7.5 The Virgo Cluster

Virgo, as the largest nearby cluster, also plays a central role in the study of the TF relations in that it is often used to define both the slope and intrinsic dispersion for the TF relations. Pierce and Tully (1988) obtained CCD photometry for the brighter cluster members in Virgo and a complete sample in the Ursa Major Cluster; the systems were found to be at about the same distance. These results are comparable to those obtained previously for the two clusters by Tully and Fisher (1977), and Mould et al. (1980). Pierce and Tully (1988) found greater dispersion in Virgo than in Ursa Major and interpreted this as the result of an infalling cloud of galaxies superposed on the Virgo Cluster core (e.g., Fig. 12), whereas Kraan-Korteweg et al. (1988; hereafter KKCT) interpreted the dispersion as an intrinsic property of the TF relations. They also suggested that the large values of H₀ found by Aaronson and collaborators for more distant clusters were the result of selection biases producing both artificially small observed dispersions for the TF relations, as well as resulting in systematically low distances via ``Malmquist effects'' as discussed in Sec. 7.2.5.

Figure 12. Distance-velocity diagram for the 6° region of the Virgo cluster from Pierce and Tully (1989), and from data given by Kraan-Kortweg et al. (1988) shown as the upper and lower panels, respectively. The data are from independent data sets and independent calibrations. The superimposed lines represent envelopes of predicted peculiar velocities for a Virgo mass constrained by a virial analysis of the cluster (Tully and Shaya 1984). The evidence for a background contamination of the Virgo cluster sample is clear, with the mean distance for the virialized core of the cluster being ~ 15.5 Mpc. The background group with low-velocity dispersion at a distance of ~ 25 Mpc is also localized on the sky and has been referred to as the S' of B cloud.

Pierce and Tully (1992b) have recently obtained CCD photometry for the fainter members of Virgo. Their sample includes essentially all the galaxies used by KKCT. The data are presented here in the lower panels of Figure 11. Clearly, KKCT were correct in their assertion that the dispersion in the TF relations increases when the fainter sample is added. In Figure 12 the individual distance estimates are shown against radial velocity for both the Pierce and Tully (1992b) and the KKCT data (and calibrations). The superimposed lines are the predicted envelopes in velocity that a galaxy infalling into the cluster for the first time will follow (Tully and Shaya 1984). The concentration of points with a distance of ~ 21 Mpc is evident in both data sets, suggestive of a ``background cloud''. Note that the distance range with the highest velocity dispersion corresponds to ~ 15-16 Mpc, or a distance modulus of ~ 31.0. This is also the distance obtained for the brighter members by Pierce and Tully (1988) who interpreted this region of the diagram as the virialized core of the cluster. The ``background cloud'' defines a distinct, well-known distribution when projected on the sky: the S' or B cloud (de Vaucouleurs 1961). Figure 13 shows the distribution of these ``background candidates'' projected upon the sky.

Figure 13. The ``background candidates'' projected upon the sky. The superimposed contours illustrate the density distribution of all Virgo galaxies with 500 V0 1500 km s-1 taken from Huchra (1985).