The Correlation of Redshift with Magnitude and Morphology in the Coma Cluster

Abstract

Redshifts and magnitudes for 93 faint galaxies in the core of the Coma Cluster are now available. The sample is nearly complete to seventeenth nuclear region magnitude (central 4.8 arc second spot). Nuclear region magnitudes show a good correlation with redshift in the sense that galaxies fall in bands sloping fainter with increasing redshift. A magnitude decrease of 0.6 magnitudes per 1000 km /sec redshift increase is observed. The deviation from the expected random correlation of nuclear region magnitude and redshift is significant at a probability less than 10 -3 by x2 testing. It is further observed that nearly all of the higher redshift galaxies in the Coma Cluster are nonelliptical. This is not expected from a normal dynamical model and is remarkably similar to the pattern in the Virgo Cluster where the spiral galaxies show a distinctly greater redshift that the ellipticals. The observed redshift -magnitude pattern reality is further tested by comparison with two outlying concentrations of galaxies in the Coma Cluster which show nearly identical patterns slightly shifted in redshift. The pattern is also used to predict the m -logz diagram for field galaxies. Observed field galaxies are shown to fit the predicted pattern and to deviate from the normal velocity dispersion model. After allowance for magnitude and redshift uncertainties it is shown that a residual true Doppler velocity dispersion in the Coma Cluster cannot exceed about 140 km /sec, and the cluster is not dynamically unstable. In the presence of an intrinsic unexplained redshift which produced the observed pattern, the mean redshift of a group of galaxies need not be the same as the Hubble velocity of the cluster. Systematic trends in the deviation of the Hubble velocity from the mean redshift are possible due to selection with distance and may explain the local apparent anisotropy in the Hubble law.