Testing a prediction of the merger origin of early-type galaxies: a
  correlation between stellar populations and asymmetry
  One of the key predictions of the merger hypothesis for the origin of
early-type (elliptical and lenticular) galaxies is that tidally-induced
asymmetric structure should correlate with signatures of a relatively young
stellar population. Such a signature was found by Schweizer and Seitzer (1992;
AJ, 104, 1039) at roughly 4sigma confidence. In this paper, we revisit this
issue with a nearly ten-fold larger sample of 0.01<z<0.03 galaxies selected
from the Two Micron All-Sky Survey and the Sloan Digital Sky Survey. We
parameterize tidal structure using a repeatable algorithmic measure of
asymmetry, and correlate this with color offset from the early-type galaxy
color-magnitude relation. We recover the color offset-asymmetry correlation;
furthermore, we demonstrate observationally for the first time that this effect
is driven by a highly-significant trend towards younger ages at higher
asymmetry values. We present a simple model for the evolution of early-type
galaxies through gas-rich major and minor mergers that reproduces their
observed build-up from z=1 to the present day and the distribution of
present-day colors and ages. We show using this model that if both stellar
populations and asymmetry were ideal `clocks' measuring the time since last
major or minor gas-rich interaction, then we would expect a rather tight
correlation between age and asymmetry. We suggest that the source of extra
scatter is natural diversity in progenitor star formation history, gas content,
and merger mass ratio, but quantitative confirmation of this conjecture will
require sophisticated modeling. We conclude that the asymmetry-age correlation
is in basic accord with the merger hypothesis, and indicates that an important
fraction of the early-type galaxy population is affected by major or minor
mergers at cosmologically-recent times.
