Astronomy & Astrophysics manuscript no. aa c© ESO 2019
August 20, 2019
The blue plume population in dwarf spheroidal galaxies:
genuine blue stragglers or young stellar population?⋆
Y. Momany1, E.V. Held1, I. Saviane2, S. Zaggia1, L. Rizzi3, and M. Gullieuszik1
1 INAF: Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: yazan.almomany,enrico.held,marco.gullieuszik,simone.zaggia@oapd.inaf.it
2 European Southern Observatory, A. de Cordova 3107, Santiago, Chile
e-mail: isaviane@eso.org
3 Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
e-mail: rizzi@ifa.hawaii.edu
Received December 23, 2006; accepted April 3, 2007
ABSTRACT
Aims. Blue stragglers (BSS) in the Milky Way field and globular/open clusters are thought to be the product of either primordial or
collisional binary systems. In the context of dwarf spheroidal galaxies it is hard to firmly disentangle a genuine BSS population from
young main sequence (MS) stars tracing a ∼ 1 − 2 Gyr old star forming episode.
Methods. Assuming that their blue plume populations are made of BSS, we estimate the BSS frequency (FBSSHB ; as normalized to the
horizontal branch star counts) for 8 Local Group non star-forming dwarf galaxies, using a compilation of ground and space based
photometry.
Results. (i) The BSS frequency in dwarf galaxies, at any given MV , is always higher than that in globular clusters of similar lumi-
nosities; (ii) the BSS frequency for the lowest luminosity dwarf galaxies is in excellent agreement with that observed in the Milky
Way halo and open clusters; and most interestingly (iii) derive a statistically significant FBSSHB −MV anti-correlation for dwarf galaxies,
similar to that observed in globular clusters.
Conclusions. The low density, almost collision-less, environments of our dwarf galaxy sample allow us to infer (i) their very low
dynamical evolution; (ii) a negligible production of collisional BSS; and consequently (iii) that their blue plumes are mainly made of
primordial binaries. The dwarf galaxies FBSSHB −MV anti-correlation can be used as a discriminator: galaxies obeying the anti-correlation
are more likely to possess genuine primordial BSS rather than young main sequence stars.
Key words. Galaxies: dwarf – globular clusters: general – blue stragglers – stars: evolution
1. Introduction
In the context of Galactic Globular clusters studies, blue strag-
glers (BSS: a hotter and bluer extension of normal main se-
quence stars) represent the highest manifestation of the inter-
play between stellar evolution and stellar dynamics (Meylan &
Heggie 1997). The origin of BSS is sought as either primordial
binaries coeval with the cluster formation epoch, or to a continu-
ous production (in successive epochs) of collisional binaries due
to dynamical collisions/encounters experienced by single/binary
stars throughout the life of the cluster. Ever since their identifi-
cation in (Sandage 1953), BSS have been subject of many pho-
tometric and a handful of spectroscopic (Ferraro et al. 2006, and
references therein) studies. Nevertheless, BSS remain quite dif-
ficult to understand in the light of a single comprehensive sce-
nario. Indeed, it is sometimes necessary to invoke both the pri-
mordial and collisional mechanisms to explain the BSS distribu-
tion in the same cluster. For example the bimodal distribution of
BSS in 47Tuc (Ferraro et al. 2004; highly peaked in the cluster
center, rapidly decreasing at intermediate radii, and finally ris-
ing again at larger radii) can be interpreted as evidence of two
formation scenarios at work: primordial binaries at the cluster
Send offprint requests to: Y. Momany
⋆ Based on archival ESO and HST data.
periphery (where it is easier for them to survive) and collisional
binaries at the center (where it is easier to form).
A spectroscopic survey by Preston & Sneden (2000) of
Milky Way field blue metal-poor stars suggested that over 60%
of their sample is made up by binaries, and that at least 50% of
their blue metal-poor sample are BSS. Piotto et al. (2004) pre-
sented a homogeneous compilation of ∼ 3000 BSS (based on
HST observations of 56 globular cluster), and derived a signif-
icant and rather puzzling anti-correlation between the BSS spe-
cific frequency and the cluster total absolute luminosity (mass).
That is to say that more massive clusters are surprisingly BSS
deficient, as if their higher collision rate had no correlation with
the production of collisional BSS. Another puzzling observable
is that the BSS frequency in Milky Way (MW) field is at least an
order of magnitude larger than that of globular clusters. Recently
De Marchi et al. (2006) presented a photometric compilation for
Galactic open clusters with −6 ≤ MV ≤ −3, and confirmed
an extension of the BSS frequency-MV anti-correlation to the
open clusters regime. In an attempt to explain these observa-
tional trends, Davies et al. (2004) envisage that while the num-
ber of BSS produced via collisions tends to increase with clus-
ter mass, becoming the dominant formation channel for clus-
ters with MV ≤ −8.8, the BSS number originating from pri-
mordial binaries should decrease with increasing cluster mass.
Accounting for these two opposite trends and binary evolution,
http://arxiv.org/abs/0704.1430v1
2 Y. Momany et al.: The blue plume population in dwarf spheroidal galaxies:
Davies et al. (2004) models are able to re-produce the observed
BSS population, whose total number seems independent of the
cluster mass.
Color-magnitude diagrams (CMD) of typically old dwarf
spheroidal galaxies like Ursa Minor (Feltzing et al. 1999 and
Wyse et al. 2002) show the presence of a well-separated blue
plume of stars that very much resembles an old BSS population,
as that observed in globular and open clusters. However, in the
context of dwarf galaxies one cannot exclude that blue plume
stars may include genuinely young main sequence (MS) stars,
i.e. a residual star forming activity (e.g. Held 2005, and refer-
ences therein). The BSS-young MS ambiguity is hard to resolve,
and has been discussed before for Carina (Hurley-Keller et al.
1998), Draco (Aparicio et al. 2001), and Ursa Minor (Carrera et
al. 2002).
In order to investigate this ambiguity, in this paper we
measure the BSS frequency in the dwarf spheroidal galaxy
Leo II and collect BSS counts in 8 other galaxies. Dwarf
spheroidals/irregulars in which there is current or recent (≤ 500
Myr) star formation are not considered. For example, the Fornax
dwarf is known to possess a young population of ∼ 200 Myr
(e.g. Saviane et al. 2000) with young MS stars brighter than the
horizontal branch (HB) level. Therefore Fornax cannot be con-
sidered in this paper. The Sagittarius dwarf spheroidal, on the
other hand, shows an extended blue plume, yet it does not ex-
ceed the HB level, and is included in the study. We also consider
the case of the Carina dwarf and re-derive its BSS frequency.
As discussed in Hurley-Keller et al. (1998) and Monelli et al.
(2003), Carina shows evidence of star formation in recent epochs
(∼ 1 Gyr). However, Carina represents the first case in which the
MS-BSS ambiguity in dwarf galaxies has been addressed: a BSS
frequency was derived and compared with that in Galactic glob-
ular clusters. It is also a case in which the “youngest MS stars”
do not exceed the HB luminosity level; i.e. Carina does not pos-
sess a significant recent star formation rate. As we shall see in
Sect. 3.1, the Carina BSS frequency will be most useful for com-
parison with other dwarf galaxies of similar luminosities.
The BSS frequency is therefore collected for Sagittarius,
Sculptor, Leo II, Sextans, Ursa Minor, Draco, Carina, Ursa
Major and Boötes and these compared internally, and externally
(with that reported for Galactic halo, open and globular clusters).
This allows us to address the dependence of the BSS frequency
on environment from a wider perspective.
2. BSS frequency data points
The dwarf galaxies we study in this paper span a large range of
distances (∼ 25 Kpc for Sagittarius to ∼ 200 Kpc for Leo II).
This basically precludes the availability of one homogeneous
and large-area imaging data-set reaching 1 − 2 magnitudes be-
low the old MS turn-offs (see however the recent HST/WFPC2
archival survey of dwarf galaxies by Holtzman et al. 2006).
Thus estimating the BSS frequency for dwarf galaxies, unfor-
tunately, must rely on a compilation from various sources. We
present new reductions of archival imaging from ESO/NTT and
HST/WFPC2 for Leo II, and ESO/2.2m BVI Pre-Flames 1◦× 1◦
WFI mosaic for Sagittarius1. These were reduced and calibrated
following the standard recipes in Held et al. (1999) and Momany
et al. (2001, 2002 and 2005). For the remaining dwarf galaxies
we estimate the BSS frequency from either public photometric
catalogs (Sextans by Lee et al.2003) or published photometry
kindly provided by the authors (Ursa Minor by Carrera et al.
1 Excluding the inner 14.′ × 14.′ region around M54.
Fig. 1. Left panels display the HST CMDs of Leo II, upon which
we highlight the BSS and HB selection boxes and 2.5, 1.2 and 1
Gyr, [Fe/H]= −1.3 isochrones from Girardi et al. (2002). Right
upper panel displays the ESO/NTT diagram of Leo II along with
1.0 and 0.8 Gyr isochrones highlighting the extension of the ver-
tical clump sequence (dashed lines mark the MS phase while
continuous lines track the post-MS evolution). Also plotted are
the RR Lyrae stars from Siegel & Majewski (2000) and one (as-
terisk) of the 4 anomalous Cepheids (the remaining 3 are outside
the NTT field). Lower right panel displays the ESO/2.2m CMD
of Sagittarius highlighting the Galactic contamination and the
extension of its blue plume.
2002, Draco by Aparicio et al. 2001, Sculptor by Rizzi et al.
2003, Ursa Major by Willman et al. 2005, Boötes by Belokurov
et al. 2006 and Carina by Monelli et al. 2003). All the photomet-
ric catalogs extend to and beyond the galaxy half light radius;
i.e. we cover a significant fraction of the galaxies and therefore
the estimated BSS frequency should not be affected by specific
spatial gradients, if present. The only exception is that relative to
Sagittarius. With a core radius of ∼ 3.7◦, the estimated BSS fre-
quency of our 1◦ square degree field refers to less than 3.5% areal
coverage of Sagittarius, or a conservative ∼ 6% of the stellar
populations. Nevertheless, our Sagittarius catalog is one among
very few wide-field available catalogs of Sagittarius that reach
the BSS magnitude level with an appropriate completeness level,
and it is worthwhile to employ it in this BSS analysis.
A delicate aspect of performing star counts is estimating
the Galactic foreground/background contribution in the covered
area. To compute it in a homogeneous way we made use of the
Trilegal code (Girardi et al. 2005) that provides synthetic stel-
lar photometry of the Milky Way components (disk, halo, and
bulge). Star counts were performed on the simulated diagrams
(using the same selection boxes) and subtracted from the ob-
served HB and BSS star counts. We calculate the specific fre-
quency of BSS (normalizing the number of BSS to the HB) as:
FBSSHB = log(NBSS/NHB). We remind the reader that uncertain-
Y. Momany et al.: The blue plume population in dwarf spheroidal galaxies: 3
ties in the (i) photometric incompleteness correction, (ii) fore-
ground/background subtraction, (iii) possible overlap between
old and intermediate age stellar population around the HB level,
and (iv) confusion between BSS and normal MS stars, are all
unavoidable problems and affect the present and similar stud-
ies. The reported error bars account for the propagation of the
Poisson errors on the star counts, but mostly reflect the depen-
dence on the uncertainty in properly defining the HB and BSS
selection boxes.
2.1. Dwarf galaxies with a non-standard BSS population
The Leo II dwarf might be a prototype of a galaxy whose blue
plume properties differ from the classical BSS sequence found
in globular clusters. Figure 1 displays the HST/NTT CMDs of
Leo II upon which we highlight the BSS and HB selection boxes.
The Leo II BSS specific frequency was estimated from the HST
diagram, whose coverage is comparable with the galaxy half
light radius, about ∼ 2 arcmin. The most notable feature is the
detection of a vertical extension in correspondence of the red
HB region. Stars forming this sequence are usually called verti-
cal clump stars (VC, see Gallart et al. 2005). These are helium-
burning stars of few hundred Myr to ∼ 1 Gyr old population
whose progenitors are to be searched in the blue plume. Indeed,
the relatively large area covered by the NTT shows a CMD with
a well defined VC sequence that can be matched by ∼ 1 Gyr
isochrones. Since in the context of dwarf galaxies one cannot
exclude the presence of an extended star formation, the detec-
tion of VC stars (as is also the case for Draco, see Aparicio et al.
2001) would suggest that the blue plume population may well
hide a genuinely young main sequence. This possibility will be
further investigated in a detailed reconstruction of the star for-
mation history of Leo II (Rizzi et al in prep.).
A second diagnostic in support of a non-standard BSS se-
quence lies in the very extension of the Leo II blue plume. The
luminosity function of blue stragglers in globular clusters has
been found to increase from a luminosity cutoff at MV ∼ 1.9
toward the ancient MS turn-off at MV ∼ 4.0 (Sarajedini & Da
Costa 1991, Fusi Pecci et al. 1992). In the case of Leo II, the
cutoff luminosity should correspond to V ≃ 23.8, whereas we
observe it at brighter magnitudes (V ≃ 23.0 and V ≃ 22.7 in the
HST and NTT diagrams respectively), i.e. the BSS sequence in
Leo II is more extended than that in globulars.
A third diagnostic is that relative to the ratio of BSS to blue
HB stars (7, 000 ≤ Teff ≤ 11, 500 K: the lower temperature
limit marks the blue border of the RR Lyrae instability strip
while the upper temperature limit signs the horizontal branch
truncation in dwarf spheroidal galaxies, as noted in Momany et
al. 2004, 2002). This diagnostic has been used by Hurley-Keller
et al. (1998) in favor of a genuine ∼ 1 Gyr MS population in
Carina, and similarly can be applied for Leo II. Basically, for
Galactic globular clusters Preston et al. (1994) find a BSS to
blue HB ratio of ∼ 0.6. This is however much lower than what
we estimate for Leo II: using the 4 HST/WFPC2 catalogs we
find a BSS to blue HB ratio of ∼ 9.2. A fourth diagnostic is the
BSS to VC ratio that we estimate to remain constant (∼ 8.1) for
each of the 4 WFPC2 chips, as well as for the the entire WFPC2
catalog. This particular observable does not necessarily unveil
the true nature of the blue plume in Leo II, it however confirms
a tight correlation between the BSS and VC populations.
Lastly, we recall the discovery of four intermediate age
anomalous Cepheids in Leo II (Siegel & Majewski 2000). These
are explained as due to either extremely low metallicity variables
or to mass transfer (and possibly coalescence) in a close binary
systems. However, as discussed by Siegel & Majewski (2000),
should the anomalous Cepheids in Leo II be due to mass transfer
binaries, then the number of blue stragglers (in the HST field) is
predicted to be ∼ 0.5 − 5. This is much lower than the observed
number of BSS: corrected for incompleteness, we estimate a to-
tal of 175 BSS stars in the HST CMD. Thus, accounting for: (i)
the presence of VC stars; (ii) the extension and (iii) number of
the BSS population in Leo II makes it more likely that Leo II has
experienced an extended star formation history. Similar conclu-
sions can be applied to Sagittarius (possessing an extended blue
plume whose cutoff luminosity is brighter than MV ∼ 1.9, as il-
lustrated in Fig. 1) and Draco (possessing VC stars, as discussed
in Aparicio et al. 2001).
Despite all these arguments, it remains difficult to rule out
the BSS interpretation. In this regard, it is important to note that
the strongest evidence put in favor of a recent star formation
episode in Leo II (i.e. the detection of VC stars) is a double-edge
sword. Indeed, VC stars have been detected in globular clusters
and investigators needed not to invoke a recent star formation
in these systems: the presence of the VC sequence could and
has been interpreted as due to evolved-BSS. As an example, we
consider the case of M80. Ferraro et al. (1999) derive a ratio of
the BSS to evolved-BSS (or VC) of ∼ 7. This is very close to
the BSS to VC ratio of ∼ 8 that we estimate in Leo II. Mighell
& Rich (1996) were the first to suggest that the VC sequence
in Leo II can be due to the evolved BSS in the helium-burning
phase. Should this be the case then one need not to explain the
absence of gas (Blitz & Robishaw 2000) fueling a recent star
formation in Leo II; since there should not be any.
2.2. Dwarf galaxies showing a standard BSS population
For few dwarf galaxies there is no hint for the presence of VC
stars (see the CMDs of Sextans by Lee et al. 2003 and Ursa
Minor by Carrera et al. 2002). Although foreground contamina-
tion might contribute in veiling the VC population, in general,
the absence of VC or other intermediate age indicators seemed
to indicate a rather normal BSS population in these galaxies.
One interesting anomaly, however, has been detected in the
spatial distribution of the BSS population in Sextans. Lee et al.
(2003) find that brighter BSS in Sextans are more strongly con-
centrated towards the galaxy center, while fainter BSS are lack-
ing in the central regions and follow the distribution of old stars
in the outer regions. In the context of globular clusters a sim-
ilar trend is often attributed to the higher occurrence of colli-
sional binaries in higher density environments (i.e. the center)
normally producing brighter and bluer BSS. However, the colli-
sional rate in dwarf spheroidals like Sextans is much lower than
that in globular clusters (see Sect. 4), and therefore dynamical
evolution in Sextans cannot account for a higher production of
collisional binaries. Thus, leaving aside this particular distribu-
tion anomaly, the overall blue plume properties in galaxies like
Sextans, Ursa Minor, Ursa Major and Boötes have been inter-
preted as the old BSS population.
3. BSS frequency: analysis
3.1. BSS frequency in dwarf galaxies and globular clusters
We now address the BSS frequency for our dwarf galaxies sam-
ple and make an internal comparison. For a wider perspective,
we also compare the overall BSS frequency in dwarf galaxies
with that observed in other stellar systems.
4 Y. Momany et al.: The blue plume population in dwarf spheroidal galaxies:
Fig. 2. The FBSS vs MV diagram for globular clusters (Piotto et al. 2004) open clusters (De Marchi et al. 2006) and dwarf spheroidal
galaxies. The horizontal line shows the mean BSS frequency for Milky Way field stars (Preston & Sneden 2000).
Figure 2 displays the FBSS vs MV diagram for our dwarf
galaxy sample together with the data-points of Piotto et al.
(2004) and De Marchi et al. (2006) for globular and open clus-
ters, respectively. Of the original open cluster sample we only
plot clusters for which ≥ 2 BSS stars were found. To the globular
cluster sample we add the BSS frequency of ωCen (as derived
by Ferraro et al. 2006). The anomalies in ωCen are multi-fold
and span a multiple MS, sub-giant and red giant branches (see
Bedin et al. 2004) and peculiar chemical enrichment (Piotto et
al. 2005), that are often used in favor of a dwarf galaxy origin.
Moreover we estimate the BSS frequency in 2 peculiar systems:
(i) NGC1841 (Saviane et al. 2003) the LMC most metal-poor
and most distant (∼ 10 Kpc from the LMC bar) globular cluster
that is also young and incompatible with the LMC halo rota-
tion; and (ii) NGC2419 (Momany et al. in prep.) a massive MW
cluster at ∼ 90 Kpc from the Galactic center. The three data-
points are based on deep HST/ACS, WFPC2 and ACS archival
data, respectively. Figure 2 clearly shows that, regardless of
their specific peculiarities, ωCen, NGC1841 and NGC2419 are
consistent with the general globular clusters FBSSHB − MV anti-
correlation.
Before turning our attention to the BSS frequency in dwarf
galaxies, we first comment on the case of NGC6717 and
NGC6838 (the two faintest globular clusters with the highest
FBSSHB frequency). Located at (l, b)=(13
◦,−11◦) and (50◦,−5◦),
the two globular clusters can be subject to significant bulge/disk
contamination that was not accounted for in the Piotto et al.
analysis. Trilegal simulations showed in fact that a consider-
able number of Galactic young MS stars would overlap with the
clusters BSS sequences and this can account for their rather high
BSS frequency with respect to globulars of similar MV .
Allowing for the exclusion of NGC6717 and NGC6838, it
results immediately that the lowest luminosity dwarfs (Boötes
and Ursa Major) would possess a higher FBSSHB than globular clus-
ters with similar MV . Most interestingly, their F
HB is in fact
fully compatible with that observed in open clusters. This com-
patibility between dwarf galaxies and open clusters may suggest
that there exists a “saturation” in the BSS frequency (at 0.3−0.4)
for the lowest luminosity systems. Thus, the relatively high FBSSHB
of Boötes and Ursa Major adds more evidence in favor of a dwarf
galaxy classification of the 2 systems. Indeed, although their lu-
minosities is several times fainter than Draco or Ursa Minor, the
physical size of the two galaxies (r1/2 ≃ 220 and 250 pc respec-
tively) exceeds that of more massive galaxies like Ursa Minor
(r1/2 ≃ 150 pc).
Another interesting feature is the significant difference be-
tween the BSS frequency of Carina with that derived for
dwarf galaxies with similar luminosity, i.e. Draco, Ursa Minor,
Sextans, Sculptor and Leo II. Although it is only a lower limit2,
2 It is hard to account for the BSS population originating from the
older and fainter MS turn-off.
Y. Momany et al.: The blue plume population in dwarf spheroidal galaxies: 5
the “BSS” frequency for Carina is of great help in suggesting a
threshold near which a galaxy BSS frequency might hide some
level of recent star formation. The aforementioned 5 galaxies
however have a lower BSS frequency, a hint that these galaxies
possess a normal BSS population rather than a young MS. This
confirms previous conclusions for Sextans (Lee et al. 2003) and
Ursa Minor (Carrera et al. 2002), but is in contradiction with
that of Aparicio et al. (2001) for Draco. However, the Aparicio
et al. conclusion was mainly based on the detection of the VC
stars, a feature that, as we argued, remains an ambiguous indi-
cator. Indeed, Fig. 2 shows that the BSS frequency in Draco is
very close to that of Ursa Minor, a galaxy acceptably known to
possess an old BSS population.
Lastly, leaving aside the extreme dynamical history of
Sagittarius and allowing for the uncertainties (due to the heavy
Galactic contamination and the relatively small sampled popula-
tions) it turns out that its blue plume-HB frequency is (i) lower
than that of a recently star-forming galaxy like Carina, and most
interestingly; (ii) in good agreement with the expected BSS fre-
quency as derived from the FBSSHB − MV anti-correlation for the 7
remaining galaxies in our sample. Added to the clear absence of
MS stars overlapping or exceeding the Sagittarius HB luminos-
ity level (see Fig. 1), we suggest that the Sagittarius blue plume
is a “normal” BSS sequence. As a matter of fact, Sagittarius
is probably the nearest system with the largest BSS population:
over 2600 BSS stars in the inner 1◦ × 1◦ field.
To summarize, from Fig. 2 one finds that FBSSHB in dwarf
galaxies is (i) always higher than that in globular clusters, (ii)
very close, for the lowest luminosity dwarfs, to that observed in
the MW field and open clusters, (iii) the Carina specific FBSSHB
frequency probably sets a threshold for star-forming galaxies,
and most interestingly, (iv) shows a hint of a FBSSHB − MV anti-
correlation.
3.2. A FBSSHB − MV anti-correlation for dwarf galaxies ?
We here explore the statistical significance of a possible FBSSHB −
MV correlation. The linear-correlation coefficient (Bevington
1969) for the 8 galaxies (excluding Carina) data-points is 0.984.
The corresponding probability that any random sample of un-
correlated experimental data-points would yield a correlation
coefficient of 0.984 is < 10−6. Given the greater uncertainties
associated with the Sagittarius BSS frequency, one may be in-
terested in the correlation coefficient excluding the Sagittarius
data-point. In this case, the resulting correlation coefficient re-
mains however quite high (0.972) and the probability that the
7 remaining data-points would randomly correlate is as low as
10−4. Thus, the statistical significance of the FBSSHB − MV anti-
correlation in non star-forming dwarf galaxies is quite high. We
follow the methods outlined in Feigelson & Babu (1992) and
fit least-squares linear regressions. In particular, the intercept
and slope regression coefficients were estimated through 5 lin-
ear models (see the code of Feigelson & Babu for details) the
average of which gives (a, b)=(0.699 ± 0.081, 0.070 ± 0.010)
and (a, b)=(0.631± 0.120, 0.062± 0.014) including and exclud-
ing the Sagittarius data-point, respectively. The reported errors
were estimated through Bootstrap and Jacknife simulations so
as to provide more realistic a and b errors.
However, to firmly establish this FBSSHB − MV anti-correlation
one needs to increase the dwarf galaxies sample, in particular at
the two luminosity extremes. Unfortunately there are not many
non star-forming dwarf galaxies with −13.3 ≤ MV ≤ − 10.1
(c.f. table 14 in Mateo 1998), and few exceptions may come from
Fig. 3. The BSS frequency as a function of the half light radius
(panel a), the central surface brightness (panel b), the velocity
dispersion (panel c) and the stellar collision factor (panel d). See
text for details.
deeper imaging of galaxies like And I and And II. On the other
hand, more Local Group dwarf galaxies are being discovered in
the low luminosity regime (−8.0 ≤ MV ≤ −5.0). Deeper imag-
ing of recently discovered galaxies like Com, CVn IIm, Her and
Leo IV (Belokurov et al. 2007) and Willman 1 (Willman et al.
2006) are needed to estimate their BSS frequency. The impor-
tance of these low-luminosity galaxies is easily understood once
we exclude Boötes and Ursa Major from the BSS frequency cor-
relation analysis. In this case, the correlation coefficient for the
5 remaining data-points is found to drop to 0.901 having a prob-
ability of random correlation as high as 1.5 × 10−1. Thus, a final
word on the FBSSHB − MV anti-correlation must await for more
data-points at both luminosity extremes.
4. Discussion and Conclusions
For a sample of 8 non star-forming dwarf galaxies, we have
tested the hypothesis that the blue plume populations are made of
a genuine BSS population (as that observed in open and globular
clusters) and estimated their frequency with respect to HB stars.
Should this assumption be incorrect (and the blue plume pop-
ulation is made of young MS stars) then one would not expect
an anti-correlation between the galaxies total luminosity (mass)
and the blue plume frequency, but rather a correlation between
the two. Instead, and within the limits of this and similar analy-
sis, we detect a statistically significant anti-correlation between
FBSSHB and MV . A similar anti-correlation has been reported for
Galactic open and globular clusters.
A positive detection of vertical clump stars does not provide
a clear-cut evidence in favor of a recent star formation episode
in a dwarf galaxy. This is because the vertical clump population
has been detected in globular clusters and one cannot exclude
6 Y. Momany et al.: The blue plume population in dwarf spheroidal galaxies:
that these are evolved-BSS. Thus, the main difference between
the blue plume (in non star-forming dwarf galaxies) and the BSS
sequence (in globulars clusters) is that regarding their number.
Should a dwarf galaxy “obey” the FBSSHB − MV anti-correlation
displayed by our sample then its blue plume population is prob-
ably made of blue stragglers.
Do dwarf galaxies harbor a significant population of col-
lisional binaries ? The answer is no. This relies on the intrin-
sic properties of dwarf galaxies and the consequent difference
with those of globular clusters. Indeed, it is enough to recall
that the central luminosity density of a dwarf galaxy (e.g. Ursa
Minor: 0.006 L⊙ pc
−3 at MV = −8.9) is several orders of mag-
nitudes lower than that found in a typical globular cluster (e.g.
NGC7089 ∼ 8000 L⊙ pc
−3 at MV = −9.0). This implies that
the collisional parameter of dwarf galaxies is very low, and un-
ambiguously point to the much slower dynamical evolution of
dwarf galaxies. To further emphasize this last point, we search
for FBSSHB dependencies on other dwarf galaxies structural param-
eters. Figure 3 plots the BSS frequency as a function of the half
light radius (panel a), the central surface brightness (panel b),
the velocity dispersion (panel c) and the stellar collision factor
(panel d)3. Panels a to c plot the FBSSHB as a function of observed
globular/dwarf galaxies quantities. Panel a shows that globular
clusters and dwarf spheroidals form 2 quite distinct families.
This is further confirmed in panel b, although the central sur-
face brightness distribution might suggest a FBSSHB connection of
the two. Panel c shows a correlation between FBSSHB and the cen-
tral velocity dispersion for globular clusters. This reflects the
known globular cluster fundamental plane relations, as shown
by Djorgovski (1995). Despite the similarities in their velocity
dispersion, dwarf galaxies form a separate group from globular
clusters, showing systematically higher FBSSHB .
In panel d we show FBSSHB as a function of a calculated quan-
tity: the stellar specific collision parameter (log Γ⋆: the num-
ber of collisions per star per year). More specifically, following
Piotto et al. (2004), for both globular clusters and dwarf galax-
ies we estimate log Γ⋆ from the central surface density and the
system core size. The mean collisional parameter of the 9 stud-
ied galaxies is ≃ −19. The lowest value is that for Sagittarius
with log Γ⋆ ≃ −20.2, and this is due to its very extended galaxy
core. Compared with the mean value of −14.8 for the globular
clusters sample (see also the lower panel of Fig. 1 in Piotto et
al. 2004), the estimated number of collisions per star per year
in a dwarf spheroidal is 10−5 times lower. This almost precludes
the occurrence of collisional binaries in dwarf galaxies, and one
may conclude that genuine BSS sequences in dwarf galaxies are
mainly made of primordial binaries.
Not all primordial binaries, now present in a dwarf galaxy,
turn into or are already in the form of BSS. In particular, it is
the low exchange encounter probabilities in environments like
the Galactic halo or dwarf galaxies that guarantees a friendly
environment and a slower consumption/evolution of primordial
binary systems. The BSS production (via evolution off the MS of
the primary and the consequent mass-transfer to the secondary
that may become a BSS) is still taking place in the present epoch
3 For globular clusters we make use of the Trager et al. (1995) and
Pryor & Meylan (1993) tables, whereas for dwarf spheroidals we use
the tables from Mateo (1998) and updated velocity dispersions from
recent measurements (Sculptor: Tolstoy et al. 2004, Sextans: Walker et
al. 2006), Carina: Koch et al. 2007, Boötes: Muñoz et al. 2006, Ursa
Minor: Muñoz et al. 2005, Ursa Major: Kleyna et al. 2005, Sagittarius:
Zaggia et al. 2004, Draco: Muñoz et al. 2005; Carina: Muñoz et al.
2006b).
and this can explain the high frequency of primordial BSS in
dwarf galaxies as well as the Galactic halo.
Lastly, it is interesting to note how the BSS frequency in
the low-luminosity dwarfs and open clusters (log(NBSS/NHB) ∼
0.3 − 0.4) is very close to that derived for the Galactic halo
(log(NBSS/NHB) ∼ 0.6) by Preston & Sneden. The latter value
however has been derived relying on a composite sample of only
62 blue metal-poor stars that are (i) distributed at different line
of sights; (ii) at different distances; and most importantly, (iii)
for which no observational BSS-HB star-by-star correspondence
can be established. Thus, allowing for all these uncertainties in
the field BSS frequency (see also the discussion in Ferraro et
al. 2006), it is safe to conclude that the observed open clusters-
dwarf galaxies BSS frequency sets a realistic, and observational
upper limit to the primordial BSS frequency in stellar systems.
Acknowledgements. We thank Alvio Renzini and Giampaolo Piotto for useful
discussions that helped improve this paper. We are also grateful to Belokurov
V., Willman B., Carrera R., Monelli M. and Aparicio A. for kindly providing us
their photometric catalogs.
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http://arxiv.org/abs/astro-ph/0603486
	Introduction
	BSS frequency data points
	Dwarf galaxies with a non-standard BSS population
	Dwarf galaxies showing a standard BSS population
	BSS frequency: analysis
	BSS frequency in dwarf galaxies and globular clusters
	 A FBSSHB- MV anti-correlation for dwarf galaxies ?
	Discussion and Conclusions

  Abridged... Blue stragglers (BSS) are thought to be the product of either
primordial or collisional binary systems. In the context of dwarf spheroidal
galaxies it is hard to firmly disentangle a genuine BSS population from young
main sequence (MS) stars tracing a ~1-2 Gyr old star forming episode. Assuming
that their blue plume populations are made of BSS, we estimate the BSS
frequency for 8 Local Group non star-forming dwarf galaxies, using a
compilation of ground and space based photometry.
  Our results can be summarized as follows: (i) The BSS frequency in dwarf
galaxies, at any given Mv, is always higher than that in globular clusters of
similar luminosities; (ii) the BSS frequency for the lowest luminosity dwarf
galaxies is in excellent agreement with that observed in the Milky Way halo;
and most interestingly (iii) derive a statistically significant
anti-correlation between the BSS frequency and the galaxy Mv.
  The low density, almost collision-less, environments of our dwarf galaxy
sample allow us to infer (i) their very low dynamical evolution; (ii) a
negligible production of collisional BSS; and consequently (iii) that their
blue plumes are mainly made of primordial binaries. The dwarf galaxies
anti-correlation can be used as a discriminator: galaxies obeying the
anti-correlation are more likely to possess genuine primordial BSS rather than
young main sequence stars.

Introduction
In the context of Galactic Globular clusters studies, blue strag-
glers (BSS: a hotter and bluer extension of normal main se-
quence stars) represent the highest manifestation of the inter-
play between stellar evolution and stellar dynamics (Meylan &
Heggie 1997). The origin of BSS is sought as either primordial
binaries coeval with the cluster formation epoch, or to a continu-
ous production (in successive epochs) of collisional binaries due
to dynamical collisions/encounters experienced by single/binary
stars throughout the life of the cluster. Ever since their identifi-
cation in (Sandage 1953), BSS have been subject of many pho-
tometric and a handful of spectroscopic (Ferraro et al. 2006, and
references therein) studies. Nevertheless, BSS remain quite dif-
ficult to understand in the light of a single comprehensive sce-
nario. Indeed, it is sometimes necessary to invoke both the pri-
mordial and collisional mechanisms to explain the BSS distribu-
tion in the same cluster. For example the bimodal distribution of
BSS in 47Tuc (Ferraro et al. 2004; highly peaked in the cluster
center, rapidly decreasing at intermediate radii, and finally ris-
ing again at larger radii) can be interpreted as evidence of two
formation scenarios at work: primordial binaries at the cluster
Send offprint requests to: Y. Momany
⋆ Based on archival ESO and HST data.
periphery (where it is easier for them to survive) and collisional
binaries at the center (where it is easier to form).
A spectroscopic survey by Preston & Sneden (2000) of
Milky Way field blue metal-poor stars suggested that over 60%
of their sample is made up by binaries, and that at least 50% of
their blue metal-poor sample are BSS. Piotto et al. (2004) pre-
sented a homogeneous compilation of ∼ 3000 BSS (based on
HST observations of 56 globular cluster), and derived a signif-
icant and rather puzzling anti-correlation between the BSS spe-
cific frequency and the cluster total absolute luminosity (mass).
That is to say that more massive clusters are surprisingly BSS
deficient, as if their higher collision rate had no correlation with
the production of collisional BSS. Another puzzling observable
is that the BSS frequency in Milky Way (MW) field is at least an
order of magnitude larger than that of globular clusters. Recently
De Marchi et al. (2006) presented a photometric compilation for
Galactic open clusters with −6 ≤ MV ≤ −3, and confirmed
an extension of the BSS frequency-MV anti-correlation to the
open clusters regime. In an attempt to explain these observa-
tional trends, Davies et al. (2004) envisage that while the num-
ber of BSS produced via collisions tends to increase with clus-
ter mass, becoming the dominant formation channel for clus-
ters with MV ≤ −8.8, the BSS number originating from pri-
mordial binaries should decrease with increasing cluster mass.
Accounting for these two opposite trends and binary evolution,
http://arxiv.org/abs/0704.1430v1
2 Y. Momany et al.: The blue plume population in dwarf spheroidal galaxies:
Davies et al. (2004) models are able to re-produce the observed
BSS population, whose total number seems independent of the
cluster mass.
Color-magnitude diagrams (CMD) of typically old dwarf
spheroidal galaxies like Ursa Minor (Feltzing et al. 1999 and
Wyse et al. 2002) show the presence of a well-separated blue
plume of stars that very much resembles an old BSS population,
as that observed in globular and open clusters. However, in the
context of dwarf galaxies one cannot exclude that blue plume
stars may include genuinely young main sequence (MS) stars,
i.e. a residual star forming activity (e.g. Held 2005, and refer-
ences therein). The BSS-young MS ambiguity is hard to resolve,
and has been discussed before for Carina (Hurley-Keller et al.
1998), Draco (Aparicio et al. 2001), and Ursa Minor (Carrera et
al. 2002).
In order to investigate this ambiguity, in this paper we
measure the BSS frequency in the dwarf spheroidal galaxy
Leo II and collect BSS counts in 8 other galaxies. Dwarf
spheroidals/irregulars in which there is current or recent (≤ 500
Myr) star formation are not considered. For example, the Fornax
dwarf is known to possess a young population of ∼ 200 Myr
(e.g. Saviane et al. 2000) with young MS stars brighter than the
horizontal branch (HB) level. Therefore Fornax cannot be con-
sidered in this paper. The Sagittarius dwarf spheroidal, on the
other hand, shows an extended blue plume, yet it does not ex-
ceed the HB level, and is included in the study. We also consider
the case of the Carina dwarf and re-derive its BSS frequency.
As discussed in Hurley-Keller et al. (1998) and Monelli et al.
(2003), Carina shows evidence of star formation in recent epochs
(∼ 1 Gyr). However, Carina represents the first case in which the
MS-BSS ambiguity in dwarf galaxies has been addressed: a BSS
frequency was derived and compared with that in Galactic glob-
ular clusters. It is also a case in which the “youngest MS stars”
do not exceed the HB luminosity level; i.e. Carina does not pos-
sess a significant recent star formation rate. As we shall see in
Sect. 3.1, the Carina BSS frequency will be most useful for com-
parison with other dwarf galaxies of similar luminosities.
The BSS frequency is therefore collected for Sagittarius,
Sculptor, Leo II, Sextans, Ursa Minor, Draco, Carina, Ursa
Major and Boötes and these compared internally, and externally
(with that reported for Galactic halo, open and globular clusters).
This allows us to address the dependence of the BSS frequency
on environment from a wider perspective.
2. BSS frequency data points
The dwarf galaxies we study in this paper span a large range of
distances (∼ 25 Kpc for Sagittarius to ∼ 200 Kpc for Leo II).
This basically precludes the availability of one homogeneous
and large-area imaging data-set reaching 1 − 2 magnitudes be-
low the old MS turn-offs (see however the recent HST/WFPC2
archival survey of dwarf galaxies by Holtzman et al. 2006).
Thus estimating the BSS frequency for dwarf galaxies, unfor-
tunately, must rely on a compilation from various sources. We
present new reductions of archival imaging from ESO/NTT and
HST/WFPC2 for Leo II, and ESO/2.2m BVI Pre-Flames 1◦× 1◦
WFI mosaic for Sagittarius1. These were reduced and calibrated
following the standard recipes in Held et al. (1999) and Momany
et al. (2001, 2002 and 2005). For the remaining dwarf galaxies
we estimate the BSS frequency from either public photometric
catalogs (Sextans by Lee et al.2003) or published photometry
kindly provided by the authors (Ursa Minor by Carrera et al.
1 Excluding the inner 14.′ × 14.′ region around M54.
Fig. 1. Left panels display the HST CMDs of Leo II, upon which
we highlight the BSS and HB selection boxes and 2.5, 1.2 and 1
Gyr, [Fe/H]= −1.3 isochrones from Girardi et al. (2002). Right
upper panel displays the ESO/NTT diagram of Leo II along with
1.0 and 0.8 Gyr isochrones highlighting the extension of the ver-
tical clump sequence (dashed lines mark the MS phase while
continuous lines track the post-MS evolution). Also plotted are
the RR Lyrae stars from Siegel & Majewski (2000) and one (as-
terisk) of the 4 anomalous Cepheids (the remaining 3 are outside
the NTT field). Lower right panel displays the ESO/2.2m CMD
of Sagittarius highlighting the Galactic contamination and the
extension of its blue plume.
2002, Draco by Aparicio et al. 2001, Sculptor by Rizzi et al.
2003, Ursa Major by Willman et al. 2005, Boötes by Belokurov
et al. 2006 and Carina by Monelli et al. 2003). All the photomet-
ric catalogs extend to and beyond the galaxy half light radius;
i.e. we cover a significant fraction of the galaxies and therefore
the estimated BSS frequency should not be affected by specific
spatial gradients, if present. The only exception is that relative to
Sagittarius. With a core radius of ∼ 3.7◦, the estimated BSS fre-
quency of our 1◦ square degree field refers to less than 3.5% areal
coverage of Sagittarius, or a conservative ∼ 6% of the stellar
populations. Nevertheless, our Sagittarius catalog is one among
very few wide-field available catalogs of Sagittarius that reach
the BSS magnitude level with an appropriate completeness level,
and it is worthwhile to employ it in this BSS analysis.
A delicate aspect of performing star counts is estimating
the Galactic foreground/background contribution in the covered
area. To compute it in a homogeneous way we made use of the
Trilegal code (Girardi et al. 2005) that provides synthetic stel-
lar photometry of the Milky Way components (disk, halo, and
bulge). Star counts were performed on the simulated diagrams
(using the same selection boxes) and subtracted from the ob-
served HB and BSS star counts. We calculate the specific fre-
quency of BSS (normalizing the number of BSS to the HB) as:
FBSSHB = log(NBSS/NHB). We remind the reader that uncertain-
Y. Momany et al.: The blue plume population in dwarf spheroidal galaxies: 3
ties in the (i) photometric incompleteness correction, (ii) fore-
ground/background subtraction, (iii) possible overlap between
old and intermediate age stellar population around the HB level,
and (iv) confusion between BSS and normal MS stars, are all
unavoidable problems and affect the present and similar stud-
ies. The reported error bars account for the propagation of the
Poisson errors on the star counts, but mostly reflect the depen-
dence on the uncertainty in properly defining the HB and BSS
selection boxes.
2.1. Dwarf galaxies with a non-standard BSS population
The Leo II dwarf might be a prototype of a galaxy whose blue
plume properties differ from the classical BSS sequence found
in globular clusters. Figure 1 displays the HST/NTT CMDs of
Leo II upon which we highlight the BSS and HB selection boxes.
The Leo II BSS specific frequency was estimated from the HST
diagram, whose coverage is comparable with the galaxy half
light radius, about ∼ 2 arcmin. The most notable feature is the
detection of a vertical extension in correspondence of the red
HB region. Stars forming this sequence are usually called verti-
cal clump stars (VC, see Gallart et al. 2005). These are helium-
burning stars of few hundred Myr to ∼ 1 Gyr old population
whose progenitors are to be searched in the blue plume. Indeed,
the relatively large area covered by the NTT shows a CMD with
a well defined VC sequence that can be matched by ∼ 1 Gyr
isochrones. Since in the context of dwarf galaxies one cannot
exclude the presence of an extended star formation, the detec-
tion of VC stars (as is also the case for Draco, see Aparicio et al.
2001) would suggest that the blue plume population may well
hide a genuinely young main sequence. This possibility will be
further investigated in a detailed reconstruction of the star for-
mation history of Leo II (Rizzi et al in prep.).
A second diagnostic in support of a non-standard BSS se-
quence lies in the very extension of the Leo II blue plume. The
luminosity function of blue stragglers in globular clusters has
been found to increase from a luminosity cutoff at MV ∼ 1.9
toward the ancient MS turn-off at MV ∼ 4.0 (Sarajedini & Da
Costa 1991, Fusi Pecci et al. 1992). In the case of Leo II, the
cutoff luminosity should correspond to V ≃ 23.8, whereas we
observe it at brighter magnitudes (V ≃ 23.0 and V ≃ 22.7 in the
HST and NTT diagrams respectively), i.e. the BSS sequence in
Leo II is more extended than that in globulars.
A third diagnostic is that relative to the ratio of BSS to blue
HB stars (7, 000 ≤ Teff ≤ 11, 500 K: the lower temperature
limit marks the blue border of the RR Lyrae instability strip
while the upper temperature limit signs the horizontal branch
truncation in dwarf spheroidal galaxies, as noted in Momany et
al. 2004, 2002). This diagnostic has been used by Hurley-Keller
et al. (1998) in favor of a genuine ∼ 1 Gyr MS population in
Carina, and similarly can be applied for Leo II. Basically, for
Galactic globular clusters Preston et al. (1994) find a BSS to
blue HB ratio of ∼ 0.6. This is however much lower than what
we estimate for Leo II: using the 4 HST/WFPC2 catalogs we
find a BSS to blue HB ratio of ∼ 9.2. A fourth diagnostic is the
BSS to VC ratio that we estimate to remain constant (∼ 8.1) for
each of the 4 WFPC2 chips, as well as for the the entire WFPC2
catalog. This particular observable does not necessarily unveil
the true nature of the blue plume in Leo II, it however confirms
a tight correlation between the BSS and VC populations.
Lastly, we recall the discovery of four intermediate age
anomalous Cepheids in Leo II (Siegel & Majewski 2000). These
are explained as due to either extremely low metallicity variables
or to mass transfer (and possibly coalescence) in a close binary
systems. However, as discussed by Siegel & Majewski (2000),
should the anomalous Cepheids in Leo II be due to mass transfer
binaries, then the number of blue stragglers (in the HST field) is
predicted to be ∼ 0.5 − 5. This is much lower than the observed
number of BSS: corrected for incompleteness, we estimate a to-
tal of 175 BSS stars in the HST CMD. Thus, accounting for: (i)
the presence of VC stars; (ii) the extension and (iii) number of
the BSS population in Leo II makes it more likely that Leo II has
experienced an extended star formation history. Similar conclu-
sions can be applied to Sagittarius (possessing an extended blue
plume whose cutoff luminosity is brighter than MV ∼ 1.9, as il-
lustrated in Fig. 1) and Draco (possessing VC stars, as discussed
in Aparicio et al. 2001).
Despite all these arguments, it remains difficult to rule out
the BSS interpretation. In this regard, it is important to note that
the strongest evidence put in favor of a recent star formation
episode in Leo II (i.e. the detection of VC stars) is a double-edge
sword. Indeed, VC stars have been detected in globular clusters
and investigators needed not to invoke a recent star formation
in these systems: the presence of the VC sequence could and
has been interpreted as due to evolved-BSS. As an example, we
consider the case of M80. Ferraro et al. (1999) derive a ratio of
the BSS to evolved-BSS (or VC) of ∼ 7. This is very close to
the BSS to VC ratio of ∼ 8 that we estimate in Leo II. Mighell
& Rich (1996) were the first to suggest that the VC sequence
in Leo II can be due to the evolved BSS in the helium-burning
phase. Should this be the case then one need not to explain the
absence of gas (Blitz & Robishaw 2000) fueling a recent star
formation in Leo II; since there should not be any.
2.2. Dwarf galaxies showing a standard BSS population
For few dwarf galaxies there is no hint for the presence of VC
stars (see the CMDs of Sextans by Lee et al. 2003 and Ursa
Minor by Carrera et al. 2002). Although foreground contamina-
tion might contribute in veiling the VC population, in general,
the absence of VC or other intermediate age indicators seemed
to indicate a rather normal BSS population in these galaxies.
One interesting anomaly, however, has been detected in the
spatial distribution of the BSS population in Sextans. Lee et al.
(2003) find that brighter BSS in Sextans are more strongly con-
centrated towards the galaxy center, while fainter BSS are lack-
ing in the central regions and follow the distribution of old stars
in the outer regions. In the context of globular clusters a sim-
ilar trend is often attributed to the higher occurrence of colli-
sional binaries in higher density environments (i.e. the center)
normally producing brighter and bluer BSS. However, the colli-
sional rate in dwarf spheroidals like Sextans is much lower than
that in globular clusters (see Sect. 4), and therefore dynamical
evolution in Sextans cannot account for a higher production of
collisional binaries. Thus, leaving aside this particular distribu-
tion anomaly, the overall blue plume properties in galaxies like
Sextans, Ursa Minor, Ursa Major and Boötes have been inter-
preted as the old BSS population.
3. BSS frequency: analysis
3.1. BSS frequency in dwarf galaxies and globular clusters
We now address the BSS frequency for our dwarf galaxies sam-
ple and make an internal comparison. For a wider perspective,
we also compare the overall BSS frequency in dwarf galaxies
with that observed in other stellar systems.
4 Y. Momany et al.: The blue plume population in dwarf spheroidal galaxies:
Fig. 2. The FBSS vs MV diagram for globular clusters (Piotto et al. 2004) open clusters (De Marchi et al. 2006) and dwarf spheroidal
galaxies. The horizontal line shows the mean BSS frequency for Milky Way field stars (Preston & Sneden 2000).
Figure 2 displays the FBSS vs MV diagram for our dwarf
galaxy sample together with the data-points of Piotto et al.
(2004) and De Marchi et al. (2006) for globular and open clus-
ters, respectively. Of the original open cluster sample we only
plot clusters for which ≥ 2 BSS stars were found. To the globular
cluster sample we add the BSS frequency of ωCen (as derived
by Ferraro et al. 2006). The anomalies in ωCen are multi-fold
and span a multiple MS, sub-giant and red giant branches (see
Bedin et al. 2004) and peculiar chemical enrichment (Piotto et
al. 2005), that are often used in favor of a dwarf galaxy origin.
Moreover we estimate the BSS frequency in 2 peculiar systems:
(i) NGC1841 (Saviane et al. 2003) the LMC most metal-poor
and most distant (∼ 10 Kpc from the LMC bar) globular cluster
that is also young and incompatible with the LMC halo rota-
tion; and (ii) NGC2419 (Momany et al. in prep.) a massive MW
cluster at ∼ 90 Kpc from the Galactic center. The three data-
points are based on deep HST/ACS, WFPC2 and ACS archival
data, respectively. Figure 2 clearly shows that, regardless of
their specific peculiarities, ωCen, NGC1841 and NGC2419 are
consistent with the general globular clusters FBSSHB − MV anti-
correlation.
Before turning our attention to the BSS frequency in dwarf
galaxies, we first comment on the case of NGC6717 and
NGC6838 (the two faintest globular clusters with the highest
FBSSHB frequency). Located at (l, b)=(13
◦,−11◦) and (50◦,−5◦),
the two globular clusters can be subject to significant bulge/disk
contamination that was not accounted for in the Piotto et al.
analysis. Trilegal simulations showed in fact that a consider-
able number of Galactic young MS stars would overlap with the
clusters BSS sequences and this can account for their rather high
BSS frequency with respect to globulars of similar MV .
Allowing for the exclusion of NGC6717 and NGC6838, it
results immediately that the lowest luminosity dwarfs (Boötes
and Ursa Major) would possess a higher FBSSHB than globular clus-
ters with similar MV . Most interestingly, their F
HB is in fact
fully compatible with that observed in open clusters. This com-
patibility between dwarf galaxies and open clusters may suggest
that there exists a “saturation” in the BSS frequency (at 0.3−0.4)
for the lowest luminosity systems. Thus, the relatively high FBSSHB
of Boötes and Ursa Major adds more evidence in favor of a dwarf
galaxy classification of the 2 systems. Indeed, although their lu-
minosities is several times fainter than Draco or Ursa Minor, the
physical size of the two galaxies (r1/2 ≃ 220 and 250 pc respec-
tively) exceeds that of more massive galaxies like Ursa Minor
(r1/2 ≃ 150 pc).
Another interesting feature is the significant difference be-
tween the BSS frequency of Carina with that derived for
dwarf galaxies with similar luminosity, i.e. Draco, Ursa Minor,
Sextans, Sculptor and Leo II. Although it is only a lower limit2,
2 It is hard to account for the BSS population originating from the
older and fainter MS turn-off.
Y. Momany et al.: The blue plume population in dwarf spheroidal galaxies: 5
the “BSS” frequency for Carina is of great help in suggesting a
threshold near which a galaxy BSS frequency might hide some
level of recent star formation. The aforementioned 5 galaxies
however have a lower BSS frequency, a hint that these galaxies
possess a normal BSS population rather than a young MS. This
confirms previous conclusions for Sextans (Lee et al. 2003) and
Ursa Minor (Carrera et al. 2002), but is in contradiction with
that of Aparicio et al. (2001) for Draco. However, the Aparicio
et al. conclusion was mainly based on the detection of the VC
stars, a feature that, as we argued, remains an ambiguous indi-
cator. Indeed, Fig. 2 shows that the BSS frequency in Draco is
very close to that of Ursa Minor, a galaxy acceptably known to
possess an old BSS population.
Lastly, leaving aside the extreme dynamical history of
Sagittarius and allowing for the uncertainties (due to the heavy
Galactic contamination and the relatively small sampled popula-
tions) it turns out that its blue plume-HB frequency is (i) lower
than that of a recently star-forming galaxy like Carina, and most
interestingly; (ii) in good agreement with the expected BSS fre-
quency as derived from the FBSSHB − MV anti-correlation for the 7
remaining galaxies in our sample. Added to the clear absence of
MS stars overlapping or exceeding the Sagittarius HB luminos-
ity level (see Fig. 1), we suggest that the Sagittarius blue plume
is a “normal” BSS sequence. As a matter of fact, Sagittarius
is probably the nearest system with the largest BSS population:
over 2600 BSS stars in the inner 1◦ × 1◦ field.
To summarize, from Fig. 2 one finds that FBSSHB in dwarf
galaxies is (i) always higher than that in globular clusters, (ii)
very close, for the lowest luminosity dwarfs, to that observed in
the MW field and open clusters, (iii) the Carina specific FBSSHB
frequency probably sets a threshold for star-forming galaxies,
and most interestingly, (iv) shows a hint of a FBSSHB − MV anti-
correlation.
3.2. A FBSSHB − MV anti-correlation for dwarf galaxies ?
We here explore the statistical significance of a possible FBSSHB −
MV correlation. The linear-correlation coefficient (Bevington
1969) for the 8 galaxies (excluding Carina) data-points is 0.984.
The corresponding probability that any random sample of un-
correlated experimental data-points would yield a correlation
coefficient of 0.984 is < 10−6. Given the greater uncertainties
associated with the Sagittarius BSS frequency, one may be in-
terested in the correlation coefficient excluding the Sagittarius
data-point. In this case, the resulting correlation coefficient re-
mains however quite high (0.972) and the probability that the
7 remaining data-points would randomly correlate is as low as
10−4. Thus, the statistical significance of the FBSSHB − MV anti-
correlation in non star-forming dwarf galaxies is quite high. We
follow the methods outlined in Feigelson & Babu (1992) and
fit least-squares linear regressions. In particular, the intercept
and slope regression coefficients were estimated through 5 lin-
ear models (see the code of Feigelson & Babu for details) the
average of which gives (a, b)=(0.699 ± 0.081, 0.070 ± 0.010)
and (a, b)=(0.631± 0.120, 0.062± 0.014) including and exclud-
ing the Sagittarius data-point, respectively. The reported errors
were estimated through Bootstrap and Jacknife simulations so
as to provide more realistic a and b errors.
However, to firmly establish this FBSSHB − MV anti-correlation
one needs to increase the dwarf galaxies sample, in particular at
the two luminosity extremes. Unfortunately there are not many
non star-forming dwarf galaxies with −13.3 ≤ MV ≤ − 10.1
(c.f. table 14 in Mateo 1998), and few exceptions may come from
Fig. 3. The BSS frequency as a function of the half light radius
(panel a), the central surface brightness (panel b), the velocity
dispersion (panel c) and the stellar collision factor (panel d). See
text for details.
deeper imaging of galaxies like And I and And II. On the other
hand, more Local Group dwarf galaxies are being discovered in
the low luminosity regime (−8.0 ≤ MV ≤ −5.0). Deeper imag-
ing of recently discovered galaxies like Com, CVn IIm, Her and
Leo IV (Belokurov et al. 2007) and Willman 1 (Willman et al.
2006) are needed to estimate their BSS frequency. The impor-
tance of these low-luminosity galaxies is easily understood once
we exclude Boötes and Ursa Major from the BSS frequency cor-
relation analysis. In this case, the correlation coefficient for the
5 remaining data-points is found to drop to 0.901 having a prob-
ability of random correlation as high as 1.5 × 10−1. Thus, a final
word on the FBSSHB − MV anti-correlation must await for more
data-points at both luminosity extremes.
4. Discussion and Conclusions
For a sample of 8 non star-forming dwarf galaxies, we have
tested the hypothesis that the blue plume populations are made of
a genuine BSS population (as that observed in open and globular
clusters) and estimated their frequency with respect to HB stars.
Should this assumption be incorrect (and the blue plume pop-
ulation is made of young MS stars) then one would not expect
an anti-correlation between the galaxies total luminosity (mass)
and the blue plume frequency, but rather a correlation between
the two. Instead, and within the limits of this and similar analy-
sis, we detect a statistically significant anti-correlation between
FBSSHB and MV . A similar anti-correlation has been reported for
Galactic open and globular clusters.
A positive detection of vertical clump stars does not provide
a clear-cut evidence in favor of a recent star formation episode
in a dwarf galaxy. This is because the vertical clump population
has been detected in globular clusters and one cannot exclude
6 Y. Momany et al.: The blue plume population in dwarf spheroidal galaxies:
that these are evolved-BSS. Thus, the main difference between
the blue plume (in non star-forming dwarf galaxies) and the BSS
sequence (in globulars clusters) is that regarding their number.
Should a dwarf galaxy “obey” the FBSSHB − MV anti-correlation
displayed by our sample then its blue plume population is prob-
ably made of blue stragglers.
Do dwarf galaxies harbor a significant population of col-
lisional binaries ? The answer is no. This relies on the intrin-
sic properties of dwarf galaxies and the consequent difference
with those of globular clusters. Indeed, it is enough to recall
that the central luminosity density of a dwarf galaxy (e.g. Ursa
Minor: 0.006 L⊙ pc
−3 at MV = −8.9) is several orders of mag-
nitudes lower than that found in a typical globular cluster (e.g.
NGC7089 ∼ 8000 L⊙ pc
−3 at MV = −9.0). This implies that
the collisional parameter of dwarf galaxies is very low, and un-
ambiguously point to the much slower dynamical evolution of
dwarf galaxies. To further emphasize this last point, we search
for FBSSHB dependencies on other dwarf galaxies structural param-
eters. Figure 3 plots the BSS frequency as a function of the half
light radius (panel a), the central surface brightness (panel b),
the velocity dispersion (panel c) and the stellar collision factor
(panel d)3. Panels a to c plot the FBSSHB as a function of observed
globular/dwarf galaxies quantities. Panel a shows that globular
clusters and dwarf spheroidals form 2 quite distinct families.
This is further confirmed in panel b, although the central sur-
face brightness distribution might suggest a FBSSHB connection of
the two. Panel c shows a correlation between FBSSHB and the cen-
tral velocity dispersion for globular clusters. This reflects the
known globular cluster fundamental plane relations, as shown
by Djorgovski (1995). Despite the similarities in their velocity
dispersion, dwarf galaxies form a separate group from globular
clusters, showing systematically higher FBSSHB .
In panel d we show FBSSHB as a function of a calculated quan-
tity: the stellar specific collision parameter (log Γ⋆: the num-
ber of collisions per star per year). More specifically, following
Piotto et al. (2004), for both globular clusters and dwarf galax-
ies we estimate log Γ⋆ from the central surface density and the
system core size. The mean collisional parameter of the 9 stud-
ied galaxies is ≃ −19. The lowest value is that for Sagittarius
with log Γ⋆ ≃ −20.2, and this is due to its very extended galaxy
core. Compared with the mean value of −14.8 for the globular
clusters sample (see also the lower panel of Fig. 1 in Piotto et
al. 2004), the estimated number of collisions per star per year
in a dwarf spheroidal is 10−5 times lower. This almost precludes
the occurrence of collisional binaries in dwarf galaxies, and one
may conclude that genuine BSS sequences in dwarf galaxies are
mainly made of primordial binaries.
Not all primordial binaries, now present in a dwarf galaxy,
turn into or are already in the form of BSS. In particular, it is
the low exchange encounter probabilities in environments like
the Galactic halo or dwarf galaxies that guarantees a friendly
environment and a slower consumption/evolution of primordial
binary systems. The BSS production (via evolution off the MS of
the primary and the consequent mass-transfer to the secondary
that may become a BSS) is still taking place in the present epoch
3 For globular clusters we make use of the Trager et al. (1995) and
Pryor & Meylan (1993) tables, whereas for dwarf spheroidals we use
the tables from Mateo (1998) and updated velocity dispersions from
recent measurements (Sculptor: Tolstoy et al. 2004, Sextans: Walker et
al. 2006), Carina: Koch et al. 2007, Boötes: Muñoz et al. 2006, Ursa
Minor: Muñoz et al. 2005, Ursa Major: Kleyna et al. 2005, Sagittarius:
Zaggia et al. 2004, Draco: Muñoz et al. 2005; Carina: Muñoz et al.
2006b).
and this can explain the high frequency of primordial BSS in
dwarf galaxies as well as the Galactic halo.
Lastly, it is interesting to note how the BSS frequency in
the low-luminosity dwarfs and open clusters (log(NBSS/NHB) ∼
0.3 − 0.4) is very close to that derived for the Galactic halo
(log(NBSS/NHB) ∼ 0.6) by Preston & Sneden. The latter value
however has been derived relying on a composite sample of only
62 blue metal-poor stars that are (i) distributed at different line
of sights; (ii) at different distances; and most importantly, (iii)
for which no observational BSS-HB star-by-star correspondence
can be established. Thus, allowing for all these uncertainties in
the field BSS frequency (see also the discussion in Ferraro et
al. 2006), it is safe to conclude that the observed open clusters-
dwarf galaxies BSS frequency sets a realistic, and observational
upper limit to the primordial BSS frequency in stellar systems.
Acknowledgements. We thank Alvio Renzini and Giampaolo Piotto for useful
discussions that helped improve this paper. We are also grateful to Belokurov
V., Willman B., Carrera R., Monelli M. and Aparicio A. for kindly providing us
their photometric catalogs.
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http://arxiv.org/abs/astro-ph/0603486
	Introduction
	BSS frequency data points
	Dwarf galaxies with a non-standard BSS population
	Dwarf galaxies showing a standard BSS population
	BSS frequency: analysis
	BSS frequency in dwarf galaxies and globular clusters
	 A FBSSHB- MV anti-correlation for dwarf galaxies ?
	Discussion and Conclusions
