A&A 399, 489-496 (2003)
DOI: 10.1051/0004-6361:20021810
L. P. Bassino 1,2 - S. A. Cellone 1 - J. C. Forte 1,2 - B. Dirsch 3
1 - Facultad de Ciencias Astronómicas y Geofísicas,
Universidad Nacional de La Plata, Paseo del Bosque S/N,
1900-La Plata, Argentina
2 -
Consejo Nacional de Investigaciones Científicas y
Técnicas (CONICET), Argentina
3 -
Universidad de Concepción, Departamento de Física,
Casilla 160-C, Concepción, Chile
Received 24 July 2002 / Accepted 22 November 2002
Abstract
We present the results of a search for globular clusters in
the surroundings of 15 low surface brightness dwarf galaxies belonging
to the Fornax Cluster, which was carried out on CCD images obtained with
the C and T1 filters of the Washington photometric system.
The globular cluster candidates show an extended and probably
bimodal (C-T1) color
distribution, which is inconsistent with the presence of a single
population of metal-poor clusters detected in several dwarf galaxies.
The surface number density of these candidates shows no
concentration towards the respective dwarf galaxies, in whose outskirts
they have been identified.
On the contrary, if we split the candidates in two groups according
to their projected distances to the center of the Fornax Cluster, those
located closer to the center show a higher projected density
than those located farther from it. These results suggest that
the potential globular clusters might not be bound to the dwarf
galaxies. Alternatively, these globulars could form part of the very
peripheral regions of NGC 1399 (the central galaxy of the Fornax
Cluster) or even belong to the intracluster medium.
Key words: Galaxy: globular clusters: general - galaxies: star clusters - galaxies: dwarf - galaxies: clusters: individual Fornax
There are fewer investigations of globular cluster systems in dwarf
galaxies that are not in the Local Group. Miller and collaborators
(Miller et al. 1998a,b; Miller 1999; Lotz et al. 2001) carried out
a survey with images from the Wide Field Planetary
Camera 2 of the Hubble Space Telescope (FOV
6 arcmin2)
to analyze the properties of globular clusters and nuclei
of dwarf elliptical galaxies (dEs) in the Fornax and Virgo Clusters
and the Leo Group. They include about 20 dEs from the Fornax Cluster
but none of them are in common with the present work.
They show that the globular cluster specific frequency
(number of globular clusters with respect
to the parent galaxy's luminosity) of the dEs is in the range 2-6;
the luminosity function of the globular cluster candidates is consistent
with a Gaussian with peak at
mag and dispersion
mag (assuming a distance modulus of 31.4 for
the Fornax Cluster); and most of the globular cluster (V-I) colors are
similar to those of the metal-poor globular cluster population
(also Ashman & Bird 1993).
The globular cluster system of the luminous dE, N galaxy
NGC 3115 DW1 was studied by Durrell et al. (1996a) and Puzia et al. (2000)
who obtained mean metallicities
and -1, respectively,
and they both agreed on a specific frequency
= 4.9.
Durrell et al. (1996b) obtained, on the basis of Washington photometry, a
low mean metallicity,
,
for the globular cluster systems
of two dE galaxies in the Virgo Cluster and they suggested that the dwarf
galaxies globular cluster systems present a similar range of metallicities
(
to -1) as globular clusters in the halos of spiral
galaxies, in agreement with Ashman & Bird (1993).
Turning to the Local Group, Minniti et al. (1996) constructed a master-dE galaxy
combining the data from
globular cluster systems of several dEs in this Group; they found
an old and metal-poor globular cluster population whose metallicity
distribution matched the one of the Milky Way halo globulars.
The abovementioned bimodality in the color distribution of globular clusters in elliptical galaxies, is closely related to the formation of the globular clusters and a variety of scenarios have been proposed (for reviews on the subject see, e.g., Ashman & Zepf 1998 or Harris 2001). Ashman & Zepf (1992) and Zepf & Ashman (1993) predicted this bimodal metallicity distribution of globulars in elliptical galaxies as a result of gas-rich mergers; they proposed that the blue population originally belonged to the progenitor galaxies and the red population formed during the merger. The numerical simulations from Bekki et al. (2002) showed that dissipative mergers create new globular clusters but they were not able to reproduce properly the bimodal metallicity distribution observed in elliptical galaxies. A different point of view was exposed, e.g., by Forbes et al. (1997) who found a correlation between the mean metallicity of the metal-rich globular clusters and the parent galaxy luminosity, which suggested that they share the same chemical enrichment process, while the mean metallicity of the metal-poor ones seems to be independent of the galaxy luminosity. They proposed that the bimodality originated in two phases of globular cluster formation from gas with different metallicities, and that most of them formed "in situ''. The HST study of 17 early-type galaxies by Larsen et al. (2001) showed a correlation between the colors of both, blue and red globular clusters populations, with the B-luminosity and central velocity dispersion of the host galaxy, and concluded that their observations support globular cluster formation "in situ'', in the protogalaxy potential well. Alternatively, Forbes & Forte (2001) analyzed the relation between the mean color of blue and red globulars with the galaxy velocity dispersion and suggested that red globular clusters share a common origin with the host galaxy and blue ones seem to have formed quite independently; according to Côté et al. (1998) these blue globular clusters may have been captured from other galaxies by merger processes or tidal stripping. The idea of the accretion of dwarf galaxies into the cD halo of NGC 1399, the stripping of their gas and globular clusters and the formation of new clusters from this gas poses a different origin for the red globular clusters (Hilker et al. 1999). Burgarella et al. (2001) analyzed the blue globular cluster populations from 47 galaxies and found no correlation between their mean metallicity, which is very similar for all these systems, and the galaxy properties (luminosity, velocity dispersion, etc.); they proposed that the metal-poor globular clusters may have formed from gas fragments of similar metallicity, as already suggested by Ashman & Bird (1993), and located within the dark halo of the galaxy. More recently, the semianalytic model by Beasley et al. (2002) assumed that the metal-poor globular clusters formed in protogalactic fragments and the metal-rich ones originated in the gas-rich mergers of such fragments that occurred later.
Assuming the presence of globular clusters inside clusters of
galaxies, an alternative
scenario is proposed by White (1987) and West et al. (1995),
who pointed to the possible existence of a population of globular clusters
that are not bound to individual galaxies; instead, they are supposed to
move freely in the central regions of the galaxy clusters. These
intracluster globular clusters may be the result of interactions or mergers
between the galaxies, or they may have formed precisely in the
environment of a galaxy cluster without any parent galaxy.
The kinematic analysis by Minniti et al. (1998) and by Kissler-Patig et al. (1999) also
suggested that some globular
clusters may be associated with the gravitational potential of the galaxy
cluster and not solely with NGC 1399.
On the other hand, Grillmair et al. (1999) found no evidence of intergalactic
globular clusters in an HST/WFPC image at a radial distance of about
from NGC 1399, but due to the small
field of view, they were not able to rule out their existence.
Several objections against the intraclusters were raised by Harris et al. (1998)
who tried to explain by means of their existence the supposed high
specific frequency of M87, the central Virgo galaxy; but the latest
values of
obtained for NGC 1399 by Ostrov et al. (1998) and Dirsch et al. (2002b)
showed that it is not so high (
and 5.1,
respectively).
In favor of the existence of intergalactic material, Theuns & Warren (1997),
Mendez et al. (1997), and Ciardullo et al. (1998) presented evidence for the presence
of intergalactic planetary
nebulae within the Fornax and Virgo Clusters, while Ferguson et al. (1998)
reported several hundreds of intracluster red giants in Virgo. Some
globular clusters may have been stripped with them from other
cluster galaxies if this is their origin (Harris 2001).
In this paper, we analyze the characteristics of globular cluster candidates found near dwarf galaxies in the Fornax Cluster and its connection with the above mentioned scenarios. It is organized as follows: Sect. 2 describes the observations and the adopted criteria for the globular cluster candidates' selection. In Sect. 3 we analyze the color distribution, luminosity function and spatial distribution of the candidates. Finally, a summary of the results and a discussion on their implications are provided in Sect. 4. Preliminary results of this work have been presented by Bassino et al. (2002).
CCD images of 15 fields centered on low surface brightness (LSB)
galaxies in the Fornax Cluster were obtained (with the original purpose
of studying the LSB galaxies, Cellone et al.
1994; Cellone & Forte 1996) with
the 0.90-m and 1.50-m telescopes at CTIO (Chile),
during two observing runs in October 1989 and November 1990,
and using the C and T1 filters of the Washington photometric
system. The dwarf galaxies are listed in Table 1: the first column
gives their FCC numbers (Ferguson 1989) and the second one gives the
respective angular distances from NGC 1399,
which will be considered as the center of the Cluster.
The dwarfs are distributed in
angular distance from 12
up to 175
from NGC 1399
(see Fig. 1); as we will adopt a distance modulus of 31.35
for the Fornax Cluster throughout this paper (Madore et al. 1999), that
corresponds to projected distances ranging between 65 and
950 kpc from the cluster center. The fields sizes range from 10 to
17 arcmin2; hence, they have the advantage of being larger
than the ones studied by Miller et al. (1998a) to search for cluster candidates.
However, they are not
as deep as the HST images: we identify cluster candidates up to
22 mag while Miller et al. reach
25 mag
(equivalent to
24.5 mag, according to the relation
between these magnitudes obtained from Geisler 1996 and Cellone et al. 1994).
For a detailed description of the observations we refer to
Cellone et al. (1994).
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Figure 1:
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In order to identify the globular cluster candidates, we selected the point sources within certain ranges of colors and magnitudes. The analysis of each frame was carried out in the following steps:
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Figure 2: Color-magnitude diagram for all the detected sources (crosses) and the globular cluster candidates (open circles). Dotted lines represent the limits of the globular cluster selection. |
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Figure 3: (C-T1) color distribution: the dashed line corresponds to the raw data for the globular cluster candidates, the dot-dashed line to the comparison field and the solid line shows the result after the background subtraction. Dotted lines represent the limits of the globular cluster selection. The metallicity scale (Geisler & Forte 1990) is given on top. |
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Figure 2 shows the color-magnitude diagram of all the identified sources (about 350), and the ones selected as globular cluster candidates (75). The mean photometric errors for the globular cluster candidates are 0.09 mag in T1 and 0.15 mag in (C-T1).
We concentrate on the following features of the globular cluster candidates: their color distribution, their luminosity function, and their spatial distribution. Due to the small number of candidates found around each dwarf, we will analyze the characteristics of all of them together.
The (C-T1) color distribution is shown in Fig. 3; it is
an histogram smoothed by means of a Gaussian with a dispersion comparable
to the errors in (C-T1). The color-metallicity calibration from
Geisler & Forte (1990) has also been included in this figure. In order to take
into account the background contamination we have used a comparison
field located at
north-east from the cluster center, which has
already been used by Dirsch et al. (2002b) and whose color distribution is also
shown in Fig. 3, after normalizing
for the different field sizes corresponding to the dwarfs' images
and to the background's one. The comparison field was observed with
C and
filters instead of C and T1, but according to
Geisler (1996), the Kron-Cousins R and the Washington T1 magnitudes
are similar, to a high degree of accuracy, for the color range
considered in this paper. The comparison field is not very far
from FCC 76, the dwarf with the largest projected distance from
NGC 1399 in the sample; thus, the contribution
from the background may be probably overestimated and so we may be
underestimating the surface densities obtained after this correction.
The surface density of globular cluster
candidates for each dwarf field, after the background-correction, is
given in the last column of Table 1.
The color distribution of the raw data, as can be seen in
Fig. 3, is extended and appears to be bimodal
(see also the color-magnitude diagram displayed in Fig. 2).
In order to quantify this apparent bimodality in a statistical way,
we have applied to the raw data the KMM test which helps to detect
and evaluate
bimodality in datasets (see Ashman et al. 1994 for a description of the
test and its application). The results of the test indicate
that two Gaussians with means at 1.18 and 1.86 mag can be fit to
the set of (C-T1) values, and assuming that both Gaussians have the
same covariance (homoscedastic fitting) we obtain a dispersion
= 0.2 mag for them. The hypothesis that this (C-T1)
distribution is unimodal rather than bimodal is rejected at a confidence
level of 100% according to the KMM test. It must be taken into
account that the number of candidates is small (75 objects) but, according
to the study of the KMM algorithm sensitivity performed by
Ashman et al. (1994), there is a sufficient large separation in the means of
the two Gaussians (3.4
)
to be able to obtain a
significant rejection of the unimodal hypothesis.
The result of the background subtraction is also shown in Fig. 3,
where the corrected color distribution appears to be bimodal too, with
two possible peaks that would be located at
1.2
and 1.9 mag,
corresponding to metallicities [Fe/H]
and 0.1,
respectively (Geisler & Forte 1990).
However, it is not possible to apply the KMM test to this corrected
distribution because the
number of candidates is smaller than 50, and Ashman et al. (1994) state that
in this cases the test does not provide a reliable result for
detecting bimodality. Anyway, even though we cannot confirm statistically
the bimodality,
it is clear that we do not find a single population of metal-poor
globular cluster candidates around the dwarfs, as happens in the
cases already mentioned in the Introduction; instead, an extended
distribution that expands over the whole metallicity range, from
metal-poor to metal-rich populations, is detected.
In spite of the apparent symmetry in the corrected color distribution
displayed in Fig. 3, we cannot be sure that the number of
these probable metal-poor and metal-rich candidates are similar
because we do not
have a complete area sample. Figure 1 shows that there
are more dwarfs near to NGC 1399 than far from it.
We have not attempted to search for radial globular clusters
color gradients with respect to the dwarfs or to the Fornax Cluster
center due to the small sample we are considering.
Whether or not two globular cluster populations should be expected in
dwarf galaxies is still unclear.
All the globular clusters around dwarfs in the Local
Group studied by Minniti et al. (1996) had metallicities that correspond to
a metal-poor population, with [Fe/H] .
They suggested
that the dE galaxies included in their study seem to have formed no
metal-rich globular clusters. In turn, Durrell et al. (1996b) found a possible
bimodality in the colors of
the globular clusters of the Virgo dwarf VCC 1254, which they
speculate might correspond to two phases of globular cluster formation.
With regard to the results for the NGC 1399 globular
cluster system, Ostrov et al. (1998) identified globular clusters
located between
and
from the galaxy center
finding two globular cluster populations with
1.3 and 1.8 mag, respectively. In turn,
Dirsch et al. (2002b) performed a wide-field study of this system
and showed that the innermost sample, between
and
,
was clearly bimodal with peaks at
1.3 and 1.75 mag;
at larger radii, from
up to
,
the metal-poor population
became dominant without changing the position of the blue peak.
We plot the luminosity function (background-corrected) in Fig. 4:
an histogram of the number of globular cluster candidates vs. T1,
where we use an upper limiting magnitude,
T1 = 21.5 mag, omitting
the last bin where the
completeness of the red candidates is more seriously affected.
A Gaussian distribution, calculated with the parameters obtained
by Ostrov et al. (1998) fitting the luminosity function of the NGC 1399
globular cluster system, is included for comparison.
Our sample covers
only 7% of the area under that Gaussian and, after background
subtraction, we are left with 37 globular cluster candidates with magnitudes
19 < T1 < 21.5. Although it is a small sample, we attempt to
compare it with the number of globular clusters that we should have found,
in a similar area, if they belonged to the dwarf galaxies' globular
cluster systems.
We calculate the total integrated brightness of our sample's dEs by
means of the T1 total integrated magnitudes of the dwarfs given by
Cellone et al. (1994), the adopted distance modulus to the Fornax Cluster and
the relation between V and T1 magnitudes mentioned above; thus, we
obtain a total integrated visual brightness for the sample of -18.8 mag.
As the range of specific frequencies proposed for dEs is
-6 (Miller et al. 1998a; Elmegreen 1999),
we estimate that we
should have found between 4 and 13 dwarfs' globular cluster candidates,
within the mentioned T1 range. By comparison, we have identified
three to ten times more globular cluster candidates than what is inferred
from the
values.
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Figure 4:
Luminosity function (background-corrected) for the globular cluster
candidates. The dashed line represents a Gaussian with
< T1 > = 23.3 mag and
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If the globular cluster candidates are bound to the respective
dwarf galaxies, the projected density of globular clusters vs.
galactocentric distance is expected to increase towards the center.
This behavior can be clearly seen in the Fig. 5 of Lotz et al. (2001),
which shows the summed radial distribution of globular cluster
candidates from a sample of 51 dEs.
Figure 5 depicts the surface number
density of globular clusters (background-corrected), calculated in
concentric annuli
around each of the dwarfs and summed over all of them.
These surface densities are estimated as follows. First,
a set of 20
wide annuli is established around each dwarf,
taking into account the different scales of the images.
For each image, annuli with more than 60% of their area outside
the frame limits were discarded, thus leading to the 160
limit in angular distance.
Then, for each dwarf, the number of globular candidates is estimated
within each annulus, applying a completeness correction to the annuli
lying partly outside the corresponding frame. Afterwards, the counts
for each annulus are
background-corrected, subtracting the density of the background field
multiplied by the area of the corresponding complete annulus. Finally,
the background-corrected counts are summed over the annuli defined by
the same angular distance from each dwarf, and divided by its complete
area.
As can be seen in Fig. 5, no concentration towards the center
is evident.
In order to demonstrate this statistically, we compare this observed
distribution with a uniform distribution, that is, one with a constant
number density calculated as the same number of globular clusters,
scattered
across the same total area. The result of a
test performed
between them, indicates that the observed distribution is statistically
consistent with being drawn from a uniform distribution at a significance
level of 89%. Under this evidence, it is not possible to assert
that these globular cluster candidates are bound to the respective dwarfs
as they show no concentration to the dwarf centers,
although we cannot confirm this hypothesis without the aid of radial
velocities.
We must
take into account that the studied radial distribution shown in
Fig. 5 extends up to 14 kpc from the center of the dEs.
The radial density profiles from Lotz et al. (2001) reach almost zero
value at shorter distances from the dwarfs, between 1.1 and
8.7 kpc, according to the dwarf's exponential scalelength they use
and the distance modulus we have adopted,
while the summed radial distribution of the globular cluster system for
11 Virgo dEs
by Durrell et al. (1996b) extends to only 2.5 kpc. Both results indicate that the
globular cluster systems of dwarfs are rather compact.
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Figure 5: Surface density distribution of the globular cluster candidates with respect to the dwarf galaxies. Errors are based on Poisson statistics. |
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Figure 6:
Surface density of the globular cluster candidates
(background-corrected) with respect to the Fornax Cluster center
(NGC 1399) grouped according to its angular distance from it
(less than or greater than 80![]() |
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With regard to the color distribution of the globular cluster candidates, it is interesting to note that we do not find a single population, the metal-poor one, as seems to be the common case for dwarf galaxies (Durrell et al. 1996b; Miller et al. 1998a,b; Miller 1999), but an extended distribution, which appears to be bimodal though we cannot prove it statistically due to the small sample involved. In addition, if we take into account the specific frequency estimated for dwarf galaxies (Miller et al. 1998a; Elmegreen 1999) and the luminosity function of the globular cluster candidates, we should have found significantly fewer globular cluster candidates than we actually do.
According to the projected density of the potential globular clusters,
they show no concentration towards the dwarfs while they do show
concentration with respect to the center of the cluster. These results
led us to speculate that the globular cluster candidates may not be
associated to the dwarf galaxies themselves. We are then left with
three possibilities: first, that these globular cluster candidates
belong to the globular cluster system
of NGC 1399; second, they may be moving freely throughout
the potential well of the cluster, without being bound to
any galaxy in particular; or third, that they are a mix of both.
In the first case, we should be accepting that the globular cluster
system of NGC 1399
is much more extended and numerous than previously thought. According to
this hypothesis, Fig. 6 suggests that there should be clusters
up to at least an intermediate angular distance of 80 (a projected
distance of about 430 kpc) from the central galaxy; the CCD study over
the largest area around NGC 1399 was performed by Dirsch et al. (2002b),
which extends up to 22
,
and showed that the globular cluster
system extends over a radial projected distance of more than 100 kpc.
The total number (background corrected) of globular clusters within
the area covered by our observations can be roughly estimated as about
550 clusters, by extrapolating their luminosity function over the
whole range of T1 magnitudes. Thus, the number of clusters
that should be distributed within a circular area of radius 80
around NGC 1399 may be calculated, just taking into account
the ratio of the areas, as several 104 clusters. For
comparison, the total number of globular clusters associated with
galaxies in the Fornax Cluster may also be roughly estimated as
follows. The blue magnitude of all the galaxies included in the
Ferguson (1989) catalogue is B = 8.3 mag; adopting for them a mean
color index
mag and the distance modulus mentioned
above, we obtain an absolute visual magnitude
MV = -23.8. If we
assume a "typical'' specific frequency
we conclude
that about
globular clusters should be associated
with galaxies in Fornax. It is interesting to note that the number of
globular clusters that we found within a circular area of radius
80
around NGC 1399 is of the same order o larger than
the estimated number of globulars associated to galaxies in the whole
Fornax Cluster.
It is also likely that some globular clusters might have escaped from its parent galaxies and, after that, remained within the potential well of the Fornax Cluster as a whole (see, for instance, Kissler-Patig et al. 1999). White (1987) proposed that the distribution of the stripped globular clusters within the cluster will follow the same density profile as the galaxies and they might form a kind of envelope around the central galaxy. Alternative origins for the intraclusters are mentioned by West et al. (1995), who speculate that they might have formed "in situ'', without a parent galaxy, or during mergers of sub-systems with a high gas content.
Deeper images are required to clarify this picture and a new survey in the Fornax Cluster is in progress. The true nature of these candidates might be confirmed by means of spectra.
Acknowledgements
We wish to thank the referee, Dr. M. Kissler-Patig, for his comments which helped to improve the present paper. We are also grateful to S. D. Abal, M. C. Fanjul and R. E. Martínez for technical assistance. This work was partially supported by grants from CONICET and Fundación Antorchas, Argentina.