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For refcode 1995AJ....110..129K:
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1995AJ....110..129K THE LOCAL MERGER RATE OF DISK GALAXIES WILLIAM C. KEEL AND WENTAO WU Department of Physics and Astronomy, University of Alabama, Fox 870324, Tuscaloosa, Alabama 35487-0324 Received 1994 December 28; revised 1995 March 29 ABSTRACT We consider the local population of galaxy merger remnants, more specifically the remnants of disk-disk mergers for which there are diagnostic features (such as tails) that might be catalogued in a fairly complete manner. We estimate the current rate of mergers for such galaxies by comparing the luminosity function for mergers with those of spiral-spiral pairs (their most common precursors) and for all disk galaxies, incorporating the effects of changing crossing time with luminosity. These are connected via the time scale for mergers, which we estimate from the dynamical properties of our sample and published n-body models. We derive a merger rate for spirals in pairs, at the present epoch, of 4.2 per Hubble time (2.8 over the age of the Universe at critical density, or extrapolated to all spirals, 0.33 per Hubble time). Given this large difference, depending on which precursor population is considered, a two-component model seems most appropriate to assess the effects of mergers on galaxy evolution - one for galaxies originally in pairs or small groups, and a second for galaxies which have never been in environments so susceptible to merging. Distinguishing the initial contributions of each to the overall galaxy populations will require samples at high redshift. We also evaluate the history of several indicators of observable star formation, using the dynamical state of each remnant as an age estimate. With large scatter, we see evidence that star formation builds up to the time of merger rather slowly, with a rapid decline thereafter. For most systems, dust seems to be so important in obscuring much of the starburst's optical radiation that the luminosity increase of a merger over its initial components does not have a profound effect on either our derived merger rate or on higher-redshift counts. This rapid fading means that mergers per se can be at most a minor contributor to the Butcher-Oemler effect; most blue galaxies in systems fated to merge will be observed during the interactions that precede the final merging event. Further observations of the dynamical state of high-redshift systems are clearly needed to assess the time- integrated impact of merging on the overall galaxy population; the crude estimates given per Hubble time must be lower limits under most conditions. An Appendix describes the luminosity function of "field" galaxies of various morphological types, derived from a new analysis of the CfA survey in the B_t_^0^ system.
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