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 Letter

Cote, Patrick

Forman, William

Jones, Christine

Marzke, Ronald O.

West, Michael J.

English

arXiv

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

West, M J

Côté, P

Jones, C

Forman, W R

Marzke, R O

CERN Document Server

as
tr
o-
ph
/9
50
81
41
   
29
 A
ug
 1
99
5
Intracluster Globular Clusters
Michael J. West,
1
Patrick Co^te,
2
Christine Jones,
3
William Forman
3
and Ronald O. Marzke
2
ABSTRACT
Globular cluster populations of supergiant elliptical galaxies are known to vary
widely, from extremely populous systems like that of UGC 9799, the centrally domi-
nant 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 pop-
ulation of globular clusters that are not bound to individual galaxies, but rather move
freely thoughout the cores of clusters of galaxies. Such intracluster globular clusters may
have originated as tidally stripped debris from galaxy interactions and mergers, or alter-
natively they may have formed in situ in some scenarios of globular cluster formation.
Subject headings: galaxies: clusters: general { galaxies: elliptical and lenticular, cD |
galaxies: star clusters { X-rays: galaxies
accepted for publication in the Astrophysical Journal Letters
1
Department of Astronomy & Physics, Saint Mary's University, Halifax, NS B3H 3C3, Canada
2
Dominion AstrophysicalObservatory, Herzberg Institute of Astrophysics, National Research Council, 5071West SaanichRoad, Victoria,
BC V8X 4M6, Canada
2
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
1
1. Introduction
One of the outstanding problems in globular clus-
ter (hereafter GC) research is the origin of the ex-
tremely populous GC systems around many, though
not all, supergiant elliptical galaxies. A useful mea-
sure of a galaxy's GC population is its specic globular
cluster frequency, S
N
, dened as
S
N
= N
t
 10
0:4(M
V
+15)
(1)
where N
t
is the total number of globular clusters and
M
V
is the absolute visual magnitude of the galaxy
(Harris & van den Bergh 1981). To date, specic fre-
quencies have been measured for 14 brightest cluster
galaxies (hereafter BCGs); these data are summarized
in Table 1.
4
A wide range of specic frequencies is
seen, from S
N
' 4 to S
N
' 20. The lower limit of
S
N
' 4   5 for BCGs is similar to the typical value
found for most normal elliptical galaxies in dense en-
vironments (e.g., Harris 1991). This suggests that
low-S
N
BCGs have normal GC populations character-
istic of elliptical galaxies in general, whereas high-S
N
BCGs have anomalously rich GC populations. The
prototypical high-S
N
galaxy is M87 in the Virgo clus-
ter, which possesses more than 15,000 GCs, roughly
three times as many GCs per unit luminosity as any
other Virgo elliptical, and nearly an order of magni-
tude more than typical eld galaxies.
The reason for the dichotomy between high-S
N
and
low-S
N
BCGs is unknown at present. No obvious
correlation exists between S
N
and any intrinsic prop-
erty of either the BCG or the galaxy cluster in which
it resides (Harris et al. 1995). As Figure 1 shows,
for example, S
N
appears to be independent of cluster
X-ray temperature, which is proportional to the to-
tal cluster mass. Furthermore, no galaxy cluster has
been found to have more than one high-S
N
galaxy.
In this Letter we propose that the observed S
N
variations of BCGs point strongly to the existence of a
population of intracluster globular clusters (hereafter
IGCs) that are not bound to individual galaxies, but
rather move freely throughout the potential wells of
galaxy clusters. We note that this idea is not a new
one; this possibility has previously been considered,
4
NGC 4874 and NGC 4889 in the Coma cluster (Abell 1656)
have been included since both are supergiant elliptical galaxies
which are clearly the dominant cluster members. For this same
reason both M87 (NGC 4486) and M49 (NGC 4472) in the
Virgo cluster have also been included.
Figure 1. S
N
versus galaxy cluster X-ray
temperature T
X
. S
N
values for NGC 5629
and NGC 5424 are upper limits.
and subsequently rejected, by a number of authors
over the years (e.g., Fabian, Nulsen & Canizares 1984;
van den Bergh 1984; Harris 1986; Muzzio 1987). How-
ever, as we shall show below, a simple model based
on the IGC hypothesis can account quite successfully
for the high-S
N
phenomenon.
2. Intracluster Globular Clusters: A Model
We propose that the apparent excess of GCs asso-
ciated with high-S
N
galaxies originates from a popu-
lation of intracluster globular clusters. We construct
a quantitative model based on the following specic
assumptions:
 There exists a population of intracluster globular
clusters in all galaxy clusters. The total number
of IGCs depends on the cluster mass
N
IGC
/M
cl
(2)
with richer galaxy clusters having more IGCs
than poorer clusters.
 The distribution of IGCs follows the cluster
mass distribution. A convenient functional form
which satisfactorily describes the projected dis-
tribution of galaxies and the dark matter compo-
nent indicated by gravitational lens studies is the
2
King (1962) approximation of a bounded isother-
mal sphere
(r) =

o
(1 + r
2
=r
2
c
)
(3)
where (r) is the projected mass density at a
distance r from the galaxy cluster center, 
o
is
the central density, and r
c
is the core radius.
 All BCGs galaxies are born with an intrinsic GC
population of S
N
' 4, similar to that of other
elliptical galaxies. Galaxies with S
N

> 4 have
gained an additional \excess" GC component.
The number of excess GCs can be calculated us-
ing Equation 1,
N
excess
= N
t
  4 10
 0:4(M
v
+15)
(4)
Values of N
excess
for each BCG are listed in Ta-
ble 2.
 A galaxy may be surrounded by a halo of IGCs,
the number of which will depend on its position
in the host galaxy cluster.
Combining Equations 2 and 3, it is straightforward
to estimate the projected local density of IGCs at any
particular location in a cluster as

IGC
/
M
cl
(1 + r
2
=r
2
c
)
(5)
Accurate dynamical determinations of cluster masses
from optical data are somewhat problematic due to
the prevalence of substructure in many clusters of
galaxies (see West 1994 for a review). A less ambigu-
ous method is to use X-ray determinations of cluster
masses. Under the assumption that the hot X-ray
gas is in hydrostatic equilibrium in the cluster gravi-
tational potential well, the X-ray temperature is ex-
pected to be linearly proportional to the total clus-
ter mass (T
X
/ M
cl
), which upon substitution into
Equation 5 yields

IGC
/
T
X
(1 + r
2
=r
2
c
)
(6)
The cluster core radius, r
c
determines how cen-
trally concentrated the IGC population is. Until re-
cently it was thought that galaxy clusters possess
core radii on the order of several hundred h
 1
kpc.
However mounting observational evidence has shown
that the mass distribution in the centers of clusters
is much steeper than previously realized (e.g., Beers
& Tonry 1986; Merrield & Kent 1989; Gerbal et al.
1993). The most direct probe comes from observa-
tions of gravitational lensing, which have shown that
the dark matter distribution is quite centrally con-
centrated, with typical core radii r
c
' 20   50h
 1
kpc (e.g., Kneib et al. 1993; Mellier, Fort & Kneib
1993; Miralda-Escude 1995; Smail et al. 1995). As
a rst approximation, we shall adopt a core radius
r
c
' 30h
 1
kpc for all clusters.
A natural consequence of this model is that BCGs,
which usually reside at the dynamical centers of their
parent clusters, are likely to be surrounded by ex-
tended halos of IGCs. Hence most of the excess
GCs associated with high-S
N
galaxies may belong
to a background population of intracluster globulars.
Note, however, that location in a rich galaxy cluster
is a necessary, but not sucient, condition for pro-
ducing high-S
N
galaxies. If a galaxy is signicantly
oset from the dynamical center of the cluster then
it will not be surrounded by appreciable numbers of
IGCs, since 
IGC
falls o rapidly with distance from
the cluster center. Likewise, a BCG residing at the
center of a poor cluster is unlikely to be a high-S
N
system simply because there are fewer IGCs in low
mass galaxy clusters.
3. Results
Equation 6 can be used to estimate the local IGC
density around each of the BGCs in Table 1. Three
pieces of information are required: the X-ray temper-
ature of the host galaxy cluster, its dynamical center
and the position of the galaxy relative to this center.
X-ray temperatures for all clusters in Table 1 were
obtained from the compilation by David et al. (1993),
with the exception of the M87 and M49 subclusters,
which were taken from Bohringer et al. (1994). Three
of the poorest clusters in Table 1 do not have mea-
sured temperatures owing to their low X-ray lumi-
nosities (MKW 12, AWM 3, Abell 2666). In those
cases, X-ray temperatures were estimated using the
observed cluster velocity dispersion (from Beers et
al. 1995 and Scoddegio et al. 1995) together with
the well-known correlation between X-ray tempera-
ture and velocity dispersion (Lubin & Bahcall 1993).
Cluster centroids were also determined from X-ray
observations, primarily the catalogue of Jones & For-
man (1995), together with ROSAT observations of
Virgo (Bohringer et al. 1994), Coma (Vikhlinin, For-
3
Figure 2. N
excess
, the observed number of
\excess" globular clusters associated with a
BCG, versus the estimated local density of
intracluster globular clusters, 
IGC
.
man & Jones 1994) and Fornax (Rangarajan et al.
1995). The projected distance, r, of the BCG from
the cluster center is listed in Table 2 along with the
computed value of 
IGC
. For those clusters with sub-
structure, the distance of the BCG from the nearest
subcluster centroid was used. For the three poor clus-
ters without published X-ray maps, the BCG was as-
sumed to reside at the cluster center.
A plot of N
excess
versus the estimated local IGC
density, 
IGC
, is shown in Figure 2. A correlation
between these two quantities is clearly seen { those
BCGs located in the densest cluster environments in-
variably have the largest excess GC populations. This
correlation is all the more remarkable when one con-
siders the large uncertainties in the observational data
(S
N
values are typically uncertain by a factor of two)
and the simplicity of the model employed here; the
true correlation is presumably even stronger.
Closer examination of several galaxies serves to il-
lustrate the important features of the IGC model.
The galaxy with the largest measured specic fre-
quency, UGC 9799 (S
N
= 20), resides at the dynam-
ical center of a rich galaxy cluster, Abell 2052. NGC
7768 (S
N
= 4) also resides at the center of its par-
ent cluster, however Abell 2666 is much poorer than
Abell 2052, and hence would be expected to have few,
if any, IGCs. The disparity between the specic fre-
quencies of the two supergiant elliptical galaxies in
the Coma cluster can now also be easily understood;
NGC 4874 (S
N
= 14) is located much closer to the
cluster X-ray centroid than is NGC 4889 (S
N
= 7),
and thus should be surrounded by a greater density
of IGCs.
4. The Origin of Intracluster Globular Clus-
ters
What is the origin of the proposed population of
IGCs?
One possibility is that they are tidally-stripped
debris from galaxy collisions which has accumulated
at the bottom of galaxy cluster potential wells (e.g.,
White 1987; Muzzio 1987). However, as van den
Bergh (1984) has pointed out, a problem with this
idea is that one would expect halo stars and GCs to be
stripped in equal proportions during galaxy interac-
tions, and hence accretion of such material by BCGs
should not result in a net increase of S
N
. Yet be-
cause of the mass-dependence of dynamical friction,
it is possible that stripped GCs would settle much
more readily toward the cluster center than stripped
halo stars.
Alternatively, it is possible that IGCs might have
formed in situ in the intracluster environments of
galaxy clusters. In some GC formation scenarios (e.g.,
West 1993; Harris & Pudritz 1994), the eciency of
globular cluster formation is expected to depend sen-
sitively on the local matter density. In the cores of
galaxy clusters, the local density may have been suf-
ciently high to allow the birth of IGCs without the
need for a parenting galaxy. It has also been sug-
gested that IGCs might condense out of cooling ows
(Fabian et al. 1984), although no correlation exists
between S
N
and present-day cooling ow rate (Har-
ris et al. 1995). Another speculative possibility might
be that IGCs form during mergers of gas-rich subclus-
ters of galaxies.
5. Conclusions
We have shown that a simple model based on the
hypothesis of a population of intracluster globular
clusters can explain the origin of the high-S
N
phe-
nomenon, and the variation of GC populations among
brightest cluster galaxies. High-S
N
galaxies did not
form GCs more eciently than other galaxies, but
rather they have inherited an additional population
4
of IGCs thanks to their fortuitous location at the dy-
namical centers of rich galaxy clusters.
Additional support for this idea comes from the
large velocity dispersions of GC systems around M87
(Mould et al. 1987) and NGC 1399 (Grillmair et al.
1994), which suggest that these GCs are bound to the
potential well of the galaxy cluster as a whole.
Any successful model should strive not only to ex-
plain existing observations, but also to make predic-
tions which can be tested. In this spirit, we have
selected a number of nearby (z  0:04) BCGs whose
GC populations have not yet been measured, and we
attempt to predict in Table 3 which of these should be
high-S
N
systems based on the X-ray temperatures of
their parent clusters and the galaxy's location relative
to the cluster centroid.
Finally, we note that intracluster globular clusters
should be detectable with deep, high-resolution im-
ages of the cores of rich galaxy clusters, which pro-
vides the most stringent test of the IGC model pro-
posed here. Such a population may also oer a valu-
able tool for extragalactic research by providing a new
and independent tracer of the dark matter distribu-
tion in the cores of galaxy clusters.
M.J.W. was supported by the NSERC of Canada.
C.J. and W.F. were supported by the Smithsonian In-
stitution and the AXAF Science Center NASA Con-
tract NAS8-39073.
5
Table 1
Globular Cluster Populations of Brightest Cluster Galaxies
Galaxy Cluster z M
V
N
t
S
N
kT (keV) Reference
NGC 4486 (M87) Virgo 15 Mpc -22.7 16000 14 2.4 Harris 1986
NGC 4472 (M49) Virgo 15 Mpc -22.9 7400 5 1.2 Harris 1986
NGC 1399 Fornax 15 Mpc -21.2 4800 16 1.1 Bridges et al. 1991
NGC 3311 A1060 0.012 -22.7 19000 15 3.9 McLaughlin et al. 1995
NGC 5629 AWM3 0.015 -21.7 < 2000 < 5 0:4
a
Bridges & Hanes 1994
NGC 5424 MKW12 0.019 -21.6 < 2000 < 5 0:7
a
Bridges & Hanes 1994
NGC 4073 MKW4 0.020 -23.1 7200 4 1.7 Bridges & Hanes 1994
NGC 3842 A1367 0.021 -23.2 14000 7 3.7 Butterworth & Harris 1992
NGC 4874 A1656 0.023 -22.7 17000 14 8.3 Blakeslee & Tonry 1995
NGC 4889 A1656 0.023 -23.2 13000 7 8.3 Blakeslee & Tonry 1995
NGC 7768 A2666 0.028 -22.9 6000 4 1:2
a
Harris et al. 1995
NGC 6166 A2199 0.031 -23.6 10000 4 4.5 Pritchet & Harris 1990
UGC 9799 A2052 0.035 -23.4 46000 20 3.1 Harris et al. 1995
UGC 9958 A2107 0.042 -23.4 30000 13 4.2 Harris et al. 1995
a
Estimated X-ray temperature.
6
Table 2
Intracluster Globular Cluster Populations
Galaxy N
excess
r(h
 1
kpc) 
IGC
NGC 4486 (M87) 11200 0 2.4
NGC 4472 (M49) 1600 0 1.2
NGC 1399 3600 0 1.1
NGC 3311 13900 18 2.9
NGC 5629 0
a
0 0.4
NGC 5424 0
a
0 0.7
NGC 4073 0 0 1.7
NGC 3842 6400 60 0.7
NGC 4874 12200 45 2.6
NGC 4889 5300 105 0.6
NGC 7768 0 0 1.2
NGC 6166 0 33 2.0
UGC 9799 37000 0 3.1
UGC 9958 21000 6 4.0
a
S
N
= 4 has been assumed.
7
Table 3
Predictions of the IGC Model
High-S
N
Galaxies Low-S
N
Galaxies
MCG-02-12-039 (Abell 496) NGC 2329 (Abell 569)
CGCG 077-097 (Abell 2063) NGC 2832 (Abell 779)
NGC 4696 (Abell 3526) NGC 5400 (MKW 5)
NGC 5920 (MKW 3S) NGC 5718 (MKW 8)
8
REFERENCES
Beers, T.C., & Tonry, J.L., 1986, ApJ, 300, 557.
Blakeslee, J.P., & Tonry, J.L., 1995, ApJ, 442, 579.
Bohringer, H., et al., 1994, Nature, 368, 828.
Bridges, T.J., & Hanes, D.A., 1994, ApJ, 431, 625.
Bridges, T.J., Hanes, D.A., & Harris, W.E., 1991, AJ,
101, 469.
Butterworth, S.T., & Harris, W.E., 1992, AJ, 103,
1828.
David, L.P., et al., 1993, ApJ, 412, 479.
Fabian, et al., 1984, Nature, 310, 733.
Gerbal, D., Durret, F., Lima-Neto, G., & Lachieze-
Rey, M., 1992, A&A, 253, 77.
Grillmair, C.J., et al., 1994, ApJ, 422, L9.
Harris, W.E., 1986, AJ, 91, 822.
Harris, W.E., 1991, ARA&A, 29, 543
Harris, W.E., & van den Bergh, S., 1981, AJ, 86, 1627
Harris, W.E., & Pudritz, R.E., 1994, ApJ, 429, 177.
Harris, W.E., Pritchet, C.J., & McClure, R.D., 1995,
ApJ, 441, 120
Jones, C., & Forman, W., 1995, in preparation.
King, I.R., 1962, AJ, 67, 471
Kneib, J.-P., Mellier, Y., Fort, B., & Mathez, G.,
1993, A&A, 273, 367
Lubin, L.M., & Bahcall, N.A., 1993, ApJ, 415, L17
McLaughlin, D.E., et al., 1995, AJ, 109, 1033.
Mellier, Y., Fort, B., & Kneib, J.-P., 1993, ApJ, 407,
33.
Merrield, M.R., & Kent, S.M., 1989, AJ, 98, 351
Miralda-Escude, J., 1995, ApJ, 438, 514.
Mould, J.R., Oke, J.B., & Nemec, J.M., 1987, AJ, 92,
53.
Muzzio, J.C., 1987, PASP, 99, 245.
Pritchet, C.J., & Harris, W.E., ApJ, 335, 410.
Rangarajan, F.V.N., Fabian, A.C., Forman, W.R., &
Jones, C., 1995, MNRAS, 272, 665.
Smail, I., et al., 1995, preprint.
Scodeggio, M., Solanes, J.M., Giovanelli, R., &
Haynes, M.P., 1995, ApJ, 444, 41.
van den Bergh, S., 1984, PASP, 96, 236
Vikhlinin, A., Forman, W., & Jones, C., 1994, ApJ,
435, 162.
West, M.J., 1993, MNRAS, 265, 755.
West, M.J., 1994, in Clusters of Galaxies, ed. F. Dur-
ret et al. (Gif-sur-Yvette Cedex: Editions Fron-
tiers), p. 23.
White, R.E., 1987, MNRAS, 227, 185
This 2-column preprint was prepared with the AAS L
A
T
E
X
macros v4.0.
9


Intracluster globular clusters

Intracluster Globular Clusters

Intracluster Globular Clusters

Abstract

Similar works

Full text

Available Versions

CERN Document Server