| Annu. Rev. Astron. Astrophys. 1991. 29:
499-541
Copyright © 1991 by Annual Reviews. All rights reserved
|
3.6 Observations of Clusters
Clusters of galaxies are the largest structures in the universe that
unambiguously appear dynamically bound. They can thus provide estimates
of the mass distribution over scales on the order of several Mpc. The
density perturbations that give rise to the formation of clusters and
allow them to decouple from the Hubble expansion have very long growth
times, and most clusters have not yet attained the degree of dynamical
equilibrium that is accompanied by a smooth, spheroidal appearance.
Thus, the understanding of their dynamical state necessitates having
rich kinematical information in order to determine cluster morphology
and membership, and eventually to disentangle substructures from one
another and yet obtain statistically sound estimates of the
characteristics of each. Although important contributions were made
through the years, especially in the study of rich clusters like Coma,
until the relatively recent work of
Kent & Gunn (1982)
for the Coma cluster and
Kent & Sargent (1983)
for the
Perseus cluster, no truly
detailed study was available. Several other clusters have since been
studied in comparable detail, i.e. with samples that include a few
hundred redshifts per cluster, among them the nearby systems Virgo,
Centaurus, and Hydra I = A1060 and (thanks to multiobject spectrographs)
several more distant clusters.
COMA, PERSEUS, AND VIRGO
Kent & Gunn (1982)
assembled a data base of about 300 radial
velocities in Coma, complete to mcgcg = 15.7 within
3° of the cluster
center, and to mcgcg = 15.0 within 6°. Their
discussion of distribution
models for the galaxies and for the mass indicated a core radius of
170-200 h-1 kpc (8.5'-10'), a total mass within the
inner 3.5 h-1 Mpc
(~ 3°) of 1.5 x 1015 h-1
M, a mass to
light ratio M / LB ~ 360 h, and
that the matter distribution was not significantly different from that
of the galaxies.
Hughes (1989)
critically analyzed their models and
those presented in more recent work. This analysis confirmed the results
of Kent & Gunn (1982).
In spite of the low galactic latitude and high extinction of the
Perseus cluster - which exceeds half a magnitude at
the cluster center -
187 redshifts of cluster members were available to
Kent & Sargent (1983).
At 5470 km s-1, Perseus has the highest velocity dispersion
among nearby clusters (1300 km s-1). Its global parameters are
comparable to those of Coma: a core radius of 170 h-1
kpc (11'), a
virial mass of 1.7 x 1015 h-1
M, and a mass
to light ratio M / LV ~ 600 h.
In the case of Virgo, a large redshift data base has currently been
accumulated. The major recent contributions have been made by
Karachentsev &
Karachentseva (1982),
Huchra (1985),
and Hoffman et al
(1987,
1989).
Huchra compiled existing data and completed a sample of
471 galaxies within a 6° radius from M87 to a limiting magnitude of
mcgcg = 15.5.
Binggeli et al (1985)
produced a catalog of 2096 galaxies
in an area centered on the Virgo cluster, based on 2.5-m duPont
telescope plates. Using this catalog, Hoffman and coworkers
(1989 and
refs. therein) observed (at 21-cm at Arecibo) all dwarf irregular
galaxies brighter than bT = 17.0 - a sample of nearly
300 objects. H I
observations are also available of all 100 or so bright spirals deemed
to be cluster members. Overall, redshifts are known for over 400 Virgo cluster members, which constitute the highest
level of detail and
completeness recorded for any cluster.
Binggeli et al
(1987) have
written a comprehensive morphological and kinematical study of this cluster.
OTHER CLUSTERS
A detailed study of the Centaurus cluster (based on about 180 cluster
members) was presented in a series of three important papers by Lucey et
al (1986
and refs. therein). A2670 has been thoroughly studied by
Sharples et al (1988),
on the basis of a sample of 220 cluster members.
Many other clusters have been studied in some detail data sets
typically include a few dozen members per cluster. Notable among them
are: the results of
Fitchett & Merritt
(1988)
for Hydra I (A1060, based
on redshift data of 95 cluster members as given by
Richter 1987), the
work by
Fabricant et al (1989)
on A2256 (86 cluster redshifts), those by
Chapman et al (1988)
on A194 (74 cluster members),
Ostriker et al (1988)
on A539 (86 members),
Cristiani et al (1987)
on Klemola 22,
Soucail et al (1988)
on A370,
Metcalfe et al (1987)
on Shapley 8,
Dixon et al (1989)
on A2197 and A2199 (based on redshift data of
Gregory & Thompson
1984,
1986), and
(Bothun et al 1983)
on Cancer. Other contributions of
considerable interest, albeit of somewhat less wealth in the data base,
are available for the clusters A1146
(Melnick & Quintana
1985),
A1142
(Geller et al 1984),
the Eridanus cluster
(Willmer et al 1989),
A744
(Kurtz et al 1985),
A1099 and A1016
(Chapman et al 1987),
AC103
(Sharples et al 1985),
A262
(Giovanelli et al
1982),
A347 and A1367
(Moss et al 1988),
NGC 5846
(Haynes & Giovanelli
1990b),
and Klemola 27
(Richter 1984).
Significant samples including many clusters have been
collected by
Vettolani et al
(1990),
Proust et al (1987,
1988),
Rhee & Katgert (1988),
Green et al
(1988), and
Owen et al (1988).
Struble & Rood
(1987) and
Andernach (1990)
have prepared recent compilations of
redshifts and velocity dispersions for Abell clusters.
Two recent contributions stand out for their statistical wealth. To
complement
Dressler's (1980)
morphological study, Dressler & Schectman
(1988a,
b)
have measured 1268 redshifts in 15 clusters. Most recently,
Zabludoff et al (1990)
presented an analysis of a compilation of 3250
redshifts for galaxies in 69 nearby Abell clusters (including 359 new
redshifts).
MULTIPLEXING
The high concentration of bright galaxies found in cluster cores is
ideally suited for the opportunities offered by multislit and multifiber
devices. Thus,
Colless & Hewett (1987)
measured 604 radial velocities in
14 rich southern clusters - ranging in redshift from 14,000 to 44,000 km
s-1. This approach is representative of modern cluster
surveys, such as those of
Teague et al (1990),
Mazure et al (1989),
Batuski and coworkers (personal communication), and
Guzzo et al (1990).
Teague et al (1990)
obtained 1034 redshifts (805 cluster members) in a sample of 10 southern
clusters, with samples of more than 100 in 3 of them.
Guzzo et al (1990)
are completing a redshift survey of 150 clusters extracted from the
Edinburgh/Durham Southern Galaxy Catalogue
(Heydon-Dumbleton et
al 1989),
carried out with the EFOSC spectrograph in multislit mode at ESO and
the Autofib multifiber system at the AAT. This effort should result
in a sample of over 2000 redshifts. Such a rapidly growing cluster data
base will not only lead to an improved understanding of cluster
structure, dynamics, and evolution, but also to a much sounder
determination of the clustering properties of the universe at the
largest scalelengths.