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A comparative study of globular cluster systems in UGC 9799 and NGC 1129


Abstract and Figures

We present a preliminary analysis of HST-WFPC2 observations of globular cluster systems in the two brightest galaxies, UGC 9799 (cD) and NGC 1129 (non-cD), located in the center of rich clusters.
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arXiv:astro-ph/0109248v1 17 Sep 2001
Extragalactic Star Clusters
IAU Symposium Series, Vol. Vol. 207, 2001
Eva K. Grebel, Doug Geisler, and Dante Minniti, eds.
A Comparative Study of Globular Cluster Systems in
UGC 9799 and NGC 1129
Myung Gyoon Lee, Eunhyeuk Kim
Astronomy Program, SEES, Seoul National University, Seoul, 151-742,
Doug Geisler
Departamento de F´ısica, Grupo de Astronom´ıa, Universidad de
Concepci´on, Casilla 160-C, Concepci´on, Chile
Terry Bridges
Anglo-Australian Observatory, P.O.Box 296, Epping, NSW 1710,
Keith Ashman
Department of Physic and Astronomy, University of Kansas, Lawrence,
KS 66045-2151, USA
Abstract. We present a preliminary analysis of HST-WFPC2 observa-
tions of globular cluster systems in the two brightest galaxies, UGC 9799
(cD) and NGC 1129 (non-cD), located in the center of rich clusters.
UGC 9799 is a cD galaxy located in the center of the massive Abell 2052
cluster at z=0.035, and is known to h ave the largest number of the globular clus-
ters (N(total) 46, 000) and the highest specific frequency of globular clusters
from the ground-based observations (S
= 20 ± 6, Harris, Pritchet, & McClure
1995). On the other hand, NGC 1129 is a giant, but not cD galaxy, located in the
center of a rich cluster AWM7 at z=0.018. Its globular cluster s y s tem has not
yet been studied. The foreground reddenings are known to be E(V I) = 0.051
for UGC 9799 and E(V I) = 0.159 for NGC 1129. We adopt the redshift
distance modulus (m M)
= 36.0 for UGC 9799 and (m M )
= 34.5 for
NGC 1129 based on the Hubble constant of H
= 65 k m/s/Mpc.
Deep images of these galaxies were obtained using the HST-WFPC2 with
F 555W (V ) and F 814W (I) filters. We have obtained th e photometry of the
point sources in the images where bright galaxies were subtracted, using the
HSTphot package (Dolphin 2000) and the image classification parameters. Our
photometry reaches V 27.2 mag and I 26.0 mag with 50 % completeness.
Figure 1 displays the color-magnitude diagrams of the point sources in UGC 9799
and NGC 1129. In Figure 1 there is seen a vertical structure at 0.8 < (V I) <
1.5 extending up to I 23 mag, which represents the globular clusters in these
galaxies. Faint blue objects are mostly background compact galaxies.
Figure 2 shows that the (V I)
color distribu tions of the bright point
sources with V < 26.5 mag. In Figure 2 the dominant peaks are due to the glob-
2 Lee et al.
Figure 1. Color-magnitude diagrams of th e point sources in UGC
9799 (a) and NGC 1129 (b). The boxes represent the color and mag-
nitude ranges for the bright blue and red globular cluster candidates
with V < 26.5 mag.
Figure 2. (V I)
color distributions of the bright point sources
(mostly globular clusters) with V < 26.5 mag in UGC 9799 and NGC
Globular Clusters in UGC 9799 AND NGC 1129 3
ular clusters and the color distrib ution of the globular clusters in both galaxies
are similarly bimodal: a blue peak at (V I)
= 1.07 ([Fe/H] = –0.8) and a
red peak at (V I)
= 1.17 ([Fe/H] = –0.4) for UGC 9799, and a blue peak at
(V I)
= 1.02 ([Fe/H] = –1.1) and a red peak at (V I)
= 1.17 ([Fe/H] =
–0.4) for NGC 1129. The number of the bright globular clusters with V < 26.5
mag we find is 860 for UGC 9799 and 1,060 for NGC 1129. The ratio of the
number of the blue globular clusters (BGC) and that of the r ed globular clusters
(RGC) for UGC 9799 is derived to be N(BRC)/N(RGC) = 1.8, h igher than that
of NGC 1129, N(BRC)/N(RGC) =1.3.
The surface number density profiles of the globular clusters show that the
globular clusters in both galaxies are spatially more extended than those of the
stellar halo, and the mean colors of the globular clusters are bluer th an those of
the stellar halo. The RGCs are found to be more centrally concentrated than
the RGCs in both galaxies.
Luminosity functions of the globular clusters (GCLFs) are derived after
background subtraction and incompleteness correction, but they do not reach
the turnovers which are expected to be at V 28.7 mag for UGC 9799 and
V 27.5 mag for NGC 1129. We estimate the total number of the globular
clusters from the GCLFs, obtaining N(total)=10, 000 ± 700 for UGC 9799 and
N(total)=7, 000 ± 700 for NGC 1129. T he total number of the globular clusters
in UGC 9799 derived in this study is much smaller than that derived from the
ground-based observation by Harris et al. (1995).
From the integrated photometry of the galaxies the total magnitudes of the
galaxies are estimated to be V = 12.10 mag and I = 10.77 m ag for UGC 9799
(r < 63
), and V = 10.80 mag and I = 9.41 mag for NGC 1129 (r < 80
Absolute total magnitudes of the galaxies are derived to be M
= 24.02 m ag
and M
= 25.30 m ag for UGC 9799, and M
= 24.08 m ag and M
= 25.32
mag for NGC 1129, show ing that both galaxies belong to the brightest galaxies.
Finally we estimate the specific frequency of the globular clusters, S
× 10
= 2.5 ± 0.2 for UGC 9799 and S
= 1.7 ± 0.2 for NGC 1129.
These values are significantly lower than those for normal elliptical galaxies,
and the value for UGC 9799 is much lower than that based on the ground-based
observation (Harris et al. 1995). If we use the total magnitudes of the galaxies
given in the literature (M
= 23.4 mag for UGC 9799, M
= 22.88 mag for
NGC 1129), we get S
= 4.4 ± 0.3 for UGC 9799 and S
= 5.0 ± 0.5 for NGC
1129. This result is not consistent with the intracluster globular cluster model
that suggests that the globular clusters are not bound to individual galaxies,
but bound to the gravitational potential of the clusters (West et al. 1995).
Acknowledgments. This research is supported in part by the MOST/KISTEP
International Collaboration Research Program (1-99-009).
Dolphin, A. E. 2000, PASP, 112, 1383
Harris, W. Pritchet, C. J., & McClure, R . D. 1995, ApJ, 441, 120
West, M. J., ote, P., Jones, C., Forman, W., & Marzke, R. O. 1995, AJ, 453,
... Our Galaxy and M31 are included for comparison. The sources of the data are: Forbes & Forte (2001), Kundu & Whitmore (2001a,b), Larsen et al. (2001), , Lee et al. (2002) for globular clusters, Tonry et al. (2001) and Jensen et al. (2003) for distances of the galaxies, Golev & Prugniel (1998) for M g 2 of the galaxies, and Barmby et al. (2000) for the globular clusters in M31 and our Galaxy. There are 42 early-type galaxies (except for our Galaxy and M31) in the sample, and 22 to 28 elliptical galaxies with HST observations among them were used for linear fitting of the data (represented by the solid line). ...
... The dashed line in (a) represent the one-to-one relation, and the dashed line in (c) represents the color-magnitude relation of host galaxies which is also plotted in (f) as a solid line. Sources of the data: Barmby et al. (2000), Larsen et al. (2001), Forbes & Forte (2001), Kundu & Whitmore (2001a,b), Lee et al. (2002), Golev &Prugniel (1998), andTonry et al. (2001). (Barmby et al. 2000). ...
I review the current status of understanding when, how long, and how giant elliptical galaxies formed, focusing on the globular clusters. Several observational evidences show that massive elliptical galaxies formed at $z>2$ ($>10$ Gyr ago). Giant elliptical galaxies show mostly a bimodal color distribution of globular clusters, indicating a factor of $\approx 20$ metallicity difference between the two peaks. The red globular clusters (RGCs) are closely related with the stellar halo in color and spatial distribution, while the blue globular clusters (BGCs) are not. The ratio of the number of the RGCs and that of the BGCs varies depending on galaxies. It is concluded that the BGCs might have formed 12--13 Gyr ago, while the RGCs and giant elliptical galaxies might have formed similarly 10-11 Gyr ago. It remains now to explain the existence of a gap between the RGC formation epoch and the BGC formation epoch, and the rapid metallicity increase during the gap ($\Delta t \approx 2$ Gyr). If hierarchical merging can form a significant number of giant elliptical galaxies $>10$ Gyr ago, several observational constraints from stars and globular clusters in elliptical galaxies can be explained.
Full-text available
Globular cluster populations of supergiant elliptical galaxies are known to vary widely, from extremely populous systems like that of UGC 9799, the centrally dominant galaxy in Abell 2052, to globular-cluster-poor galaxies such as NGC 5629 in Abell 2666. Here we propose that these variations point strongly to the existence of a population of globular clusters that are not bound to individual galaxies, but rather move freely throughout the cores of clusters of galaxies. Such intracluster globular clusters may have originated as tidally stripped debris from galaxy interactions and mergers, or alternatively they may have formed in situ in some scenarios of globular cluster formation. Comment: 9 pages, uuencoded compressed postscript. Accepted for publication in the Astrophysical Journal Letters
We present new CCD photometry from the Canada-France-Hawaii (CFH) telescope for three cD galaxies that are the centrally dominant objects in the rich Abell clusters A2052, A2107, and A2666. Photometry to I(lim) = 25.0 reveals the existence of globular cluster populations around all three of these: the first two appear to have M87-like high specific frequencies around SN is approximately equal to 16, but the third has a normal or subnormal value SN is approximately equal to 3. With these new data, globular cluster systems have now been searched for in a total of 12 centrally dominant cD galaxies in a wide range of galaxy clusters. We find that neither SN nor the cluster population are correlated with any known property of the hot intracluster gas that usually surrounds such galaxies (cooling flow rate, gas temperature, total X-ray luminosity, etc.). From this, we conclude that cooling flows at the present epoch (i.e., in the last few Gyr) have little or nothing to do with globular cluster formation. The recently formed globular clusters in the central regions of such galaxies as NGC 1275 seem, instead, to be the result of occasional starburst phenomena. The vastly larger old-cluster populations filling the outer halos of many cD galaxies must be the result of an intensive phase of cluster formation during the very early protogalactic epoch.
HSTphot, a photometry package designed to handle the undersampled PSFs found in WFPC2 images, is introduced and described, as well as some of the considerations that have to be made in order to obtain accurate PSF-fitting stellar photometry with WFPC2 data. Tests of HSTphot's internal reliability are made using multiple observations of the same field, and tests of external reliability are made by comparing with DoPHOT reductions of the same data. Comment: 33 pages, 7 figures Accepted for publication in October 2000 PASP
  • M J West
  • P Côte
  • C Jones
  • W Forman
  • R O Marzke
West, M. J., Côte, P., Jones, C., Forman, W., & Marzke, R. O. 1995, AJ, 453, L77
  • W Harris
  • C J Pritchet
  • R D Mcclure
Harris, W. Pritchet, C. J., & McClure, R. D. 1995, ApJ, 441, 120