Structure of the Virgo Cluster.

(The following information/discussion was weaned from the landmark paper: STUDIES OF THE VIRGO CLUSTER, by Binggeli, Sandage and Tammann; The Astronomical Journal, Vol. 90, No.9, Jan., 1984-1985, and [2], and have been quoted and reproduced here to provide an in-depth description of the subject matter. It would not have been worthwhile to rewrite the information contained herein. All cited references are accessible in the original paper. SHL). (1.) The structure of the Virgo Cluster (herafter VC) is complex; it is generally classified as an irregular cluster, as opposed to regular clusters (Coma, as a prototype [Abell, 1975]). Regular clusters have been presumed to be more evolved and relaxed, and thus offer more promise for their understanding than an irregular cluster, such as Virgo, which at closer look, falls apart into several clouds of distinct structural and kinematical properties (deVaucouleurs/'s 1962, 1973). (2.) Because of its large angular extent, the VC is difficult to map photographically at a higher angular resolution. The faintest known cluster galaxies in the early surveys were relatively bright ([15.0m), and their counterparts could easily be detected. The reason the fainter members were unknown was that the surface brightness (sfc) of these members was below the then, conventional detection limit of the scopes originally used. This mapping difficulty was overcome with the very large field of the 100" duPont telescope and the deep Las Campas survey of the VC. With a true cluster distance modulus of (m-M)vc= 31.70 (Ho=50km/sec-1), about 1300 members and 500 possible member galaxies have been surveyed (we will assign varying limitiations on our project by surveying galaxies below ~14.7m...ed). No other cluster has its galaxian content recorded to this level of detail and completeness. With this data we now have the chance to analyze a typical cluster of galaxies, no longer neglecting the dwarf galaxies, which one believes dominate (in number) the population of every cluster of galaxies. DEFINITION AND OUTLINE OF THE CLUSTER. The 1277 members and 574 possible members (possible members are galaxies for which membership criteria do not allow a clear-cut distinction between "member and "background") are determined because five substructures clearly emerge form the galaxy distribution. They include the two major components of the VC, here denoted as the M87 cluster, or A cluster and the M49 cluster or B cluster (note...clusters A and B are imbedded in a large elongated halo which include a third concentration here called the C cluster, which surrounds M59 (NGC 4621) and is connected by a bar-like isophote [lines connecting points of equal light intensity] with cluster A. This bar is composed of bright, preferentially early-type galaxies. The feature is less pronounced in the isopleths [equal population contours] because of the large number of dwarf galaxies moving in the potential of cluster A. For the same reason, cluster B appears stronger in the isophotes than in the isopleths). Three smaller concentrations are the W cloud, and the W'group in the southwest and the M cloud in the northwest (note....the notation W and W' originates from deVaucouleurs, [1961]. The designation M is from Ftaclas et. al [1984], who rediscovered Shapley and Ames [1929] cluster "b" by a velocity analysis). The last three groups are more distant than the VC; this evidence is from various sources.... The magnitude distribution of the galaxies in the area W, W', and M clouds begins ~1-2 magnitudes below the brightest galaxies in the A cluster (M87) and cluster B (M49). Morphological comparisons suggest that this is not due to lower luminosities of the entities in the three clouds, rather the clouds are roughly two times more distant than the VC proper. These values are only estimates because models infer a spherically symmetric infall of the abovementioned clouds towards either the A ,or B cluster. The Virgo Cluster consists of two main concentrations, the cluster near M87 (NGC 4486), and the cluster around M49 (NGC 4472). Cluster A is the dominent feature of the VC. It contains five times as many galaxies as cluster B. Cluster A is dominated by early type galaxies, whereas cluster B consists mainly of late-type members. The systematic differance between early types and late types has disappeared in the larger sample available here. However, there remains the fact that the mean velocity of cluster B is marginally smaller than that of the cluster A, which may be taken that the latter cluster is falling from behind, towards cluster A (note...Quite unexpectedly, neither primary galaxy [M87/M49] lies at the center of its cluster, not even in the luminosity-weighted distribution. This discrepency is particularly severe of M87 and provides the principal clue that suggests the VC is dynamically young). DENSITY AND POPULATION STRUCTURE. In this section the question of how the different morphological types of galaxies are distributed in the VC. In the most general terms, we ask for the galaxy density p(x,T) as a function of location in the cluster, and of type, neglecting a further differentiation with respect to galaxian luminosity. This function approached here in a number of ways, each showing the same trends with increasing generality. (1.) E and SO galaxies form the inner core of the VC. These galaxies are strongly concentrated around M87. They are the bright galaxies in cluster A. (Note: the SO and E types in this region seem to be distributed along the EW axis. This apparent chain of E galaxies was first identified by Arp (1968), who also showed it to be aligned with the jet of M87). (2.) dE and dSO galaxies: dE galaxies greatly outnumber every other galaxy type in the VC. Their distribution is similiar to that of the early type (E+SO) giants, showing also a strong concentration towards cluster A, which is in sharp contrast to the distribution of late type (S+Irr) galaxies (Irr=Im's and BCD's) However, the dwarf's show a wider distribution in position than the giants, and clusters B and C were they are just barely defined. An interesting difference emerges by distinquishing between bright and faint, and between nucleated (de,N) and non-nucleated (dE), dwarf ellipticals. Faint dwarfs are more dispersed than the bright ones and non-nucleated dwarfs are more dispersed than nucleated ones. The two effects undoubtely reflect the same trend, because the frequency of nucleated dwarfs is strongly correlated with galaxian magnitude; bright dwarfs are usually nucleated, whilst faint ones are not.... The de,N galaxies are as strongly concentrated as the E and SO types, and thusly, give the best outline shape of the VC! (note...the ten or so strongest dwarf nuclei in the VC are all found very close to either M87 and M49, suggesting that the brightness of the nucleus is highly correlated with environmental galaxy density. If this is more than a statistical fluctuation, it is of course a clue of major proportion to galaxy formation or evolution, not now understood)[many newer theories might have resolved this dilemna...ed] dSo galaxies are distributed in an EW bar around [delta]=+12 degrees, which again roughly coincides with the symmetry axis that spans from cluster A to cluster C.The dE/Im galaxies, on the other hand, which constitute those dwarfs where a clear distinction between dE and the Im could not be achieved, are widely dispersed, essentially like the spirals and irregulars. (3.) Spirals and Irregular galaxies are scattered over the whole face of the cluster, making it difficult to see any clear boundary in their distribution. Obviously,these galaxies occupy a wider area and presumably larger volume than the early type galaxies. However, there can be no question that on the whole they belong to the cluster proper. This is demonstrated by their concentration towards the cluster center and by their radial velocity, which agrees to within 17 km/sec with that of the early types. The wider velocity distribution of the spirals and Im's shows, however, that they are in a different stage of dynamical evolution than the early type galaxies. Previous discussions have suggested that many Virgo spirals and Im's are presently falling towards the cluster core (deVaucouleurs, 1982; Tully and Shaya 1985; Shaya 1986). It is important to note that Clusters A and B are in fact well defined by late type galaxies. The separation into spirals and Im galaxies shows that cluster B is equally well populated by spirals and Im's while cluster A contains only a compact, isolated group of spirals; it appears as a larger hole in the distribution of irregulars. An outer shell of the late type galaxies surrounds cluster A, separated from it by a region of low, late type density. In summation, the two most obvious morphological features of the Virgo cluster are then: Late type galaxies are more dispersed than the early type galaxies, and cluster A is dominated by early types, cluster B by late types. VELOCITY STRUCTURE. At this point in the program, only the (projected) spatial distribution of the Virgo galaxies have been considered. In this section, a description is given of the one dimensional velocity structure exhibited by the cluster members: SAMPLE MEAN VELOCITY (km/sec) VELOCITY DISPERSION* No. All Members 1112+/- 40 757 354 without dE & dSO 1079+/- 48 800 274 E 1036+/- 111 606 30 SO 1050+/- 75 523 49 E & SO 1045+/- 63 556 79 dE 1258+/- 561 58 within 2 deg.of M87 1436+/- 108 538 25 outside " " " 1123+/- 94 539 33 dSO 1133+/- 129 605 22 dE & dSO 1223+/- 64 576 80 E,SO,dE,& dSO 1134+/- 45 573 159 Spiral 1062+/- 84 931 123 Irr 1062+/- 109 771 50 Spiral & Irr 1062+/- 68 888 173 Cluster A 1061+/- 83 760 83 Cluster B 963+/- 81 390 23 M87 1258+/- 10 M49 969+/- 11 *Velocity dispersion= random velocities of particles about their mean velocity. The "particles" may be atoms in a gas, stars in a cluster, or galaxy, or galaxies in a cluster. Several important features can be deduced from the above table....early type galaxies (E,SO, dE, & dSO) have a narrow velocity distribution with a dispersion of ~573 km/sec, while late type spirals (spiral, Irr) exhibit a broad distribution with ~888 km/sec. This means that late type spirals are not only more dispersed in space, but also in velocity. The differance between the velocity (mean) of clusters A and B= 98+/- 116 km/sec is only barely significant. The available data suggests, however, that cluster B is more distant than cluster A by 0.4m, or ~5 Mpc. If so, the lower velocity of cluster B requires infall relative to cluster A, from the far side. However, this must remain only a suggestion because the relative distances are quite uncertain, and put cluster B behind that of cluster A, which might inflate the intrinsic luminosity of M49 still further beyond that of M87.(note...The situation with clusters A, and B is reminiscent of that of the Centaurus cluster which is comprised of two velocity components, separated by ~1500 km/sec. Despite this great velocity differance, on the basis of luminosity functions and other galaxian characteristics, conclude that the two components probably lie within one cluster...this bears a good resemblance to that of the Virgo Cluster). Cluster A, being the dominant concentration, can be considered as the Virgo Cluster proper. Once cluster B is removed, the remainder appears fairly regular and symmetric. A ring count reveal a dense core of early type galaxies containing M87, and an extended halo of mixed types. Both core and halo are elongated in an E-NW direction. To the E is a third concentration, C (around M59). It may be part of the elongation of cluster A, which has a long eastern tail. The best outline of cluster A is given by the hundreds of dwarf ellipticals that act as test particles in the gravitational prolate of the cluster. Whether the elongated structure is intrisically prolate (bar-like) or oblate (disk-like) and cannot be determined from the present data. In summation...the cluster changes its appearance with each galaxy type that has been plotted. The relative frequency of early type galaxies monotonically increases with increasing local density, while that of the late type galaxies decreases. If only bright galaxies are considered, the Virgo Cluster fits well into Dressler's (1980) relation (The Dressler relation indicates that early type galaxies are more concentrated towards the center of a cluster than the late type galaxies, and further that there has been only slight kinetic mixing of the ellipticals and spirals). and we expect that the VC is also typical with respect to dwarf galaxies, to which the relation has been extended here for brevity. This fact, coupled with a parallel differance in the velocity distribution (~570 km/sec for E, SO, dE, & dSO versus ~890 km/sec for spirals & Irr), shows that early types and late types are in a different dynamical state. That they both belong to the same cluster is proven by their nearly equal velocity means, and by the fact that many spirals and irregulars-although scattered over the whole cluster area-show signs of interaction with the intergalactic medium of the cluster. (Ed. Note: The above sections were in some cases quoted directly from the paper listed in the header. Much appreciation is extended to Bruno Binggeli, Gustav Tammann, and Allan Sandage who presented this paper, so that others can reap the wealth of information that is contained within, and apply the information to their own studies).