Evolution of Elliptical Galaxies

Teresa Lynn Kroeker

Doctor of Philosophy 1998
Graduate Department of Astronomy, University of Toronto

In the first part of this thesis early-type galaxies in Arp groups are examined to determine if any recent mergers have occurred. No unusual rotation curves are found to indicate a merger event in the four out of eleven early-type galaxies with sufficient signal-to-noise ratio. Minor axis rotation is another sign of peculiar kinematics and it too is not detected in the four galaxies with strong rotation and for which the slit positions are aligned along the major and minor axes. The average velocity dispersion of this sample of Arp galaxies is 7 +/- 4% lower than other early-type galaxies, but otherwise these galaxies appear to be indistinguishable from normal galaxies. The lack of strong emission lines indicates that these galaxies have not undergone recent star formation with the exception of two galaxies which are currently interacting with other members of their group. There is no strong evidence in these Arp groups of recent mergers. These results are in agreement with other authors who find that the occurrence of recent major merger events in compact groups is rare.

The Fundamental Plane is a superior tool to either the Faber-Jackson or luminosity-size relation for the study of the luminosity and mass-to-light ratio evolution of early-type galaxies in clusters. Surface brightness selection effects will cause the Faber-Jackson relation to underestimate the luminosity evolution. These selection effects are clearly demonstrated for the cluster Abell 2390. Surface brightness selection effects will also affect the results derived from the luminosity-size relation but to a lesser degree. The Fundamental Plane analysis is insensitive to surface brightness selection effects and therefore produces reliable results.

Comparing the Fundamental Planes of the Coma cluster, a nearby cluster, and Abell 2390, a distant cluster, we find the evolution in luminosity is \Delta M_B(AB) = -0.54 +/- 0.11. The mass-to-light ratios increase by 20^{+14}_{-13}% between Abell 2390 (z=0.23) and Coma (z=0.023). This weak evolution in mass-to-light ratio favours high formation redshifts for early-type galaxies in clusters. These results are consistent with those found by other authors.

The results of this thesis support the formation of early-type galaxies at redshifts greater than 2, and seems to rule out any significant formation at redshifts less than 1.

Copied with permission by Marlene Cummins
March 5 1998