CLUSTERS of GALAXIES

Content: Virgo cluster, Hercules cluster
Adapted from The Astronomy and Astophysics Encyclopedia and B. Binggeli

1. VIRGO CLUSTER

The Virgo Cluster is the closest and best-studied great cluster of galaxies, lying at a distance of approximately 20 Mpc in the constellation of Virgo. Cosmographically, the Virgo Cluster is the nucleus of the Local Supercluster of galaxies, in whose outskirts we (in the Milky Way, in the Local Group) are situated. As early as 1784, Charles Messier noted an unusual concentration of "nebulae" in Virgo; 15 out of the 109 "Messier" objects are, in fact, Virgo Cluster galaxies, the most famous of which is Messier 87, the giant elliptical galaxy with the mysterious jet. After Edwin P. Hubble's 1923 discovery of Cepheids in M31, the true nature of the group of nebulae in Virgo as a self-gravitating system of hundreds of galaxies was soon realized, and the first systematic investigations of the Virgo Cluster (as it was subsequently called) were carried out by Harlow Shapley and Adelaide Ames. Ever since, the Virgo Cluster has been, and still is, of primary importance for extragalactic astronomy: Large numbers of equidistant galaxies of all types and luminosities can be observed here in great detail, rendering the cluster: (1) an ideal laboratory for the study of the systematic properties of galaxies and (2) a fundamental stepping stone for the cosmological distance scale.

The Virgo Cluster is a fairly poor, loosely concentrated, irregularly shaped (see Fig. 1) cluster of galaxies with a high abundance of spiral galaxies among the bright cluster members (see Table 1). It is representative of the most common class of galaxy clusters, which is characterized by these properties. Rich, dense, regularly shaped clusters of predominantly E and SO galaxies are much rarer. Nevertheless, owing to its proximity, the "mediocre" Virgo cluster could be mapped to an unsurpassed level of depth and morphological detail, rendering it presently the richest cluster of galaxies in terms of the number of known member galaxies. As Table 1 shows, dwarf galaxies, the dwarf elliptical (dE) types in particular, numerically dominate the cluster population. These stellar systems of low surface brightness are hard to detect, even in nearby Virgo. There must be thousands more extremely faint and diffuse cluster members that are still awaiting discovery.

Figure 1

Figure 1. Map of the Virgo Cluster. All cluster members are plotted with luminosity-weighted symbols. The symbol size (area) is sproportional to the luminosity of the galaxy. The magnitude scale (blue total apparent magnitudes) is given at the top of the figure. This map should be a fair representation of how the cluster appears in the sky. The two brightest galaxies, at right ascension approximately equal to 12h 28m and declination approximately equal to 12°40' and right ascension approximately equal to 12h 27m and declination approximately equal to 8°17', are M87 and M49, respectively. [Reproduced by permission from Binggeli, Tammann, and Sandage (1987), Astron. J. 94, 251].

The distribution of the presently known Virgo Cluster members is shown in Fig.1. The cluster covers a large, roughly circular sky area of approximately 10° diameter. Several subconcentrations can be distinguished. There is a major subcluster (A) of galaxies around the giant E galaxy M87, centered on right ascension appeq 12h 25m and declination appeq 13°; there is a smaller, less dense subcluster (B) around the brightest cluster member M49, centered on right ascension appeq 12h 27m and declination appeq 8°30'. A third, barely significant subclump (C) has been identified around M59, at right ascension approximately equal to 12h 40m and declination approximately equal to 12°. Although M87 is most often taken as the center of the Virgo Cluster, it is off the center (density peak) of A by approximately 1° in the direction toward C. With respect to morphological type, the elliptical and SO member galaxies are the most strongly clustered species; they constitute the "skeleton" of the cluster. The E types, in particular, are distributed preferentially (almost chain-like) along the axis A-C. Remarkably, even the jet of M87 is aligned with this fundamental cluster axis. Spiral and (dwarf) irregular galaxies, on the other hand, are scattered over the whole face of the cluster, almost without noticeable concentration.

As its irregular structure suggests, the Virgo Cluster is not in a state of dynamical equilibrium-not even in the central region, which is more surprising. There is evidence that the cluster is still in the making.

From the presently known radial velocities (redshifts) of about 350, mostly bright Virgo members, one derives a mean heliocentric, systemic velocity of the cluster of < v > appeq 1100 km s-1. Although this mean is invariant, the velocity distribution differs substantially for different galaxy types. Late-type (spiral and irregular) galaxies have a broad velocity distribution with a dispersion (standard deviation from < v >) of sigmav appeq 900 km s-1, whereas early-type (E, SO, dE, dSO) galaxies show a narrow distribution with sigmav appeq 550 km s-1. The late types are thus more dispersed, not only in space, but also in velocity. This has been taken as evidence that spiral and irregular galaxies have only recently (in the last few 109 yr) fallen, or are still in the process of falling, into the cluster from the environment: These galaxies are not yet settled down in the cluster ("dynamically relaxed") but are streaming inward and outward in the manner of a damped oscillation. Such an infall scenario is plausible, as the Local Supercluster is indeed made up of large "clouds" of spiral and irregular galaxies: One such cloud seems to be falling into the Virgo cluster at this very epoch. Likewise, the southern subcluster B may be falling into the main subcluster A.

The well-concentrated early-type galaxies of subcluster A must then be viewed as the oldest cluster members that formed in the densest part(s) of the cluster or fell into it very early on. However, these galaxies do not constitute a dynamically relaxed cluster core, as one would expect. Rather, the central part of the Virgo Cluster seems to consist of a small number of subclumps of galaxies, one of which is defined by M87 alone. In spite of its enormous mass of approximately 5 × 1013 Modot, which is indicated by its large, x-ray emitting halo of hot gas, this giant galaxy is off the cluster center in space and velocity (Delta v appeq 200 km s-1). However, as a result of "dynamical friction," the subclumps will rapidly merge. We may, in fact, be living in a very special time, shortly (appeq 109 yr) before the final formation of a relaxed cluster core in Virgo.

This is exciting but it also complicates the dynamical modeling of the Virgo Cluster. The virial theorem can no longer be applied to derive a cluster mass. Nevertheless, requiring simply that the cluster be gravitationally bound (total energy equal to 0), one gets Mtot geq 5 × 1014 Modot, and a mass-to-light ratio of M/L gtapprox 450 in solar units - which clearly indicates the dominance of dark matter.

Bright cluster members have traditionally been used to derive the Hubble constant (expansion rate of the universe), H0. In fact, almost all determinations of H0 are based on the Virgo Cluster, because it is the center of a large velocity perturbation pattern that embraces the whole supercluster, including us. The velocity of the Virgo Cluster, if referred to the centroid of the Local Group (removing the motion of the Sun in the Milky Way, removing the motion of the Galaxy in the Local Group), is < vodot > appeq 1000 km s-1; if referred to the Sun, it is < vodot > appeq 1100 km s-1. The Local Group is falling toward (but will not fall into!) the Virgo Cluster with 200-300 km s-1 (the value is debated), so the true, cosmic expansion velocity of the cluster is < vcos > appeq 1200-1300 km s-1. As the distance estimates for the Virgo Cluster range from 15-22 Mpc, one arrives at a value of the Hubble constant (H0) between 50-100 km s-1 Mpc-1. Thus the present uncertainty in H0 is essentially the difficulty in pinning down the distance to the Virgo Cluster.

Once this important problem is solved, attention is likely to shift back to the cluster as such, and the freed energy may be used to exploit this great galaxy mine for the sake of a better understanding of the formation and evolution of structure in the universe, rendering the Virgo Cluster a true probe for cosmology.

2. Recent results of studying A 2151 (HERCULES) supercluster.
Amongst the famous known voids are Coma and Hercules, which are practically devoid of matter, whereas Bootes and Perseus-Pisces are not empty (approx 10 emission line galaxies in Boo, furthermore evidence for obscuring matter).

<><>Chincarini & Rood, 1970 pointed out  in  the constellation Hercules the redshifts segregate into a small number of groups. They hypothesized that galaxies occur in groups and that the apparent field of galaxies in two dimensional distribution is the result of a superposition of such groups - model (a).

Oort, 1983 pointed out a large void in Hercules from the analysis of data from 1976 - 1981. In fact “VOID” is a natural contraction of “region devoid of galaxies”.

<>Burbidge E.M. et al., 1985, (ApJ, v.288, 82) reported slit spectrophotometry in the 305-855-nm range of 25 QSO candidates identified by slitless spectroscopy in two 1-deg-sq fields (one centered on Abell 2151 and one in a blank region) near the Hercules cluster of galaxies. In the cluster field, 14 candidates are confirmed as QSOs with z = 0.89-3.24; in the blank field, six are confirmed with z = 1.52-3.00. <> 

Freudling, W. 1990, PhDT., got a three-dimensional distribution of galaxies in the Hercules region, which contains the Hercules supercluster and an underdense region in front of the supercluster, the Hercules void. With the aid of the Tully-Fisher relation, deviations are sought from a smooth Hubble flow in that region. The observational parameters needed for the application of the Tully-Fisher relation were measured from two kinds of observations, namely H,I line spectra for 636 galaxies, which were obtained with the Arecibo telescope, used to measure redshifts, line widths, and integrated line fluxes. New I-band CCD images were obtained for 285 galaxies. Isophotal ellipses were fit to the galaxy images, where the center of the ellipses and their ellipticities are free parameters for a given major axis. The surface brightnesses along those ellipses are used to construct a luminosity profile for each galaxy. The outer ellipses, which trace the apparent ellipticity of the disk, are used to estimate the inclination of the galaxy. The total magnitude was measured with each of the ellipses, and the disk ellipticity is used to extrapolate the magnitude to a total magnitude corresponding to a disk extending to infinity. The total magnitudes in combination with the inclination-corrected H I line width are used to estimate redshift-independent distances. The observed distribution of galaxies is used to predict the gravitationally induced deviation from a smooth Hubble flow using the linear theory of gravitational perturbations. The Tully-Fisher distances in combination with the redshifts can be used as a measurement of the peculiar velocity of a galaxy. <> 

Freudling W. et al., 1992, (ApJS, v.79, 157) got a H I redshift for 218 galaxies in the region of the Hercules supercluster. In addition, spectra of 136 galaxies from selected CGCG fields are given cover the projected position of the Hercules void. Measured H I parameters from the spectra are given. This survey was conducted for application of the Tully-Fisher relation to search for streaming motion around the void in front of the supercluster. <> 

Lindner U. et al., 1995, (A&A, v.301, 329) define Supervoids as regions in the local Universe which do not contain rich clusters of galaxies. In order to investigate the distribution of galaxies in and around supervoids, they have studied the closest example, the Northern Local Void. It is defined as the region between the Local, Coma, and the Hercules superclusters, which is well covered by available redshift surveys. they find that this supervoid is not empty, but it contains small galaxy systems and poor clusters of galaxies. They study the cosmography of this void by analyzing the distribution of poor clusters of galaxies, elliptical and other galaxies in two projections and present a catalogue of voids, defined by galaxies of different morphological type and luminosity, and analyze mean diameters of voids in different environments. This analysis shows that sizes of voids and properties of void walls are related. Voids defined by poor clusters of galaxies and by bright elliptical galaxies have a mean diameter of up to 40,h^-1 Mpc. Faint late-type galaxies divide these voids into smaller voids. The faintest galaxies we can study are outlining voids with mean diameters of about 8/h Mpc. Voids located in a high-density environment are smaller than voids in low-density regions. The dependence of void diameters on the type and luminosity of galaxies, as well as on the large-scale environment shows that voids form a hierarchical system. <> 

Hopp U. et al., 1995, (A&AS, v.109, 537) present the redshift and photometric data of a survey for intrinsically faint galaxies towards three nearby voids and the Hercules supercluster. The project is aimed at finding galaxies of absolute faint magnitudes or of low surface brightness within these voids. B and R magnitudes, major and minor diameters, as well as the morphology are determined. The diameter distribution of the galaxies is discussed. Optically measured redshifts of 174 galaxies are given. Most of the galaxies found show emission lines and late-type morphology. Several have low-surface brightness features. This survey identified a higher percentage of nearby galaxies than magnitude or diameter limited surveys. <> 

Kuhn B. et al., 1997, (A&A, v.318, 405) present the results of a search for intrinsically faint galaxies towards three regions with known voids and the Hercules supercluster. The intention was to identify galaxies of low luminosity in order to find possibly a galaxy population in the voids. Within these selected fields we increased the range of observations in comparison with the recent large field surveys which revealed the non-uniform spatial distribution of galaxies. The limiting magnitude was raised by about 5mag, the limiting surface brightness by 2mag/sq.arcsec, and the limiting diameter reduced to less than 1/3. The individual observational data of the sample, published in Hopp et al. 1995 describes the search strategy and contains B and R magnitudes, apparent diameters, redshifts and galaxy types of about 200 newly identified objects. Their luminosity distribution demonstrates a relatively high percentage of dwarfish galaxies. As the essential result of the survey one have to point out that no clear indication of a void-population was found. The majority of the objects lie outside voids in regions where the already known galaxies are concentrated. Some are located in the middle or near the edges of voids. They appear to be rather isolated, their distances to the nearest neighbour are quite large. Only few of the objects seem to be real void galaxies. Even in the three nearest and rather well defined voids they do not find any hitherto unknown galaxy. <> 

Wakamatsu K. et al., 1997, (PASA, v.14, 126) study a large scale structures in nearby space (cz < 10,000 km/s). Several irregular clusters adjacent to Ophiuchus were found forming a supercluster which may be connected to the Hercules supercluster by a wall structure parallel to the local supergalactic plane (Wakamatsu et al. 1994). In front of this supercluster, an `Ophiuchus Void' is suggested (cz = 4,500 km/s). The Ophiuchus supercluster at cz=8,500 km/s is similar to the Hercules supercluster (cz=11,000 km/s), and extends north toward the latter supercluster. They have used FLAIR, the fibre-spectroscopy system on the UKST (Parker,1996) to study the bridge region between the two superclusters which covers (16:00 <= alpha <= 17:20, -25deg <= delta <= +2.5deg) and a void region, 1.5 hour west of a declination zone (-30deg <= delta <= -15deg. FLAIR is well matched to the number-density (~3/sq.deg) and magnitude limit (B<=17.0) of the survey galaxies. The region, mostly above b ~ 15deg, has star densities low enough for FLAIR use without severe crowding or contamination problems. So far 1500 redshifts for obscured galaxies have been obtained. The Ophiuchus supercluster extends at least one field north towards the Hercules supercluster and is surrounded by a diffuse, extended halo ~20deg = 30/h Mpc across. A new sparse `Libra' supercluster candidate is also detected at cz=9,000km/s, one field south of the southern edge of the Hercules supercluster. A wall structure is clearly suggested between this and the Ophiuchus supercluster. The proposed `Ophiuchus - Hercules Wall' formed by a local void in front of, and another behind the Ophiuchus and Libra superclusters, may form a structure as large as the Great Wall both in apparent size (>70deg) and physically (100,h^-1 Mpc). These two walls cross perpendicularly near Abell 2199 - the northern edge of Hercules supercluster. Any `true' 3-D orthogonality between the Ophiuchus - Hercules Wall and the Great Wall may be crucial for understanding 0.1c scale structure whilst this local contrast of galaxy distributions may strongly affect our estimation of the Local Group motion relative to the Microwave background. <> 

Karachentseva V.E. et al., 1999, (A&AS, v.135, 221) published a list of nearby dwarf galaxies towards the Local Void in Hercules-Aquila. Based on film copies of the POSS-II they inspected a wide area of approx6000sq degr in the direction of the nearest cosmic void: RA = 18h 38m, Dec = +18degr , V_0 < 1500 km/s. As a result they present a list of 78 nearby dwarf galaxy candidates which have angular diameters leq 0.5 arcmin and a mean surface brightness geq 26 mag/sq arcsec . Of them 22 are in the direction of the Local Void region. To measure their redshifts, a HI survey of these objects is undertaken on the 100 m Effelsberg telescope. <> 

Lindner U. et al., 1999, (IAUS, v.183, 185) study the Void Hierarchy in the Northern Local Void - a huge underdense region of the nearby Universe situated between the Hercules, Coma and Local Superclusters. They present an investigation of the galaxy distribution in the Northern Local Void using void statistics. In particular galaxies of different morphological type and luminosity have been studied separately and void catalogues have been compiled from three different luminosity limited galaxy samples for the first time. The approach is complementary to most other methods usually used in Large-Scale Structure studies and has the potential to detect and describe subtle structures in the galaxy distribution. They found that the resulting sets of voids form a hierarchical system: The fainter the limiting luminosity of the galaxies the smaller are the voids defined by them. Voids outlined by bright galaxies are interlaced by a fine network of faint galaxy filaments dividing them into smaller subvoids. This Void Hierarchy is an important property of the Large-Scale Structure in the Universe which constrains any realistic galaxy and structure formation scenario. In addition, this concept of Void Hierarchy may help to devise new concepts for the study of the Large-Scale Structure in the Universe. <> 

Petrov G., A. Kniazev and J. Fried, 2004 presented photometry and morphology of faint galaxies in the direction of 1600+18 in Hercules void. Coordinates of ca. 1200 faint galaxies in a field of one square degree centered at 1600+18 (1950)(Hercules void), and m(B),diameters, position angles and morphological classification are presented. The distribution of the magnitudes of the galaxies in this direction is compared with “Log Normal” and “Gauss” ones and with similar results from SDDS studies of galaxies. Major axes luminosity profiles are analysed. Some candidates for primeval galaxies - Low surface brightness galaxies were detected in the direction of the void.
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Compiled by G.T.Petrov, 2004