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

Abstract

Redshifts and magnitudes for nearly 100 galaxies in the core of the Coma Cluster are now available. The sample is nearly complete to seven- teenth nuclear region magnitude (central 4.8 arc second spot). Nearly all the higher redshift galaxies in the Coma Cluster are nonellipticals. The separation of the ellipticals from the nonellipticals is significant in both the redshift and magnitude coordinates and is shown to be a maximum in a linear combination of redshift and magnitude along a direction sloping close to 0.6 magnitude fainter per 1000 km /sec increase in redshift. The morphological separation is in the same sense as the pattern in the Virgo Cluster. When the redshift magnitude diagram is examined in the direction of the maximum morphological separation it is shown to have a strongly banded structure which is not present in any other direction. The observed redshift -magnitude pattern is compared with an outlying distinct group of galaxies in the Coma Cluster which shows a nearly identical pattern 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 residual true Doppler velocity dispersion in the Coma Cluster cannot exceed about 220 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 dependence of the Hubble constant on distance.