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Blue Ellipticals in Compact Groups

Published online by Cambridge University Press:  12 April 2016

Stephen E. Zepf  and
Bradley C. Whitmore
Stephen E. Zepf
Affiliation:
Space Telescope Science Institute and the Johns Hopkins University
Bradley C. Whitmore
Affiliation:
Space Telescope Science Institute

Abstract

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We examine the hypothesis that mergers of spiral galaxies make elliptical galaxies by studying galaxies in compact groups. We combine dynamical models of the merger-rich compact group environment with stellar evolution models and predict that roughly 15% of compact group ellipticals should be 0.15 mag bluer in B – R color than normal ellipticals. The published colors of these galaxies suggest the existence of this predicted blue population, but a normal distribution with large random errors can not be ruled out based on these data alone. However, we have new UBVRI data which confirm the blue color of the two ellipticals with blue B – R colors for which we have our own colors. This confirmation of a population of blue ellipticals indicates that interactions are occurring in compact groups, but a blue color in one index alone does not require that these ellipticals are recent products of the merger of two spirals. We demonstrate how optical spectroscopy in the blue may distinguish between a true spiral + spiral merger and the swallowing of a gas-rich system by an already formed elliptical. We also show that the sum of the luminosity of the galaxies in each group is consistent with the hypothesis that the final stage in the evolution of a compact group is an elliptical galaxy.

Type
II. Classical Observations of Multiplets
Information
International Astronomical Union Colloquium , Volume 124: Paired and Interacting Galaxies , 1990 , pp. 99 - 103
Copyright
Copyright © NASA 1990

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