as it involves fewer assumptions.
The second fact that is readily apparent in looking at Figures 6 and 7 is that with the
new measurements of magnitude and diameter, none of the galaxies in our sample meet
the criterion laid out by Bothun, et al. (1985) for an LSB galaxy. That is that none of
the galaxies in this sample have 〈µ25〉 >25 mag arcsec
−2. As Bothun, et al. (1985) listed
all of these objects as having 〈µ25〉 >25 mag arcsec
−2 using the magnitudes and diameters
provided by the original UGC measurements, this shows that the UGC measurements indeed
predicted fainter magnitudes/larger values for D25 than is found with more sophisticated
measurement techniques. Additionally, it is good to note that the trends shown in Figures 6
and 7 indicate that any galaxy which met the 〈µ25〉 >25 mag arcsec
−2 criteria would be
highly likely to also have µ(0) >23 mag arcsec−2.
In these days of digital sky surveys it is difficult to appreciate the immense undertaking
that defines the UGC catalog. Anyone who has looked at the Nilson selected galaxies on
the Palomar Observatory Sky Survey (POSS) plates with a magnifying eyepiece really has
to marvel that Nilson’s eye saw objects at least one arcminute in diameter. It is thus not
surprising that, at the ragged end of that catalog, many of the listed UGC diameters are
systematically high. Cornell, et al. (1987) made a detailed diameter comparison between
diameters as obtained from high quality CCD surface photometry and the estimates made
by Nilson (1973). They compared the diameter at the 25.0 mag arcsec−2 isophote in CCD
– 10 –
B images to the tabulated diameter in the UGC. The study, based on approximately 250
galaxies, identified two sources of systematic error (neither of which are surprising). First,
galaxies with reported diameters less than 2′ typically had D25,B as measured by the CCD
images that were 15-25% smaller. Second, Cornell, et al. (1987) found a systematic bias as a
function of surface brightness in the sense that lower surface brightness galaxies had a higher
number of overestimated diameters in the UGC than higher surface brightness galaxies. It
should also be noted that the majority of the galaxies in this study lie at low Galactic latitude.
This seems to be a perverse consequence that there is a large collection of galaxies between
7,000 – 10,000 km s−1 (where the diameter criterion in the UGC yields a relatively large
physical size) located at relatively low galactic latitude. Nominal corrections for galactic
extinction made by Bothun, et al. (1985) turned out to underestimate the extinction as
shown by later published extinction maps. In some cases, the differences were as large
as one magnitude. The combination of these facts with the very uncertain magnitudes of
many of these galaxies (see Bothun & Cornell 1990), it is not surprising that the measured
average surface brightness could easily be 1-1.5 magnitudes higher than the average surface
brightness that has been estimated from the UGC catalog parameters (roughly 40% of this
comes from systematic magnitude errors and 60% from the diameter errors discovered by
Cornell, et al. (1987)).
5. Morphology & Color
All of the galaxies observed have large sizes (3αB = 10 – 54 kpc), bright central bulges,
and well defined spiral structure (Figure 1). In most cases the galaxies can be described as
late-type systems (Sbc and later). There are, though, a number of exceptions to this rule.
Three of the galaxies, UGC 00023, UGC 07598, and UGC 11355 (Sb, Sc, and Sb galaxies,
respectively) have clear nuclear bars. UGC 08311, classified as an Sbc galaxy, is clearly in
the late stages of merging with another system. In this case the LSB classification of the
galaxy is likely bogus, as the apparently LSB disk is likely just the remnant the merging
process and will disappear as the galaxy compacts after the merging process. UGC 8904 is
given a morphological type of S? with both NED and HYPERLEDA, yet the faint spiral
arms surrounding it indicate its should be properly classified as an Sbc system. UGC 12021
is, like UGC 00023, listed as an Sb galaxy. Finally, UGC 11068 has a faint nuclear ring
which is most readily visible in the B image.
The differences between the galaxies becomes more apparent when the Hα images are
examined. Hodge & Kennicutt (1983) classify the radial distribution of H II regions in
spiral galaxies into three broad categories – galaxies with H II region surface densities which
– 11 –
decrease with increasing radius, galaxies with oscillating H II region surface densities, and
galaxies with ring-like H II density distributions. To these categories we would add a fourth,
to include those galaxies with no detectable H II regions.
The first category of Hodge & Kennicutt (1983) is also the most common, as it includes
all galaxies with generally decreasing radial densities of Hα. In the Hodge & Kennicutt (1983)
sample this category is dominated by Sc – Sm galaxies but contains all Hubble types. In our
sample, this category includes both galaxies with and without significant Hα emission in the
spiral arm regions. This group includes UGC 00023, UGC 00189, UGC 02588, UGC 02796,
UGC 03119, UGC 03308, UGC 07598, and UGC 12021. Interestingly, of the galaxies listed
above, 4/8 are Sb/Sbc galaxies and 3/8 are Sc-Sm galaxies. (The last galaxy, UGC 02588,
is an irregular galaxy.)
The second category of Hodge & Kennicutt (1983), galaxies with oscillating densities,
is dominated in their sample of Sb galaxies. Only a few of the galaxies in this sample fall
into this category, 80% of which are also Sb/Sc galaxies. These are UGC 02299, UGC 08311,
UGC 08904, UGC 11355, and UGC 11396. These galaxies all have a concentration of star
formation seen in the nuclear regions and then clumps of star formation spread through the
spiral arms, typically accompanied by diffuse Hα also spread throughout the arms.
The third category of Hodge & Kennicutt (1983) is dominated by early-type galaxies,
of which we have none in our sample. Nonetheless we have three galaxies which fall into this
category – UGC 08644, UGC 10894, and UGC 11617. All three have H II regions spread
throughout their disks, with no central concentration near the galaxies’ nuclei. In fact, the
three brightest star forming regions within UGC 08644 all lie with the spiral arms, and are
visible in all three filters. In contrast, both UGC 11617 and UGC 10894 have no bright H II
regions, but instead have a large number of diffuse H II regions, with the brightest (as listed
in Table 2) receiving that designation simply due to its size.
The fourth category of galaxies contains UGC 01362, UGC 11068, and UGC 11840,
none of which have detectable Hα. In the case of UGC 01362 and UGC 11840 this is not too
surprising as the galaxies are dominated by a bright nucleus, and their surrounding spiral
arms are extremely faint in both R and B. As a result, any Hα which may exist in the
galaxies’ disks is too diffuse to be detected. UGC 11068, though, has both a well defined
nucleus and a clear spiral structure extending out to a radius of ∼13 kpc (3α). Yet no Hα
can be detected in this galaxy. This may mean that UGC 11068 is in a transition state for
its star formation, with no ongoing star formation yet with enough recent activity that the
spiral arms remain well defined.
Perhaps the most intriguing galaxy of our sample is UGC 11355. This galaxy was
– 12 –
placed in Category 2, above, as it has a bright nucleus and clumpy disk in the Hα image.
The B and R band images of UGC 11355 show a galaxy with a simple Sbc morphology.
The Hα image, though, shows a distinct star forming ring. The ring is at a very different
inclination from the rest of the galaxy (i=49◦ for the ring and 73◦ for the galaxy as a whole),
and lies approximately 2.6 kpc in radius from the center of the galaxy, measured along the
major axis. As the B and R images show no indication of a ring morphology this indicates
unusually strong star formation in the ring. It is also useful to note the presence of a bar in
UGC 11355 – shown more clearly in Figure 8. The fact that the inclination of the ring is
significantly different from that of the rest of the galaxy suggests the ring a tidal effect due
to an interaction, such as a small satellite galaxy being cannibalized by UGC 11355, or the
influence of CGCG 143-026, 14.9′ and 68 km s−1 away.
It is interesting to note that the Hα morphology of the galaxies does not appear to
correlate with the galaxies’ color profile (Figure 3). The galaxy with the steepest slope in
the color profile is UGC 08644 which has only a few H II regions in its outer arms. The other
galaxies with steep color profiles are UGC 00023 and UGC 8904, which have a bright knot
of star formation in the nucleus and faint Hα spread throughout their arms, and UGC 11840
and UGC 11068 both of which have no detectable Hα. The galaxies with the shallowest slopes
similarly show no correlation between their color profiles and morphology. This suggests that
the current star formation in these galaxies is largely independent of the past star-formation
history, although this result should be confirmed with better, extinction-corrected, data.
6. Star Formation
Figures 10 – 17 compare the properties of the H II regions and emission of our galaxy
sample. Where possible, measurements from other samples of late-type galaxies are also
shown (Kennicutt & Kent 1983; Jansen, et al. 2000; Helmboldt, et al. 2005; Oey, et al.
2006). Examining the figures it is clear that the overall properties of our sample are similar
to those of other late-type (Sbc-Sc) galaxies. That is, the values for the individual H II region
luminosities are similar to those reported by Helmboldt, et al. (2005) and Kennicutt & Kent
(1983) (Figure 10) while the global Hα equivalent width (EW) and global star formation
rates match those seen by all three comparison samples (Figures 11, 12).
We should note that as discussed in § 3 our sample suffers from having many of the H II
regions blended together as a result of the distance to our galaxy samples. As a result, it
is highly likely that in the comparisons of the luminosities for the galaxies’ individual H II
regions the luminosities (Figure 10) from our sample are artificially higher then those in the
other sample, potentially by a factor of 3 or more. This fact does not alter the results of this
– 13 –
section, but it is the likely explanation for the slightly higher than average values found for
L3 in Figure 10.
To examine the total amount of gas found within the H II regions compared with that
found in the diffuse Hα gas, we need to determine the galaxies’ Hα diffuse fraction, defined
here as the ratio of Hα flux not found within the defined H II regions to the total Hα flux
found for the entire galaxy. Examining Figures 13 and 14, as well as Tables 2 and 4, reveals
an interesting fact – while the global SFR for these galaxies is fairly typical (0.3 – 5 M⊙/yr),
the combined SFR from the galaxies’ H II regions is a factor of 2 – 10 smaller. That is, on
average the majority of the Hα emission and thus the majority of the star formation in the
observed galaxies comes not from the bright knots of star formation but instead from the
galaxies’ diffuse Hα gas. This is in contrast to the behavior seen from typical HSB galaxies,
as evidenced by the data of Oey, et al. (2006) in Figure 13. We note that blending and
angular resolution effects appear to be relatively unimportant in estimating the fraction of
diffuse Hα emission. Oey, et al. (2006) demonstrate this by showing no systematic changes
in measured diffuse fractions as a function of distance up to almost 80 Mpc, and inclination
angle, for their sample of 100+ SINGG survey galaxies.
While at first glance the higher diffuse Hα fractions found for these galaxies seems
surprising, recent GALEX results of the outer edges of M83, a region whose environment
closely resembles that of the disks of massive LSB galaxies, also show considerable star
formation outside the H II regions in that part of the galaxy (Thilker, et al. 2005). Similarly,
Helmboldt, et al. (2005) found a slight trend with lower surface brightness galaxies having
higher diffuse fractions than their higher surface brightness counterparts.
The fact that these galaxies have higher Hα diffuse gas fractions raises an interesting
question. Typically diffuse gas is believed to be ionized by OB stars lying within density-
bounded H II regions. The problem of transporting the ionizing photons from these regions
to the diffuse gas is extreme in these cases, as there would need to be a very large number
of density-bound H II regions leaking ionizing photons to ionize the quantity of diffuse
gas seen here. (See the more detailed discussion in Hoopes, Walterbos, & Bothun 2001,
which also discusses shock heating from stellar winds and SNe as ionization sources.) An
alternative suggestion is that field OB stars are also ionizing the diffuse gas, as was suggested
by Hoopes, Walterbos, & Bothun (2001). This would imply a different stellar population
within and without the H II regions, as it would likely be the later OB types (B0–O9) which
either escape the H II regions or survive the regions’ destruction. We note Oey, King, &
Parker (2004) predict a modest increase in the fraction of field massive stars in galaxies with
the lowest absolute star-formation rates. Scheduled GALEX observations of a subset of our
observed galaxies may shed light on the underlying stellar population in the galaxies’ diffuse
– 14 –
stellar disks.
Finally, it is elucidating to look for any trends between the global and regional properties
of the galaxies and their SFR and Hα content. Figure 15 plots the galaxies’ central surface
brightness (in both B and R) against the galaxies’ total star formation rate. While the error
bars make defining any trend difficult, there certainly appears to be a decrease in the global
SFR with decreasing central surface brightness, similar to the trends seen in other studies
(e.g. van den Hock, et al. 2000; Gerritsen & de Blok 1999). Figure 16 shows the galaxies’
gas-to-luminosity ratios plotted against both their global equivalent width and diffuse Hα
fraction. In both cases, no trend can be seen with our data, although the small number of
points available make any diagnosis difficult. Combined with the other datasets, though,
we can see a general trend toward higher equivalent widths with increasing MHI/LB, but
surprisingly no trend between gas fraction and the galaxies’ diffuse Hα fraction is visible.
This lack of correlation is also seen by Oey, et al. (2006). The last trend which can be seen is a
rough correlation between the galaxies’ global color and star formation rate (Figure 11), with
redder galaxies having higher SFR, a fact which may be a reddening effect. The individual
Hα regions, however, show no such trend (Figure 17).
7. Conclusion
The sample of 19 galaxies observed for this project were chosen to be large galaxies
with low surface brightness disks. The surface brightness measurements for this sample
were obtained originally through the UGC measurements through determining the galaxies’
average surface brightness within the µ=25 mag arcsec−2 isophote. The relation employed
to determine the galaxies’ average surface brightness (Equation 7) has shown itself it be a
good predictor of a galaxy’s central surface brightness. But for a wide variety of reasons the
UGC measurements were not sufficient to insure the galaxies contained within this catalog
have true LSB disks, underscoring the difficulty in designing targeted searches for large LSB
galaxies.
Nonetheless, the sample of galaxies observed for this project have lower surface bright-
nesses than is found for a typical sample of large high surface brightness galaxies. In most
other aspects the galaxies appear fairly ‘normal’, with colors typically B−R=0.3−0.9, mor-
phological types ranging from Sb – Irr, and color gradients which typically grow bluer toward
the outer radius. However, the galaxies have both higher gas mass-to-luminosity fractions
and diffuse Hα fractions than is found in higher surface brightness samples. This raises
two questions. First, if the SFR for these galaxies has been similar to their higher sur-
face brightness counterparts through the galaxies’ life, why do the lower surface brightness
– 15 –
galaxies have higher gas mass-to-luminosity ratios? Second, why do these galaxies have a
higher fraction of ionizing photons outside the density-bounded H II regions then their higher
surface brightness counterparts?
The answer to the first question posed above likely comes from the difference between
the studied galaxies’ current and historical SFR. As these galaxies have on average and lower
metallicities (de Naray, McGaugh, & de Blok 2004; Gerritsen & de Blok 1999) than their
higher surface brightness counterparts, it is likely that the galaxies’ SFR has not remained
constant throughout the their lifetimes. Indeed the simplest explanation for the current
similar SFRs and higher gas mass-to-luminosity ratios for the studied galaxies than for
their higher surface brightness counterparts is that the galaxies’ past SFR was significantly
different than is currently seen. In fact, the measured properties would be expected if
the galaxies in this study have episodic star formation histories, with significant time (1-3
Gyr) lapsing between SF bursts, as has been conjectured for LSB galaxies in the past (e.g.
Gerritsen & de Blok 1999). Such a star formation history would help promote significant
changes in the galaxies’ mean surface brightness and allow an individual large disk galaxy
to appear as either (a) a relatively normal Hubble sequence spiral, (b) a large, lower surface
brightness disk, or (c) perhaps even a lower surface brightness disk if the time between
episodes is sufficiently large, depending on the elapsed time since the last SF burst. The
final answer to this may be found when an answer to the second question, determining why
the diffuse fractions for the studied galaxies is higher than for similar HSB galaxies, is also
found. Irregardless, what is clear is that the studied sample shows a clear bridge between
the known properties of high surface brightness galaxies and the more poorly understood
properties of their very low surface brightness counterparts, such as Malin 1.
Thanks to Joe Helmboldt for his help in getting the HIIphot program running with LSB
galaxies and to Rene Walterbos for his loan of the Hα filters. MSO acknowledges support
from the National Science Foundation, grant AST-0448893.
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This preprint was prepared with the AAS LATEX macros v5.2.
Table 1. Global Properties of Galaxies – B & R Measurements
Galaxy RAa Deca Vela Typea mb Mb D25
c 〈µ〉d mb Mb D25
c 〈µ〉d rb B−Rb ie
[J2000] [J2000]
km s−1
[mag] [Mag] [′′]
mag/′′2
[mag] [Mag] [′′]
mag/′′2
[′′] [◦]
UGC 00023 00 04 13.0 10 47 25 7787 3 14.4 (0.1) -20.7 (0.1) 71 23.4 13.0 (0.1) -22.1 (0.1) 86.7 22.4 40 1.4 (0.2) 52
UGC 00189 00 19 57.5 15 05 32 7649 7 15.0 (0.2) -20.1 (0.2) 84 24.4 13.8 (0.1) -21.3 (0.1) 116.1 23.8 57 1.2 (0.2) 67
UGC 01362 01 52 50.7 14 45 52 7918 8.8 16.9 (0.2) -18.2 (0.2) 30 24.3 15.6 (0.1) -19.5 (0.1) 42.1 23.5 23 1.3 (0.3) 0
UGC 02299 02 49 07.8 11 07 09 10253 8 15.4 (0.2) -20.3 (0.2) 59 24.0 14.5 (0.4) -21.2 (0.4) 65.1 23.3 33 0.9 (0.4) 32
UGC 02588 03 12 26.5 14 24 27 10093 9.9 15.8 (0.2) -19.9 (0.2) 39 23.6 14.7 (0.1) -20.9 (0.1) 50.1 23.0 28 1.1 (0.2) 28
UGC 02796 03 36 52.5 13 24 24 9076 4 14.8 (0.2) -20.6 (0.2) 66 23.6 13.3 (0.1) -22.1 (0.1) 94.1 22.7 28 1.5 (0.2) 57
UGC 03119 04 39 07.7 11 31 50 7851 4 14.3 (0.2) -20.8 (0.2) 71 23.7 12.4 (0.1) -22.7 (0.1) ‡ 24.1 40 1.9 (0.2) 72
UGC 03308 05 26 01.8 08 57 25 8517 6 14.3 (0.3) -21.0 (0.3) 89 23.8 14.0 (0.2) -21.3 (0.2) 88.1 23.5 48 0.3 (0.4) 28
UGC 07598 12 28 30.9 32 32 52 9041 5.9 15.3 (0.1) -20.1 (0.1) 46 23.5 14.8 (0.1) -20.6 (0.1) 66.8 22.8 33 0.5 (0.2) 22
UGC 08311 13 13 50.8 23 15 16 3451 4.1 15.5 (0.1) -17.9 (0.1) 50 23.8 14.8 (0.1) -18.5 (0.1) 62.0 23.5 33 0.7 (0.2) 26
UGC 08644 13 40 01.4 07 22 00 6983 8 16.1 (0.2) -18.8 (0.2) 43 24.2 15.3 (0.2) -19.6 (0.2) 49.4 23.5 28 0.8 (0.3) 30
UGC 08904 13 58 51.1 26 06 24 9773 3.6 15.9 (0.1) -19.7 (0.1) 43 23.8 14.9 (0.1) -20.7 (0.1) 55.5 23.4 33 1.0 (0.2) 48
UGC 10894 17 33 03.8 27 34 29 6890 4 16.0 (0.3) -18.8 (0.3) 48 24.1 14.9 (0.3) -19.9 (0.3) ‡ 23.0 28 1.1 (0.4) 57
UGC 11068 17 58 05.0 28 14 38 4127 3.2 15.0 (0.2) -18.7 (0.2) 64 24.0 13.8 (0.1) -19.9 (0.1) 89.4 23.4 57 1.2 (0.2) 0
UGC 11355 18 47 57.0 22 56 33 4360 3.5 13.9 (0.3) -19.9 (0.3) 173 24.8 12.5 (0.1) -21.3 (0.1) ‡ 23.7 82 1.4 (0.3) 73
UGC 11396 19 03 49.5 24 21 28 4441 3.5 14.8 (0.3) -19.1 (0.3) 114 24.4 13.8 (0.2) -20.1 (0.2) 78.7 23.0 33 1.0 (0.4) 59
UGC 11617 20 43 39.3 14 17 52 5119 6.1 14.9 (0.2) -19.2 (0.2) 75 24.1 13.9 (0.2) -20.3 (0.2) 86.1 23.3 40 1.1 (0.3) 58
UGC 11840 21 53 18.0 04 14 50 7986 4 16.3 (0.1) -18.9 (0.1) 24 22.9 15.3 (0.1) -19.9 (0.1) 23.3 21.9 11 1.0 (0.2) 40
UGC 12021 22 24 11.6 06 00 12 4472 3 14.7 (0.2) -19.2 (0.2) 113 24.7 13.6 (0.1) -20.2 (0.1) 130.4 23.9 57 1.1 (0.2) 63
Note. — Errors are given in parenthesis.
aRA, Dec, velocity and type information obtained from NED, the NASA Extragalacitc Database. Galaxy types are defined in de Vaucouleurs, et al. (1991).
bMagnitudes and colors were obtained at the maximum usable radius, r. Corrections applied and error estimates are described in Section 3.
cD25 are the diameters for the 25 mag arcsec
−2 isophotes.
dAverage surface brightness, as defined by Equation 7 in Section 4.
eInclinations are simply the major to minor axis ratio of the galaxies, found through isophote fitting.
‡Isophotes did not reach 25 mag arcsec−2.
– 20 –
Table 2. Global Properties of Galaxies – Hα
Galaxy Hα flux ×10−13a EWb SFRc rd
erg cm−2 s−1
UGC 00023 5 (1) 22 (8) 4 (1) 28
UGC 00189 4 (1) 63 (33) 4 (1) 23
UGC 01362 · · · · · · · · · · · ·
UGC 02299 0.41 (0.09) 38 (13) 0.7 (0.2) 6
UGC 02588 0.7 (0.3) 32 (21) 1.1 (0.5) 13
UGC 02796 2.1 (0.7) 12 (4) 2.7 (0.9) 13
UGC 03119 7 (4) 20 (12) 6 (6) 28
UGC 03308 1.0 (0.3) 22 (8) 1.1 (0.3) 11
UGC 07598 1.3 (0.3) 60. (18) 1.8 (0.4) 19
UGC 08311 3.1 (0.6) 91 (37) 0.6 (0.1) 19
UGC 08644 · · · · · · · · · · · ·
UGC 08904 0.8 (0.2) 40 (13) 1.1 (0.3) 16
UGC 10894 0.4 (1) 34 (78) 0.4 (0.1) 9
UGC 11068 · · · · · · · · · · · ·
UGC 11355 8 (2) 30 (9) 2.7 (0.6) 28
UGC 11396 2.4 (0.8) 400 (2000) 0.8 (0.2) 16
UGC 11617 3.1 (0.7) 30 (2) 0.3 (0.2) 19
UGC 11840 · · · · · · · · · · · ·
UGC 12021 4 (1) 63 (32) 1.3 (0.4) 23
UGC 12289 · · · · · · · · · · · ·
Note. — Derivation of quantities are described in Section 3. Errors
are given in parenthesis.
aTotal Hα flux found within the radius centered on the (optical)
center of the galaxy and extending to the radius given in the last column.
Errors were determined in the same manners as for magnitudes, and are
given in Section 3.
bThe equivalent width was calculated simply as the ratio of the total
Hα flux to total Hα-subtracted continuum flux.
cSFR =
1.26×1041ergs−1
; from Kennicutt, Tamblyn, & Congdon
(1994).
dRadius at which the isophotal signal-to-noise went below 1σ.
– 21 –
Table 3. Fitted Galaxy Properties
inner outer
Galaxy Fit Filterb µeff/µ0
c Reff/α
e αf Boundaryg Fith
Type a
mag arcsec−2
mag arcsec−2
[′′] [′′] Error
∗ UGC 00023 Two Disk B 19.06 (0.33) 1.71 (0.02) 21.17 (0.21) 11.05 (0.06) 6.55 0.82
∗ UGC 00023 Two Disk R 17.30 (0.27) 1.72 (0.02) 19.58 (0.16) 9.58 (0.03) 6.93 0.53
UGC 00023 Bulge/Disk B 21.48 (1.98) 4.06 (0.23) 21.47 (0.51) 11.96 (0.11) 8.94 0.89
UGC 00023 Bulge/Disk R 19.20 (1.56) 3.07 (0.14) 19.82 (0.35) 9.96 (0.05) 8.79 0.58
∗ UGC 00189 Two Disk B 20.99 (0.08) 7.75 (0.06) 24.05 (1.85) 36.90 (2.11) 28.34 0.33
∗ UGC 00189 Two Disk R 19.53 (0.09) 6.13 (0.05) 21.66 (0.67) 18.84 (0.22) 24.92 0.97
∗ UGC 01362 One Disk B · · · · · · 23.06 (0.33) 7.77 (0.10) · · · 1.78
∗ UGC 01362 One Disk R · · · · · · 21.84 (0.15) 7.85 (0.04) · · · 0.46
∗ UGC 02299 Two Disk B 20.85 (0.10) 2.00 (0.16) 22.43 (1.76) 11.24 (0.04) 6.48 1.88
∗ UGC 02299 Two Disk R 19.84 (0.28) 2.21 (0.03) 21.46 (0.38) 10.50 (0.10) 7.23 0.99
UGC 02299 Bulge/Disk B 25.10 (2.85) 17.35 (1.95) 23.23 (2.18) 12.62 (0.27) 17.01 2.08
UGC 02299 Bulge/Disk R 24.00 (1.75) 17.76 (1.24) 22.30 (1.43) 10.37 (0.19) 0.00 1.18
UGC 02588 One Disk B · · · · · · 21.50 (0.08) 5.47 (0.02) · · · 1.40
UGC 02588 One Disk R · · · · · · 20.41 (0.07) 5.79 (0.02) · · · 1.12
∗ UGC 02588 Bulge/Disk B 25.75 (2.43) 16.16 (1.75) 22.61 (0.97) 8.78 (0.08) 6.94 1.18
∗ UGC 02588 Bulge/Disk R 23.77 (2.50) 7.86 (0.75) 21.09 (0.48) 7.41 (0.04) 4.57 0.82
∗ UGC 02796 Two Disk B 19.07 (0.34) 1.89 (0.03) 21.02 (0.60) 8.75 (0.14) 6.90 0.48
∗ UGC 02796 Two Disk R 17.52 (0.27) 2.19 (0.03) 19.66 (0.41) 8.69 (0.08) 8.59 0.68
UGC 02796 Bulge/Disk B 23.07 (2.01) 13.80 (1.10) 22.54 (4.11) 8.92 (0.45) † 0.71
UGC 02796 Bulge/Disk R 21.01 (0.90) 10.26 (0.31) 20.49 (1.06) 6.76 (0.22) † 0.21
UGC 03119 One Disk B · · · · · · 19.82 (0.07) 10.35 (0.03) · · · 0.75
UGC 03119 One Disk R · · · · · · 17.92 (0.05) 9.48 (0.01) · · · 0.49
∗ UGC 03119 Bulge/Disk B 22.05 (9.17) 3.54 (0.91) 20.03 (0.31) 11.51 (0.08) 3.50 0.61
∗ UGC 03119 Bulge/Disk R 21.07 (5.81) 6.53 (1.29) 18.16 (0.24) 10.10 (0.02) 3.71 0.28
∗ UGC 03308 Two Disk B 19.83 (2.25) 0.87 (0.04) 21.70 (0.18) 15.47 (0.16) 3.47 0.35
∗ UGC 03308 Two Disk R 19.06 (1.32) 0.92 (0.03) 20.99 (0.13) 11.60 (0.06) 3.58 0.25
UGC 03308 Bulge/Disk B 18.36 (10.1) 0.43 (0.08) 21.71 (0.23) 15.66 (0.18) 3.62 0.37
UGC 03308 Bulge/Disk R 18.70 (5.77) 0.67 (0.08) 21.03 (0.18) 11.90 (0.08) 3.85 0.20
UGC 07598 One Disk B · · · · · · 21.69 (0.07) 7.96 (0.02) · · · 0.75
UGC 07598 One Disk R · · · · · · 19.82 (0.08) 6.26 (0.02) · · · 1.12
∗ UGC 07598 Bulge/Disk B 16.02 (6.98) 0.21 (0.03) 21.84 (0.13) 8.60 (0.03) 3.25 0.52
∗ UGC 07598 Bulge/Disk R 15.78 (3.90) 0.40 (0.03) 20.18 (0.21) 7.34 (0.04) 4.06 0.49
∗ UGC 08311 Two Disk B 20.82 (0.09) 2.94 (0.02) 23.29 (0.66) 14.91 (0.26) 12.50 0.48
– 22 –
Table 3—Continued
inner outer
Galaxy Fit Filterb µeff /µ0
c Reff/α
e αf Boundaryg Fith
Type a
mag arcsec−2
mag arcsec−2
[′′] [′′] Error
∗ UGC 08311 Two Disk R 20.28 (0.08) 3.22 (0.03) 22.73 (0.71) 15.81 (0.32) 13.64 0.39
∗ UGC 08644 One Disk B · · · · · · 22.56 (0.14) 8.79 (0.06) · · · 0.55
∗ UGC 08644 One Disk R · · · · · · 21.19 (0.15) 6.40 (0.04) · · · 0.78
∗ UGC 08904 Two Disk B 20.43 (0.16) 2.56 (0.02) 22.94 (0.71) 10.07 (0.15) 11.22 0.69
∗ UGC 08904 Two Disk R 18.90 (0.16) 2.20 (0.01) 22.05 (0.49) 9.67 (0.10) 11.19 0.35
UGC 08904 Bulge Only B 23.99 (0.27) 12.55 (0.09) · · · · · · · · · 1.17
UGC 08904 Bulge Only R 21.83 (0.25) 6.95 (0.04) · · · · · · · · · 0.48
∗ UGC 10894 One Disk B · · · · · · 21.65 (0.10) 7.90 (0.04) · · · 0.44
∗ UGC 10894 One Disk R · · · · · · 20.26 (0.12) 6.69 (0.04) · · · 0.42
UGC 11068 Two Disk B 20.15 (0.65) 1.15 (0.02) 22.46 (0.13) 13.65 (0.07) 4.87 0.31
UGC 11068 Two Disk R 18.67 (0.54) 1.25 (0.02) 21.02 (0.14) 11.26 (0.05) 5.23 0.54
∗ UGC 11068 Bulge/Disk B 20.36 (0.18) 1.00 (0.00) 22.53 (0.11) 14.10 (0.06) 5.31 0.38
∗ UGC 11068 Bulge/Disk R 19.89 (2.52) 1.66 (0.10) 21.18 (0.25) 11.97 (0.07) 6.23 0.42
∗ UGC 11355 Two Disk B 19.20 (0.15) 3.34 (0.02) 21.61 (0.18) 32.30 (0.24) 14.23 0.86
∗ UGC 11355 Two Disk R 17.30 (0.16) 3.01 (0.02) 19.83 (0.13) 24.07 (0.10) 13.02 0.64
UGC 11355 Bulge/Disk B 23.29 (0.95) 19.99 (0.72) 22.13 (0.56) 38.81 (0.57) 26.27 1.13
UGC 11355 Bulge/Disk R 20.70 (0.94) 10.96 (0.37) 20.19 (0.35) 26.30 (0.18) 26.30 0.93
∗ UGC 11396 Two Disk B 20.34 (1.82) 1.14 (0.05) 22.08 (0.27) 22.66 (0.39) 4.46 0.95
∗ UGC 11396 Two Disk R 19.77 (2.17) 1.23 (0.08) 20.20 (0.17) 13.04 (0.10) 2.80 1.69
UGC 11396 Bulge/Disk B 20.52 (9.40) 0.97 (0.19) 22.10 (0.35) 23.03 (0.45) 4.75 0.93
UGC 11396 Bulge/Disk R 20.09 (3.23) 1.09 (0.32) 20.21 (0.23) 13.03 (0.11) 3.03 1.69
∗ UGC 11617 One Disk B · · · · · · 21.30 (0.07) 12.83 (0.05) · · · 0.69
∗ UGC 11617 One Disk R · · · · · · 20.08 (0.08) 10.99 (0.04) · · · 0.46
∗ UGC 11840 No Fit B · · · · · · · · · · · · · · · · · ·
∗ UGC 11840 No Fit R · · · · · · · · · · · · · · · · · ·
UGC 12021 One Disk B · · · · · · 20.73 (0.06) 11.05 (0.02) · · · 1.63
UGC 12021 One Disk R · · · · · · 19.41 (0.06) 10.26 (0.02) · · · 1.58
∗ UGC 12021 Bulge/Disk B 26.49 (1.28) 23.19 (0.00) 20.88 (0.16) 11.59 (0.04) 2.37 1.54
∗ UGC 12021 Bulge/Disk R 23.85 (0.32) 23.19 (0.00) 19.96 (0.18) 11.60 (0.03) 6.23 1.01
Note. — Derivation of quantities is described in Section 3. Errors are given in parenthesis.
aThe type of fit made to the surface brightenss profile – bulge+disk, two exponential disks, or one exponential disk.
– 23 –
bOptical filter for the data described within that row.
cEffective surface brightness (R1/4 bulge fit) or central surface brightness (exponential disk fit) for the inner disk fit. See Equations 5
and 6.
dEffective radius (R1/4 bulge fit) or scale length (exponential disk fit) for the inner disk fit. See Equations 5 and 6.
eCentral surface brightness for the outer exponential disk fit.
fScale length for the outer exponential disk fit.
gBoundary between the inner and outer fits, defined by where the fitted lines cross.
hχ2 error for the fits.
∗Best fit – used for all further analysis.
†Fitted lines for the bulge and disk components do not cross.
Table 4. Properties of Hα Regions
Galaxy Regiona Hα Flux ×10−15 Hα Luminosity×1038 SFRb EWc Bd Rd B−Rd Diffusee
erg cm−2 s−1
erg s−1
M⊙ yr
[mag] [mag] [mag] [%]
UGC 00023 1 70 (14) 760 (150) 0.6 (0.1) 23 (1) 16.9 (0.1) 15.2 (0.1) 1.7 (0.2) 0.82 (0.04)
UGC 00189 1 10 (2) 110 (20) 0.09 (0.02) 30 (2) 19.9 (0.6) 18.3 (0.5) 1.7 (0.7) 0.98 (0.01)
UGC 01362 0 · · · · · · · · · · · · · · · · · · · · · · · ·
UGC 02299 1 12 (2) 240 (50) 0.19 (0.04) 20 (1) 21 (2) 21 (2) 0. (3) · · ·
UGC 02299 2 15 (3) 260 (50) 0.21 (0.04) 14 (1) 18.2 (0.2) 17.1 (0.2) 1.1 (0.3) · · ·
UGC 02299 TOTAL 22 (3) 490 (70) 0.39 (0.06) · · · · · · · · · · · · 0.45 (0.06)
UGC 02588 0 · · · · · · · · · · · · · · · · · · · · · · · ·
UGC 02796 1 74.8 (15) 900 (200) 0.71 (0.1) 10 (1) 16.5 (0.2) 14.8 (0.4) 1.7 (0.4) 0.67 (0.01)
UGC 03119 1 151 (30) 1600 (300) 1.3 (0.3) 16 (1) 18.8 (0.1) 16.7 (0.1) 2 (0.1) 0.77 (0.02)
UGC 03308 0 · · · · · · · · · · · · · · · · · · · · · · · ·
UGC 07598 1 6 (1) 80 (17) 0.07 (0.01) 13 (1) 22.2 (0.7) 21.5 (0.7) 0.6 (1) · · ·
UGC 07598 2 4.7 (0.9) 60 (12) 0.05 (0.01) 12 (1) 22 (1) 20.2 (0.9) 2 (1) · · ·
UGC 07598 3 2.4 (0.5) 32 (6) 0.03 (0.01) 13 (1) 23 (1) 21 (1) 2 (2) · · ·
UGC 07598 4 6 (1) 80 (20) 0.06 (0.01) 12 (1) 21.1 (0.8) 19.6 (0.7) 2 (1) · · ·
UGC 07598 5 3.3 (0.7) 43 (9) 0.03 (0.01) 13 (1) 23 (2) 22 (2) 2 (3) · · ·
UGC 07598 6 6 (1) 80 (20) 0.06 (0.01) 11 (1) 21 (3) 22 (3) -1 (4) · · ·
UGC 07598 7 4.1 (0.8) 50 (10) 0.04 (0.01) 11 (1) 22 (1) 21 (2) 1 (4) · · ·
UGC 07598 8 1.3 (0.3) 15 (3) 0.012 (0.003) 9 (1) 25 (3) 24 (3) 1 (3) · · ·
UGC 07598 9 32 (6) 430 (90) 0.34 (0.07) 13 (1) 18 (0.1) 16 (0.1) 1.9 (3) · · ·
UGC 07598 10 1.4 (0.3) 20. (4) 0.015 (0.003) 16 (1) 21.8 (0.5) 20.4 (0.5) 1.3 (0.5) · · ·
UGC 07598 11 10 (2) 130 (30) 0.11 (0.02) 11 (1) 21.4 (0.5) 20 (0.6) 1.4 (0.8) · · ·
UGC 07598 12 4.8 (1) 60 (10) 0.05 (0.01) 11 (1) 22 (1) 20.2 (0.9) 2 (1) · · ·
UGC 07598 13 4.4 (0.9) 60 (10) 0.05 (0.01) 12 (1) 21.8 (0.8) 20.1 (0.7) 2 (1) · · ·
UGC 07598 14 5 (1) 70 (10) 0.05 (0.01) 11 (1) 22.7 (1) 21 (1) 1 (2) · · ·
UGC 07598 TOTAL 69 (6) 1200 (100) 0.96 (0.08) · · · · · · · · · · · · 0.5 (0.1)
UGC 08311 1 7 (1) 14 (3) 0.011 (0.002) 20. (1) 21.3 (0.7) 21 (0.9) 0. (1) · · ·
UGC 08311 2 100 (20) 240 (50) 0.19 (0.04) 53 (1) 18.8 (0.4) 18.4 (0.5) 0.3 (0.6) · · ·
UGC 08311 3 7 (1) 16 (3) 0.012 (0.003) 18 (1) 20.8 (0.1) 20. (1) 1 (1) · · ·
UGC 08311 4 17 (3) 38 (8) 0.03 (0.01) 29 (1) 0.9 (0.1) 20.5 (0.9) -19.6 (0.9) · · ·
UGC 08311 5 8 (2) 18 (4) 0.015 (0.003) 25 (1) 22 (2) 22 (2) 1 (3) · · ·
UGC 08311 6 140 (30) 320 (60) 0.26 (0.05) 38 (1) 16.9 (0.1) 16.3 (0.1) 0.6 (0.1) · · ·
UGC 08311 7 15 (3) 32 (6) 0.03 (0.01) 19 (1) 18.6 (0.1) 18.1 (0.2) 0.6 (0.2) · · ·
UGC 08311 TOTAL 270 (30) 680 (80) 0.54 (0.07) · · · · · · · · · · · · 0.2 (0.1)
UGC 08644 1 5.1 (1) 44 (9) 0.04 (0.01) 17 (1) 21 (0.1) 20. (1) 1 (1) · · ·
Table 4—Continued
Galaxy Regiona Hα Flux ×10−15 Hα Luminosity×1038 SFRb EWc Bd Rd B−Rd Diffusee
erg cm−2 s−1
erg s−1
M⊙ yr
[mag] [mag] [mag] [%]
UGC 08644 2 4.8 (1) 42 (8) 0.03 (0.01) 20. (1) 22 (0.5) 21.1 (0.6) 0.9 (0.8) · · ·
UGC 08644 TOTAL 8.3 (1) 90 (10) 0.07 (0.01) · · · · · · · · · · · · · · ·
UGC 08904 1 3.4 (0.7) 47 (9) 0.04 (0.01) 10. (1) 22.4 (0.8) 22 (1) 1 (1) · · ·
UGC 08904 2 0.7 (0.1) 12 (2) 0.01 (0.009) 23 (1) 23.6 (0.5) 23.0 (0.6) 0.7 (0.8) · · ·
UGC 08904 3 1.2 (0.3) 20. (4) 0.02 (0.02) 16 (1) 24.3 (0.7) 24 (1) 1 (1) · · ·
UGC 08904 4 0.8 (0.2) 12 (2) 0.01 (0.009) 10. (1) 23.8 (0.4) 24 (1) 0. (1) · · ·
UGC 08904 5 0.7 (0.1) 9 (2) 0.01 (0.007) 10. (1) 24.5 (0.9) 24 (1) 1 (2) · · ·
UGC 08904 6 32 (6) 600 (100) 0.45 (0.09) 25 (1) 17.7 (0.1) 16.3 (0.1) 1.4 (0.1) · · ·
UGC 08904 TOTAL 33 (6) 700 (100) 0.53 (0.09) · · · · · · · · · · · · 0.6 (0.1)
UGC 10894 1 1.2 (0.2) 11 (2) 0.009 (0.002) 39 (3) 24 (2) 23 (1) 1 (2) · · ·
UGC 10894 2 1.4 (0.3) 13 (3) 0.010 (0.002) 37 (2) 22.1 (0.5) 21.3 (0.6) 0.8 (0.8) · · ·
UGC 10894 3 13 (3) 110 (20) 0.09 (0.02) 27 (1) 18.9 (0.2) 17.2 (0.1) 1.8 (0.2) · · ·
UGC 10894 4 1.9 (0.4) 18 (4) 0.014 (0.003) 31 (2) 23 (1) 22 (1) 1 (2) · · ·
UGC 10894 5 1.8 (0.4) 16 (3) 0.013 (0.003) 35 (2) 22.8 (0.7) 21.9 (0.7) 1 (1) · · ·
UGC 10894 6 2.8 (0.6) 26 (5) 0.020 (0.004) 36 (2) 22.4 (0.7) 22 (1) 0. (1) · · ·
UGC 10894 7 3.9 (0.8) 35 (7) 0.03 (0.01) 31 (2) 22.6 (0.8) 21.6 (0.8) 1 (1) · · ·
UGC 10894 TOTAL 23 (2) 230 (25) 0.18 (0.02) · · · · · · · · · · · · 0.6 (0.3)
UGC 11068 0 · · · · · · · · · · · · · · · · · · · · · · · ·
UGC 11355 1 180 (40) 600 (100) 0.48 (0.1) 17 (1) 19.7 (0.4) 18.2 (0.4) 1.5 (0.6) 0.87 (0.08)
UGC 11396 1 25 (5) 90 (20) 0.07 (0.01) 21 (1) 22 (1) 18.1 (0.2) 4 (1) 0.91 (0.01)
UGC 11617 1 9 (2) 40 (8) 0.07 (0.01) 12 (1) 21 (1) 21 (0.6) 0. (1) · · ·
UGC 11617 2 21 (4) 90 (20) 0.04 (0.01) 13 (1) 21 (2) 20. (2) 1 (3) · · ·
UGC 11617 3 13 (3) 50 (10) 0.04 (0.01) 13 (1) 22 (2) 20. (3) 1 (3) · · ·
UGC 11617 4 12 (2) 47 (9) 0.09 (0.01) 11 (1) 21 (2) 21 (1) 1 (3) · · ·
UGC 11617 5 26 (5) 110 (20) 0.03 (0.02) 14 (1) 21.5 (0.6) 21 (4) 1 (4) · · ·
UGC 11617 6 10 (2) 42 (8) 0.03 (0.01) 12 (1) 21.5 (0.2) 20.8 (0.7) 0.7 (0.7) · · ·
UGC 11617 TOTAL 69 (6) 390 (30) 0.31 (0.03) · · · · · · · · · · · · 0.8 (0.1)
UGC 11840 0 · · · · · · · · · · · · · · · · · · · · · · · ·
UGC 12021 1 13 (3) 50 (10) 0.04 (0.01) 31 (2) 19.2 (0.4) 17.8 (0.3) 1.4 (0.5) 0.97 (0.01)
Note. — Derivation of quantities is described in Section 3. Errors are given in parenthesis.
aInternal numbering scheme for the HII regions found by HIIphot.
bSFR for the region, defined as SFR =
1.26×1041erg s−1
cEquivalent width was calculated simply as the ratio of the total Hα flux to total Hα-subtracted continuum flux.
dTotal B and R magnitudes and colors within the HII regions
eDiffuse fraction found for the galaxy, defined defined here as the ratio of the fraction of Hα flux not found within the defined Hα regions to the
total Hα flux found for the entire galaxy.
†Due to both a (masked) star near the center of this galaxy and a (masked) CCD flaw (bad column) which also runs through the center of the
galaxy, a number of H II regions which should have been identified by HIIphot were not, artificially rasing the diffuse fraction on this galaxy, possibly
by as much as 20-30%.
– 27 –
Fig. 1.— Grey scale images of the observed galaxies. Figure available through the published
AJ paper or online at http://www.gb.nrao.edu/∼koneil.
Fig. 2.— Surface brightness profiles for all galaxies observed. The dash-dotted lines show
the inner fit, the dashed lines show the outer fit, and the solid lines show the combined fits.
Both the B (blue - bottom) and R (red - top) profiles are shown. Figure available through
the published AJ paper or online at http://www.gb.nrao.edu/∼koneil.
Fig. 3.— Color profiles for all the galaxies observed. Here the inner fits (when made) are
shown by a dashed line and the outer fits are shown by a solid line. Figure available through
the published AJ paper or online at http://www.gb.nrao.edu/∼koneil.
– 28 –
UGC 00189 UGC 10894
UGC 07598 UGC 08904
Fig. 4.— Example images showing the H II regions found by HIIphot for the galaxies
UGC 00189, UGC 10894, UGC 07598, and UGC 08904. The H II regions are outlines in
white. In the case of UGC 10894 two regions which were masked due to the presence of stars
can also be seen, outlined by the square white boxes.
– 29 –
18 20 22 24
µB,R(0) [mag arcsec
19 20 21 22 23 24 25
µB,R(0) [mag arcsec
Fig. 5.— (Left) Histogram showing the distribution of central surface brightnesses for the
observed galaxies. The (red) dashed line shows the R-band data and the (blue) solid line
shows the B-band data. (Right) Plot of the observed central surface brightness against scale
length of the outer disk. Here, the R-band data is demarcated by (red) open circles while
the B-band data uses (blue) filled circles.
22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5
<µB,R> [mag arcsec
22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5
<µB,R> [mag arcsec
Fig. 6.— Plots comparing the measured central surface brightnesses with the average surface
brightness for the galaxies, as defined by Equation 7 (left) and Equation 8 (right) and using
the magnitude and diameter values found herein. The R-band data is demarcated by (red)
open circles while the B-band data uses (blue) filled circles.
– 30 –
22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5
<µB,R> [mag arcsec
22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5
<µB,R> [mag arcsec
Fig. 7.— Plots comparing the measured central surface brightnesses, corrected for inclina-
tion, with the average surface brightness for the galaxies, as defined by Equation 7 (left) and
Equation 8 (right) and using the magnitude and diameter values found herein. The R-band
data is demarcated by (red) open circles while the B-band data uses (blue) filled circles.
Fig. 8.— Images of UGC 11355 with the stretch altered to show the galaxy’s nuclear bar
(left - R-band image) and star forming ring (right - Hα image). In both images the ellipse
shows the shape and size of the star forming ring. The images are 1.0′ across.
– 31 –
0 1 2 3 4
MHI/LR,B global [MO  · /LO  · ]
Fig. 9.— Gas mass to B and R-band luminosity ratios plotted against the global star
formation rate for the galaxies. The (blue) filled circles are for the B-band data and the
(red) open circles are for the R-band data.
– 32 –
−14 −16 −18 −20 −22
MB, global
S0 − Sa
Sab − Sb
Sbc−Sc
Sm−Im
Fig. 10.— Total B magnitude plotted against the average luminosity of the brightest three
Hα regions. (If less than three regions were found, the average of all H II regions was used.)
The filled (red) symbols are the data from our observations; the filled (blue) symbols are
from Helmboldt, et al. (2005); and the open (black) symbols are from Kennicutt & Kent
(1983).
– 33 –
−0.5 0.0 0.5 1.0 1.5 2.0 2.5
(B − R)global
Our Data
Helmboldt, et.al 2004
Jansen, et.al 2000
−0.5 0.0 0.5 1.0 1.5 2.0 2.5
(B−R)global
Our Data
Jansen, et.al 2000
Fig. 11.— Global color versus equivalent width (left) and star formation rate (right). To
insure any trends (or lack) remain the same, the data from this paper is shown both without
inclination correction (black) and with (gray). Note that inclination corrections are described
in Section 3. As the global SFR was not available for the Helmboldt, et al. (2005) data, it
is not shown on the right.
−17 −18 −19 −20 −21 −22 −23
MB,global
Our Data
Kennicutt & Kent 1983
Helmbolt, et.al 2004
−16 −17 −18 −19 −20 −21 −22
MB, global
Our data
Kennicutt 1983
Jansen et al. 2000
Fig. 12.— Total B magnitude plotted against the global equivalent width (left) and star
formation rate (right). As the global SFR was not available for the Helmboldt, et al. (2005)
data, it is not shown on the right.
– 34 –
36 37 38 39 40 41
log(LHalpha, eff/area) [erg s
−1 kpc−2]
Our Data
Oey, et.al
Fig. 13.— Luminosity surface brightness (Luminosity/area) plotted against the diffuse Hα
fraction for our sample and that of Oey, et al. (2006).
– 35 –
0.0 0.5 1.0 1.5 2.0
SFRtotal,region [MO  · /yr]
0 20 40 60 80 100 120 140
E.Wglobal [Å]
Fig. 14.— A comparison of regional and global star formation rate and equivalent width for
the studied galaxies. On the left is a plot of the global SFR against the total SFR found for
the individual H II regions, with a line demarcating the point where the global and regional
SFR are equal. On the right is a plot of the global EW against the average EW for the
individual H II regions.
– 36 –
19 20 21 22 23 24 25
µB,R(0) [mag arcsec
Fig. 15.— Central surface brightness versus global star formation rate for the observed
galaxies. The (red) open circles are from the R band data, while the (blue) filled circles are
for the B data.
– 37 –
0.001 0.010 0.100 1.000 10.000
MHI/LB global [MO  · /LO  · ]
Our Data
Kennicutt & Kent 1983
Helmbolt, et.al 2004
0.1 1.0 10.0
MHI/LR global [MO  · /LO  · ]
Our Data
Oey, et.al (2006)
Helmboldt, et.al (2005)
Fig. 16.— Gas mass to luminosity ratios plotted against global equivalent widths (left) and
diffuse Hα fractions (right). On the left, the (black) circles are our data, the (blue) triangles
are from Kennicutt & Kent (1983) and the (red) diamonds are from Helmboldt, et al. (2004).
On the right, the (black) circles are again our data, while the (blue) asterisks are from Oey,
et al. (2006).
– 38 –
−0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0
(B − R)region
Fig. 17.— This plot shows the regional colors versus star formation rates for the observed
galaxies.

  We present B, R, and Halpha imaging data of 19 large disk galaxies whose
properties are intermediate between classical low surface brightness galaxies
and ordinary high surface brightness galaxies. We use data taken from the
Lowell 1.8m Perkins telescope to determine the galaxies' overall morphology,
color, and star formation properties. Morphologically, the galaxies range from
Sb through Irr and include galaxies with and without nuclear bars. The colors
of the galaxies vary from B-R = 0.3 - 1.9, and most show at least a slight
bluing of the colors with increasing radius. The Halpha images of these
galaxies show an average star formation rate lower than is found for similar
samples with higher surface brightness disks. Additionally, the galaxies
studied have both higher gas mass-to-luminosity and diffuse Halpha emission
than is found in higher surface brightness samples.

Introduction
Large low surface brightness galaxies are galaxies with disk central surface brightnesses
statistically far from the Freeman (1970) value of µB(0) = 21.65 ± 0.3 mag arcsec
−2, and
whose properties are significantly removed from the dwarf galaxy category (e.g. MB < −18,
MHI > 10
9M⊙). Studies of large LSB galaxies have discovered a number of intriguing facts:
large LSB galaxies, in contrast to dwarf LSB galaxies, can exhibit molecular gas (Das,
et al. 2006; O’Neil & Schinnerer 2004; O’Neil, Schinnerer, & Hofner 2003; O’Neil, Hofner,
& Schinnerer 2000); the gas mass-to-luminosity ratios of large LSB galaxies are typically
higher than for similar high surface brightness counterparts by a factor of 2 or more (O’Neil,
et al. 2004); and, like dwarf LSB galaxies, large LSB systems are typically dark-matter
dominated (Pickering, et al. 1997; McGaugh, Rubin, & de Blok 2001). These properties,
added to their typically low metallicities (de Naray, McGaugh, & de Blok 2004; Gerritsen
& de Blok 1999), lead to the inference that even large LSB galaxies are under-evolved
compared to their high surface brightness (HSB) counterparts. Once their typically low
gas surface densities (MHI ≤ 10
21 cm−2) (Pickering, et al. 1997) and low baryonic-to-dark
matter ratios (Gurovich, et al. 2004; McGaugh, et al. 2000) are taken into account, the
question becomes less why LSB galaxies are under-evolved than how they can form stars at
all (O’Neil, Bothun, & Schombert 2000, and references therein). Yet large LSB galaxies have
the same total luminosity within them as ordinary Hubble sequence spirals (O’Neil, et al.
2004; Impey & Bothun 1997; Pickering, et al. 1997; Sprayberry, et al. 1995). On average
then, star formation cannot be too inefficient in these large LSB galaxies in spite of their
unevolved characteristics, else their integrated light would be significantly less then in their
HSB counterparts.
In an effort to better understand this enigmatic group of galaxies and their evolutionary
status, we recently conducted a 21-cm survey to discover a larger nearby sample of such
objects (O’Neil, van Driel, & Schneider 2006; O’Neil, et al. 2004). We succeeded in identi-
fying about 25 candidates within the redshift range 0.04 < z < 0.08, whose combined HI
and optical properties suggest them to be large LSB galaxies. We obtained B, R, and Hα
imaging of 19 of these galaxies at Lowell Observatory to confirm whether these candidates
are indeed LSB galaxies, and to obtain a dataset of their fundamental parameters. These
observations are presented here, and interestingly, none of the galaxies ultimately turned out
to be LSB galaxies by the strict conventional definition; we discuss this result below in § 4.
However, these galaxies still represent a sample whose surface brightnesses are below average,
and whose properties are intermediate between those of the bona fide massive LSB galaxies,
and ordinary HSB galaxies. In this work, we quantify and parameterize the fundamental
properties of this sample of large, “lower surface brightness” galaxies.
– 3 –
2. Galaxy Sample
There are three ways that disk galaxy surface brightness can be measured or quantified –
using a surface brightness profile and fitting an exponential disk to derive the central surface
brightness; measuring an average surface brightness within a given isophotal diameter; and
measuring the surface brightness of the isophote at the 1/2 light radius point (the effective
surface brightness). The latter two definitions suffer from the fact that the bulge light is
included in the surface brightness estimates, resulting in their prediction of the disk surface
brightness to be less accurate. As a result, the typical operational definition of an LSB
galaxy uses the first definition, and defines an LSB galaxy as one whose whose observed disk
central surface brightness is µB(0) ≥23.0 mag arcsec
−2. For reference, the Freeman value of
µB(0)=21.65 +/- 0.30 mag arcsec
−2 defines the distribution of central surface brightness, in
the blue band, for Hubble sequence spirals.
Regardless of the definition, without pre-existing high quality optical imaging of galaxies,
it is difficult to unambiguously identify a sample of disk galaxies that will turn out to be
LSB. With only catalog data available, one is driven to use the average surface brightness
and identify potential LSB galaxies as those whose average surface brightness is below some
threshold level.
All of the galaxies in this sample were identified as LSB by Bothun, et al. (1985) using
the magnitude and diameter values found in the Uppsala General Catalog (Nilson 1973), and
employing the general equation 〈µB〉 = mpg +5log(D)+8.63. Here, mpg is the photographic
magnitude of the galaxies, D is the diameter in arcminutes, and the constant, 8.63, is derived
from the conversion from arcminutes to arcseconds (8.89) and the conversion from mpg to mB
(-0.26, as used by Bothun, et al.) Bothun, et al. (1985) then made a cut-off to the galaxies
in their sample, requiring 〈µB〉 >24.0 mag arcsec
−2 to look for galaxies with lower surface
brightness disks, with the majority of the galaxies chosen having 〈µB〉 >25.0 mag arcsec
(The inclusion of a number of galaxies with 〈µB〉=24-25 mag arcsec
−2 was due to the 0.5mag
errors given in the UGC.)
The Bothun, et al. (1985) sample was further pared down by our desire to image large
LSB galaxies. That is, we wished to avoid the dwarf galaxy category entirely. To do this, we
required the galaxies to have MHI > 10
9 M⊙, W20 > 200 km s
−1, and/or MB < −19. These
criteria are sufficiently removed from the dwarf galaxy category to guarantee no overlap
between our sample and that category exists.
– 4 –
3. Observations & Data Reduction
Galaxies’ integrated broad-band colors represent a convolution of the mean age of the
stellar population, metallicity, and recent star formation rate; while measurements of Hα
luminosity provide a direct measure of the current star formation rate (SFR). With these
combined observations, is is possible to parameterize the current SFR relative to the overall
star formation history. As a result, these observations are widely used in many surveys
that target fundamental galaxy parameters, for example, SINGG (Meurer, et al. 2006) and
11HUGS (Kennicutt, et al. 2004), and others (e.g. Gavazzi, et al. 2006; Koopman & Kenney
2006; Helmboldt, et al. 2005).
19 galaxies were observed on 7-10 June, 2002 and 5-8 October, 2003 using the Lowell
1.8m Perkins telescope. The filter set used included Johnson B and R as well as three
Hα filters from a private set (R. Walterbos) with center frequency/bandwidths of 6650/75,
6720/35, 6760/75 Å. A 1065x1024 pixel Loral SN1259 CCD camera was used, giving a 3.3′
field of view and resolution of 0.196′′/pixel. Seeing in June, 2002, ranged from 1.8′′ - 2.4′′ and
from 1.4′′ - 2.2′′ for the October, 2003, observations. At least 3 frames, each shifted slightly
in position, were obtained for each object through each filter and were median filtered to
reduce the effect from cosmic rays, bad pixels, etc. All initial data reduction (bias and flat
field removal, image alignment, etc) was done within IRAF. The R band images were scaled
and used as the continuum images for data reduction purposes.
Corrections to the measured fluxes were made in the following way. Atmospheric extinc-
tion was obtained using the observational airmass and the atmospheric extinction coefficients
for Kitt Peak which are distributed with IRAF. Galactic extinction was corrected using the
values for E(B−V) obtained from NED, the reddening law of Seaton (1979) as parameter-
ized by Howarth (1983) (A(λ) = X(λ)E(B−V )) and assuming the case B recombination of
Osterbrock (1989) with RV=3.1 (O’Donnell 1994) (X(6563Å)=2.468). Contamination from
[NII] emission in the Hα images was corrected using the relationship derived by Jansen,
et al. (2000) and re-confirmed by Helmboldt, et al. (2004):
[NII]
= [−0.13± 0.035]MR + [−3.2 ± 0.90] ,
where MR is the absolute magnitude in the R band. Hα extinction was determined using
the equation found in Helmboldt, et al. (2004):
log (Hα)int = [−0.12± 0.048]MR + [−2.5± 0.96]
which was found through a linear least squares fitting to the A(Hα)int determined using
all galaxies in his sample with a measured Hβ flux. For this calculation, Helmboldt, et al.
– 5 –
(2004) used the Hα to Hβ ratio measured by Jansen, et al. (2000), an assumed intrinsic ratio
of Hα
=2.85 (Case B recombination and T=104 K (Osterbrock 1989)), the extinction curve
of O’Donnell (1994), and RV=3.1 No correction for internal extinction due to inclination
was made for the B and R bands. It should be noted, though, that in a number of plots
inclination corrections were made to the B and R colors and central surface brightnesses, as
noted in the Figure captions. The corrections used in these cases are:
µ(0)λcorr = µ(0)
λ − 2.5Cλlog(b/a) (1)
mλcorr = m
λ −Aλ (2)
Aλ = −2.5log
1 + e−τ
λsec(i)
+ (1− 2f)
1− e−τ
λsec(i)
τλsec(i)
Here, CR,B=1 (Verheijen 1997); (b/a) is the ratio of the minor to major axis; f = 0.1 and
τR,B=0.40, 0.81 (Tully, et al. 1998; Verheijen 1997). Finally, a correction was applied to
account for the effect of stellar absorption in the Balmer line of
Fcor = Fobs
, (4)
where Fcor is the corrected and Fobs is the observed Hα flux, We is the measured equivalent
width and Wa is the equivalent width of the Balmer absorption lines. As we do not have
measurements for Wa, we estimated Wa to be 3±1 Å, based off the values found in Oey &
Kennicutt (1993); Roennback & Bergvall (1995); McCall, Rybski, & Shields (1985). Note
that this effect is potentially stronger in the diffuse gas than in the H II regions due to
the older stellar population likely lying in the diffuse gas. As a result we may still be
underestimating the total Hα flux in the diffuse gas within the galaxies. However, as the
diffuse gas fractions for these galaxies are extremely high (see Section 6, below), it is unlikely
that this effect is high.
Global parameters and radial profiles for the galaxies were determined primarily using
the routines available in IRAF (notably ellipse) and the results are given in Tables 1 and 2.
Galaxy images, surface brightness profiles, and color profiles are given in Figures 1 – 3. In
all cases the inclination and position angle for the galaxies were determined from the best fit
values from the B & R frames. These best fit values were then used for the ellipse fitting in
all four images (B, R, Hα and continuum with Hα subtracted), a practice which insures the
color profiles are obtained accurately and are not affected by, e.g. misaligned ellipses. The
same apertures were also used for all four images, with the apertures found through allowing
ellipse to range from 1 pixel (0.196′′) until the mean value in the ellipse reaches the sky value,
– 6 –
increasing geometrically by a factor of 1.2. Sky values were found through determining the
mean value in more than 100 5×5 sq. pixel boxes in each frame. The error found for the sky
was incorporated into all magnitude and surface brightness errors, which also include errors
from the determination of the zeropoint and the errors from the N II contribution to the Hα
(in the case of the Hα magnitudes).
The B and R surface brightness profiles of all galaxies were fit using two methods. First,
the inner regions of the galaxies’ surface brightness profiles was fit using the de Vaucouleurs
r1/4 profile
Σ(r) = Σeffexp
−7.669[(r/reff )1/4−1] → µ(r) = µeff + 8.327
, (5)
and the outer regions were fit by the exponential disk profiles
Σ(r) = Σ0exp
( rα) → µ(r) = µ0 + 1.086
. (6)
Additionally, we attempted to fit a disk profile (6) to both the inner and outer regions of the
galaxies’, to determine if a two-disk fit would better match the data (Broeils & Courteau
1997; de Jong 1996). Roughly one-fourth of the galaxies (5/19) were best fit (in the χ2-sense)
by the standard bulge+disk model. Another 47% of the galaxies were best fit by the two-disk
model. Of the remaining galaxies, 21% (4 galaxies) were best fit by a single disk, and one
galaxy (UGC 11840) could not be fit by any profile. The results from the fits are shown in
Table 3 and Figure 2, and an asterisk (*) is placed next to the best fit model. Note that
in a few cases (e.g. UGC 00189) only one model is listed in the Table. This is due to the
fact that in these cases the fitting using the other model proved to be completely unrealistic.
Finally, it should be noted that in all cases the same best-fit model was used for both the B
and R data.
The color profiles were similarly fit (using an an inverse error weighting) with a line to
both the inner and outer galaxy regions (Figure 3). Here, though, the “boundary radius”
was simply taken from the surface brightness profile fits, with the “boundary radius” being
defined as the radius where the inner and outer surface brightness fits crossed. If only one
(or no) fit was made to the surface brightness profile, then only one color profile was fit. In
a number of cases the difference in slope between the inner and outer galaxy regions was
less than the least-squares error for the fit. In these cases again only one line was fit for the
color profiles.
The HIIphot program (Thilker, Braun, & Walterbos 2000) was used both to determine
the shape and number of H II regions for each galaxy and also to determine the Hα flux for
each of these regions. The fluxes from the Hα, Hα-subtracted continuum, B, and R images
– 7 –
were measured in identical corresponding apertures, which are the H II region boundaries
defined by HIIphot. While HIIphot applies an interpolation algorithm across these apertures
to estimate the diffuse background in the Hα frames, we determined the background in other
bands from the median flux in an annulus around each H II region aperture. Results from
the analysis of the H II regions are given in Table 4, and sample H II regions are shown in
Figure 4. Errors for the Hα, SFR, and EW measurements are derived from the error values
reported with HIIphot. Errors for the B and R magnitudes, and colors, are derived from the
total sky and zeropoint errors, as well as the error in positioning of the HII regions. The
diffuse fraction errors are derived both from the total Hα flux errors and also include errors
in determining the total flux within the HII regions and for the entire galaxy. Finally, it
should be mentioned that the equivalent width (EW) was calculated simply as the ratio of
the Hα flux to Hα-subtracted continuum flux for a given region (or the whole galaxy).
The large distances to the observed galaxies (40 - 100 Mpc) results in many of the H II
region being blended together. As a result, any luminosity function derived for these objects
would be necessarily skewed towards larger HII regions (see Oey, et al. 2006). This can be
seen in the analysis done by Thilker, Braun, & Walterbos (2000) wherein the dependence of
the luminosity function found for M51 was examined. There one can clearly see the increase
in the number of high luminosity regions and subsequent reduction in the number of low
luminosity regions as the galaxy is ’moved’ to increasing distances. Examining their results
also shows that while the distribution of H II region luminosities changes with distance, the
total luminosity of the H II regions, as found by HIIphot, does not change significantly as the
galaxy moves from 10 Mpc to 45 Mpc. As a result, while determining luminosity functions
for the galaxies in this paper is not feasible due to the distances involved, derivations such
as the diffuse fraction are unaffected by distance. This fact is also supported by the SINGG
survey results (Oey, et al. 2006).
4. Surface Brightness
The distribution of central surface brightnesses found for the galaxies observed is shown
in Figure 5. As is plain from that Figure, the mean measured central surface brightness for
this sample, falls short of the definitions discussed in Section 2. Indeed only 4 galaxies in our
sample meet the operational definition of LSB galaxies as having µB(0) ≥ 23 mag arcsec
If we return to the Freeman value, however, we see that the operational definition of LSB
galaxies is 4.5σ from the value for Hubble sequence spirals, making it statistically extreme.
For the sample defined here, half have central surface brightnesses at least two sigma above
the Freeman value, a definition only 2.5% of the Freeman sample meets. As a result, while
– 8 –
the sample does not meet the operation criteria for LSB galaxies, we clearly do have a sample
with lower central surface brightnesses that would be found in the average Hubble sequence
galaxies.
It should be pointed out here that the main scientific focus of Bothun, et al. (1985)
was not oriented toward producing a representative sample of LSB galaxies as detected on
photographic surveys (that focus did not occur until Schombert & Bothun 1988), but rather
toward identifying cataloged galaxies for 21-cm based redshift determinations. The galaxies
were chosen to have surface brightnesses that were too low for reliable optical spectroscopy
(assuming emission lines were not present). This was done as a test of the potentially large
problem of bias in on going optical redshift surveys in the time (see Bothun, et al. 1986).
In fact, the operational criteria for selecting the galaxies that were observed at Arecibo 20
years ago, lay in the knowledge that these cataloged galaxies were never going to be even
attempted in the optical redshift surveys of the time and this raised the very real possibility
of biased redshift distributions and an erroneous mapping of large scale structure.
In the original redshift measurements of Bothun, et al. (1985) a significant number of
candidate LSB galaxies were not detected at 21-cm within the observational redshift window
(approximately 0-12,000 km/s). Many of those non-detections would later turn out to be
intrinsically large galaxies located at redshifts beyond 12,000 km/s (see O’Neil, et al. 2004).
As we are interested here in the Hα properties of galaxies with large, relatively LSB disks,
these initial non-detections comprise the bulk of our sample.
Surface photometry of this sample not only provides detailed information regarding
the galaxies’ surface brightness and color distributions, but it also probes the efficacy of
the Bothun, et al. (1985) average surface brightness criteria for selecting LSB disks. Here,
we used the magnitudes and diameters obtained in this study (Table 1) with two different
equations for determining a galaxy’s average surface brightness within the D25 radius. The
first equation used is that of Bothun, et al. (1985)
〈µ25〉 = m25 + 5log(D25) + 3.63 (7)
and the second is a modified version of the above equation from Bottinelli, et al. (1995)
which takes the galaxies’ inclination into account:
〈µ25〉 = m25 + 5log(D25) + 3.63− 2.5log
kR−2C + (1− k)R(0.4C/K)−1
. (8)
In both equations, m25 and D25 are the magnitude and diameter (in units of 0.1
′) at the
µ=25.0 mag arcsec−2 isophote, R is the axis ratio (a/b), and C is defined as (logD/logR)
and is fixed at 0.04 (Bottinelli, et al. 1995). Finally, k (the ratio of the bulge-to-disk lumi-
nosity) and K (a measure of how the apparent diameter changes with surface brightness at
– 9 –
a given axis ratio) are dependent on the revised de Vaucouleurs morphological type (T) as
follows (Simien & de Vaucouleurs 1986; Fouqué & Paturel 1985):
T=1 → k=0.41; T=2 → k=0.32; T=3 → k=0.24; T=4 → k=0.16; T=5 → k=0.09; T=6 →
k=0.05; T=7 → k=0.02; T≥8 → k=0.0;
K = 0.12− 0.007T if T < 0; K = 0.094 if T ≥ 0.
The values for k at T≥8 are extrapolated from fitting the Simien & de Vaucouleurs (1986)
values.
The results of equations 7 and 8, plotted against the galaxies’ central surface brightness
both uncorrected and corrected for inclination, are shown in Figures 6 and 7, respectively.
The difference between the two plots is small, with neither equation doing an excellent job in
predicting when a disk’s central surface brightness will be low. The two equations (Bothun,
et al. (1985) and Bottinelli, et al. (1995)) have roughly the same fit (in the χ2 sense), which
at first appears surprising. It is likely that uncertainties in the inclination measurements and
morphological classification of the galaxies have increased the scatter in the Bottinelli, et al.
(1995) equation, increasing the scatter in an otherwise more accurate equation. As a result,
while the Bottinelli, et al. (1995) may indeed be the most accurate, the simpler equation is
equally as good to use in most circumstances as it involves fewer assumptions.
The second fact that is readily apparent in looking at Figures 6 and 7 is that with the
new measurements of magnitude and diameter, none of the galaxies in our sample meet
the criterion laid out by Bothun, et al. (1985) for an LSB galaxy. That is that none of
the galaxies in this sample have 〈µ25〉 >25 mag arcsec
−2. As Bothun, et al. (1985) listed
all of these objects as having 〈µ25〉 >25 mag arcsec
−2 using the magnitudes and diameters
provided by the original UGC measurements, this shows that the UGC measurements indeed
predicted fainter magnitudes/larger values for D25 than is found with more sophisticated
measurement techniques. Additionally, it is good to note that the trends shown in Figures 6
and 7 indicate that any galaxy which met the 〈µ25〉 >25 mag arcsec
−2 criteria would be
highly likely to also have µ(0) >23 mag arcsec−2.
In these days of digital sky surveys it is difficult to appreciate the immense undertaking
that defines the UGC catalog. Anyone who has looked at the Nilson selected galaxies on
the Palomar Observatory Sky Survey (POSS) plates with a magnifying eyepiece really has
to marvel that Nilson’s eye saw objects at least one arcminute in diameter. It is thus not
surprising that, at the ragged end of that catalog, many of the listed UGC diameters are
systematically high. Cornell, et al. (1987) made a detailed diameter comparison between
diameters as obtained from high quality CCD surface photometry and the estimates made
by Nilson (1973). They compared the diameter at the 25.0 mag arcsec−2 isophote in CCD
– 10 –
B images to the tabulated diameter in the UGC. The study, based on approximately 250
galaxies, identified two sources of systematic error (neither of which are surprising). First,
galaxies with reported diameters less than 2′ typically had D25,B as measured by the CCD
images that were 15-25% smaller. Second, Cornell, et al. (1987) found a systematic bias as a
function of surface brightness in the sense that lower surface brightness galaxies had a higher
number of overestimated diameters in the UGC than higher surface brightness galaxies. It
should also be noted that the majority of the galaxies in this study lie at low Galactic latitude.
This seems to be a perverse consequence that there is a large collection of galaxies between
7,000 – 10,000 km s−1 (where the diameter criterion in the UGC yields a relatively large
physical size) located at relatively low galactic latitude. Nominal corrections for galactic
extinction made by Bothun, et al. (1985) turned out to underestimate the extinction as
shown by later published extinction maps. In some cases, the differences were as large
as one magnitude. The combination of these facts with the very uncertain magnitudes of
many of these galaxies (see Bothun & Cornell 1990), it is not surprising that the measured
average surface brightness could easily be 1-1.5 magnitudes higher than the average surface
brightness that has been estimated from the UGC catalog parameters (roughly 40% of this
comes from systematic magnitude errors and 60% from the diameter errors discovered by
Cornell, et al. (1987)).
5. Morphology & Color
All of the galaxies observed have large sizes (3αB = 10 – 54 kpc), bright central bulges,
and well defined spiral structure (Figure 1). In most cases the galaxies can be described as
late-type systems (Sbc and later). There are, though, a number of exceptions to this rule.
Three of the galaxies, UGC 00023, UGC 07598, and UGC 11355 (Sb, Sc, and Sb galaxies,
respectively) have clear nuclear bars. UGC 08311, classified as an Sbc galaxy, is clearly in
the late stages of merging with another system. In this case the LSB classification of the
galaxy is likely bogus, as the apparently LSB disk is likely just the remnant the merging
process and will disappear as the galaxy compacts after the merging process. UGC 8904 is
given a morphological type of S? with both NED and HYPERLEDA, yet the faint spiral
arms surrounding it indicate its should be properly classified as an Sbc system. UGC 12021
is, like UGC 00023, listed as an Sb galaxy. Finally, UGC 11068 has a faint nuclear ring
which is most readily visible in the B image.
The differences between the galaxies becomes more apparent when the Hα images are
examined. Hodge & Kennicutt (1983) classify the radial distribution of H II regions in
spiral galaxies into three broad categories – galaxies with H II region surface densities which
– 11 –
decrease with increasing radius, galaxies with oscillating H II region surface densities, and
galaxies with ring-like H II density distributions. To these categories we would add a fourth,
to include those galaxies with no detectable H II regions.
The first category of Hodge & Kennicutt (1983) is also the most common, as it includes
all galaxies with generally decreasing radial densities of Hα. In the Hodge & Kennicutt (1983)
sample this category is dominated by Sc – Sm galaxies but contains all Hubble types. In our
sample, this category includes both galaxies with and without significant Hα emission in the
spiral arm regions. This group includes UGC 00023, UGC 00189, UGC 02588, UGC 02796,
UGC 03119, UGC 03308, UGC 07598, and UGC 12021. Interestingly, of the galaxies listed
above, 4/8 are Sb/Sbc galaxies and 3/8 are Sc-Sm galaxies. (The last galaxy, UGC 02588,
is an irregular galaxy.)
The second category of Hodge & Kennicutt (1983), galaxies with oscillating densities,
is dominated in their sample of Sb galaxies. Only a few of the galaxies in this sample fall
into this category, 80% of which are also Sb/Sc galaxies. These are UGC 02299, UGC 08311,
UGC 08904, UGC 11355, and UGC 11396. These galaxies all have a concentration of star
formation seen in the nuclear regions and then clumps of star formation spread through the
spiral arms, typically accompanied by diffuse Hα also spread throughout the arms.
The third category of Hodge & Kennicutt (1983) is dominated by early-type galaxies,
of which we have none in our sample. Nonetheless we have three galaxies which fall into this
category – UGC 08644, UGC 10894, and UGC 11617. All three have H II regions spread
throughout their disks, with no central concentration near the galaxies’ nuclei. In fact, the
three brightest star forming regions within UGC 08644 all lie with the spiral arms, and are
visible in all three filters. In contrast, both UGC 11617 and UGC 10894 have no bright H II
regions, but instead have a large number of diffuse H II regions, with the brightest (as listed
in Table 2) receiving that designation simply due to its size.
The fourth category of galaxies contains UGC 01362, UGC 11068, and UGC 11840,
none of which have detectable Hα. In the case of UGC 01362 and UGC 11840 this is not too
surprising as the galaxies are dominated by a bright nucleus, and their surrounding spiral
arms are extremely faint in both R and B. As a result, any Hα which may exist in the
galaxies’ disks is too diffuse to be detected. UGC 11068, though, has both a well defined
nucleus and a clear spiral structure extending out to a radius of ∼13 kpc (3α). Yet no Hα
can be detected in this galaxy. This may mean that UGC 11068 is in a transition state for
its star formation, with no ongoing star formation yet with enough recent activity that the
spiral arms remain well defined.
Perhaps the most intriguing galaxy of our sample is UGC 11355. This galaxy was
– 12 –
placed in Category 2, above, as it has a bright nucleus and clumpy disk in the Hα image.
The B and R band images of UGC 11355 show a galaxy with a simple Sbc morphology.
The Hα image, though, shows a distinct star forming ring. The ring is at a very different
inclination from the rest of the galaxy (i=49◦ for the ring and 73◦ for the galaxy as a whole),
and lies approximately 2.6 kpc in radius from the center of the galaxy, measured along the
major axis. As the B and R images show no indication of a ring morphology this indicates
unusually strong star formation in the ring. It is also useful to note the presence of a bar in
UGC 11355 – shown more clearly in Figure 8. The fact that the inclination of the ring is
significantly different from that of the rest of the galaxy suggests the ring a tidal effect due
to an interaction, such as a small satellite galaxy being cannibalized by UGC 11355, or the
influence of CGCG 143-026, 14.9′ and 68 km s−1 away.
It is interesting to note that the Hα morphology of the galaxies does not appear to
correlate with the galaxies’ color profile (Figure 3). The galaxy with the steepest slope in
the color profile is UGC 08644 which has only a few H II regions in its outer arms. The other
galaxies with steep color profiles are UGC 00023 and UGC 8904, which have a bright knot
of star formation in the nucleus and faint Hα spread throughout their arms, and UGC 11840
and UGC 11068 both of which have no detectable Hα. The galaxies with the shallowest slopes
similarly show no correlation between their color profiles and morphology. This suggests that
the current star formation in these galaxies is largely independent of the past star-formation
history, although this result should be confirmed with better, extinction-corrected, data.
6. Star Formation
Figures 10 – 17 compare the properties of the H II regions and emission of our galaxy
sample. Where possible, measurements from other samples of late-type galaxies are also
shown (Kennicutt & Kent 1983; Jansen, et al. 2000; Helmboldt, et al. 2005; Oey, et al.
2006). Examining the figures it is clear that the overall properties of our sample are similar
to those of other late-type (Sbc-Sc) galaxies. That is, the values for the individual H II region
luminosities are similar to those reported by Helmboldt, et al. (2005) and Kennicutt & Kent
(1983) (Figure 10) while the global Hα equivalent width (EW) and global star formation
rates match those seen by all three comparison samples (Figures 11, 12).
We should note that as discussed in § 3 our sample suffers from having many of the H II
regions blended together as a result of the distance to our galaxy samples. As a result, it
is highly likely that in the comparisons of the luminosities for the galaxies’ individual H II
regions the luminosities (Figure 10) from our sample are artificially higher then those in the
other sample, potentially by a factor of 3 or more. This fact does not alter the results of this
– 13 –
section, but it is the likely explanation for the slightly higher than average values found for
L3 in Figure 10.
To examine the total amount of gas found within the H II regions compared with that
found in the diffuse Hα gas, we need to determine the galaxies’ Hα diffuse fraction, defined
here as the ratio of Hα flux not found within the defined H II regions to the total Hα flux
found for the entire galaxy. Examining Figures 13 and 14, as well as Tables 2 and 4, reveals
an interesting fact – while the global SFR for these galaxies is fairly typical (0.3 – 5 M⊙/yr),
the combined SFR from the galaxies’ H II regions is a factor of 2 – 10 smaller. That is, on
average the majority of the Hα emission and thus the majority of the star formation in the
observed galaxies comes not from the bright knots of star formation but instead from the
galaxies’ diffuse Hα gas. This is in contrast to the behavior seen from typical HSB galaxies,
as evidenced by the data of Oey, et al. (2006) in Figure 13. We note that blending and
angular resolution effects appear to be relatively unimportant in estimating the fraction of
diffuse Hα emission. Oey, et al. (2006) demonstrate this by showing no systematic changes
in measured diffuse fractions as a function of distance up to almost 80 Mpc, and inclination
angle, for their sample of 100+ SINGG survey galaxies.
While at first glance the higher diffuse Hα fractions found for these galaxies seems
surprising, recent GALEX results of the outer edges of M83, a region whose environment
closely resembles that of the disks of massive LSB galaxies, also show considerable star
formation outside the H II regions in that part of the galaxy (Thilker, et al. 2005). Similarly,
Helmboldt, et al. (2005) found a slight trend with lower surface brightness galaxies having
higher diffuse fractions than their higher surface brightness counterparts.
The fact that these galaxies have higher Hα diffuse gas fractions raises an interesting
question. Typically diffuse gas is believed to be ionized by OB stars lying within density-
bounded H II regions. The problem of transporting the ionizing photons from these regions
to the diffuse gas is extreme in these cases, as there would need to be a very large number
of density-bound H II regions leaking ionizing photons to ionize the quantity of diffuse
gas seen here. (See the more detailed discussion in Hoopes, Walterbos, & Bothun 2001,
which also discusses shock heating from stellar winds and SNe as ionization sources.) An
alternative suggestion is that field OB stars are also ionizing the diffuse gas, as was suggested
by Hoopes, Walterbos, & Bothun (2001). This would imply a different stellar population
within and without the H II regions, as it would likely be the later OB types (B0–O9) which
either escape the H II regions or survive the regions’ destruction. We note Oey, King, &
Parker (2004) predict a modest increase in the fraction of field massive stars in galaxies with
the lowest absolute star-formation rates. Scheduled GALEX observations of a subset of our
observed galaxies may shed light on the underlying stellar population in the galaxies’ diffuse
– 14 –
stellar disks.
Finally, it is elucidating to look for any trends between the global and regional properties
of the galaxies and their SFR and Hα content. Figure 15 plots the galaxies’ central surface
brightness (in both B and R) against the galaxies’ total star formation rate. While the error
bars make defining any trend difficult, there certainly appears to be a decrease in the global
SFR with decreasing central surface brightness, similar to the trends seen in other studies
(e.g. van den Hock, et al. 2000; Gerritsen & de Blok 1999). Figure 16 shows the galaxies’
gas-to-luminosity ratios plotted against both their global equivalent width and diffuse Hα
fraction. In both cases, no trend can be seen with our data, although the small number of
points available make any diagnosis difficult. Combined with the other datasets, though,
we can see a general trend toward higher equivalent widths with increasing MHI/LB, but
surprisingly no trend between gas fraction and the galaxies’ diffuse Hα fraction is visible.
This lack of correlation is also seen by Oey, et al. (2006). The last trend which can be seen is a
rough correlation between the galaxies’ global color and star formation rate (Figure 11), with
redder galaxies having higher SFR, a fact which may be a reddening effect. The individual
Hα regions, however, show no such trend (Figure 17).
7. Conclusion
The sample of 19 galaxies observed for this project were chosen to be large galaxies
with low surface brightness disks. The surface brightness measurements for this sample
were obtained originally through the UGC measurements through determining the galaxies’
average surface brightness within the µ=25 mag arcsec−2 isophote. The relation employed
to determine the galaxies’ average surface brightness (Equation 7) has shown itself it be a
good predictor of a galaxy’s central surface brightness. But for a wide variety of reasons the
UGC measurements were not sufficient to insure the galaxies contained within this catalog
have true LSB disks, underscoring the difficulty in designing targeted searches for large LSB
galaxies.
Nonetheless, the sample of galaxies observed for this project have lower surface bright-
nesses than is found for a typical sample of large high surface brightness galaxies. In most
other aspects the galaxies appear fairly ‘normal’, with colors typically B−R=0.3−0.9, mor-
phological types ranging from Sb – Irr, and color gradients which typically grow bluer toward
the outer radius. However, the galaxies have both higher gas mass-to-luminosity fractions
and diffuse Hα fractions than is found in higher surface brightness samples. This raises
two questions. First, if the SFR for these galaxies has been similar to their higher sur-
face brightness counterparts through the galaxies’ life, why do the lower surface brightness
– 15 –
galaxies have higher gas mass-to-luminosity ratios? Second, why do these galaxies have a
higher fraction of ionizing photons outside the density-bounded H II regions then their higher
surface brightness counterparts?
The answer to the first question posed above likely comes from the difference between
the studied galaxies’ current and historical SFR. As these galaxies have on average and lower
metallicities (de Naray, McGaugh, & de Blok 2004; Gerritsen & de Blok 1999) than their
higher surface brightness counterparts, it is likely that the galaxies’ SFR has not remained
constant throughout the their lifetimes. Indeed the simplest explanation for the current
similar SFRs and higher gas mass-to-luminosity ratios for the studied galaxies than for
their higher surface brightness counterparts is that the galaxies’ past SFR was significantly
different than is currently seen. In fact, the measured properties would be expected if
the galaxies in this study have episodic star formation histories, with significant time (1-3
Gyr) lapsing between SF bursts, as has been conjectured for LSB galaxies in the past (e.g.
Gerritsen & de Blok 1999). Such a star formation history would help promote significant
changes in the galaxies’ mean surface brightness and allow an individual large disk galaxy
to appear as either (a) a relatively normal Hubble sequence spiral, (b) a large, lower surface
brightness disk, or (c) perhaps even a lower surface brightness disk if the time between
episodes is sufficiently large, depending on the elapsed time since the last SF burst. The
final answer to this may be found when an answer to the second question, determining why
the diffuse fractions for the studied galaxies is higher than for similar HSB galaxies, is also
found. Irregardless, what is clear is that the studied sample shows a clear bridge between
the known properties of high surface brightness galaxies and the more poorly understood
properties of their very low surface brightness counterparts, such as Malin 1.
Thanks to Joe Helmboldt for his help in getting the HIIphot program running with LSB
galaxies and to Rene Walterbos for his loan of the Hα filters. MSO acknowledges support
from the National Science Foundation, grant AST-0448893.
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This preprint was prepared with the AAS LATEX macros v5.2.
Table 1. Global Properties of Galaxies – B & R Measurements
Galaxy RAa Deca Vela Typea mb Mb D25
c 〈µ〉d mb Mb D25
c 〈µ〉d rb B−Rb ie
[J2000] [J2000]
km s−1
[mag] [Mag] [′′]
mag/′′2
[mag] [Mag] [′′]
mag/′′2
[′′] [◦]
UGC 00023 00 04 13.0 10 47 25 7787 3 14.4 (0.1) -20.7 (0.1) 71 23.4 13.0 (0.1) -22.1 (0.1) 86.7 22.4 40 1.4 (0.2) 52
UGC 00189 00 19 57.5 15 05 32 7649 7 15.0 (0.2) -20.1 (0.2) 84 24.4 13.8 (0.1) -21.3 (0.1) 116.1 23.8 57 1.2 (0.2) 67
UGC 01362 01 52 50.7 14 45 52 7918 8.8 16.9 (0.2) -18.2 (0.2) 30 24.3 15.6 (0.1) -19.5 (0.1) 42.1 23.5 23 1.3 (0.3) 0
UGC 02299 02 49 07.8 11 07 09 10253 8 15.4 (0.2) -20.3 (0.2) 59 24.0 14.5 (0.4) -21.2 (0.4) 65.1 23.3 33 0.9 (0.4) 32
UGC 02588 03 12 26.5 14 24 27 10093 9.9 15.8 (0.2) -19.9 (0.2) 39 23.6 14.7 (0.1) -20.9 (0.1) 50.1 23.0 28 1.1 (0.2) 28
UGC 02796 03 36 52.5 13 24 24 9076 4 14.8 (0.2) -20.6 (0.2) 66 23.6 13.3 (0.1) -22.1 (0.1) 94.1 22.7 28 1.5 (0.2) 57
UGC 03119 04 39 07.7 11 31 50 7851 4 14.3 (0.2) -20.8 (0.2) 71 23.7 12.4 (0.1) -22.7 (0.1) ‡ 24.1 40 1.9 (0.2) 72
UGC 03308 05 26 01.8 08 57 25 8517 6 14.3 (0.3) -21.0 (0.3) 89 23.8 14.0 (0.2) -21.3 (0.2) 88.1 23.5 48 0.3 (0.4) 28
UGC 07598 12 28 30.9 32 32 52 9041 5.9 15.3 (0.1) -20.1 (0.1) 46 23.5 14.8 (0.1) -20.6 (0.1) 66.8 22.8 33 0.5 (0.2) 22
UGC 08311 13 13 50.8 23 15 16 3451 4.1 15.5 (0.1) -17.9 (0.1) 50 23.8 14.8 (0.1) -18.5 (0.1) 62.0 23.5 33 0.7 (0.2) 26
UGC 08644 13 40 01.4 07 22 00 6983 8 16.1 (0.2) -18.8 (0.2) 43 24.2 15.3 (0.2) -19.6 (0.2) 49.4 23.5 28 0.8 (0.3) 30
UGC 08904 13 58 51.1 26 06 24 9773 3.6 15.9 (0.1) -19.7 (0.1) 43 23.8 14.9 (0.1) -20.7 (0.1) 55.5 23.4 33 1.0 (0.2) 48
UGC 10894 17 33 03.8 27 34 29 6890 4 16.0 (0.3) -18.8 (0.3) 48 24.1 14.9 (0.3) -19.9 (0.3) ‡ 23.0 28 1.1 (0.4) 57
UGC 11068 17 58 05.0 28 14 38 4127 3.2 15.0 (0.2) -18.7 (0.2) 64 24.0 13.8 (0.1) -19.9 (0.1) 89.4 23.4 57 1.2 (0.2) 0
UGC 11355 18 47 57.0 22 56 33 4360 3.5 13.9 (0.3) -19.9 (0.3) 173 24.8 12.5 (0.1) -21.3 (0.1) ‡ 23.7 82 1.4 (0.3) 73
UGC 11396 19 03 49.5 24 21 28 4441 3.5 14.8 (0.3) -19.1 (0.3) 114 24.4 13.8 (0.2) -20.1 (0.2) 78.7 23.0 33 1.0 (0.4) 59
UGC 11617 20 43 39.3 14 17 52 5119 6.1 14.9 (0.2) -19.2 (0.2) 75 24.1 13.9 (0.2) -20.3 (0.2) 86.1 23.3 40 1.1 (0.3) 58
UGC 11840 21 53 18.0 04 14 50 7986 4 16.3 (0.1) -18.9 (0.1) 24 22.9 15.3 (0.1) -19.9 (0.1) 23.3 21.9 11 1.0 (0.2) 40
UGC 12021 22 24 11.6 06 00 12 4472 3 14.7 (0.2) -19.2 (0.2) 113 24.7 13.6 (0.1) -20.2 (0.1) 130.4 23.9 57 1.1 (0.2) 63
Note. — Errors are given in parenthesis.
aRA, Dec, velocity and type information obtained from NED, the NASA Extragalacitc Database. Galaxy types are defined in de Vaucouleurs, et al. (1991).
bMagnitudes and colors were obtained at the maximum usable radius, r. Corrections applied and error estimates are described in Section 3.
cD25 are the diameters for the 25 mag arcsec
−2 isophotes.
dAverage surface brightness, as defined by Equation 7 in Section 4.
eInclinations are simply the major to minor axis ratio of the galaxies, found through isophote fitting.
‡Isophotes did not reach 25 mag arcsec−2.
– 20 –
Table 2. Global Properties of Galaxies – Hα
Galaxy Hα flux ×10−13a EWb SFRc rd
erg cm−2 s−1
UGC 00023 5 (1) 22 (8) 4 (1) 28
UGC 00189 4 (1) 63 (33) 4 (1) 23
UGC 01362 · · · · · · · · · · · ·
UGC 02299 0.41 (0.09) 38 (13) 0.7 (0.2) 6
UGC 02588 0.7 (0.3) 32 (21) 1.1 (0.5) 13
UGC 02796 2.1 (0.7) 12 (4) 2.7 (0.9) 13
UGC 03119 7 (4) 20 (12) 6 (6) 28
UGC 03308 1.0 (0.3) 22 (8) 1.1 (0.3) 11
UGC 07598 1.3 (0.3) 60. (18) 1.8 (0.4) 19
UGC 08311 3.1 (0.6) 91 (37) 0.6 (0.1) 19
UGC 08644 · · · · · · · · · · · ·
UGC 08904 0.8 (0.2) 40 (13) 1.1 (0.3) 16
UGC 10894 0.4 (1) 34 (78) 0.4 (0.1) 9
UGC 11068 · · · · · · · · · · · ·
UGC 11355 8 (2) 30 (9) 2.7 (0.6) 28
UGC 11396 2.4 (0.8) 400 (2000) 0.8 (0.2) 16
UGC 11617 3.1 (0.7) 30 (2) 0.3 (0.2) 19
UGC 11840 · · · · · · · · · · · ·
UGC 12021 4 (1) 63 (32) 1.3 (0.4) 23
UGC 12289 · · · · · · · · · · · ·
Note. — Derivation of quantities are described in Section 3. Errors
are given in parenthesis.
aTotal Hα flux found within the radius centered on the (optical)
center of the galaxy and extending to the radius given in the last column.
Errors were determined in the same manners as for magnitudes, and are
given in Section 3.
bThe equivalent width was calculated simply as the ratio of the total
Hα flux to total Hα-subtracted continuum flux.
cSFR =
1.26×1041ergs−1
; from Kennicutt, Tamblyn, & Congdon
(1994).
dRadius at which the isophotal signal-to-noise went below 1σ.
– 21 –
Table 3. Fitted Galaxy Properties
inner outer
Galaxy Fit Filterb µeff/µ0
c Reff/α
e αf Boundaryg Fith
Type a
mag arcsec−2
mag arcsec−2
[′′] [′′] Error
∗ UGC 00023 Two Disk B 19.06 (0.33) 1.71 (0.02) 21.17 (0.21) 11.05 (0.06) 6.55 0.82
∗ UGC 00023 Two Disk R 17.30 (0.27) 1.72 (0.02) 19.58 (0.16) 9.58 (0.03) 6.93 0.53
UGC 00023 Bulge/Disk B 21.48 (1.98) 4.06 (0.23) 21.47 (0.51) 11.96 (0.11) 8.94 0.89
UGC 00023 Bulge/Disk R 19.20 (1.56) 3.07 (0.14) 19.82 (0.35) 9.96 (0.05) 8.79 0.58
∗ UGC 00189 Two Disk B 20.99 (0.08) 7.75 (0.06) 24.05 (1.85) 36.90 (2.11) 28.34 0.33
∗ UGC 00189 Two Disk R 19.53 (0.09) 6.13 (0.05) 21.66 (0.67) 18.84 (0.22) 24.92 0.97
∗ UGC 01362 One Disk B · · · · · · 23.06 (0.33) 7.77 (0.10) · · · 1.78
∗ UGC 01362 One Disk R · · · · · · 21.84 (0.15) 7.85 (0.04) · · · 0.46
∗ UGC 02299 Two Disk B 20.85 (0.10) 2.00 (0.16) 22.43 (1.76) 11.24 (0.04) 6.48 1.88
∗ UGC 02299 Two Disk R 19.84 (0.28) 2.21 (0.03) 21.46 (0.38) 10.50 (0.10) 7.23 0.99
UGC 02299 Bulge/Disk B 25.10 (2.85) 17.35 (1.95) 23.23 (2.18) 12.62 (0.27) 17.01 2.08
UGC 02299 Bulge/Disk R 24.00 (1.75) 17.76 (1.24) 22.30 (1.43) 10.37 (0.19) 0.00 1.18
UGC 02588 One Disk B · · · · · · 21.50 (0.08) 5.47 (0.02) · · · 1.40
UGC 02588 One Disk R · · · · · · 20.41 (0.07) 5.79 (0.02) · · · 1.12
∗ UGC 02588 Bulge/Disk B 25.75 (2.43) 16.16 (1.75) 22.61 (0.97) 8.78 (0.08) 6.94 1.18
∗ UGC 02588 Bulge/Disk R 23.77 (2.50) 7.86 (0.75) 21.09 (0.48) 7.41 (0.04) 4.57 0.82
∗ UGC 02796 Two Disk B 19.07 (0.34) 1.89 (0.03) 21.02 (0.60) 8.75 (0.14) 6.90 0.48
∗ UGC 02796 Two Disk R 17.52 (0.27) 2.19 (0.03) 19.66 (0.41) 8.69 (0.08) 8.59 0.68
UGC 02796 Bulge/Disk B 23.07 (2.01) 13.80 (1.10) 22.54 (4.11) 8.92 (0.45) † 0.71
UGC 02796 Bulge/Disk R 21.01 (0.90) 10.26 (0.31) 20.49 (1.06) 6.76 (0.22) † 0.21
UGC 03119 One Disk B · · · · · · 19.82 (0.07) 10.35 (0.03) · · · 0.75
UGC 03119 One Disk R · · · · · · 17.92 (0.05) 9.48 (0.01) · · · 0.49
∗ UGC 03119 Bulge/Disk B 22.05 (9.17) 3.54 (0.91) 20.03 (0.31) 11.51 (0.08) 3.50 0.61
∗ UGC 03119 Bulge/Disk R 21.07 (5.81) 6.53 (1.29) 18.16 (0.24) 10.10 (0.02) 3.71 0.28
∗ UGC 03308 Two Disk B 19.83 (2.25) 0.87 (0.04) 21.70 (0.18) 15.47 (0.16) 3.47 0.35
∗ UGC 03308 Two Disk R 19.06 (1.32) 0.92 (0.03) 20.99 (0.13) 11.60 (0.06) 3.58 0.25
UGC 03308 Bulge/Disk B 18.36 (10.1) 0.43 (0.08) 21.71 (0.23) 15.66 (0.18) 3.62 0.37
UGC 03308 Bulge/Disk R 18.70 (5.77) 0.67 (0.08) 21.03 (0.18) 11.90 (0.08) 3.85 0.20
UGC 07598 One Disk B · · · · · · 21.69 (0.07) 7.96 (0.02) · · · 0.75
UGC 07598 One Disk R · · · · · · 19.82 (0.08) 6.26 (0.02) · · · 1.12
∗ UGC 07598 Bulge/Disk B 16.02 (6.98) 0.21 (0.03) 21.84 (0.13) 8.60 (0.03) 3.25 0.52
∗ UGC 07598 Bulge/Disk R 15.78 (3.90) 0.40 (0.03) 20.18 (0.21) 7.34 (0.04) 4.06 0.49
∗ UGC 08311 Two Disk B 20.82 (0.09) 2.94 (0.02) 23.29 (0.66) 14.91 (0.26) 12.50 0.48
– 22 –
Table 3—Continued
inner outer
Galaxy Fit Filterb µeff /µ0
c Reff/α
e αf Boundaryg Fith
Type a
mag arcsec−2
mag arcsec−2
[′′] [′′] Error
∗ UGC 08311 Two Disk R 20.28 (0.08) 3.22 (0.03) 22.73 (0.71) 15.81 (0.32) 13.64 0.39
∗ UGC 08644 One Disk B · · · · · · 22.56 (0.14) 8.79 (0.06) · · · 0.55
∗ UGC 08644 One Disk R · · · · · · 21.19 (0.15) 6.40 (0.04) · · · 0.78
∗ UGC 08904 Two Disk B 20.43 (0.16) 2.56 (0.02) 22.94 (0.71) 10.07 (0.15) 11.22 0.69
∗ UGC 08904 Two Disk R 18.90 (0.16) 2.20 (0.01) 22.05 (0.49) 9.67 (0.10) 11.19 0.35
UGC 08904 Bulge Only B 23.99 (0.27) 12.55 (0.09) · · · · · · · · · 1.17
UGC 08904 Bulge Only R 21.83 (0.25) 6.95 (0.04) · · · · · · · · · 0.48
∗ UGC 10894 One Disk B · · · · · · 21.65 (0.10) 7.90 (0.04) · · · 0.44
∗ UGC 10894 One Disk R · · · · · · 20.26 (0.12) 6.69 (0.04) · · · 0.42
UGC 11068 Two Disk B 20.15 (0.65) 1.15 (0.02) 22.46 (0.13) 13.65 (0.07) 4.87 0.31
UGC 11068 Two Disk R 18.67 (0.54) 1.25 (0.02) 21.02 (0.14) 11.26 (0.05) 5.23 0.54
∗ UGC 11068 Bulge/Disk B 20.36 (0.18) 1.00 (0.00) 22.53 (0.11) 14.10 (0.06) 5.31 0.38
∗ UGC 11068 Bulge/Disk R 19.89 (2.52) 1.66 (0.10) 21.18 (0.25) 11.97 (0.07) 6.23 0.42
∗ UGC 11355 Two Disk B 19.20 (0.15) 3.34 (0.02) 21.61 (0.18) 32.30 (0.24) 14.23 0.86
∗ UGC 11355 Two Disk R 17.30 (0.16) 3.01 (0.02) 19.83 (0.13) 24.07 (0.10) 13.02 0.64
UGC 11355 Bulge/Disk B 23.29 (0.95) 19.99 (0.72) 22.13 (0.56) 38.81 (0.57) 26.27 1.13
UGC 11355 Bulge/Disk R 20.70 (0.94) 10.96 (0.37) 20.19 (0.35) 26.30 (0.18) 26.30 0.93
∗ UGC 11396 Two Disk B 20.34 (1.82) 1.14 (0.05) 22.08 (0.27) 22.66 (0.39) 4.46 0.95
∗ UGC 11396 Two Disk R 19.77 (2.17) 1.23 (0.08) 20.20 (0.17) 13.04 (0.10) 2.80 1.69
UGC 11396 Bulge/Disk B 20.52 (9.40) 0.97 (0.19) 22.10 (0.35) 23.03 (0.45) 4.75 0.93
UGC 11396 Bulge/Disk R 20.09 (3.23) 1.09 (0.32) 20.21 (0.23) 13.03 (0.11) 3.03 1.69
∗ UGC 11617 One Disk B · · · · · · 21.30 (0.07) 12.83 (0.05) · · · 0.69
∗ UGC 11617 One Disk R · · · · · · 20.08 (0.08) 10.99 (0.04) · · · 0.46
∗ UGC 11840 No Fit B · · · · · · · · · · · · · · · · · ·
∗ UGC 11840 No Fit R · · · · · · · · · · · · · · · · · ·
UGC 12021 One Disk B · · · · · · 20.73 (0.06) 11.05 (0.02) · · · 1.63
UGC 12021 One Disk R · · · · · · 19.41 (0.06) 10.26 (0.02) · · · 1.58
∗ UGC 12021 Bulge/Disk B 26.49 (1.28) 23.19 (0.00) 20.88 (0.16) 11.59 (0.04) 2.37 1.54
∗ UGC 12021 Bulge/Disk R 23.85 (0.32) 23.19 (0.00) 19.96 (0.18) 11.60 (0.03) 6.23 1.01
Note. — Derivation of quantities is described in Section 3. Errors are given in parenthesis.
aThe type of fit made to the surface brightenss profile – bulge+disk, two exponential disks, or one exponential disk.
– 23 –
bOptical filter for the data described within that row.
cEffective surface brightness (R1/4 bulge fit) or central surface brightness (exponential disk fit) for the inner disk fit. See Equations 5
and 6.
dEffective radius (R1/4 bulge fit) or scale length (exponential disk fit) for the inner disk fit. See Equations 5 and 6.
eCentral surface brightness for the outer exponential disk fit.
fScale length for the outer exponential disk fit.
gBoundary between the inner and outer fits, defined by where the fitted lines cross.
hχ2 error for the fits.
∗Best fit – used for all further analysis.
†Fitted lines for the bulge and disk components do not cross.
Table 4. Properties of Hα Regions
Galaxy Regiona Hα Flux ×10−15 Hα Luminosity×1038 SFRb EWc Bd Rd B−Rd Diffusee
erg cm−2 s−1
erg s−1
M⊙ yr
[mag] [mag] [mag] [%]
UGC 00023 1 70 (14) 760 (150) 0.6 (0.1) 23 (1) 16.9 (0.1) 15.2 (0.1) 1.7 (0.2) 0.82 (0.04)
UGC 00189 1 10 (2) 110 (20) 0.09 (0.02) 30 (2) 19.9 (0.6) 18.3 (0.5) 1.7 (0.7) 0.98 (0.01)
UGC 01362 0 · · · · · · · · · · · · · · · · · · · · · · · ·
UGC 02299 1 12 (2) 240 (50) 0.19 (0.04) 20 (1) 21 (2) 21 (2) 0. (3) · · ·
UGC 02299 2 15 (3) 260 (50) 0.21 (0.04) 14 (1) 18.2 (0.2) 17.1 (0.2) 1.1 (0.3) · · ·
UGC 02299 TOTAL 22 (3) 490 (70) 0.39 (0.06) · · · · · · · · · · · · 0.45 (0.06)
UGC 02588 0 · · · · · · · · · · · · · · · · · · · · · · · ·
UGC 02796 1 74.8 (15) 900 (200) 0.71 (0.1) 10 (1) 16.5 (0.2) 14.8 (0.4) 1.7 (0.4) 0.67 (0.01)
UGC 03119 1 151 (30) 1600 (300) 1.3 (0.3) 16 (1) 18.8 (0.1) 16.7 (0.1) 2 (0.1) 0.77 (0.02)
UGC 03308 0 · · · · · · · · · · · · · · · · · · · · · · · ·
UGC 07598 1 6 (1) 80 (17) 0.07 (0.01) 13 (1) 22.2 (0.7) 21.5 (0.7) 0.6 (1) · · ·
UGC 07598 2 4.7 (0.9) 60 (12) 0.05 (0.01) 12 (1) 22 (1) 20.2 (0.9) 2 (1) · · ·
UGC 07598 3 2.4 (0.5) 32 (6) 0.03 (0.01) 13 (1) 23 (1) 21 (1) 2 (2) · · ·
UGC 07598 4 6 (1) 80 (20) 0.06 (0.01) 12 (1) 21.1 (0.8) 19.6 (0.7) 2 (1) · · ·
UGC 07598 5 3.3 (0.7) 43 (9) 0.03 (0.01) 13 (1) 23 (2) 22 (2) 2 (3) · · ·
UGC 07598 6 6 (1) 80 (20) 0.06 (0.01) 11 (1) 21 (3) 22 (3) -1 (4) · · ·
UGC 07598 7 4.1 (0.8) 50 (10) 0.04 (0.01) 11 (1) 22 (1) 21 (2) 1 (4) · · ·
UGC 07598 8 1.3 (0.3) 15 (3) 0.012 (0.003) 9 (1) 25 (3) 24 (3) 1 (3) · · ·
UGC 07598 9 32 (6) 430 (90) 0.34 (0.07) 13 (1) 18 (0.1) 16 (0.1) 1.9 (3) · · ·
UGC 07598 10 1.4 (0.3) 20. (4) 0.015 (0.003) 16 (1) 21.8 (0.5) 20.4 (0.5) 1.3 (0.5) · · ·
UGC 07598 11 10 (2) 130 (30) 0.11 (0.02) 11 (1) 21.4 (0.5) 20 (0.6) 1.4 (0.8) · · ·
UGC 07598 12 4.8 (1) 60 (10) 0.05 (0.01) 11 (1) 22 (1) 20.2 (0.9) 2 (1) · · ·
UGC 07598 13 4.4 (0.9) 60 (10) 0.05 (0.01) 12 (1) 21.8 (0.8) 20.1 (0.7) 2 (1) · · ·
UGC 07598 14 5 (1) 70 (10) 0.05 (0.01) 11 (1) 22.7 (1) 21 (1) 1 (2) · · ·
UGC 07598 TOTAL 69 (6) 1200 (100) 0.96 (0.08) · · · · · · · · · · · · 0.5 (0.1)
UGC 08311 1 7 (1) 14 (3) 0.011 (0.002) 20. (1) 21.3 (0.7) 21 (0.9) 0. (1) · · ·
UGC 08311 2 100 (20) 240 (50) 0.19 (0.04) 53 (1) 18.8 (0.4) 18.4 (0.5) 0.3 (0.6) · · ·
UGC 08311 3 7 (1) 16 (3) 0.012 (0.003) 18 (1) 20.8 (0.1) 20. (1) 1 (1) · · ·
UGC 08311 4 17 (3) 38 (8) 0.03 (0.01) 29 (1) 0.9 (0.1) 20.5 (0.9) -19.6 (0.9) · · ·
UGC 08311 5 8 (2) 18 (4) 0.015 (0.003) 25 (1) 22 (2) 22 (2) 1 (3) · · ·
UGC 08311 6 140 (30) 320 (60) 0.26 (0.05) 38 (1) 16.9 (0.1) 16.3 (0.1) 0.6 (0.1) · · ·
UGC 08311 7 15 (3) 32 (6) 0.03 (0.01) 19 (1) 18.6 (0.1) 18.1 (0.2) 0.6 (0.2) · · ·
UGC 08311 TOTAL 270 (30) 680 (80) 0.54 (0.07) · · · · · · · · · · · · 0.2 (0.1)
UGC 08644 1 5.1 (1) 44 (9) 0.04 (0.01) 17 (1) 21 (0.1) 20. (1) 1 (1) · · ·
Table 4—Continued
Galaxy Regiona Hα Flux ×10−15 Hα Luminosity×1038 SFRb EWc Bd Rd B−Rd Diffusee
erg cm−2 s−1
erg s−1
M⊙ yr
[mag] [mag] [mag] [%]
UGC 08644 2 4.8 (1) 42 (8) 0.03 (0.01) 20. (1) 22 (0.5) 21.1 (0.6) 0.9 (0.8) · · ·
UGC 08644 TOTAL 8.3 (1) 90 (10) 0.07 (0.01) · · · · · · · · · · · · · · ·
UGC 08904 1 3.4 (0.7) 47 (9) 0.04 (0.01) 10. (1) 22.4 (0.8) 22 (1) 1 (1) · · ·
UGC 08904 2 0.7 (0.1) 12 (2) 0.01 (0.009) 23 (1) 23.6 (0.5) 23.0 (0.6) 0.7 (0.8) · · ·
UGC 08904 3 1.2 (0.3) 20. (4) 0.02 (0.02) 16 (1) 24.3 (0.7) 24 (1) 1 (1) · · ·
UGC 08904 4 0.8 (0.2) 12 (2) 0.01 (0.009) 10. (1) 23.8 (0.4) 24 (1) 0. (1) · · ·
UGC 08904 5 0.7 (0.1) 9 (2) 0.01 (0.007) 10. (1) 24.5 (0.9) 24 (1) 1 (2) · · ·
UGC 08904 6 32 (6) 600 (100) 0.45 (0.09) 25 (1) 17.7 (0.1) 16.3 (0.1) 1.4 (0.1) · · ·
UGC 08904 TOTAL 33 (6) 700 (100) 0.53 (0.09) · · · · · · · · · · · · 0.6 (0.1)
UGC 10894 1 1.2 (0.2) 11 (2) 0.009 (0.002) 39 (3) 24 (2) 23 (1) 1 (2) · · ·
UGC 10894 2 1.4 (0.3) 13 (3) 0.010 (0.002) 37 (2) 22.1 (0.5) 21.3 (0.6) 0.8 (0.8) · · ·
UGC 10894 3 13 (3) 110 (20) 0.09 (0.02) 27 (1) 18.9 (0.2) 17.2 (0.1) 1.8 (0.2) · · ·
UGC 10894 4 1.9 (0.4) 18 (4) 0.014 (0.003) 31 (2) 23 (1) 22 (1) 1 (2) · · ·
UGC 10894 5 1.8 (0.4) 16 (3) 0.013 (0.003) 35 (2) 22.8 (0.7) 21.9 (0.7) 1 (1) · · ·
UGC 10894 6 2.8 (0.6) 26 (5) 0.020 (0.004) 36 (2) 22.4 (0.7) 22 (1) 0. (1) · · ·
UGC 10894 7 3.9 (0.8) 35 (7) 0.03 (0.01) 31 (2) 22.6 (0.8) 21.6 (0.8) 1 (1) · · ·
UGC 10894 TOTAL 23 (2) 230 (25) 0.18 (0.02) · · · · · · · · · · · · 0.6 (0.3)
UGC 11068 0 · · · · · · · · · · · · · · · · · · · · · · · ·
UGC 11355 1 180 (40) 600 (100) 0.48 (0.1) 17 (1) 19.7 (0.4) 18.2 (0.4) 1.5 (0.6) 0.87 (0.08)
UGC 11396 1 25 (5) 90 (20) 0.07 (0.01) 21 (1) 22 (1) 18.1 (0.2) 4 (1) 0.91 (0.01)
UGC 11617 1 9 (2) 40 (8) 0.07 (0.01) 12 (1) 21 (1) 21 (0.6) 0. (1) · · ·
UGC 11617 2 21 (4) 90 (20) 0.04 (0.01) 13 (1) 21 (2) 20. (2) 1 (3) · · ·
UGC 11617 3 13 (3) 50 (10) 0.04 (0.01) 13 (1) 22 (2) 20. (3) 1 (3) · · ·
UGC 11617 4 12 (2) 47 (9) 0.09 (0.01) 11 (1) 21 (2) 21 (1) 1 (3) · · ·
UGC 11617 5 26 (5) 110 (20) 0.03 (0.02) 14 (1) 21.5 (0.6) 21 (4) 1 (4) · · ·
UGC 11617 6 10 (2) 42 (8) 0.03 (0.01) 12 (1) 21.5 (0.2) 20.8 (0.7) 0.7 (0.7) · · ·
UGC 11617 TOTAL 69 (6) 390 (30) 0.31 (0.03) · · · · · · · · · · · · 0.8 (0.1)
UGC 11840 0 · · · · · · · · · · · · · · · · · · · · · · · ·
UGC 12021 1 13 (3) 50 (10) 0.04 (0.01) 31 (2) 19.2 (0.4) 17.8 (0.3) 1.4 (0.5) 0.97 (0.01)
Note. — Derivation of quantities is described in Section 3. Errors are given in parenthesis.
aInternal numbering scheme for the HII regions found by HIIphot.
bSFR for the region, defined as SFR =
1.26×1041erg s−1
cEquivalent width was calculated simply as the ratio of the total Hα flux to total Hα-subtracted continuum flux.
dTotal B and R magnitudes and colors within the HII regions
eDiffuse fraction found for the galaxy, defined defined here as the ratio of the fraction of Hα flux not found within the defined Hα regions to the
total Hα flux found for the entire galaxy.
†Due to both a (masked) star near the center of this galaxy and a (masked) CCD flaw (bad column) which also runs through the center of the
galaxy, a number of H II regions which should have been identified by HIIphot were not, artificially rasing the diffuse fraction on this galaxy, possibly
by as much as 20-30%.
– 27 –
Fig. 1.— Grey scale images of the observed galaxies. Figure available through the published
AJ paper or online at http://www.gb.nrao.edu/∼koneil.
Fig. 2.— Surface brightness profiles for all galaxies observed. The dash-dotted lines show
the inner fit, the dashed lines show the outer fit, and the solid lines show the combined fits.
Both the B (blue - bottom) and R (red - top) profiles are shown. Figure available through
the published AJ paper or online at http://www.gb.nrao.edu/∼koneil.
Fig. 3.— Color profiles for all the galaxies observed. Here the inner fits (when made) are
shown by a dashed line and the outer fits are shown by a solid line. Figure available through
the published AJ paper or online at http://www.gb.nrao.edu/∼koneil.
– 28 –
UGC 00189 UGC 10894
UGC 07598 UGC 08904
Fig. 4.— Example images showing the H II regions found by HIIphot for the galaxies
UGC 00189, UGC 10894, UGC 07598, and UGC 08904. The H II regions are outlines in
white. In the case of UGC 10894 two regions which were masked due to the presence of stars
can also be seen, outlined by the square white boxes.
– 29 –
18 20 22 24
µB,R(0) [mag arcsec
19 20 21 22 23 24 25
µB,R(0) [mag arcsec
Fig. 5.— (Left) Histogram showing the distribution of central surface brightnesses for the
observed galaxies. The (red) dashed line shows the R-band data and the (blue) solid line
shows the B-band data. (Right) Plot of the observed central surface brightness against scale
length of the outer disk. Here, the R-band data is demarcated by (red) open circles while
the B-band data uses (blue) filled circles.
22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5
<µB,R> [mag arcsec
22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5
<µB,R> [mag arcsec
Fig. 6.— Plots comparing the measured central surface brightnesses with the average surface
brightness for the galaxies, as defined by Equation 7 (left) and Equation 8 (right) and using
the magnitude and diameter values found herein. The R-band data is demarcated by (red)
open circles while the B-band data uses (blue) filled circles.
– 30 –
22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5
<µB,R> [mag arcsec
22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5
<µB,R> [mag arcsec
Fig. 7.— Plots comparing the measured central surface brightnesses, corrected for inclina-
tion, with the average surface brightness for the galaxies, as defined by Equation 7 (left) and
Equation 8 (right) and using the magnitude and diameter values found herein. The R-band
data is demarcated by (red) open circles while the B-band data uses (blue) filled circles.
Fig. 8.— Images of UGC 11355 with the stretch altered to show the galaxy’s nuclear bar
(left - R-band image) and star forming ring (right - Hα image). In both images the ellipse
shows the shape and size of the star forming ring. The images are 1.0′ across.
– 31 –
0 1 2 3 4
MHI/LR,B global [MO  · /LO  · ]
Fig. 9.— Gas mass to B and R-band luminosity ratios plotted against the global star
formation rate for the galaxies. The (blue) filled circles are for the B-band data and the
(red) open circles are for the R-band data.
– 32 –
−14 −16 −18 −20 −22
MB, global
S0 − Sa
Sab − Sb
Sbc−Sc
Sm−Im
Fig. 10.— Total B magnitude plotted against the average luminosity of the brightest three
Hα regions. (If less than three regions were found, the average of all H II regions was used.)
The filled (red) symbols are the data from our observations; the filled (blue) symbols are
from Helmboldt, et al. (2005); and the open (black) symbols are from Kennicutt & Kent
(1983).
– 33 –
−0.5 0.0 0.5 1.0 1.5 2.0 2.5
(B − R)global
Our Data
Helmboldt, et.al 2004
Jansen, et.al 2000
−0.5 0.0 0.5 1.0 1.5 2.0 2.5
(B−R)global
Our Data
Jansen, et.al 2000
Fig. 11.— Global color versus equivalent width (left) and star formation rate (right). To
insure any trends (or lack) remain the same, the data from this paper is shown both without
inclination correction (black) and with (gray). Note that inclination corrections are described
in Section 3. As the global SFR was not available for the Helmboldt, et al. (2005) data, it
is not shown on the right.
−17 −18 −19 −20 −21 −22 −23
MB,global
Our Data
Kennicutt & Kent 1983
Helmbolt, et.al 2004
−16 −17 −18 −19 −20 −21 −22
MB, global
Our data
Kennicutt 1983
Jansen et al. 2000
Fig. 12.— Total B magnitude plotted against the global equivalent width (left) and star
formation rate (right). As the global SFR was not available for the Helmboldt, et al. (2005)
data, it is not shown on the right.
– 34 –
36 37 38 39 40 41
log(LHalpha, eff/area) [erg s
−1 kpc−2]
Our Data
Oey, et.al
Fig. 13.— Luminosity surface brightness (Luminosity/area) plotted against the diffuse Hα
fraction for our sample and that of Oey, et al. (2006).
– 35 –
0.0 0.5 1.0 1.5 2.0
SFRtotal,region [MO  · /yr]
0 20 40 60 80 100 120 140
E.Wglobal [Å]
Fig. 14.— A comparison of regional and global star formation rate and equivalent width for
the studied galaxies. On the left is a plot of the global SFR against the total SFR found for
the individual H II regions, with a line demarcating the point where the global and regional
SFR are equal. On the right is a plot of the global EW against the average EW for the
individual H II regions.
– 36 –
19 20 21 22 23 24 25
µB,R(0) [mag arcsec
Fig. 15.— Central surface brightness versus global star formation rate for the observed
galaxies. The (red) open circles are from the R band data, while the (blue) filled circles are
for the B data.
– 37 –
0.001 0.010 0.100 1.000 10.000
MHI/LB global [MO  · /LO  · ]
Our Data
Kennicutt & Kent 1983
Helmbolt, et.al 2004
0.1 1.0 10.0
MHI/LR global [MO  · /LO  · ]
Our Data
Oey, et.al (2006)
Helmboldt, et.al (2005)
Fig. 16.— Gas mass to luminosity ratios plotted against global equivalent widths (left) and
diffuse Hα fractions (right). On the left, the (black) circles are our data, the (blue) triangles
are from Kennicutt & Kent (1983) and the (red) diamonds are from Helmboldt, et al. (2004).
On the right, the (black) circles are again our data, while the (blue) asterisks are from Oey,
et al. (2006).
– 38 –
−0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0
(B − R)region
Fig. 17.— This plot shows the regional colors versus star formation rates for the observed
galaxies.
