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,
Departamento de F´ısica, Grupo de Astronom´ıa, Universidad de
Concepci´on, Casilla 160-C, Concepci´on, Chile
Anglo-Australian Observatory, P.O.Box 296, Epping, NSW 1710,
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
= −25.30 m ag for UGC 9799, and M
= −24.08 m ag and M
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
= 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).
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