Exploring Infrared Properties of Giant Low Surface Brightness
Galaxies
Nurur Rahman1, Justin H. Howell, George Helou, Joseph M. Mazzarella, and Brent
Buckalew
Infrared Processing and Analysis Center (IPAC)/Caltech,
Mail Code 100-22, 770 S. Wilson Avenue,
Pasadena, CA 91125, USA
nurur@ipac.caltech.edu
ABSTRACT
We present analysis of Spitzer Space Telescope observations of the three low
surface brightness (LSB) optical giant galaxies Malin 1, UGC 6614 and UGC
9024. Mid- and far-infrared morphology, spectral energy distributions, and in-
tegrated colors are used to derive the dust mass, dust-to-gas mass ratio, total
infrared luminosity, and star formation rate (SFR). We also investigate UGC
6879, which is intermediate between high surface brightness (HSB) and LSB
galaxies. The 8 µm images indicate that polycyclic aromatic hydrocarbon (PAH)
molecules are present in the central regions of all three metal-poor LSB galax-
ies. The diffuse optical disks of Malin 1 and UGC 9024 remain undetected at
mid- and far-infrared wavelengths. The dustiest of the three LSB galaxies, UGC
6614, has infrared morphology that varies significantly with wavelength; 160 µm
(cool) dust emission is concentrated in two clumps on the NE and NW sides of
a distinct ring seen in the 24 and 8 µm images (and a broken ring at 70 µm) at
a radius of ∼40′′ (18 kpc) from the galaxy center. The 8 and 24 µm emission is
co-spatial with Hα emission previously observed in the outer ring of UGC 6614.
The estimated dust-to-gas ratios, from less than 10−3 to 10−2, support previous
indications that the LSB galaxies are relatively dust poor compared to the HSB
galaxies. The total infrared luminosities are approximately 1/3 to 1/2 the blue
band luminosities, suggesting that old stellar populations are the primary source
of dust heating in these LSB objects. The SFR estimated from the infrared data
ranges ∼0.01− 0.88 M⊙ yr−1, consistent with results from optical studies.
Subject headings: galaxies: spirals - galaxies: dust - galaxies: ISM - galaxies:
structure - galaxies: individual (Malin 1, UGC 6614, UGC 6879, UGC 9024)
1National Research Council (NRC) Postdoc Fellow
http://arxiv.org/abs/0704.1483v1
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1. Introduction
The unprecedented imaging and spectroscopic sensitivity and higher spatial resolution
of the Spitzer Space Telescope (SST; Werner et al. 2004) compared to past IR missions
such as the Infrared Astronomical Satellite (IRAS; Neugebauer et al. 1984) and the Infrared
Space Observatory (ISO; Kessler et al. 1996) provide a unique opportunity to probe the
inter-stellar medium (ISM) of optically faint sources such as low surface brightness (LSB)
galaxies. The goal of this study is to use the Spitzer data to analyze the IR properties of
three LSB optical giants: Malin 1, UGC 6614, and UGC 9024. This is the first time we are
able to view these galaxies in ∼3-160 µm wavelength range.
The LSB galaxies are usually defined as diffuse spiral disks with low B-band central
surface brightness (e.g. µB,0 ≥ 23 mag arcsec−2; Bothun et al. 1997; Impey & Bothun 1997).
These galaxies are either blue (B-V . 0.5) or red (B-V & 0.8) in color (O’Neil et al. 1997),
metal poor ([O/H] . 1/3Z⊙; McGaugh 1994; de Blok & van der Hulst 1998b; de Naray et al.
2004), rich in neutral hydrogen (H I) (Schombert et al. 1992; O’Neil et al. 2004), deficient
in H II emission (McGaugh et al. 1995; de Naray et al. 2004), and have low star formation
rate (SFR) . 0.1 M⊙ yr
−1 (van den Hoek et al. 2000; Burkholder et al. 2001). The majority
of these galaxies lack molecular (CO) gas (Schombert et al. 1990; de Blok & van der Hulst
1998a); only a handful have been reported to have molecular emission (O’Neil & Schinnerer
2004; Matthews et al. 2005; Das et al. 2006).The observed properties suggest that LSB disks
are relatively unevolved systems and may have a different evolutionary history compared to
their high surface brightness (HSB) counterparts (McGaugh 1994; van den Hoek et al. 2000;
Vallenari et al. 2005; Zackrisson et al. 2005).
Most of our knowledge regarding the composition and structure of the ISM of LSB spirals
comes from optical (Impey et al. 1996) and H I surveys (O’Neil et al. 2004). These surveys
have demonstrated that the neutral hydrogen is by far the dominant component of the ISM
in these galaxies (∼95% by mass; Matthews 2005). While we have improved understanding
of the gaseous component of the ISM from decade long H I surveys, our knowledge of the
inter-stellar dust, the component of the ISM radiating from mid-IR (∼8 µm) through sub-
millimeter (∼850 µm) wavelengths, is still very limited. Because of the scarcity of information
in this wavelength range, complementary observational facts such as low metal abundance
(McGaugh 1994), strong similarities in optical and near-IR morphology (Bergvall et al. 1999;
Bell et al. 2000), transparency of the stellar disks (O’Neil et al. 1998; Matthews et al. 1999),
and deficiency in molecular emission (Schombert et al. 1990; de Blok & van der Hulst 1998a)
have been used to probe the ISM of LSB galaxies. All these observations lead to a general
consensus that the LSB disks are deficient in dust and molecular gas.
Given that LSB spirals comprise ∼50% of the local disk galaxy population (McGaugh
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et al. 1995b), they deserve equal attention as their HSB cousins. To develop a consistent
picture of the local galaxy populations it is therefore necessary to probe each population
at all wavelength regimes as have been done in most cases for HSB galaxies. Previous long
wavelength studies on LSB galaxies involved a few cases in sub-millimeter and millimeter
wavelengths (de Blok & van der Hulst 1998b; Pickering & van der Hulst 1999; O’Neil &
Schinnerer 2004; Matthews et al. 2005; Das et al. 2006). Hoeppe et al. (1994) made
the first attempt to investigate long wavelength (60 µm, 100 µm, and 20 cm) properties of
LSB dwarfs; however, no study has been made in the mid-IR and far-IR for LSB disks. In
this study we have made the first attempt to explore ∼3-160 µm properties of three LSB
giants: Malin 1, UGC 6614, and UGC 9024, recently observed by the SST. We focus on the
IR morphology to probe extent of dust in the ISM, on the SEDs for the IR energy budget
and dust content, and on the IR colors to establish the dust temperature of the ISM. The
organization of this paper is as follows: we describe observation and data reduction in section
§2 and present our results in section §3. Discussions and conclusions are given in section §4.
2. Observation and Data Reduction
We describe the Infrared Array Camera (IRAC; Fazio et al. 2004) and Multiband
Imaging Photometer for Spitzer (MIPS; Rieke et al. 2004) imaging data for Malin 1, UGC
6614, and UGC 9024. These extended disk galaxies with radial scale length hr,R > 5 kpc
are observed as part of a larger guaranteed time observing program (Spitzer PID #62). The
program also includes two LSB galaxies (UGC 5675 and UGC 6151) with 2 kpc < hr,R <
3 kpc, an edge-on disk (UGC 6879) with hr,R ∼2.5 kpc, and a HSB dwarf (UGC 10445)
with hr,R ∼1 kpc. The central brightness of UGC 6879 is µB,0 ∼ 20.4 mag arcsec−2. A
simple correction for inclination, using µ0,face−on = µ0,observed+2.5 log(a/b) brings its central
brightness to ∼ 21.71 mag arcsec−2. It is lower than the conventional choice (µ0 ≈ 23 mag
arcsec−2) but close to the Freeman value (µ0 ∼ 21.65 mag arcsec−2; Freeman 1970). The
central brightness of this galaxy falls in between the range of LSB and HSB galaxies and
hence, we include it as a representative of the intermediate class. The properties of UGC
5675 and UGC 6151 will be explored in a forthcoming paper. The readers are refereed to
Hinz et al. (2006) for an analysis of UGC 10445.
The IRAC 3.6, 4.5, 5.8, and 8 µm images and the MIPS 24, 70, and 160 µm images
were acquired, respectively, in the mapping and photometry modes. The IRAC images were
reduced with the standard Spitzer Science Center data pipeline, and aligned, re-sampled, and
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combined into a mosaic image using the Mopex1 software. The MIPS 24 µm data required
the use of the self-calibration procedure described in the MIPS Data Handbook2 to remove
latent image artifacts. The corrected images were then combined into a mosaic using Mopex.
Time filtering and column filtering was applied to the 70 µm images using IDL routines
created by D. Fadda. The filtered images were then combined using Mopex. The 160 µm
images were combined into a mosaic using Mopex. The IRAC spatial resolution is ∼2′′ for
all bands. The MIPS spatial resolutions are 6
, and 40
for the respective bands.
Sky subtraction was carried out through the use of multiple sky apertures placed near
the source which do not overlap with the faintest isophotes visible from the galaxy. For
each galaxy we measured flux densities from the sky subtracted images within the aperture
covering the entire galaxy. The flux density contributed by foreground stars within a galaxy
aperture was removed by measuring each such star in a small aperture and subtracting
the result from the total flux within the galaxy aperture. The calibration uncertainty in
the IRAC flux densities is at the level of ∼10% (Reach et al. 2005; Dale et al. 2005).
Aperture corrections have been applied to all IRAC flux densities (T. H. Jarrett 2006; private
communication). The MIPS flux density calibration uncertainties are 10% at 24 µm and 20%
at 70 and 160 µm.
Near-IR (1.3, 1.7, and 2.2 µm) flux densities from the Two Micron All Sky Survey
(2MASS; Jarrett et al. 2000), upper limits on IRAS flux densities, and those derived from
the IRAC and MIPS bands are given in Table 1. Basic properties of the galaxies obtained
from the literature, the NASA/IPAC Extragalactic Database (NED), the Lyon-Meudon Ex-
tragalactic Database (LEDA), and derived in this study are summarized in Table 2. The
IRAS flux densities for UGC 6614 and UGC 6879 were computed using the SCANPI tool
available from IRSA as linked via NED where IRAS flux density limits represent SCANPI’s
inband total measurement, fν(t). No IRAS detections were available for Malin 1 and UGC
9024. Distances for the galaxies are estimated from heliocentric radial velocity after correct-
ing for the local group, infall into the Virgo cluster, the Great Attractor, and the Shapley
supercluster following the Mould et al. (2000) flow model.
1http://ssc.spitzer.caltech.edu/postbcd/
2http://ssc.spitzer.caltech.edu/mips/dh/
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2.1. Contamination from Galactic Cirrus
A basic concern about faint extragalactic sources with highly diffuse disk structure is
confusion by foreground Galactic “cirrus” emission. This is especially critical in the case of
far-IR cool sources (defined below) such as LSB galaxies. The far-IR ratio of typical local
cirrus is S60µm/S100µm ≤ 0.2 (Gautier 1986). UGC 6614 and UGC 6879 are well above this
limit (Fig. 8c). From its Galactic latitude b ∼ +22o and the observed S70µm/S160µm ratio, it
is reasonably safe to assume that UGC 9024 has not been affected by cirrus emission. In the
absence of far-IR information it is uncertain for Malin 1. However, its high galactic latitude
b ∼ +14◦ can be used to argue against any cirrus contamination.
We should also stress the fact that whether there is cirrus in the foreground is less
significant as compared to whether the cirrus in the foreground varies on scales of the IRAC
and MIPS field of view. If its spatial variation is negligible across the IRAC and MIPS
mosaic, it gets subtracted out as “sky” in the data reduction process.
3. Results
In this section we present IR morphology, SEDs, and IR colors of LSB galaxies. Using
this information we estimate dust mass (Md), dust-to-(atomic) gas ratio (D), total infrared
luminosity (LTIR), and star formation rate (SFR). To obtain a qualitative assessment of the
observed properties of LSB disks compared to their HSB counterparts, we take the Spitzer
Infrared Nearby Galaxy Survey (SINGS; Kennicutt et al. 2003) sample as a representative
sample of local HSB galaxies. This sample contains 75 galaxies of various Hubble types as
well as dwarfs and irregular galaxies, and thus making it a suitable reference for comparative
analysis with LSB spirals.
3.1. Infrared Morphology
The IR emission beyond ∼25 µm is dominated by the inter-stellar dust under various
heating conditions. On the other hand, mid-IR (∼5-25 µm) emission marks the transition
from stellar photospheres to inter-stellar dust dominating the emission. Whereas the mor-
phology of a galaxy at 3.6 and 4.5 µm represents the stellar disk, at ∼5 µm and beyond it
shows the structure of the ISM. The IRAC 3.6 and 4.5 µm bands are sensitive to the under-
lying stellar populations typically consisting of red giants and old stars. In some galaxies 3.6
µm band is also known to contain emission from a hot dust component (Bernard et al. 1994;
Hunt et al. 2002; Lu et al. 2003; Helou et al. 2004). The hot dust is also visible in the other
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IRAC bands. While 3.6 µm is only sensitive to hot dust near the sublimation temperatures
(∼1000 K), the longer wavelength IRAC bands can detect dust at lower temperatures down
to several hundred Kelvin.
The IRAC 5.8 and 8 µm bands are primarily sensitive to the PAH emission at 6.2, 7.7,
and 8.6 µm (Puget & Leger 1989; Helou et al. 2000; Lu et al. 2003). The PAH is the hot
component of the inter-stellar dust with effective temperature Td > 100 K stochastically ex-
cited to high energy levels by stellar photons. Stellar photospheric emission also contributes
to these two IRAC wavebands. The fraction of stellar emission at 5.8 and 8 µm, respectively,
are ∼40% and ∼20% (Helou et al. 2004; Pahre et al. 2004).
The emission detected by the MIPS bands are coming from dust grains with different
size distributions. Very small grains (∼1-10 nm) emit in the mid-IR region (&15 µm),
intermediate between thermal equilibrium and single-photon heating. Large, classical grains
(∼100-200 nm) are in thermal equilibrium with the radiation field and responsible for far-IR
emission (Desert et al. 1990). To begin with we should bear in mind that the demarcation
lines among various heating environments are ad-hoc and we assume the following effective
color temperature ranges for large grains in thermal equilibrium: warm (40 K . Td . 100
K), cool (20 K . Td . 30 K), cold (10 K . Td . 20 K), and very cold (10 K . Td).
The Solan Digital Sky Survey (SDSS) images of target galaxies are shown in Fig. 1.
The Spitzer images of the LSB galaxies are shown in Fig. 2 (Malin 1), Fig. 3 (UGC 6614),
and Fig. 4 (UGC 9024). UGC 6879 is shown in Fig. 5. Galaxy images are shown using
Gaussian equalization (Ishida 2004). The images are oriented such that north is up and east
is to the left. Malin 1 is too faint to be detected by the MIPS 70 and 160 µm channels. The
other three galaxies were detected by all IRAC and MIPS bands. We do not show 5.8 µm
images in these figures since the morphological appearances of each of these galaxies at 5.8
µm closely follow that at 8 µm. For all galaxies, the IRAC 4.5 and 8 µm images are shown
without subtracting stellar photospheric emission. The contours represent surface brightness
with intervals of
10 where the lowest level is 4σ above the background. The lowest level
of contours at different bands are 0.04 (3.6 µm), 0.05 (4.5 µm), 0.30 (8 µm), 0.08 (24 µm),
1.6 (70 µm), and 2.1 (160 µm) expressed in MJy/Sr.
The giant optical disks in Malin 1 and UGC 9024 appear as point sources in the IRAC
images. While we detect the stellar bulges of these galaxies, their optically diffuse disks
remain undetected long-ward of the IRAC bands. This suggests that the low surface bright-
ness structures at larger radii might be photometrically distinct components rather than
smoothed extensions of the normal inner disks (see Barth 2007 for a discussion on Malin 1
based on Hubble data). That the disks appear in the B-band but are undetected at 3.6 µm
suggest that these disks have a small population of young stars rather than a large popu-
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lation of old stars. The bulge spectrum of Malin 1 is consistent with a predominantly old
stellar population (Impey & Bothun 1989). For both of these galaxies, the mid-IR emissions
at 8 and 24 µm are concentrated in the central few kpc, within a region of 12′′ (20 kpc)
radius for Malin 1 and 24′′ (5 kpc) radius for UGC 9024.
Undetected far-IR emission from the disk of Malin 1 implies that it contains either very
cold (Td < 10 K) cirrus-like dust emitting in the submillimeter and millimeter wavelengths
or it lacks cold dust altogether and contains only neutral gas. For UGC 9024, 70 µm emission
comes from the central region but 160 µm emission is very hard to measure because of large
scale diffuse emission in the field. This result in an upper limit of S160 µm < 268 mJy.
The optical morphology of UGC 6614 shows a massive bulge and spiral structure. A
thin ring ∼40′′ from the core is prominent in Hα (McGaugh et al. 1995). The 3.6 and
4.5 µm emission is spread over the entire disk of this galaxy. The 3.6 µm image shows a
discernible spiral arm pattern closely resembling the optical morphology. At 4.5 µm this
feature disappears and the disk shrinks in radius showing only its inner region. This galaxy
appears markedly different at 5.8 and 8 µm compared to the other IRAC bands. The 8 µm
morphology suggests that the PAH emission is coming from two distinct regions: the central
bulge and an outer ring surrounding the bulge. The 8 µm morphology closely traces the
Hα image. The MIPS 24 µm morphology is similar to the 8 µm PAH emission although
the outer ring appears a bit more disjointed at this band. The dust emission at 24 µm is
coming mostly (∼70%) from the central disk. The lower resolution image at 70 µm indicates
that only ∼25% of the dust emission is coming from the central part of the galaxy, with the
remaining ∼75% emission co-spatial with the ring of radius ∼40′′. Surprisingly a dumbbell
shaped region is the dominant source (∼90%) of 160 µm emission; these two peaks are
located on the NE and NW sides of the ring. The far-IR images of this galaxy show a small,
localized region within the ring, SW from the center. Whether or not this region coincide
with the spatial location showing the CO emission in this galaxy disk (Das et al. 2006) is
not entirely clear. We will investigate this in the forthcoming paper.
UGC 6879 is an edge on spiral with a red central part and a blue (optical) disk. This
radial color gradient is perhaps related to greater concentrations of dust in the nucleus than in
the overall disk. Being a transitional disk, with a central surface brightness in between HSB
and LSB spirals, it is not unexpected to find that UGC 6879 is a strong IR emitting source
compared to the LSB galaxies. Both PAH emission and warm dust (24 µm ) emission show
spatial variation along the disk. These emissions peak at the central region and diminish
toward the edge.
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3.2. Infrared Diagnostics
3.2.1. IR Spectral Energy Distributions
The flux densities obtained from the 2MASS and IRAS archives and those estimated
from the IRAC and MIPS images are given in Table 1. These flux densities are used to
construct the observed infrared SEDs of LSB galaxies as shown in Fig. 6. The open and
solid circles represent, respectively, 2MASS and IRAC data. The open triangles represent
the MIPS data whereas the IRAS upper limits for UGC 6614 and UGC 6879 are shown by
filled triangles. Since Malin 1 was undetected by the MIPS far-IR channels we show the
detection limits at 70 and 160 µm for the total integration time (∼252 sec. and ∼42 sec.
respectively). We include four SINGS galaxies with different ISM properties for comparison.
These are NGC 0337 (a normal star forming galaxy), NGC 2798 (a star burst galaxy), NGC
2976 (a normal galaxy with nuclear H II region), and NGC 3627 (a Seyfert galaxy). They are
shown, respectively, by dotted, dashed, dashed-dotted, and long dashed lines. The motive
is to make a visual comparison between SEDs of LSB and HSB galaxies. All flux densities
are normalized by the 3.6 µm flux density.
There are several noticeable features in these SEDs. First, the amplitude of the mid-IR
and far-IR dust emissions of LSB galaxies are lower compared to those of HSB galaxies. An
obvious and expected result is that the LSB galaxies have less dust content and hence have
lower IR emission. Second, Malin 1 is deficient in the integrated 8 µm emission compared
to its 4.5 µm emission. This is quite opposite for other LSB galaxies and more like the
SED of an elliptical galaxy (see Fig. 4 in Pahre et al. 2004). Third, the 24 µm emission
is suppressed in both Malin 1 and UGC 9024. For UGC 6614 it is slightly above the IRAC
bands. This feature suggests that the ISM of UGC 9024 lacks warm dust emission, and
is made mostly of cool dust. Fourth, the SEDs show a tendency to turnover at relatively
longer wavelength, a signature that the low density and low surface brightness ISM have low
radiation intensity. On the other hand, the shape of the SED of UGC 9024 is quite similar
to those of the representative HSB galaxies. We discuss this in detail in section §3.3
3.2.2. Dust Mass
Dust mass is frequently estimated by fitting the far-IR peak by a single temperature
modified blackbody function (Hildebrand 1983). However, the inability of a single tempera-
ture or even two temperature blackbody function to fit the observed flux densities suggests
that a more sophisticated model of the IR SED is needed. The global SED models of Dale
et al. (2001) and Dale & Helou (2002; hereafter DH02) provide a robust treatment of the
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multiple grain populations that contribute to the IR emission in a galaxy. This model allows
a realistic derivation of dust mass since it combines information from the full range of heating
environments (∼10K-1000K). Previous studies have shown that dust mass is underestimated
by a factor of ∼5-10 for quiescent galaxies (i.e. IR cool) if one simply fits the far-IR and
sub-millimeter continuum data points with a simple single temperature black body instead
of exploiting information from the full range of the SED.
Figure 7 shows the fits to the observed SEDs obtained from the DH02 model (solid
line). The dashed and dotted lines represent, respectively, empirical stellar SED (Pahre
et al. 2004) and stellar synthesis model prediction fitted only to the 2MASS fluxes from
Vazquez & Leitherer (2005). We use the model fit (solid line) to estimate Md and D noting
that DH02 model does not provide ideal fits to very extended, low density, diffuse disks like
Malin 1 and UGC 9024. However, within the measurement and observational uncertainties
the model fits provide useful insight. A rigorous and detailed treatment of infrared SEDs of
LSB disks will be presented in a future study.
The estimated mass is given in Table 2. We find that the ISM of UGC 6614 has the
highest amount of dust with dust-to-gas ratio, D ∼0.01. Both Malin 1 and UGC 9024 are
∼3 times less dusty than UGC 6614. Given that IR emission coming only from the central
regions of the latter two galaxies, it is not surprising that they show low dust content. In
a recent study Das et al. (2006) detected CO(1-0) emission localized in a specific region on
the disk of UGC 6614. They estimated molecular gas mass (MH2 ∼ 2.8 × 108M⊙) which is
almost equal to the total dust content (Md ∼ 2.6 × 108 M⊙) that we measure distributed
over the bulge and disk. The difference between D and dust-to-(total) gas mass is negligible.
The (systematic) calibration uncertainty in the observed flux densities and the uncer-
tainty in the distance estimates result in a ∼30% uncertainty in Md. Additional uncertainty
comes from the mass absorption coefficient. The different sizes used to measure IR and
H I fluxes will also attribute additional uncertainty in D. Besides, considering that the
long-wavelength end of the SED is poorly constrained the estimate of overall dust mass
and gas-to-dust ratio is a bit uncertain. All of these errors compound to make Md and D
uncertain by a factor of ∼2 or more.
3.2.3. Infrared Luminosity
DH02 proposed a simple relation to compute total IR luminosity using the MIPS bands
(see Eq. 4 in DH02). Due to uncertainties in the MIPS flux densities for Malin 1 and UGC
9024 we use the empirical relation given by Calzetti et al. (2005) to estimate LTIR. Calzetti
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et al. relate flux densities at 8 µm and 24 µm to derive LTIR for M 51, a normal star forming
galaxy. Estimated total IR luminosities are given in Table 2 with estimated uncertainty of
∼35%. We find comparable IR luminosity for both Malin 1 and UGC 6614. UGC 9024 is the
least luminous because of its suppressed 24 µm warm dust emission. In spite of its border
line HSB nature, the IR output of UGC 6879 resembles a normal quiescent galaxy.
We also estimate LTIR using DH02 model fits. We follow Sanders & Mirabel (1996) to
define LTIR where flux densities at 12, 25, 60, and 100 µm are obtained from the model SEDs
by interpolation. The result is presented in Table 2. Interestingly, these two estimates are
within a factor of ∼2.5 where the model estimates are always higher. The largest difference
is noted for UGC 9024 which is a result of relatively poor fit to the data.
Within the uncertainty, the IR energy output of LSB galaxies are smaller by a factor
of a few compared to their B-band luminosities LB. The infrared-to-blue ratio, LTIR/LB,
compares the luminosity processed by dust to that of escaping starlight (see Table 2). The
ratio ranges from <0.01 (in quiescent galaxies) to ∼100 (in ultra-luminous galaxies). It can
be used to characterize optical depth of a system composed of dust and stars as well as
recent (∼100 Myr) SFR to the long term (∼1 Gyr) average rate. The ratio is in the range
0.3-0.5 (see Table 2) indicating that the current level of star formation is low, a consistent
result with previous studies on star formation in the LSB ISM (van den Hoek et al. 2000;
Burkholder 2001). However, there is a potential degeneracy in this parameter and one can
only make an indirect assessment of the ISM of a galaxy if this degeneracy can be lifted.
The fact that LTIR/LB is less than unity can arise from two different physical conditions. In
one hand, a galaxy may be undergoing intense heating by young stars (large LB) but have
very little neutral ISM left (less IR emission) resulting in low LTIR/LB. On the other hand,
a quiescent galaxy may generate most of its IR emission in H I clouds heated by older stellar
populations and will display a similarly low LTIR/LB (Helou 2000).
3.2.4. Star Formation Rate
A widely used recipe for estimating SFR from IR luminosity is given by Kennicutt
(1998). However, far-IR luminosity in Kennicutt model is based on IRAS data. Without a
proper calibration between IRAS and MIPS far-IR flux densities the uncertainty will loom
large in the SFR estimate. We use a new SFR estimator, derived recently by Alonso-Herrero
et al. (2006; hereafter AH06) using 24 µm flux density. Our estimates are given in Table 2.
The error associated with this SFR is ∼10%.
van den Hoek et al. (2000) derived a current SFR ranging ∼0.02-0.2 M⊙ yr−1 from
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I-band photometry for a sample of LSB galaxies generally found in the field. Their estimate
agree within a factor of two with our results based on infrared data. For Malin 1 and UGC
6614 the infrared SFR are ∼0.38 M⊙ yr−1 and ∼0.88 M⊙ yr−1 , respectively, whereas it is
∼0.01 M⊙ yr−1 for UGC 9024. The fact that these galaxies have low dust content indicates
that extinction is less likely to cause the difference between SFR derived from IR and optical
data. The higher SFR of UGC 6614 compared to the other two galaxies is consistent with
the fact that this galaxy has a prominent Hα morphology which indicate modest level of
current star formation. A very low SFR for UGC 9024 implies that more light is scattered
off from the central disk than being absorbed by the ISM.
The SFR of these three LSB galaxies, as derived from IR data, are thus significantly
below the rate ∼5-10 M⊙ yr−1 derived for the HSB galaxies (Kennicutt 1998) but consider-
ably larger than the rate ∼0.001-0.01 M⊙ yr−1 observed typically in dwarf irregular galaxies
(Hunter & Elmegreen 2004).
The star formation efficiency (SFE), quantified by LTIR/MHI, is also shown in Table
2. This measure represents the amount of unprocessed gas available to be consumed in
subsequent star formation. As expected the LSB galaxies have a SFE that is ≤ 1/5 of the
SFE of HSB galaxies.
3.3. Infrared Colors
Panels 8a and 8b show, respectively, mid-IR colors and the well known PAH-metallicity
connection. In these diagrams we highlight two extreme classes (in terms of their IR SEDs)
of HSB galaxies: the dwarf systems represented by the decimal numerals and the massive
elliptical galaxies represented by the roman numerals. All of these galaxies are obtained
from the SINGS sample.
Panel 8a compares PAH emission in various classes of galaxies. Metal-rich HSB galaxies
preferentially show high ratio in S8µm/S24µm and low ratios of non-stellar 4.5-to-8 µm dust
emission. Metal poor (e.g. dwarfs and irregulars) and extremely metal poor HSB galaxies
such as blue compact dwarfs (BCDs), show the opposite trend and fall within the PAH-
deficient box (Engelbracht et al. 2005; hereafter E05). Note that E05 only looked at star-
forming galaxies. “Red and dead” ellipticals should have little or no PAH emission, yet are
nowhere near the PAH-deficient region. So this region will not necessarily contain all PAH
deficient galaxies. Also note that the result of E05 could also arise from selection bias since
recent studies have shown that dwarf galaxies are not necessarily PAH defficient systems
(Rosenberg et al. 2006).
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Panel 8b, on the other hand, illustrates the PAH-Z connection in HSB galaxies. E05
showed that galaxies with low PAH emission have relatively unpolluted ISMs. They noted
a sharp boundary between galaxy metallicity with and without PAH emission, although the
trend may have been affected by selection bias. We show this trend in panel 8b, where the
PAH deficient galaxies reside inside the dashed region and galaxies with higher metallicites
avoid two regions in the diagram which are shown by the dotted lines. We are interested to
see where the LSB galaxies fit in these diagrams.
Panel 8a shows that the LSB galaxies are different than the PAH-deficient dwarf galaxies
in terms of their mid-IR colors. In the color space, LSB galaxies occupy a region similar to
HSB galaxies and reside significantly farther (> 3σ, along the horizontal axis) from PAH-
deficient galaxies. While UGC 6614 stays right in the middle of the locus, both Malin 1 and
UGC 9024 falls in the edge because of the shapes of their SEDs at 8 and 24 µm. The mid-IR
colors of these galaxies closely resemble those of elliptical galaxies which is surprising given
that their apparent different star formation histories.
The LSB galaxies are metal poor with Z ≤ 1/3Z⊙ (McGaugh 1994; de Bloke & van
der Hulst 1998b; de Naray et a. 2004). McGaugh (1994) provided an estimate of oxygen
abundance for UGC 6614 but it is highly uncertain. Following the general trend shown by
the LSB galaxies we assign one-third solar value to Malin 1 and a solar value to UGC 6879.
The full range and the median values of published oxygen abundance are shown for UGC
6614 and UGC 9024.Having very limited information it is, therefore, extremely difficult to
explore the PAH-metallicity connection for these galaxies. We are interested in the question
whether LSB galaxies being low Z systems, will appear close to the PAH-deficient box or
will they fall in the region shunned both by HSB dwarfs and HSB spirals with extended disks
(dotted regions; Fig 8b)? While it is tempting to give more weight to the latter case, only
three data points with large errors are insufficient to derive any trend. A larger sample of
LSB galaxies with 8 µm detections is needed to shed more light on this topic.
Panel 8c shows the connection between mid-IR and far-IR colors which basically describe
the nature of dust emission at these wavelengths. In this panel, the far-IR color of Malin 1
is shown with respect to a far-IR flat SED, i.e. same flux density at 70 and 160 µm. For the
other two galaxies the diagram shows rather low S70µm/S160µm and high S8µm/S24µm ratios.
While low far-IR color suggest that they are IR cool sources, the mid-IR color may be linked
with the destruction of very small grains (but not the PAH molecules) and thus can be used
as a parameter which can signal evolutionary stages of an ISM. A higher value of S8µm/S24µm
can mean that S8µm is high (large amount of PAH) or that S24µm is low (little or no warm
dust). Ellipticals are in the latter category (see Pahre et al. 2004) whereas LSB systems are
in the first category. It should be noted that the 24 µm emission is very closely associated
– 13 –
with H II regions (Helou et al. 2004). Therefore, the lack of emission at this wavelength is
more likely a deficiency in H II region which is quite consistent with the Hα images of LSB
galaxies (McGaugh et al. 1995; de Naray et al. 2004).
The vertical arrow in panel 8d represents a probable range of IRAS far-IR color for UGC
9024 assuming that Spitzer far-IR ratio is the same as the IRAS far-IR ratio. The sequence of
IR colors can be associated with a progression toward greater dust heating intensity and thus
with a sequence of star formation activity. The cool end of the color sequence corresponds to
cool diffuse H I medium and quiescent molecular clouds, whereas the warm end corresponds
to the colors of H II regions, starbursts, and galaxies with higher LTIR/LB ratios. Although
the IR nature of these three LSB galaxies are explicit in this diagram, the interesting feature
is that they are not the extreme cases in terms of IR SEDs as shown by some of the SINGS
galaxies.
Two primary sources have been proposed to explain the heating of the dust which
produces IR luminosity in spiral galaxies - massive young (OB) stars and associated H II
regions (Helou et al. 1985; Devereux & Young 1990, 1993), and non-ionizing A and later stars
(Lonsdale & Helou 1986; Walterbos & Schwering 1987; Bothun et al. 1989). Some authors,
however, suggest contribution from both sources (Smith 1982; Habing et al. 1984; Rice et
al. 1990; Sauvage & Thuan 1992; Smith et al. 1994; Devereux et al. 1994). The dominance
of the heating source is, therefore, governed by the availability of discrete and dense star
forming regions in the ISM. While observational evidence suggests that the global IR emission
from luminous IR spiral galaxies provides a measure of high mass SFR (Kennicutt 1998),
the diffuse IR emission in quiescent galaxies is caused mainly by the thermal radiation of
the interstellar dust heated by the interstellar radiation field (ISRF) (Jura 1982; Mezner et
al. 1982; Cox et al. 1986; Jura et al. 1987).
The dust color temperature (Td) deduced for galaxies from the IRAS 60 and 100 µm
flux densities are typically 25-40 K assuming emissivity index β = 2 (Rahman et al. 2006).
This range is similar to the temperature of dust in Galactic star-forming regions (Wynn-
Williams & Becklin 1974; Scoville & Good 1989), and considerably greater than the 15-20K
temperatures expected for dust heated by the ambient ISRF (Jura 1982; Mezner et al. 1982;
Cox et al. 1986; Jura et al. 1987). The color temperature derived from the MIPS 70 and
160 µm of LSB galaxies ranges ∼17-21 K.
From a statistical study of a large sample of optically selected galaxies, Bothun et al.
(1989) demonstrated that in the absence of UV radiation, far-IR color ratio of S60µm/S100µm ≤
0.3 can result from dust which is heated by old stars. Galaxies with S60µm/S100µm ≈
0.3 − 0.5 requires a steadily increasing proportion of UV heated dust, while galaxies with
S60µm/S100µm ≥ 0.5 are entirely dominated by the UV heated dust. From panel 8d we
– 14 –
find that the LSB galaxies have cool effective dust temperatures and therefore lack intense
heating from massive stars.
It should be mentioned here that in the IR color analysis we did not subtract the stellar
contributions from the 8 and 24 µm measurements to exactly reproduce the E05 diagram.
Assuming conservatively that the 3.6 µm emission is coming from the stellar photosphere, the
stellar contributions obtained from the empirical stellar SED of Pahre et al. (2004) derived
for early type galaxies are ∼59%, ∼38%, and ∼19% at 4.5, 5.8, and 8 µm, respectively.
The stellar contribution at 24 µm emission is ∼5% or less. Applying corrections to the
corresponding flux densities do not change our results since these corrections systematically
shift the parameters in the color space. Note that we use the same set of numbers for
each galaxy ignoring the variation in amount of stellar contributions from galaxy to galaxy.
However, error due to this is negligible. To find stellar contributions, preference is given
to the empirical SED than any stellar synthesis model because of unknown SFHs and poor
metallicity constraints for LSB galaxies.
In summary, from the four panels of Fig. 8 we conclude that: (a) Malin 1, UGC 6614,
and the intermediate HSB/LSB object UGC 6879 have mid-IR colors similar to quiescent
HSB disk galaxies; UGC 9024 falls in the region of HSB elliptical galaxies in this color plane.
(b) There is insufficient data to conclude whether LSB galaxies have PAH emission properties
significantly different from HSB galaxies with comparably low metallicities. Observations
of many more LSB galaxies are needed to settle this. (c) Available far-IR detections and
upper limits indicate that LSB galaxies are far-IR cool sources. The dust temperatures
derived from the MIPS 70 and 160 µm of LSB galaxies ranges Td ∼17-21 K, similar to many
quiescent HSB spiral and elliptical galaxies.
3.4. Molecular ISM
The LSB galaxies are rich in neutral hydrogen (H I). Molecular hydrogen (H2) gas
inferred from CO emission has been detected in only a handful of such galaxies (Matthews
& Gao 2001; O’Neil & Schinnerer 2004; Matthews et al. 2005; Das et al. 2006). The low
rate of CO detection has been attributed to an ISM with low dust content and a low surface
density of neutral gas. Dust opacity is crucial for the formation and survival of molecules
since it provides them necessary shielding from the ISRF. A larger column density is needed
to self-shield the H2 molecule. A low density and less dusty environment exposes H2 to UV
photons which can easily dissociate these molecules. The low star formation and far-IR cool
nature of LSB galaxies implies lower energy density of the radiation field and consequently
lower dissociation of CO and H2 (de Blok & van der Hulst 1998b).
– 15 –
The deficiency of molecular gas detections in LSB galaxies may point to a dynamical
condition such as the absence of large-scale instability in the disk preventing formation of
giant molecular clouds (Mihos et al. 1997). Local instabilities may lead to cloud condensation
resulting in localized star formation which may escape detection by current observations (de
Blok & van der Hulst 1998b). Local instability in the disk invokes energetic phenomena
such as SN explosions and the frequency of such occurrence in LSB galaxies is low (Hoeppe
et al. 1994; McGaugh 1994). The enhanced cooling by molecules is crucial in the onset of
instability of molecular clouds. Therefore, the effect of less efficient cooling of the ISM can
also prevent local instabilities. Long cooling time leads to higher cloud temperatures and
thus makes it difficult for a cold molecular phase to exist (Gerritson & de Blok 1999).
If the low density ISM truly lacks H2 and CO molecules, what other types of molecule
can exist in this environment? Are they very cold or very warm molecules of known types?
To which dust components do they belong? Along with millimeter wavelength observations,
can we use currently available IR data to shed light into these questions? One of the three
LSB galaxies, UGC 6614, has been reported to have CO(1-0) emission at a certain localized
region in the disk (Das et al. 2006), while the other two galaxies have not been observed in
this wavelength. On the other hand, the Spitzer observation of UGC 6614 shows the presence
of enhanced PAH emission on the bulge and almost entirely along the outer disk. This 8 µm
emission, however, is concentrated in the central regions of the other two galaxies.
A possible source of the origin of PAH molecules is the dense, high temperature, carbon-
rich ([C]/[O] > 1) environment of circumstellar envelopes surrounding mass-losing AGB car-
bon stars. PAH formation by stars with more normal, oxygen-rich, photospheric abundances
([C]/[O] < 1) will be negligible because nearly all the available carbon is bound up in the CO
molecules (Latter 1991). Therefore, if the stellar population responsible for the enrichment
of the ISM are dominantly old carbon-rich stars, the warm PAH molecules will be ubiquitous
resulting in lower abundance of CO molecules. While a detailed investigation is beyond the
scope of this study, we believe the observed PAH emission and lack of CO emission holds
the potential clue to probe not only the ISM but also to better understand the SFHs in LSB
galaxies.
3.5. Mid-IR Photometry of UGC 6614
The optical spectra of large LSB disks show an unexpected high occurrence of low-level
active galactic nuclei (AGN) type activity (Spraybarry et al. 1995; Schombert 1998). UGC
6614, a optical giant LSB galaxy, is suspected to harbor a weak AGN from the optical
spectrum (Schombert 1998) and from excess emissions at millimeter (Das et al. 2006) and
– 16 –
radio wavelengths (Condon et al. 2000). Integrated mid-IR photometry provides a robust
technique to identify AGN in HSB galaxies where AGN tend to be redder than normal star
forming galaxies in the mid-IR (Lacy et al. 2004; Stern et al. 2005).
To determine whether one can use a similar technique to detect AGN signatures in a
LSB bulge, we analyze the IRAC colors ([3.6]-[4.5] vs. [5.8]-[8.0]) of UGC 6614. The colors
are measured for two regions of radius ∼1 and ∼5.5 kpc, respectively, encircling the galaxy
center (Fig. 9; left panel). We find that in the color space, along the vertical [3.6]-[4.5] axis,
the galaxy resides well outside the Stern et al “AGN box”. Although the lower end of the
AGN box is within the error bar of this galaxy, its overall mid-IR color put it in a region
occupied mostly by star forming galaxies suggesting that broad band colors may not be an
efficient tracer for weak AGNs.
The contribution from an AGN to the measured IRAC fluxes can be estimated by
combining image subtraction with assumptions about the nature of the SED coming from
starlight and AGN emission. We use Pahre et al. (2004) for stellar flux ratios, and a ν−1
power law SED for AGN (Clavel et al. 2000). Following the procedure of Howell et al.
(2007), a 4.5 µm image of the non-stellar emission was constructed (Fig. 9; right panel).
Unlike the procedure of E05 which simply measures S4.5 − αS3.6, this procedure includes
an additional factor to account for the contribution of non-stellar emission to the 3.6 µm
image. The non-stellar flux density measured this way indicates that, within a 12′′ aperture,
the AGN contributes ∼12% of the light at 4.5 µm and ∼6% at 3.6 µm. At 8 µm, starlight
and the AGN each contribute ∼35% of the light, with PAHs contributing the remaining.
Note that the selected aperture includes only the bulge of the galaxy, excluding the spiral
arm/ring structure.
UGC 6614 illustrates that although [3.6]-[4.5] color can identify strong AGN, weaker
AGN will not be clearly separated from pure stellar sources. The procedure of E05, measuring
S4.5µm−αS3.6µm, will identify regions of non-stellar emission but will not provide quantitative
picture of stellar emission. Given reasonable assumptions the procedure of Howell et al.
allows a quantitative decomposition of the stellar and non-stellar flux densities.
4. Summary and Conclusions
The Spitzer observations of the three optical giant low surface brightness galaxies Malin
1, UGC 6614, and UGC 9024 have been examined to study the mid and far-IR morphology,
spectral energy distributions, and IR color to estimate dust mass, dust-to-(atomic) gas mass
ratio, total IR luminosity, and star formation rate (SFR). We also investigate UGC 6879,
– 17 –
which is intermediate between HSB and LSB galaxies.
The 8 µm images indicate that polycyclic aromatic hydrocarbon (PAH) molecules are
present in the central regions of all three metal-poor LSB galaxies. The diffuse optical
disks of Malin 1 and UGC 9024 remain undetected at mid- and far-infrared wavelengths.
The dustiest of the three LSB galaxies, UGC 6614, has infrared morphology that varies
significantly with wavelength; 160 µm (cool) dust emission is concentrated in two clumps
on the NE and NW sides of a distinct ring seen in the 24 and 8 µm images (and a broken
ring at 70 µm) at a radius of ∼40′′ (18 kpc) from the galaxy center. The 8 and 24 µm
emission is co-spatial with Hα emission previously observed in the outer ring of UGC 6614.
The estimated dust-to-gas ratios, from less than 10−3 to 10−2, support previous indications
that LSB galaxies are relatively dust poor compared to HSB galaxies. The total infrared
luminosities are approximately 1/3 to 1/2 the blue band luminosities, suggesting that old
stellar populations are the primary source of dust heating in these LSB objects. The SFR
estimated from the infrared data ranges ∼0.01− 0.88 M⊙ yr−1, consistent with results from
optical studies. The mid-IR colors of UGC 6614 shows the presence of a weak AGN at the
central bulge.
Questions can be raised such as what is the most viable reason for these LSB galaxies
to have LTIR/LB < 1? To answer this question we first note that observables such as stellar
populations and SED shapes can be used to break the degeneracy in infrared-to-blue ratio
(Helou 2000). That LSB galaxies have low infrared-to-blue luminosities, stellar populations
spanning a wide range of mean ages, are not dominated by OB stars (McGaugh 1994), have
less dust than the HSB galaxies (see Fig. 6), and are IR cool sources (see Fig. 8) suggest a
composite scenario. The LSB disks are less dusty and the older stellar populations are the
primary source of the IR emission from their ISMs.
The presence of PAH emission in these three galaxies indicates that the ISM of the region
contributing to the emission in these galaxies have significant amount of carbon enrichment
over cosmic time and the ISRF in the ISM must have been significantly weak and thus it is
unable to reduce the strength of PAH emission. In other words, the small grains are more
exposed to the ISRF so that their destruction rate is larger than for PAH molecules.
The detection of mid and far-IR emission from a larger sample will be crucial to un-
derstand the properties of ISM in LSB galaxies and probing their star formation histories.
This will have a significant effect on analytical modeling of galaxy formation and evolution,
the role of different galaxy populations in observed number counts and possibly metallicity
effects in the observed number counts. Whether star formation in LSB disks occurred in a
continuous fashion but with a low rate, or in an exponentially decreasing rate, or as sporadic
bursts with quiescent periods in between is still a matter of debate. Since each type of for-
– 18 –
mation history will lead to a stellar population that could be traced by optical photometry
(i.e. blue or red), the formation scenario must follow the route where the ISM would be in a
state having significant carbon enrichment with substantial amount of dust. The constraints
coming from SST such as mid-IR 8 µm emission and moderate dust mass could be used as
probes to understand the nature of LSB spirals.
Metal poor HSB objects such as blue compact dwarf galaxies are PAH deficient systems
(E05; Wu et al. 2006). Their SEDs are markedly different in the ∼5-15 µm wavelength range
compared to metal rich HSB galaxies. The LSB galaxies are metal poor but have substantial
PAH emission. Although these galaxies are not the extreme cases in metal deficiency such
as BCDs, they fill an interesting niche among local populations, distinct from HSB dwarfs
and from HSB regular galaxies. They may also represent a significant fraction of the galaxy
population at earlier epochs (Zackrisson et al. 2005), and therefore, may have important
implication in the interpretation of galaxy number counts in the infrared/submillimeter as
well as in the visible and near-IR wavelengths.
Previous analytical studies suggest that metal abundances have profound implications
on galaxy number counts observed at 24, 70, and 160 µm (Lagache et al. 2003; Dale et
al. 2005). To date, in analytical models metallicity effects have been incorporated in an
ad hoc manner by artificially manipulating SEDs of HSB galaxies in the wavelength range
mentioned above (Lagache et al. 2003). When template SEDs of many nearby metal-poor
LSB galaxies become available, one can incorporate these in galaxy evolution models as an
independent class along with various other classes such as normal star forming, starburst,
luminous and ultra-luminous, and AGNs to understand the observed galaxy number counts
and the origin of the IR background.
The anonymous referee is thanked for constructive comments and suggestions. We
happily thank D. Dale for his model fits. We also thank Y. Wu, B. R. Brandl, J. R. Houck
for helpful communications. We acknowledge useful discussions from A. Blain, G. D. Bothun,
and S. S. McGaugh on LSB galaxy population. One of us (NR) gratefully acknowledges the
support of a Research Associateship administered by Oak Ridge Associated Universities
(ORAU) during this research. This research has made use of the NASA/IPAC Extragalactic
Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of
Technology, USA under contract with the National Aeronautics and Space Administration,
and the LEDA database in France. This study is based on observations made with the Spitzer
Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute
of Technology under NASA contract 1407. This study has made use of data products from
the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts
and IPAC/Caltech, funded by NASA and the National Science Foundation. This study also
– 19 –
acknowledges use of data products from Solan Digital Sky Survey.
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Fig. 1.— The Solan Digitial Sky Survey (SDSS) composite images of target galaxies. In each
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Malin 1
Fig. 2.— The Spitzer view of Malin 1. The IRAC 3.6, 4.5, and 8 µm images are at left and the
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– 22 –
UGC 6614
Fig. 3.— The Spitzer view of UGC 6614. The position of 24 µm peak emission is shown by the
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– 23 –
UGC 6879
Fig. 4.— The Spitzer view of UGC 9024. The position of 24 µm peak emission is shown by the
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– 24 –
UGC 9024
Fig. 5.— The Spitzer view of UGC 6879. The B-band central surface brightness µB,0 of this
galaxy is intermediate between LSB and HSB galaxies.
– 25 –
Fig. 6.— The observed SEDs of LSB galaxies and UGC 6879 at near, mid, and far-IR wavelengths.
The 2MASS, IRAC, and MIPS points are shown by the open circles, filled circles, and open triangles,
respectively. The IRAS upper limits are shown by the filled triangles. For all galaxies the flux
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was undetected by the MIPS far-IR channels and hence the detection limits are shown for the total
integration time (∼252 sec. at 70 µm and ∼42 sec. at 160 µm).
– 26 –
Fig. 7.— The observed SEDs of LSB galaxies and UGC 6879 using the DH02 model. The symbol
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The dotted lines represents the stellar synthesis model prediction from Vazquez & Leitherer (2005)
fitted only to the 2MASS fluxes. The dust mass is estimated using the fitted SEDs (solid line).
– 27 –
Fig. 8.— The mid and far-IR color-color diagrams highlighting the LSB spirals (shown by open
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– 28 –
Fig. 9.— Left panel: The IRAC colors for UGC 6614 as shown by solid symbols. The triangle
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This preprint was prepared with the AAS LATEX macros v5.2.
– 33 –
Table 1. Flux densities from 2MASS, IRAS, and Spitzer
Source λ(µm) Malin 1 UGC 6614 UGC 6879 UGC 9024
Sν r Sν r Sν r Sν r
(mJy) (arcsec) (mJy) (arcsec) (mJy) (arcsec) (mJy) (arcsec)
2MASS 1.3 3.93 10 26.80 26 21.10 63 2.67 13
2MASS 1.7 3.41 10 35.50 26 40.90 63 3.62 13
2MASS 2.2 2.00 10 28.00 26 28.70 63 2.65 13
IRAC 3.6 1.61 12 22.31 60 16.03 75 1.94 24
IRAC 4.5 1.10 12 13.74 60 9.93 75 1.18 24
IRAC 5.8 0.73 12 14.11 60 14.73 75 1.57 24
IRAC 8 0.74 12 16.33 60 25.78 75 2.38 24
IRAS 12 – – <60 – <60 – – –
MIPS 24 0.57 30 22 60 28.35 75 1.00 30
IRAS 25 – – <170 – <330 – – –
IRAS 60 – – <150 – <360 – – –
MIPS 70 <5 – 147 60 447 75 98 60
IRAS 100 – – <430 – <1080 – – –
MIPS 160 <25 – 677 60 1529 75 <268 60
Note. — The IRAC flux densities are aperture corrected. Malin 1 was un-
detected by the MIPS far-IR channels and hence the detection limits at 70
and 160 µm are shown for the total integration time: ∼252 sec. at 70 µm
and ∼42 sec. at 160 µm. In general, r represents the radius of a circu-
lar aperture which is used to estimate the flux density. However, the 2MASS
flux densities are given for “total” elliptical aperture radius in the archive
(http://irsa.ipac.caltech.edu/applications/2MASS/PubGalPS) where elliptical ra-
dius represents the semi-major axis. UGC 6879 is edge-on and for this galaxy we
use “effective radius”, defined by r =
ab, where a and b are semi-major and
minor axes.
http://irsa.ipac.caltech.edu/applications/2MASS/PubGalPS
– 34 –
Table 2. Basic Properties of LSB galaxies
Property Malin 1 UGC 6614 UGC 6879∗ UGC 9024 Reference
Type S? SAa SABc SAab 1,2
redshift, z 0.0834 0.0237 0.009 0.0088 1,2
DA (Mpc) 320.47 93.51 35.87 34.86 1,2
inclination, i 16.1o 34o 90o 32.2o 1,2
D25 (arcsec) 16.15 75.54 87.75 104.26 1,2
D (kpc) 25.09 34.24 15.25 17.62 1,2
mB (mag) 17.92 14.12 13.40 17.11 1,2
MB (mag) -19.83 -20.70 -19.52 -15.64 1,2
µB,0 (mag arcsec
−2) 26.50 24.50 20.40 24.71 4,5,6
hr,R (kpc) 73.3 16.00 2.50 7.47 4,5
Yo <8.66 7.35-9.16 – 7.98-8.35 3,9
log(LB/L⊙) 9.94 10.29 9.83 8.21 1,2,5
log(MHI/M⊙) 10.66 10.42 9.08 9.40 5,7,8
log(MH2/M⊙) – 8.45 – – 10
log(Md/M⊙) <8.14 8.42 7.81 <6.94 11
D <0.003 0.01 0.053 <0.004 11
log(LTIR/L⊙) 9.49 9.72 9.05 7.90 11
log(L
/L⊙) <9.50 9.80 9.28 <8.30 11
LTIR/LB 0.35 0.27 0.17 0.49 11
SFR (M⊙ yr
−1) 0.38 0.88 0.20 0.01 11
LTIR/MHI (L⊙/M⊙) 0.07 0.20 0.93 0.03 11
Note. — ∗UGC 6879 is not strictly a LSB galaxy. Its central surface brightness µB,0 is
intermediate between LSB and HSB galaxies. We assume ΩM = 0.3, ΩΛ = 0.7, and H0 = 75 km
sec−1 Mpc−1 to be consistent with the literature. Notation: DA is the angular diameter distance;
D25 is the B-band 25 mag arcsec
−2 isophotal diameter; D is the physical diameter; mB and MB ,
respectively, are B-band apparent and absolute magnitude corrected for Galactic extinction,
internal extinction, and K-correction; µB,0 is the B-band central surface brightness; disk scale
length hr,R is in kpc; the solar value of the oxygen abundance Yo = 12 + log[O/H] ∼ 8.66 from
Asplund et al. (2004); MHI is neutral hydrogen mass; MH2 is molecular hydrogen mass; Md is
dust mass; the dust-to-gas ratio is D = Md/MHI; total IR luminosity LTIR is estimated from
– 35 –
Calzetti et al. (2005); total IR luminosity L
is estimated from the DH02 model fit; star
formation rate SFR is calculated from Alfonso-Herrero et al. (2006).
References: 1) LEDA; 2) NED; 3) McGaugh 1994; 4) Sprayberry et al. 1995; 5) Impey et al.
1996; 6) McGaugh & de Blok 1998; 7) Matthews et al. 2001; 8) Sauty et al. 2003; 9) de Naray
et al. 2004; 10) Das et al. 2006; 11) This study.
	Introduction
	Observation and Data Reduction
	Contamination from Galactic Cirrus
	Results
	Infrared Morphology
	Infrared Diagnostics
	IR Spectral Energy Distributions
	Dust Mass
	Infrared Luminosity
	Star Formation Rate
	Infrared Colors
	Molecular ISM
	Mid-IR Photometry of UGC 6614
	Summary and Conclusions

  Abridged: We present analysis of Spitzer Space Telescope observations of the
three low surface brightness (LSB) optical giant galaxies Malin 1, UGC 6614 and
UGC 9024. Mid- and far-infrared morphology, spectral energy distributions, and
integrated colors are used to derive the dust mass, dust-to-gas mass ratio,
total infrared luminosity, and star formation rate (SFR). The 8 micron images
indicate that polycyclic aromatic hydrocarbon molecules are present in the
central regions of all three metal-poor LSB galaxies. The diffuse optical disks
of Malin 1 and UGC 9024 remain undetected at mid- and far-infrared wavelengths.
The dustiest of the three LSB galaxies, UGC 6614, has infrared morphology that
varies significantly with wavelength. The 8 and 24 micron emission is
co-spatial with H\alpha emission previously observed in the outer ring of UGC
6614. The estimated dust-to-gas ratios, from less than 10^{-3} to 10^{-2},
support previous indications that the LSB galaxies are relatively dust poor
compared to the HSB galaxies. The total infrared luminosities are approximately
1/3 to 1/2 the blue band luminosities, suggesting that old stellar populations
are the primary source of dust heating in these LSB objects. The SFR estimated
from the infrared data ranges ~0.01-0.88 M_sun yr^{-1}, consistent with results
from optical studies.

Introduction
The unprecedented imaging and spectroscopic sensitivity and higher spatial resolution
of the Spitzer Space Telescope (SST; Werner et al. 2004) compared to past IR missions
such as the Infrared Astronomical Satellite (IRAS; Neugebauer et al. 1984) and the Infrared
Space Observatory (ISO; Kessler et al. 1996) provide a unique opportunity to probe the
inter-stellar medium (ISM) of optically faint sources such as low surface brightness (LSB)
galaxies. The goal of this study is to use the Spitzer data to analyze the IR properties of
three LSB optical giants: Malin 1, UGC 6614, and UGC 9024. This is the first time we are
able to view these galaxies in ∼3-160 µm wavelength range.
The LSB galaxies are usually defined as diffuse spiral disks with low B-band central
surface brightness (e.g. µB,0 ≥ 23 mag arcsec−2; Bothun et al. 1997; Impey & Bothun 1997).
These galaxies are either blue (B-V . 0.5) or red (B-V & 0.8) in color (O’Neil et al. 1997),
metal poor ([O/H] . 1/3Z⊙; McGaugh 1994; de Blok & van der Hulst 1998b; de Naray et al.
2004), rich in neutral hydrogen (H I) (Schombert et al. 1992; O’Neil et al. 2004), deficient
in H II emission (McGaugh et al. 1995; de Naray et al. 2004), and have low star formation
rate (SFR) . 0.1 M⊙ yr
−1 (van den Hoek et al. 2000; Burkholder et al. 2001). The majority
of these galaxies lack molecular (CO) gas (Schombert et al. 1990; de Blok & van der Hulst
1998a); only a handful have been reported to have molecular emission (O’Neil & Schinnerer
2004; Matthews et al. 2005; Das et al. 2006).The observed properties suggest that LSB disks
are relatively unevolved systems and may have a different evolutionary history compared to
their high surface brightness (HSB) counterparts (McGaugh 1994; van den Hoek et al. 2000;
Vallenari et al. 2005; Zackrisson et al. 2005).
Most of our knowledge regarding the composition and structure of the ISM of LSB spirals
comes from optical (Impey et al. 1996) and H I surveys (O’Neil et al. 2004). These surveys
have demonstrated that the neutral hydrogen is by far the dominant component of the ISM
in these galaxies (∼95% by mass; Matthews 2005). While we have improved understanding
of the gaseous component of the ISM from decade long H I surveys, our knowledge of the
inter-stellar dust, the component of the ISM radiating from mid-IR (∼8 µm) through sub-
millimeter (∼850 µm) wavelengths, is still very limited. Because of the scarcity of information
in this wavelength range, complementary observational facts such as low metal abundance
(McGaugh 1994), strong similarities in optical and near-IR morphology (Bergvall et al. 1999;
Bell et al. 2000), transparency of the stellar disks (O’Neil et al. 1998; Matthews et al. 1999),
and deficiency in molecular emission (Schombert et al. 1990; de Blok & van der Hulst 1998a)
have been used to probe the ISM of LSB galaxies. All these observations lead to a general
consensus that the LSB disks are deficient in dust and molecular gas.
Given that LSB spirals comprise ∼50% of the local disk galaxy population (McGaugh
– 3 –
et al. 1995b), they deserve equal attention as their HSB cousins. To develop a consistent
picture of the local galaxy populations it is therefore necessary to probe each population
at all wavelength regimes as have been done in most cases for HSB galaxies. Previous long
wavelength studies on LSB galaxies involved a few cases in sub-millimeter and millimeter
wavelengths (de Blok & van der Hulst 1998b; Pickering & van der Hulst 1999; O’Neil &
Schinnerer 2004; Matthews et al. 2005; Das et al. 2006). Hoeppe et al. (1994) made
the first attempt to investigate long wavelength (60 µm, 100 µm, and 20 cm) properties of
LSB dwarfs; however, no study has been made in the mid-IR and far-IR for LSB disks. In
this study we have made the first attempt to explore ∼3-160 µm properties of three LSB
giants: Malin 1, UGC 6614, and UGC 9024, recently observed by the SST. We focus on the
IR morphology to probe extent of dust in the ISM, on the SEDs for the IR energy budget
and dust content, and on the IR colors to establish the dust temperature of the ISM. The
organization of this paper is as follows: we describe observation and data reduction in section
§2 and present our results in section §3. Discussions and conclusions are given in section §4.
2. Observation and Data Reduction
We describe the Infrared Array Camera (IRAC; Fazio et al. 2004) and Multiband
Imaging Photometer for Spitzer (MIPS; Rieke et al. 2004) imaging data for Malin 1, UGC
6614, and UGC 9024. These extended disk galaxies with radial scale length hr,R > 5 kpc
are observed as part of a larger guaranteed time observing program (Spitzer PID #62). The
program also includes two LSB galaxies (UGC 5675 and UGC 6151) with 2 kpc < hr,R <
3 kpc, an edge-on disk (UGC 6879) with hr,R ∼2.5 kpc, and a HSB dwarf (UGC 10445)
with hr,R ∼1 kpc. The central brightness of UGC 6879 is µB,0 ∼ 20.4 mag arcsec−2. A
simple correction for inclination, using µ0,face−on = µ0,observed+2.5 log(a/b) brings its central
brightness to ∼ 21.71 mag arcsec−2. It is lower than the conventional choice (µ0 ≈ 23 mag
arcsec−2) but close to the Freeman value (µ0 ∼ 21.65 mag arcsec−2; Freeman 1970). The
central brightness of this galaxy falls in between the range of LSB and HSB galaxies and
hence, we include it as a representative of the intermediate class. The properties of UGC
5675 and UGC 6151 will be explored in a forthcoming paper. The readers are refereed to
Hinz et al. (2006) for an analysis of UGC 10445.
The IRAC 3.6, 4.5, 5.8, and 8 µm images and the MIPS 24, 70, and 160 µm images
were acquired, respectively, in the mapping and photometry modes. The IRAC images were
reduced with the standard Spitzer Science Center data pipeline, and aligned, re-sampled, and
– 4 –
combined into a mosaic image using the Mopex1 software. The MIPS 24 µm data required
the use of the self-calibration procedure described in the MIPS Data Handbook2 to remove
latent image artifacts. The corrected images were then combined into a mosaic using Mopex.
Time filtering and column filtering was applied to the 70 µm images using IDL routines
created by D. Fadda. The filtered images were then combined using Mopex. The 160 µm
images were combined into a mosaic using Mopex. The IRAC spatial resolution is ∼2′′ for
all bands. The MIPS spatial resolutions are 6
, and 40
for the respective bands.
Sky subtraction was carried out through the use of multiple sky apertures placed near
the source which do not overlap with the faintest isophotes visible from the galaxy. For
each galaxy we measured flux densities from the sky subtracted images within the aperture
covering the entire galaxy. The flux density contributed by foreground stars within a galaxy
aperture was removed by measuring each such star in a small aperture and subtracting
the result from the total flux within the galaxy aperture. The calibration uncertainty in
the IRAC flux densities is at the level of ∼10% (Reach et al. 2005; Dale et al. 2005).
Aperture corrections have been applied to all IRAC flux densities (T. H. Jarrett 2006; private
communication). The MIPS flux density calibration uncertainties are 10% at 24 µm and 20%
at 70 and 160 µm.
Near-IR (1.3, 1.7, and 2.2 µm) flux densities from the Two Micron All Sky Survey
(2MASS; Jarrett et al. 2000), upper limits on IRAS flux densities, and those derived from
the IRAC and MIPS bands are given in Table 1. Basic properties of the galaxies obtained
from the literature, the NASA/IPAC Extragalactic Database (NED), the Lyon-Meudon Ex-
tragalactic Database (LEDA), and derived in this study are summarized in Table 2. The
IRAS flux densities for UGC 6614 and UGC 6879 were computed using the SCANPI tool
available from IRSA as linked via NED where IRAS flux density limits represent SCANPI’s
inband total measurement, fν(t). No IRAS detections were available for Malin 1 and UGC
9024. Distances for the galaxies are estimated from heliocentric radial velocity after correct-
ing for the local group, infall into the Virgo cluster, the Great Attractor, and the Shapley
supercluster following the Mould et al. (2000) flow model.
1http://ssc.spitzer.caltech.edu/postbcd/
2http://ssc.spitzer.caltech.edu/mips/dh/
– 5 –
2.1. Contamination from Galactic Cirrus
A basic concern about faint extragalactic sources with highly diffuse disk structure is
confusion by foreground Galactic “cirrus” emission. This is especially critical in the case of
far-IR cool sources (defined below) such as LSB galaxies. The far-IR ratio of typical local
cirrus is S60µm/S100µm ≤ 0.2 (Gautier 1986). UGC 6614 and UGC 6879 are well above this
limit (Fig. 8c). From its Galactic latitude b ∼ +22o and the observed S70µm/S160µm ratio, it
is reasonably safe to assume that UGC 9024 has not been affected by cirrus emission. In the
absence of far-IR information it is uncertain for Malin 1. However, its high galactic latitude
b ∼ +14◦ can be used to argue against any cirrus contamination.
We should also stress the fact that whether there is cirrus in the foreground is less
significant as compared to whether the cirrus in the foreground varies on scales of the IRAC
and MIPS field of view. If its spatial variation is negligible across the IRAC and MIPS
mosaic, it gets subtracted out as “sky” in the data reduction process.
3. Results
In this section we present IR morphology, SEDs, and IR colors of LSB galaxies. Using
this information we estimate dust mass (Md), dust-to-(atomic) gas ratio (D), total infrared
luminosity (LTIR), and star formation rate (SFR). To obtain a qualitative assessment of the
observed properties of LSB disks compared to their HSB counterparts, we take the Spitzer
Infrared Nearby Galaxy Survey (SINGS; Kennicutt et al. 2003) sample as a representative
sample of local HSB galaxies. This sample contains 75 galaxies of various Hubble types as
well as dwarfs and irregular galaxies, and thus making it a suitable reference for comparative
analysis with LSB spirals.
3.1. Infrared Morphology
The IR emission beyond ∼25 µm is dominated by the inter-stellar dust under various
heating conditions. On the other hand, mid-IR (∼5-25 µm) emission marks the transition
from stellar photospheres to inter-stellar dust dominating the emission. Whereas the mor-
phology of a galaxy at 3.6 and 4.5 µm represents the stellar disk, at ∼5 µm and beyond it
shows the structure of the ISM. The IRAC 3.6 and 4.5 µm bands are sensitive to the under-
lying stellar populations typically consisting of red giants and old stars. In some galaxies 3.6
µm band is also known to contain emission from a hot dust component (Bernard et al. 1994;
Hunt et al. 2002; Lu et al. 2003; Helou et al. 2004). The hot dust is also visible in the other
– 6 –
IRAC bands. While 3.6 µm is only sensitive to hot dust near the sublimation temperatures
(∼1000 K), the longer wavelength IRAC bands can detect dust at lower temperatures down
to several hundred Kelvin.
The IRAC 5.8 and 8 µm bands are primarily sensitive to the PAH emission at 6.2, 7.7,
and 8.6 µm (Puget & Leger 1989; Helou et al. 2000; Lu et al. 2003). The PAH is the hot
component of the inter-stellar dust with effective temperature Td > 100 K stochastically ex-
cited to high energy levels by stellar photons. Stellar photospheric emission also contributes
to these two IRAC wavebands. The fraction of stellar emission at 5.8 and 8 µm, respectively,
are ∼40% and ∼20% (Helou et al. 2004; Pahre et al. 2004).
The emission detected by the MIPS bands are coming from dust grains with different
size distributions. Very small grains (∼1-10 nm) emit in the mid-IR region (&15 µm),
intermediate between thermal equilibrium and single-photon heating. Large, classical grains
(∼100-200 nm) are in thermal equilibrium with the radiation field and responsible for far-IR
emission (Desert et al. 1990). To begin with we should bear in mind that the demarcation
lines among various heating environments are ad-hoc and we assume the following effective
color temperature ranges for large grains in thermal equilibrium: warm (40 K . Td . 100
K), cool (20 K . Td . 30 K), cold (10 K . Td . 20 K), and very cold (10 K . Td).
The Solan Digital Sky Survey (SDSS) images of target galaxies are shown in Fig. 1.
The Spitzer images of the LSB galaxies are shown in Fig. 2 (Malin 1), Fig. 3 (UGC 6614),
and Fig. 4 (UGC 9024). UGC 6879 is shown in Fig. 5. Galaxy images are shown using
Gaussian equalization (Ishida 2004). The images are oriented such that north is up and east
is to the left. Malin 1 is too faint to be detected by the MIPS 70 and 160 µm channels. The
other three galaxies were detected by all IRAC and MIPS bands. We do not show 5.8 µm
images in these figures since the morphological appearances of each of these galaxies at 5.8
µm closely follow that at 8 µm. For all galaxies, the IRAC 4.5 and 8 µm images are shown
without subtracting stellar photospheric emission. The contours represent surface brightness
with intervals of
10 where the lowest level is 4σ above the background. The lowest level
of contours at different bands are 0.04 (3.6 µm), 0.05 (4.5 µm), 0.30 (8 µm), 0.08 (24 µm),
1.6 (70 µm), and 2.1 (160 µm) expressed in MJy/Sr.
The giant optical disks in Malin 1 and UGC 9024 appear as point sources in the IRAC
images. While we detect the stellar bulges of these galaxies, their optically diffuse disks
remain undetected long-ward of the IRAC bands. This suggests that the low surface bright-
ness structures at larger radii might be photometrically distinct components rather than
smoothed extensions of the normal inner disks (see Barth 2007 for a discussion on Malin 1
based on Hubble data). That the disks appear in the B-band but are undetected at 3.6 µm
suggest that these disks have a small population of young stars rather than a large popu-
– 7 –
lation of old stars. The bulge spectrum of Malin 1 is consistent with a predominantly old
stellar population (Impey & Bothun 1989). For both of these galaxies, the mid-IR emissions
at 8 and 24 µm are concentrated in the central few kpc, within a region of 12′′ (20 kpc)
radius for Malin 1 and 24′′ (5 kpc) radius for UGC 9024.
Undetected far-IR emission from the disk of Malin 1 implies that it contains either very
cold (Td < 10 K) cirrus-like dust emitting in the submillimeter and millimeter wavelengths
or it lacks cold dust altogether and contains only neutral gas. For UGC 9024, 70 µm emission
comes from the central region but 160 µm emission is very hard to measure because of large
scale diffuse emission in the field. This result in an upper limit of S160 µm < 268 mJy.
The optical morphology of UGC 6614 shows a massive bulge and spiral structure. A
thin ring ∼40′′ from the core is prominent in Hα (McGaugh et al. 1995). The 3.6 and
4.5 µm emission is spread over the entire disk of this galaxy. The 3.6 µm image shows a
discernible spiral arm pattern closely resembling the optical morphology. At 4.5 µm this
feature disappears and the disk shrinks in radius showing only its inner region. This galaxy
appears markedly different at 5.8 and 8 µm compared to the other IRAC bands. The 8 µm
morphology suggests that the PAH emission is coming from two distinct regions: the central
bulge and an outer ring surrounding the bulge. The 8 µm morphology closely traces the
Hα image. The MIPS 24 µm morphology is similar to the 8 µm PAH emission although
the outer ring appears a bit more disjointed at this band. The dust emission at 24 µm is
coming mostly (∼70%) from the central disk. The lower resolution image at 70 µm indicates
that only ∼25% of the dust emission is coming from the central part of the galaxy, with the
remaining ∼75% emission co-spatial with the ring of radius ∼40′′. Surprisingly a dumbbell
shaped region is the dominant source (∼90%) of 160 µm emission; these two peaks are
located on the NE and NW sides of the ring. The far-IR images of this galaxy show a small,
localized region within the ring, SW from the center. Whether or not this region coincide
with the spatial location showing the CO emission in this galaxy disk (Das et al. 2006) is
not entirely clear. We will investigate this in the forthcoming paper.
UGC 6879 is an edge on spiral with a red central part and a blue (optical) disk. This
radial color gradient is perhaps related to greater concentrations of dust in the nucleus than in
the overall disk. Being a transitional disk, with a central surface brightness in between HSB
and LSB spirals, it is not unexpected to find that UGC 6879 is a strong IR emitting source
compared to the LSB galaxies. Both PAH emission and warm dust (24 µm ) emission show
spatial variation along the disk. These emissions peak at the central region and diminish
toward the edge.
– 8 –
3.2. Infrared Diagnostics
3.2.1. IR Spectral Energy Distributions
The flux densities obtained from the 2MASS and IRAS archives and those estimated
from the IRAC and MIPS images are given in Table 1. These flux densities are used to
construct the observed infrared SEDs of LSB galaxies as shown in Fig. 6. The open and
solid circles represent, respectively, 2MASS and IRAC data. The open triangles represent
the MIPS data whereas the IRAS upper limits for UGC 6614 and UGC 6879 are shown by
filled triangles. Since Malin 1 was undetected by the MIPS far-IR channels we show the
detection limits at 70 and 160 µm for the total integration time (∼252 sec. and ∼42 sec.
respectively). We include four SINGS galaxies with different ISM properties for comparison.
These are NGC 0337 (a normal star forming galaxy), NGC 2798 (a star burst galaxy), NGC
2976 (a normal galaxy with nuclear H II region), and NGC 3627 (a Seyfert galaxy). They are
shown, respectively, by dotted, dashed, dashed-dotted, and long dashed lines. The motive
is to make a visual comparison between SEDs of LSB and HSB galaxies. All flux densities
are normalized by the 3.6 µm flux density.
There are several noticeable features in these SEDs. First, the amplitude of the mid-IR
and far-IR dust emissions of LSB galaxies are lower compared to those of HSB galaxies. An
obvious and expected result is that the LSB galaxies have less dust content and hence have
lower IR emission. Second, Malin 1 is deficient in the integrated 8 µm emission compared
to its 4.5 µm emission. This is quite opposite for other LSB galaxies and more like the
SED of an elliptical galaxy (see Fig. 4 in Pahre et al. 2004). Third, the 24 µm emission
is suppressed in both Malin 1 and UGC 9024. For UGC 6614 it is slightly above the IRAC
bands. This feature suggests that the ISM of UGC 9024 lacks warm dust emission, and
is made mostly of cool dust. Fourth, the SEDs show a tendency to turnover at relatively
longer wavelength, a signature that the low density and low surface brightness ISM have low
radiation intensity. On the other hand, the shape of the SED of UGC 9024 is quite similar
to those of the representative HSB galaxies. We discuss this in detail in section §3.3
3.2.2. Dust Mass
Dust mass is frequently estimated by fitting the far-IR peak by a single temperature
modified blackbody function (Hildebrand 1983). However, the inability of a single tempera-
ture or even two temperature blackbody function to fit the observed flux densities suggests
that a more sophisticated model of the IR SED is needed. The global SED models of Dale
et al. (2001) and Dale & Helou (2002; hereafter DH02) provide a robust treatment of the
– 9 –
multiple grain populations that contribute to the IR emission in a galaxy. This model allows
a realistic derivation of dust mass since it combines information from the full range of heating
environments (∼10K-1000K). Previous studies have shown that dust mass is underestimated
by a factor of ∼5-10 for quiescent galaxies (i.e. IR cool) if one simply fits the far-IR and
sub-millimeter continuum data points with a simple single temperature black body instead
of exploiting information from the full range of the SED.
Figure 7 shows the fits to the observed SEDs obtained from the DH02 model (solid
line). The dashed and dotted lines represent, respectively, empirical stellar SED (Pahre
et al. 2004) and stellar synthesis model prediction fitted only to the 2MASS fluxes from
Vazquez & Leitherer (2005). We use the model fit (solid line) to estimate Md and D noting
that DH02 model does not provide ideal fits to very extended, low density, diffuse disks like
Malin 1 and UGC 9024. However, within the measurement and observational uncertainties
the model fits provide useful insight. A rigorous and detailed treatment of infrared SEDs of
LSB disks will be presented in a future study.
The estimated mass is given in Table 2. We find that the ISM of UGC 6614 has the
highest amount of dust with dust-to-gas ratio, D ∼0.01. Both Malin 1 and UGC 9024 are
∼3 times less dusty than UGC 6614. Given that IR emission coming only from the central
regions of the latter two galaxies, it is not surprising that they show low dust content. In
a recent study Das et al. (2006) detected CO(1-0) emission localized in a specific region on
the disk of UGC 6614. They estimated molecular gas mass (MH2 ∼ 2.8 × 108M⊙) which is
almost equal to the total dust content (Md ∼ 2.6 × 108 M⊙) that we measure distributed
over the bulge and disk. The difference between D and dust-to-(total) gas mass is negligible.
The (systematic) calibration uncertainty in the observed flux densities and the uncer-
tainty in the distance estimates result in a ∼30% uncertainty in Md. Additional uncertainty
comes from the mass absorption coefficient. The different sizes used to measure IR and
H I fluxes will also attribute additional uncertainty in D. Besides, considering that the
long-wavelength end of the SED is poorly constrained the estimate of overall dust mass
and gas-to-dust ratio is a bit uncertain. All of these errors compound to make Md and D
uncertain by a factor of ∼2 or more.
3.2.3. Infrared Luminosity
DH02 proposed a simple relation to compute total IR luminosity using the MIPS bands
(see Eq. 4 in DH02). Due to uncertainties in the MIPS flux densities for Malin 1 and UGC
9024 we use the empirical relation given by Calzetti et al. (2005) to estimate LTIR. Calzetti
– 10 –
et al. relate flux densities at 8 µm and 24 µm to derive LTIR for M 51, a normal star forming
galaxy. Estimated total IR luminosities are given in Table 2 with estimated uncertainty of
∼35%. We find comparable IR luminosity for both Malin 1 and UGC 6614. UGC 9024 is the
least luminous because of its suppressed 24 µm warm dust emission. In spite of its border
line HSB nature, the IR output of UGC 6879 resembles a normal quiescent galaxy.
We also estimate LTIR using DH02 model fits. We follow Sanders & Mirabel (1996) to
define LTIR where flux densities at 12, 25, 60, and 100 µm are obtained from the model SEDs
by interpolation. The result is presented in Table 2. Interestingly, these two estimates are
within a factor of ∼2.5 where the model estimates are always higher. The largest difference
is noted for UGC 9024 which is a result of relatively poor fit to the data.
Within the uncertainty, the IR energy output of LSB galaxies are smaller by a factor
of a few compared to their B-band luminosities LB. The infrared-to-blue ratio, LTIR/LB,
compares the luminosity processed by dust to that of escaping starlight (see Table 2). The
ratio ranges from <0.01 (in quiescent galaxies) to ∼100 (in ultra-luminous galaxies). It can
be used to characterize optical depth of a system composed of dust and stars as well as
recent (∼100 Myr) SFR to the long term (∼1 Gyr) average rate. The ratio is in the range
0.3-0.5 (see Table 2) indicating that the current level of star formation is low, a consistent
result with previous studies on star formation in the LSB ISM (van den Hoek et al. 2000;
Burkholder 2001). However, there is a potential degeneracy in this parameter and one can
only make an indirect assessment of the ISM of a galaxy if this degeneracy can be lifted.
The fact that LTIR/LB is less than unity can arise from two different physical conditions. In
one hand, a galaxy may be undergoing intense heating by young stars (large LB) but have
very little neutral ISM left (less IR emission) resulting in low LTIR/LB. On the other hand,
a quiescent galaxy may generate most of its IR emission in H I clouds heated by older stellar
populations and will display a similarly low LTIR/LB (Helou 2000).
3.2.4. Star Formation Rate
A widely used recipe for estimating SFR from IR luminosity is given by Kennicutt
(1998). However, far-IR luminosity in Kennicutt model is based on IRAS data. Without a
proper calibration between IRAS and MIPS far-IR flux densities the uncertainty will loom
large in the SFR estimate. We use a new SFR estimator, derived recently by Alonso-Herrero
et al. (2006; hereafter AH06) using 24 µm flux density. Our estimates are given in Table 2.
The error associated with this SFR is ∼10%.
van den Hoek et al. (2000) derived a current SFR ranging ∼0.02-0.2 M⊙ yr−1 from
– 11 –
I-band photometry for a sample of LSB galaxies generally found in the field. Their estimate
agree within a factor of two with our results based on infrared data. For Malin 1 and UGC
6614 the infrared SFR are ∼0.38 M⊙ yr−1 and ∼0.88 M⊙ yr−1 , respectively, whereas it is
∼0.01 M⊙ yr−1 for UGC 9024. The fact that these galaxies have low dust content indicates
that extinction is less likely to cause the difference between SFR derived from IR and optical
data. The higher SFR of UGC 6614 compared to the other two galaxies is consistent with
the fact that this galaxy has a prominent Hα morphology which indicate modest level of
current star formation. A very low SFR for UGC 9024 implies that more light is scattered
off from the central disk than being absorbed by the ISM.
The SFR of these three LSB galaxies, as derived from IR data, are thus significantly
below the rate ∼5-10 M⊙ yr−1 derived for the HSB galaxies (Kennicutt 1998) but consider-
ably larger than the rate ∼0.001-0.01 M⊙ yr−1 observed typically in dwarf irregular galaxies
(Hunter & Elmegreen 2004).
The star formation efficiency (SFE), quantified by LTIR/MHI, is also shown in Table
2. This measure represents the amount of unprocessed gas available to be consumed in
subsequent star formation. As expected the LSB galaxies have a SFE that is ≤ 1/5 of the
SFE of HSB galaxies.
3.3. Infrared Colors
Panels 8a and 8b show, respectively, mid-IR colors and the well known PAH-metallicity
connection. In these diagrams we highlight two extreme classes (in terms of their IR SEDs)
of HSB galaxies: the dwarf systems represented by the decimal numerals and the massive
elliptical galaxies represented by the roman numerals. All of these galaxies are obtained
from the SINGS sample.
Panel 8a compares PAH emission in various classes of galaxies. Metal-rich HSB galaxies
preferentially show high ratio in S8µm/S24µm and low ratios of non-stellar 4.5-to-8 µm dust
emission. Metal poor (e.g. dwarfs and irregulars) and extremely metal poor HSB galaxies
such as blue compact dwarfs (BCDs), show the opposite trend and fall within the PAH-
deficient box (Engelbracht et al. 2005; hereafter E05). Note that E05 only looked at star-
forming galaxies. “Red and dead” ellipticals should have little or no PAH emission, yet are
nowhere near the PAH-deficient region. So this region will not necessarily contain all PAH
deficient galaxies. Also note that the result of E05 could also arise from selection bias since
recent studies have shown that dwarf galaxies are not necessarily PAH defficient systems
(Rosenberg et al. 2006).
– 12 –
Panel 8b, on the other hand, illustrates the PAH-Z connection in HSB galaxies. E05
showed that galaxies with low PAH emission have relatively unpolluted ISMs. They noted
a sharp boundary between galaxy metallicity with and without PAH emission, although the
trend may have been affected by selection bias. We show this trend in panel 8b, where the
PAH deficient galaxies reside inside the dashed region and galaxies with higher metallicites
avoid two regions in the diagram which are shown by the dotted lines. We are interested to
see where the LSB galaxies fit in these diagrams.
Panel 8a shows that the LSB galaxies are different than the PAH-deficient dwarf galaxies
in terms of their mid-IR colors. In the color space, LSB galaxies occupy a region similar to
HSB galaxies and reside significantly farther (> 3σ, along the horizontal axis) from PAH-
deficient galaxies. While UGC 6614 stays right in the middle of the locus, both Malin 1 and
UGC 9024 falls in the edge because of the shapes of their SEDs at 8 and 24 µm. The mid-IR
colors of these galaxies closely resemble those of elliptical galaxies which is surprising given
that their apparent different star formation histories.
The LSB galaxies are metal poor with Z ≤ 1/3Z⊙ (McGaugh 1994; de Bloke & van
der Hulst 1998b; de Naray et a. 2004). McGaugh (1994) provided an estimate of oxygen
abundance for UGC 6614 but it is highly uncertain. Following the general trend shown by
the LSB galaxies we assign one-third solar value to Malin 1 and a solar value to UGC 6879.
The full range and the median values of published oxygen abundance are shown for UGC
6614 and UGC 9024.Having very limited information it is, therefore, extremely difficult to
explore the PAH-metallicity connection for these galaxies. We are interested in the question
whether LSB galaxies being low Z systems, will appear close to the PAH-deficient box or
will they fall in the region shunned both by HSB dwarfs and HSB spirals with extended disks
(dotted regions; Fig 8b)? While it is tempting to give more weight to the latter case, only
three data points with large errors are insufficient to derive any trend. A larger sample of
LSB galaxies with 8 µm detections is needed to shed more light on this topic.
Panel 8c shows the connection between mid-IR and far-IR colors which basically describe
the nature of dust emission at these wavelengths. In this panel, the far-IR color of Malin 1
is shown with respect to a far-IR flat SED, i.e. same flux density at 70 and 160 µm. For the
other two galaxies the diagram shows rather low S70µm/S160µm and high S8µm/S24µm ratios.
While low far-IR color suggest that they are IR cool sources, the mid-IR color may be linked
with the destruction of very small grains (but not the PAH molecules) and thus can be used
as a parameter which can signal evolutionary stages of an ISM. A higher value of S8µm/S24µm
can mean that S8µm is high (large amount of PAH) or that S24µm is low (little or no warm
dust). Ellipticals are in the latter category (see Pahre et al. 2004) whereas LSB systems are
in the first category. It should be noted that the 24 µm emission is very closely associated
– 13 –
with H II regions (Helou et al. 2004). Therefore, the lack of emission at this wavelength is
more likely a deficiency in H II region which is quite consistent with the Hα images of LSB
galaxies (McGaugh et al. 1995; de Naray et al. 2004).
The vertical arrow in panel 8d represents a probable range of IRAS far-IR color for UGC
9024 assuming that Spitzer far-IR ratio is the same as the IRAS far-IR ratio. The sequence of
IR colors can be associated with a progression toward greater dust heating intensity and thus
with a sequence of star formation activity. The cool end of the color sequence corresponds to
cool diffuse H I medium and quiescent molecular clouds, whereas the warm end corresponds
to the colors of H II regions, starbursts, and galaxies with higher LTIR/LB ratios. Although
the IR nature of these three LSB galaxies are explicit in this diagram, the interesting feature
is that they are not the extreme cases in terms of IR SEDs as shown by some of the SINGS
galaxies.
Two primary sources have been proposed to explain the heating of the dust which
produces IR luminosity in spiral galaxies - massive young (OB) stars and associated H II
regions (Helou et al. 1985; Devereux & Young 1990, 1993), and non-ionizing A and later stars
(Lonsdale & Helou 1986; Walterbos & Schwering 1987; Bothun et al. 1989). Some authors,
however, suggest contribution from both sources (Smith 1982; Habing et al. 1984; Rice et
al. 1990; Sauvage & Thuan 1992; Smith et al. 1994; Devereux et al. 1994). The dominance
of the heating source is, therefore, governed by the availability of discrete and dense star
forming regions in the ISM. While observational evidence suggests that the global IR emission
from luminous IR spiral galaxies provides a measure of high mass SFR (Kennicutt 1998),
the diffuse IR emission in quiescent galaxies is caused mainly by the thermal radiation of
the interstellar dust heated by the interstellar radiation field (ISRF) (Jura 1982; Mezner et
al. 1982; Cox et al. 1986; Jura et al. 1987).
The dust color temperature (Td) deduced for galaxies from the IRAS 60 and 100 µm
flux densities are typically 25-40 K assuming emissivity index β = 2 (Rahman et al. 2006).
This range is similar to the temperature of dust in Galactic star-forming regions (Wynn-
Williams & Becklin 1974; Scoville & Good 1989), and considerably greater than the 15-20K
temperatures expected for dust heated by the ambient ISRF (Jura 1982; Mezner et al. 1982;
Cox et al. 1986; Jura et al. 1987). The color temperature derived from the MIPS 70 and
160 µm of LSB galaxies ranges ∼17-21 K.
From a statistical study of a large sample of optically selected galaxies, Bothun et al.
(1989) demonstrated that in the absence of UV radiation, far-IR color ratio of S60µm/S100µm ≤
0.3 can result from dust which is heated by old stars. Galaxies with S60µm/S100µm ≈
0.3 − 0.5 requires a steadily increasing proportion of UV heated dust, while galaxies with
S60µm/S100µm ≥ 0.5 are entirely dominated by the UV heated dust. From panel 8d we
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find that the LSB galaxies have cool effective dust temperatures and therefore lack intense
heating from massive stars.
It should be mentioned here that in the IR color analysis we did not subtract the stellar
contributions from the 8 and 24 µm measurements to exactly reproduce the E05 diagram.
Assuming conservatively that the 3.6 µm emission is coming from the stellar photosphere, the
stellar contributions obtained from the empirical stellar SED of Pahre et al. (2004) derived
for early type galaxies are ∼59%, ∼38%, and ∼19% at 4.5, 5.8, and 8 µm, respectively.
The stellar contribution at 24 µm emission is ∼5% or less. Applying corrections to the
corresponding flux densities do not change our results since these corrections systematically
shift the parameters in the color space. Note that we use the same set of numbers for
each galaxy ignoring the variation in amount of stellar contributions from galaxy to galaxy.
However, error due to this is negligible. To find stellar contributions, preference is given
to the empirical SED than any stellar synthesis model because of unknown SFHs and poor
metallicity constraints for LSB galaxies.
In summary, from the four panels of Fig. 8 we conclude that: (a) Malin 1, UGC 6614,
and the intermediate HSB/LSB object UGC 6879 have mid-IR colors similar to quiescent
HSB disk galaxies; UGC 9024 falls in the region of HSB elliptical galaxies in this color plane.
(b) There is insufficient data to conclude whether LSB galaxies have PAH emission properties
significantly different from HSB galaxies with comparably low metallicities. Observations
of many more LSB galaxies are needed to settle this. (c) Available far-IR detections and
upper limits indicate that LSB galaxies are far-IR cool sources. The dust temperatures
derived from the MIPS 70 and 160 µm of LSB galaxies ranges Td ∼17-21 K, similar to many
quiescent HSB spiral and elliptical galaxies.
3.4. Molecular ISM
The LSB galaxies are rich in neutral hydrogen (H I). Molecular hydrogen (H2) gas
inferred from CO emission has been detected in only a handful of such galaxies (Matthews
& Gao 2001; O’Neil & Schinnerer 2004; Matthews et al. 2005; Das et al. 2006). The low
rate of CO detection has been attributed to an ISM with low dust content and a low surface
density of neutral gas. Dust opacity is crucial for the formation and survival of molecules
since it provides them necessary shielding from the ISRF. A larger column density is needed
to self-shield the H2 molecule. A low density and less dusty environment exposes H2 to UV
photons which can easily dissociate these molecules. The low star formation and far-IR cool
nature of LSB galaxies implies lower energy density of the radiation field and consequently
lower dissociation of CO and H2 (de Blok & van der Hulst 1998b).
– 15 –
The deficiency of molecular gas detections in LSB galaxies may point to a dynamical
condition such as the absence of large-scale instability in the disk preventing formation of
giant molecular clouds (Mihos et al. 1997). Local instabilities may lead to cloud condensation
resulting in localized star formation which may escape detection by current observations (de
Blok & van der Hulst 1998b). Local instability in the disk invokes energetic phenomena
such as SN explosions and the frequency of such occurrence in LSB galaxies is low (Hoeppe
et al. 1994; McGaugh 1994). The enhanced cooling by molecules is crucial in the onset of
instability of molecular clouds. Therefore, the effect of less efficient cooling of the ISM can
also prevent local instabilities. Long cooling time leads to higher cloud temperatures and
thus makes it difficult for a cold molecular phase to exist (Gerritson & de Blok 1999).
If the low density ISM truly lacks H2 and CO molecules, what other types of molecule
can exist in this environment? Are they very cold or very warm molecules of known types?
To which dust components do they belong? Along with millimeter wavelength observations,
can we use currently available IR data to shed light into these questions? One of the three
LSB galaxies, UGC 6614, has been reported to have CO(1-0) emission at a certain localized
region in the disk (Das et al. 2006), while the other two galaxies have not been observed in
this wavelength. On the other hand, the Spitzer observation of UGC 6614 shows the presence
of enhanced PAH emission on the bulge and almost entirely along the outer disk. This 8 µm
emission, however, is concentrated in the central regions of the other two galaxies.
A possible source of the origin of PAH molecules is the dense, high temperature, carbon-
rich ([C]/[O] > 1) environment of circumstellar envelopes surrounding mass-losing AGB car-
bon stars. PAH formation by stars with more normal, oxygen-rich, photospheric abundances
([C]/[O] < 1) will be negligible because nearly all the available carbon is bound up in the CO
molecules (Latter 1991). Therefore, if the stellar population responsible for the enrichment
of the ISM are dominantly old carbon-rich stars, the warm PAH molecules will be ubiquitous
resulting in lower abundance of CO molecules. While a detailed investigation is beyond the
scope of this study, we believe the observed PAH emission and lack of CO emission holds
the potential clue to probe not only the ISM but also to better understand the SFHs in LSB
galaxies.
3.5. Mid-IR Photometry of UGC 6614
The optical spectra of large LSB disks show an unexpected high occurrence of low-level
active galactic nuclei (AGN) type activity (Spraybarry et al. 1995; Schombert 1998). UGC
6614, a optical giant LSB galaxy, is suspected to harbor a weak AGN from the optical
spectrum (Schombert 1998) and from excess emissions at millimeter (Das et al. 2006) and
– 16 –
radio wavelengths (Condon et al. 2000). Integrated mid-IR photometry provides a robust
technique to identify AGN in HSB galaxies where AGN tend to be redder than normal star
forming galaxies in the mid-IR (Lacy et al. 2004; Stern et al. 2005).
To determine whether one can use a similar technique to detect AGN signatures in a
LSB bulge, we analyze the IRAC colors ([3.6]-[4.5] vs. [5.8]-[8.0]) of UGC 6614. The colors
are measured for two regions of radius ∼1 and ∼5.5 kpc, respectively, encircling the galaxy
center (Fig. 9; left panel). We find that in the color space, along the vertical [3.6]-[4.5] axis,
the galaxy resides well outside the Stern et al “AGN box”. Although the lower end of the
AGN box is within the error bar of this galaxy, its overall mid-IR color put it in a region
occupied mostly by star forming galaxies suggesting that broad band colors may not be an
efficient tracer for weak AGNs.
The contribution from an AGN to the measured IRAC fluxes can be estimated by
combining image subtraction with assumptions about the nature of the SED coming from
starlight and AGN emission. We use Pahre et al. (2004) for stellar flux ratios, and a ν−1
power law SED for AGN (Clavel et al. 2000). Following the procedure of Howell et al.
(2007), a 4.5 µm image of the non-stellar emission was constructed (Fig. 9; right panel).
Unlike the procedure of E05 which simply measures S4.5 − αS3.6, this procedure includes
an additional factor to account for the contribution of non-stellar emission to the 3.6 µm
image. The non-stellar flux density measured this way indicates that, within a 12′′ aperture,
the AGN contributes ∼12% of the light at 4.5 µm and ∼6% at 3.6 µm. At 8 µm, starlight
and the AGN each contribute ∼35% of the light, with PAHs contributing the remaining.
Note that the selected aperture includes only the bulge of the galaxy, excluding the spiral
arm/ring structure.
UGC 6614 illustrates that although [3.6]-[4.5] color can identify strong AGN, weaker
AGN will not be clearly separated from pure stellar sources. The procedure of E05, measuring
S4.5µm−αS3.6µm, will identify regions of non-stellar emission but will not provide quantitative
picture of stellar emission. Given reasonable assumptions the procedure of Howell et al.
allows a quantitative decomposition of the stellar and non-stellar flux densities.
4. Summary and Conclusions
The Spitzer observations of the three optical giant low surface brightness galaxies Malin
1, UGC 6614, and UGC 9024 have been examined to study the mid and far-IR morphology,
spectral energy distributions, and IR color to estimate dust mass, dust-to-(atomic) gas mass
ratio, total IR luminosity, and star formation rate (SFR). We also investigate UGC 6879,
– 17 –
which is intermediate between HSB and LSB galaxies.
The 8 µm images indicate that polycyclic aromatic hydrocarbon (PAH) molecules are
present in the central regions of all three metal-poor LSB galaxies. The diffuse optical
disks of Malin 1 and UGC 9024 remain undetected at mid- and far-infrared wavelengths.
The dustiest of the three LSB galaxies, UGC 6614, has infrared morphology that varies
significantly with wavelength; 160 µm (cool) dust emission is concentrated in two clumps
on the NE and NW sides of a distinct ring seen in the 24 and 8 µm images (and a broken
ring at 70 µm) at a radius of ∼40′′ (18 kpc) from the galaxy center. The 8 and 24 µm
emission is co-spatial with Hα emission previously observed in the outer ring of UGC 6614.
The estimated dust-to-gas ratios, from less than 10−3 to 10−2, support previous indications
that LSB galaxies are relatively dust poor compared to HSB galaxies. The total infrared
luminosities are approximately 1/3 to 1/2 the blue band luminosities, suggesting that old
stellar populations are the primary source of dust heating in these LSB objects. The SFR
estimated from the infrared data ranges ∼0.01− 0.88 M⊙ yr−1, consistent with results from
optical studies. The mid-IR colors of UGC 6614 shows the presence of a weak AGN at the
central bulge.
Questions can be raised such as what is the most viable reason for these LSB galaxies
to have LTIR/LB < 1? To answer this question we first note that observables such as stellar
populations and SED shapes can be used to break the degeneracy in infrared-to-blue ratio
(Helou 2000). That LSB galaxies have low infrared-to-blue luminosities, stellar populations
spanning a wide range of mean ages, are not dominated by OB stars (McGaugh 1994), have
less dust than the HSB galaxies (see Fig. 6), and are IR cool sources (see Fig. 8) suggest a
composite scenario. The LSB disks are less dusty and the older stellar populations are the
primary source of the IR emission from their ISMs.
The presence of PAH emission in these three galaxies indicates that the ISM of the region
contributing to the emission in these galaxies have significant amount of carbon enrichment
over cosmic time and the ISRF in the ISM must have been significantly weak and thus it is
unable to reduce the strength of PAH emission. In other words, the small grains are more
exposed to the ISRF so that their destruction rate is larger than for PAH molecules.
The detection of mid and far-IR emission from a larger sample will be crucial to un-
derstand the properties of ISM in LSB galaxies and probing their star formation histories.
This will have a significant effect on analytical modeling of galaxy formation and evolution,
the role of different galaxy populations in observed number counts and possibly metallicity
effects in the observed number counts. Whether star formation in LSB disks occurred in a
continuous fashion but with a low rate, or in an exponentially decreasing rate, or as sporadic
bursts with quiescent periods in between is still a matter of debate. Since each type of for-
– 18 –
mation history will lead to a stellar population that could be traced by optical photometry
(i.e. blue or red), the formation scenario must follow the route where the ISM would be in a
state having significant carbon enrichment with substantial amount of dust. The constraints
coming from SST such as mid-IR 8 µm emission and moderate dust mass could be used as
probes to understand the nature of LSB spirals.
Metal poor HSB objects such as blue compact dwarf galaxies are PAH deficient systems
(E05; Wu et al. 2006). Their SEDs are markedly different in the ∼5-15 µm wavelength range
compared to metal rich HSB galaxies. The LSB galaxies are metal poor but have substantial
PAH emission. Although these galaxies are not the extreme cases in metal deficiency such
as BCDs, they fill an interesting niche among local populations, distinct from HSB dwarfs
and from HSB regular galaxies. They may also represent a significant fraction of the galaxy
population at earlier epochs (Zackrisson et al. 2005), and therefore, may have important
implication in the interpretation of galaxy number counts in the infrared/submillimeter as
well as in the visible and near-IR wavelengths.
Previous analytical studies suggest that metal abundances have profound implications
on galaxy number counts observed at 24, 70, and 160 µm (Lagache et al. 2003; Dale et
al. 2005). To date, in analytical models metallicity effects have been incorporated in an
ad hoc manner by artificially manipulating SEDs of HSB galaxies in the wavelength range
mentioned above (Lagache et al. 2003). When template SEDs of many nearby metal-poor
LSB galaxies become available, one can incorporate these in galaxy evolution models as an
independent class along with various other classes such as normal star forming, starburst,
luminous and ultra-luminous, and AGNs to understand the observed galaxy number counts
and the origin of the IR background.
The anonymous referee is thanked for constructive comments and suggestions. We
happily thank D. Dale for his model fits. We also thank Y. Wu, B. R. Brandl, J. R. Houck
for helpful communications. We acknowledge useful discussions from A. Blain, G. D. Bothun,
and S. S. McGaugh on LSB galaxy population. One of us (NR) gratefully acknowledges the
support of a Research Associateship administered by Oak Ridge Associated Universities
(ORAU) during this research. This research has made use of the NASA/IPAC Extragalactic
Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of
Technology, USA under contract with the National Aeronautics and Space Administration,
and the LEDA database in France. This study is based on observations made with the Spitzer
Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute
of Technology under NASA contract 1407. This study has made use of data products from
the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts
and IPAC/Caltech, funded by NASA and the National Science Foundation. This study also
– 19 –
acknowledges use of data products from Solan Digital Sky Survey.
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– 20 –
Fig. 1.— The Solan Digitial Sky Survey (SDSS) composite images of target galaxies. In each
image north is up and east is to the left. The field of view is 2.5′ × 2.5′ with 0.4′′ resolution.
Note that the diffuse disks of Malin 1 and UGC 9024 are barely visible in these images. To
help visualize these extended disks readers are referred to Barth (2007) for I-band Hubble image
of Malin 1 and the following website for deeper B-band images of UGC 6614 and UGC 9024
(http://zebu.uoregon.edu/sb2.html).
http://zebu.uoregon.edu/sb2.html
– 21 –
Malin 1
Fig. 2.— The Spitzer view of Malin 1. The IRAC 3.6, 4.5, and 8 µm images are at left and the
MIPS 24, 70, and 160 µm images are at right. The IRAC 4.5 and 8 µm images are shown without
subtracting stellar photospheric emission. In each image north is up and east is to the left. The
field of view is 2.5′ × 2.5′ in all bands. Pixel sizes are 0.61′′ for the IRAC bands and 1.8′′, 4.0′′, and
8.0′′ for the MIPS 24, 70, and 160 µm bands, respectively. Galaxies from Figs. 3-5 are presented
in a similar manner. There is no detection of Malin 1 at 70 and 160 µm and hence the position
of 24 µm peak emission is shown by the “+” sign at these bands. The contours represent surface
brightness (MJy/Sr) with intervals of
10 where the lowest level is 4σ above the background. See
text for values of the lowest contour levels at different bands.
– 22 –
UGC 6614
Fig. 3.— The Spitzer view of UGC 6614. The position of 24 µm peak emission is shown by the
“+” sign at 160 µm image.
– 23 –
UGC 6879
Fig. 4.— The Spitzer view of UGC 9024. The position of 24 µm peak emission is shown by the
“+” sign at 160 µm image.
– 24 –
UGC 9024
Fig. 5.— The Spitzer view of UGC 6879. The B-band central surface brightness µB,0 of this
galaxy is intermediate between LSB and HSB galaxies.
– 25 –
Fig. 6.— The observed SEDs of LSB galaxies and UGC 6879 at near, mid, and far-IR wavelengths.
The 2MASS, IRAC, and MIPS points are shown by the open circles, filled circles, and open triangles,
respectively. The IRAS upper limits are shown by the filled triangles. For all galaxies the flux
densities are normalized at 3.6 µm. Dotted, dashed, dashed-dotted, and long dashed lines are used,
respectively, to show the SEDs of: NGC 0337 (normal star forming galaxy), NGC 2798 (starburst
galaxy), NGC 2976 (galaxy with nuclear H II region), and NGC 3627 (seyfert II galaxy). Malin 1
was undetected by the MIPS far-IR channels and hence the detection limits are shown for the total
integration time (∼252 sec. at 70 µm and ∼42 sec. at 160 µm).
– 26 –
Fig. 7.— The observed SEDs of LSB galaxies and UGC 6879 using the DH02 model. The symbol
styles are similar to Fig. 6. The dashed represents the empirical stellar SED of Pahre et al. (2004).
The dotted lines represents the stellar synthesis model prediction from Vazquez & Leitherer (2005)
fitted only to the 2MASS fluxes. The dust mass is estimated using the fitted SEDs (solid line).
– 27 –
Fig. 8.— The mid and far-IR color-color diagrams highlighting the LSB spirals (shown by open
stars) with respect to different classes of HSB galaxies (shown by black dots). The SINGS (Kenni-
cutt et al. 2003 ) sample galaxies are taken as the representative of local HSB galaxies. This sample
contains various types of galaxies such as dwarfs, normal starforming, starburst, Syfert I, Seyfert
II and ellipticals. In all panels galaxies represented by the decimal numerals are dwarfs systems
whereas those shown by the roman numerals are ellipticals. The rest of the points (black dots)
represent other population types where one galaxy from each population are shown by the open
circle: NGC 0337 (normal star forming galaxy), NGC 2798 (starburst galaxy), NGC 2976 (galaxy
with nuclear H II region), and NGC 3627 (Seyfert II galaxy). In panel (a) α ≈ 0.58 is the stellar
contribution at 4.5 µm estimated from the empirically derived stellar SED of Pahre et al. (2004).
The regions covered by the dotted boxes (panel b) are from a similar diagram of Eangelbracht et
al. (2005). No HSB galaxies (dwarfs or extended disks) occupy these regions. Far-IR color of
Malin 1 (panel c) is shown with respect to a flat far-infrared SED. The vertical arrow in panel (d)
represents a probable range of IRAS color for UGC 9024. A few galaxies shown by numerals do not
appear in panel (d) because of a lack of IRAS data. Representative error bars based on calibration
uncertainty are shown.
– 28 –
Fig. 9.— Left panel: The IRAC colors for UGC 6614 as shown by solid symbols. The triangle
(square) represents the color measured within a central region of radius of 2′′ (12′′); this corresponds
to a physical radius of ∼1 kpc (∼5.5 kpc). The circle represents integrated flux from the entire
galaxy (Table 1 and 2). In this diagram, stars occupy a locus around (0,0) with various exceptions
(Stern et al. 2005). Star forming galaxies mostly lie along the the horizontal axis depending on
the amount of 8 µm PAH emission. UGC 6879 is shown by open symbols to highlight the behavior
of non-AGN type galaxies. A typical error bar is shown at the bottom. Right panel: an image
(radius 12′′) of non-stellar emission at 4.5 µm after taking into account the contribution of similar
emission at 3.6 µm image. See text for detail.
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This preprint was prepared with the AAS LATEX macros v5.2.
– 33 –
Table 1. Flux densities from 2MASS, IRAS, and Spitzer
Source λ(µm) Malin 1 UGC 6614 UGC 6879 UGC 9024
Sν r Sν r Sν r Sν r
(mJy) (arcsec) (mJy) (arcsec) (mJy) (arcsec) (mJy) (arcsec)
2MASS 1.3 3.93 10 26.80 26 21.10 63 2.67 13
2MASS 1.7 3.41 10 35.50 26 40.90 63 3.62 13
2MASS 2.2 2.00 10 28.00 26 28.70 63 2.65 13
IRAC 3.6 1.61 12 22.31 60 16.03 75 1.94 24
IRAC 4.5 1.10 12 13.74 60 9.93 75 1.18 24
IRAC 5.8 0.73 12 14.11 60 14.73 75 1.57 24
IRAC 8 0.74 12 16.33 60 25.78 75 2.38 24
IRAS 12 – – <60 – <60 – – –
MIPS 24 0.57 30 22 60 28.35 75 1.00 30
IRAS 25 – – <170 – <330 – – –
IRAS 60 – – <150 – <360 – – –
MIPS 70 <5 – 147 60 447 75 98 60
IRAS 100 – – <430 – <1080 – – –
MIPS 160 <25 – 677 60 1529 75 <268 60
Note. — The IRAC flux densities are aperture corrected. Malin 1 was un-
detected by the MIPS far-IR channels and hence the detection limits at 70
and 160 µm are shown for the total integration time: ∼252 sec. at 70 µm
and ∼42 sec. at 160 µm. In general, r represents the radius of a circu-
lar aperture which is used to estimate the flux density. However, the 2MASS
flux densities are given for “total” elliptical aperture radius in the archive
(http://irsa.ipac.caltech.edu/applications/2MASS/PubGalPS) where elliptical ra-
dius represents the semi-major axis. UGC 6879 is edge-on and for this galaxy we
use “effective radius”, defined by r =
ab, where a and b are semi-major and
minor axes.
http://irsa.ipac.caltech.edu/applications/2MASS/PubGalPS
– 34 –
Table 2. Basic Properties of LSB galaxies
Property Malin 1 UGC 6614 UGC 6879∗ UGC 9024 Reference
Type S? SAa SABc SAab 1,2
redshift, z 0.0834 0.0237 0.009 0.0088 1,2
DA (Mpc) 320.47 93.51 35.87 34.86 1,2
inclination, i 16.1o 34o 90o 32.2o 1,2
D25 (arcsec) 16.15 75.54 87.75 104.26 1,2
D (kpc) 25.09 34.24 15.25 17.62 1,2
mB (mag) 17.92 14.12 13.40 17.11 1,2
MB (mag) -19.83 -20.70 -19.52 -15.64 1,2
µB,0 (mag arcsec
−2) 26.50 24.50 20.40 24.71 4,5,6
hr,R (kpc) 73.3 16.00 2.50 7.47 4,5
Yo <8.66 7.35-9.16 – 7.98-8.35 3,9
log(LB/L⊙) 9.94 10.29 9.83 8.21 1,2,5
log(MHI/M⊙) 10.66 10.42 9.08 9.40 5,7,8
log(MH2/M⊙) – 8.45 – – 10
log(Md/M⊙) <8.14 8.42 7.81 <6.94 11
D <0.003 0.01 0.053 <0.004 11
log(LTIR/L⊙) 9.49 9.72 9.05 7.90 11
log(L
/L⊙) <9.50 9.80 9.28 <8.30 11
LTIR/LB 0.35 0.27 0.17 0.49 11
SFR (M⊙ yr
−1) 0.38 0.88 0.20 0.01 11
LTIR/MHI (L⊙/M⊙) 0.07 0.20 0.93 0.03 11
Note. — ∗UGC 6879 is not strictly a LSB galaxy. Its central surface brightness µB,0 is
intermediate between LSB and HSB galaxies. We assume ΩM = 0.3, ΩΛ = 0.7, and H0 = 75 km
sec−1 Mpc−1 to be consistent with the literature. Notation: DA is the angular diameter distance;
D25 is the B-band 25 mag arcsec
−2 isophotal diameter; D is the physical diameter; mB and MB ,
respectively, are B-band apparent and absolute magnitude corrected for Galactic extinction,
internal extinction, and K-correction; µB,0 is the B-band central surface brightness; disk scale
length hr,R is in kpc; the solar value of the oxygen abundance Yo = 12 + log[O/H] ∼ 8.66 from
Asplund et al. (2004); MHI is neutral hydrogen mass; MH2 is molecular hydrogen mass; Md is
dust mass; the dust-to-gas ratio is D = Md/MHI; total IR luminosity LTIR is estimated from
– 35 –
Calzetti et al. (2005); total IR luminosity L
is estimated from the DH02 model fit; star
formation rate SFR is calculated from Alfonso-Herrero et al. (2006).
References: 1) LEDA; 2) NED; 3) McGaugh 1994; 4) Sprayberry et al. 1995; 5) Impey et al.
1996; 6) McGaugh & de Blok 1998; 7) Matthews et al. 2001; 8) Sauty et al. 2003; 9) de Naray
et al. 2004; 10) Das et al. 2006; 11) This study.
	Introduction
	Observation and Data Reduction
	Contamination from Galactic Cirrus
	Results
	Infrared Morphology
	Infrared Diagnostics
	IR Spectral Energy Distributions
	Dust Mass
	Infrared Luminosity
	Star Formation Rate
	Infrared Colors
	Molecular ISM
	Mid-IR Photometry of UGC 6614
	Summary and Conclusions
