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Many recent observations and numerical simulations suggest that nearbymassive, early-type galaxies were formed through a "two-phase" process. In theproposed second phase, the extended stellar envelope was accumulated throughmany dry mergers. However, details of the past merger history of present-dayellipticals, such as the typical merger mass ratio, are difficult to constrainobservationally. Within the context and assumptions of the two-phase formationscenario, we propose a straightforward method, using photometric data alone, toestimate the average mass ratio of mergers that contributed to the build-up ofmassive elliptical galaxies. We study a sample of nearby massive ellipticalgalaxies selected from the Carnegie-Irvine Galaxy Survey, using two-dimensionalanalysis to decompose their light distribution into an inner, denser componentplus an extended, outer envelope, each having a different optical color. Thecombination of these two substructures accurately recovers the negative colorgradient exhibited by the galaxy as whole. The color difference between the twocomponents (<\Delta(B-V)> ~ 0.10 mag; <\Delta(B-R)> ~ 0.14 mag), based on theslope of the M_stellar-color relation for nearby early-type galaxies, can betranslated into an estimate of the average mass ratio of the mergers. The roughestimate, 1:5 to 1:10, is consistent with the expectation of the two-phaseformation scenario, suggesting that minor mergers were largely responsible forbuilding up to the outer stellar envelope of present-day massive ellipticals.With the help of accurate photometry, large sample size, and more choices ofcolors promised by ongoing and future surveys, the approach proposed here canreveal more insights into the growth of massive galaxies during the last fewGyr.

The Carnegie-Irvine Galaxy Survey. IV. A Method to Determine the Average Mass Ratio of Mergers That Built Massive Elliptical Galaxies