THE GALAXY-CLUSTER-SUPERCLUSTER CONNECTION
| Abstract | Résumé |
| Surveys undertaken over the past few decades to map the distribution of galaxies in the nearby universe have revealed a cosmic landscape of wondrous diversity, with richly populated galaxy clusters, filamentary and sheet-like superclusters and enormous voids. One of the most striking features of the large-scale distribution of galaxies is its filamentary appearance, with long, quasi-linear arrangements of galaxies extending over tens and perhaps even hundreds of Megaparsecs. In this article I will review observational and theoretical evidence which suggests that the filamentary distribution of matter on supercluster scales exerts a profound influence on the formation of clusters and their member galaxies. | Les études entreprises au cours des dernières décennies dans le but de cartographier la distribution de galaxies dans l'univers proche ont révelé un paysage cosmique d'une extraordinaire diversité, montrant des riches amas de galaxies, des super-amas filamentaires ou aplatis ainsi que d'immenses vides. Une des caractéristiques les plus frappantes de la distribution galactique à grande échelle est son aspect filamentaire, lequel dévoile de longs et quasi-linéaires arrangements de galaxies qui s'étendent sur des dizaines, voire même des centaines de mégaparsecs. Dans cet article, je passerai en revue les preuves théoriques et observationnelles qui suggèrent que cette distribution filamentaire de matière à l'échelle des super-amas, soit un facteur important dans la formation des amas et de leurs galaxies membres. |
§ 1. Coma's Cosmic Congruence
The Coma cluster's environs have been discussed in the astronomical literature for nearly as long as the cluster itself. In 1785, William Herschel discovered what he called "the nebulous stratum of Coma Berenices" and remarked that
"I have fully ascertained the existence and direction of this stratum for more than 30 degrees of a great circle and found it to be almost everywhere equally rich in fine nebulae."Unbeknownst to Herschel, these were the first observations of a large-scale galaxy distribution, more than a century before the true nature of extragalactic nebulae was even known.
Figure 1. The distribution of 3649 galaxies contained in the RC3 (de Vaucouleurs et al. 1991). To highlight features in the galaxy distribution symbol size is proportional to local galaxy density. Blue circles denote Abell clusters with redshifts z < 0.03.
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A closeup view of a 100 square degree region centered on the Coma cluster is shown in Figure 2. It is evident that the cluster is embedded in an elongated swath of galaxies extending along a projected position angle of roughly 70 - 80 degrees. Gregory & Thompson's (1978) pioneering redshift survey of Coma's environs showed that this apparent feature in the projected galaxy distribution is in fact a genuine three-dimensional bridge linking Coma with its nearest neighbouring cluster, Abell 1367, which lies some 22 Mpc away (a Hubble constant of 100 km/sec/Mpc will be used throughout). A few years later the famed CfA redshift survey (de Lapparent, Geller & Huchra 1986; Geller & Huchra 1989) mapped the galaxy distribution around Coma in exquisite detail and provided evidence of a majestic 150 Mpc long structure sweeping across the entire survey region. |
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In 1982 Bruno Binggeli made a very interesting discovery. He found that the major axes of rich clusters of galaxies are not randomly oriented, but instead exhibit a remarkable tendency to "point" towards neighbouring clusters. This alignment effect had been anticipated by Einasto, Joveer & Saar (1980) who noted that "the orientations of clusters in superclusters is a conspicuous morphological property of superclusters." This tendency for clusters of galaxies to have preferred orientations with respect to their surroundings has since been confirmed by numerous other studies (e.g., West 1989; Plionis 1994). It is readily seen in the Coma cluster, whose major axis (as determined from the distribution of either its member galaxies or its hot intracluster gas) lies along the same 70 - 80 degree position angle as the prominent supercluster filament that surrounds it.
| This apparent congruence between the Coma cluster and its supercluster environs also extends to galactic scales. It has been known for some time that central dominant galaxies in clusters exhibit a marked tendency to share the same major axis orientation as their parent cluster (e.g., Sastry 1968; Binggeli 1982; Porter et al. 1991; West 1994), and hence they also tend to "point" along large-scale filaments. In the case of the Coma cluster, the orientation of the two central dominant galaxies, NGC 4889 and NGC 4874, as well as the projected separation vector between them, all share the same preferred 70 - 80 degree position angle. |  |
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Further examination reveals that this galaxy alignment effect is
not limited to NGC 4889 and NGC 4874.
Many of the brightest elliptical
galaxies in Coma also reflect this special 70 - 80 degree orientation
(see Table 1 below), a coincidence which was
first noted more than 50 years ago by Brown (1939).
Hence it appears that the Coma cluster and its member galaxies have been greatly influenced by its supercluster surroundings. A more detailed review can be found in West (1998a). |
Table 1. Coma Galaxies
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§ 2. Virgo's Virtual Vector
The apparent galaxy-cluster-supercluster connection described in the previous section is not unique to the Coma cluster. A similar effect is seen in the nearby Virgo cluster. Figure 4 shows the distribution of Virgo galaxies on the plane of the sky. Virgo's giant ellipticals, which are denoted by the red symbols, have a remarkably linear configuration which is oriented along a projected position angle of roughly 120 degrees, a fact which was first pointed out by Arp (1968). ROSAT X-ray observations also show the Virgo cluster to be elongated in this same direction (Bohringer et al. 1994).
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Furthermore, Virgo's principal axis points in the direction of Abell 1367,
a rich cluster which lies some 50 Mpc away, along a
projected position angle of 125 degrees.
This raises the intriguing possibility that Virgo, Abell 1367 and Coma
are all members of a common filamentary network,
an idea suggested 15 years ago by Zeldovich, Einasto & Shandarin (1982).
Such a filamentary bridge of material connecting
Virgo and Abell 1367 can be seen in Figure 5,
which plots the distribution of galaxy groups from the CfA redshift
survey (Ramella et al. 1997).
Interestingly, the principal axis traced by Virgo's giant
ellipticals appears to be a genuine
three-dimensional linear structure (West 1998b).
Table 2 lists all Virgo ellipticals
whose distances have been measured to date using the technique of
surface brightness fluctuations pioneered by John Tonry
and collaborators (Tonry, Ajhar & Luppino 1990; Tonry et al. 1997).
This redshift-independent distance estimator has allowed Virgo's
true three-dimensional structure to be resolved for the first time.
Comparison of Figure 4 and Table 2 reveals a striking correlation
between right ascension and galaxy distance, in the sense that the
galaxies in the eastern-most part of the chain are systematically
nearer to us than those in the western part.
Table 2. Virgo galaxies
Position Angle SBF distance
NGC 4374 (M84)
NGC 4406 (M86)
NGC 4486 (M87)
NGC 4552 (M89)
NGC 4621 (M59)
NGC 4649 (M60)
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Additional evidence of a Virgo-A1367 connection
comes from Abell 1367's orientation, which is
elongated along a position angle of 135 degrees, and hence
in the general direction of the Virgo cluster
(compare Figures 4 and 6).
Like Coma, there is also a strong correlation between the direction of Virgo's principal axis and the orientations of its brightest elliptical galaxies, with most having major axis position angles near 120 - 130 degrees (see Table 2). It is intriguing that M87's famous jet also emanates along this same 120 degree position angle; could this be more than merely a coincidence? A more detailed discussion can be found in West (1998b). |
Figure 6. ROSAT X-ray image of Abell 1367, kindly provided by David Davis. Note the elongation of the cluster, which points in the direction of the Virgo cluster. Click on the image to see a larger version. |
The alignments of galaxies, clusters and superclusters must surely be an important clue about how these objects formed. I believe that the most likely explanation involves a new twist on an old idea: giant elliptical galaxies and clusters are built by mergers, with the merger process itself being highly anisotropic.
Let's consider cluster alignments first. Substructure is a common feature of many, perhaps most, clusters of galaxies. Current estimates suggest that at least 30-50% of rich clusters exhibit multiple lumps in their galaxy and gas distributions. Such subclusters are the building blocks from which clusters of galaxies are assembled. The prevalence of substructure in clusters today provides compelling evidence that cluster formation is a relatively recent event and is probably still an ongoing process.
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Christine Jones, Bill Forman and I
(West, Jones & Forman 1995) showed that the distribution of subclusters
in clusters traces the surrounding
filamentary distribution of matter on supercluster scale.
We interpreted this as evidence
that cluster formation proceeds via anisotropic infall of material
sheparded along these filaments. Built by a series of subcluster
mergers that occur along preferred directions, clusters of galaxies
naturally develop orientations that reflect
the surrounding filamentary pattern of superclustering.
In this way the matter distribution on supercluster scales
influences the properties of clusters.
In the Coma cluster, the arrangement of its multiple subclusters clearly reflects the orientation of the surrounding supercluster filament. As the figure to the right shows, there is a large subcluster associated with NGC 4839 which appears to be falling into the cluster along the direction of the Coma-A1367 filament. The distribution of other subclusters shows the same effect (e.g., Mellier et al. 1988). This suggests that Coma has probably been built by mergers of subclusters which infall along the prominent filament in which it is embedded. | 
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Theoretical work, much of it done here in Canada, has shown that
anisotropic infall of material along filaments is likely to be a
common feature of many models of structure formation.
Dick Bond of the Canadian Institute for
Theoretical Astrophysics and collaborators have
developed an elegant analytical
framework to describe the evolution of the "cosmic web"
of interconnected filaments and sheets of material
(e.g., Bond 1987; Bond, Pogosyan & Kofman 1996).
This work has shown that cluster alignments are expected
to arise quite naturally from the dynamical evolution of filaments
whose seeds were imprinted on the primordial matter distribution.
Cosmological N-body simulations of structure formation confirm this idea.
Figure 8 below shows a computer simulation of a region in a
cold dark matter universe that was performed by
Hugh Couchman of the University of Western Ontario.
Such simulations show that cluster formation is driven by the
flow of material along filaments.
| Figure 8. N-body simulation of the distribution of luminous matter in a universe dominated by cold dark matter, kindly provided by Hugh Couchman (see Couchman 1997). Note the interlaced network of filaments which bridges the rich galaxy clusters and channel material into them. Clusters are often located at the vertices where multiple filaments merge; the cluster orientation is determined by the direction of the most prominent filament. Only a portion of the image is shown here; click on it to see it in its entirety. |
What about the alignments of giant ellipticals? Again, I think that anisotropic mergers are implicated. It has often been suggested that giant elliptical galaxies formed from mergers of smaller galaxies. As evidence one can cite the following:
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Keith Ashman & Steve Zepf (1992) proposed that such bimodal distributions
may arise from mergers of gas-rich spirals,
with the mergers acting
as a catalyst for the formation of new (metal-rich) globular clusters.
More recently, Pat Côté, Ron Marzke (both formerly of the Dominion Astrophysical Observatory) and I have shown (Côté, Marzke & West 1998) that bimodal globular cluster metallicity distributions will also result quite naturally from capture of globular clusters by other galaxies, either through mergers or tidal stripping, without requiring new globular clusters to be formed in the process. In either case, a plausible argument can be made that the bimodal globular cluster metallicity distributions of giant elliptical galaxies are a consequence of a previous history of mergers. | 
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Assuming that giant elliptical galaxies have indeed formed by mergers
(for a dissenting view see van den Bergh 1992 and 1995), then it
is natural to expect that these mergers will occur
preferentially along the direction defined by the cluster principal axis.
Again, computer simulations confirm this idea.
Figures 10 and 11 show two simulations of the formation of giant
elliptical galaxies in clusters beginning from realistic
cosmological initial conditions, one by Ray Carlberg of the
University of Toronto and the other by John Dubinski of CITA.
In both cases a massive galaxy forms not by random isotropic
mergers but instead by mergers which tend to
occur along a preferred axis whose orientation is
dictated by the surrounding large-scale filamentary structure.
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Figure 10 (left). Computer simulation of the formation of a giant elliptical
galaxy at the centre of a rich cluster, kindly provided
by Ray Carlberg. This has been extracted from a larger million-particle
simulation of a universe dominated by cold dark matter
(see Carlberg 1994 for details). Each box is approximately
4 Mpc on a side. Time sequence is denoted by redshift z.
Only those particles that end up in the final galaxy are shown here.
Figure 11 (below). Computer simulation of the formation of a giant elliptical galaxy by John Dubinski. Note the highly anisotropic nature of the merger process in both simulations, and how this is reflected by the shape and orientation of the galaxy. |
§ 4. Conclusions
The observed
alignments of giant elliptical galaxies and clusters of galaxies with their
supercluster surroundings indicates a truly remarkable coherence of
structures over roughly four orders of magnitude in mass.
As this review has hopefully shown, the galaxy-cluster-supercluster
connection suggests that the formation of galaxies and
clusters has been driven by a process of anisotropic mergers
of material which infalls along filaments.
§ 5. Acknowledgements:
Thanks to Hugh Couchman, Ray Carlberg, John Dubinski, Christine Jones and David Davis for permission to reproduce their images here. I also thank Cheryl Samsel for a careful reading of this article and for comments which helped improve its presentation. The Digitized Sky Survey POSS-II images shown in Figure 3 were produced at the Space Telescope Science Institute under U.S. Government grant NAC W-2166. The author's work presented here was supported by a research grant from NSERC.
Finally, on behalf of all of us here at Saint Mary's University
I would like to extend a hearty Maritimes welcome to all
CASCA members to come visit us in Halifax when we
host the 1999 CASCA meeting!
§ 6. References
Michael West is an Associate Professor in the Department of
Astronomy & Physics at Saint Mary's University in Halifax,
Nova Scotia. He received his PhD in Astronomy from Yale University
in 1987. Prior to coming to Saint Mary's in 1994 he held positions
at the University of Michigan, the Canadian Institute for Theoretical
Astrophysics, and Leiden University in the Netherlands. His research
interests include both observational and theoretical work on clusters
of galaxies, galaxy formation, cosmology and globular clusters.
