We analyze the projected axial ratio distribution, p(b/a), of galaxies that were spectroscopically selected from the Sloan Digital Sky Survey (DR6) to have low star-formation rates. For these quiescent galaxies we find a rather abrupt change in p(b/a) at a stellar mass of ~10^{11} M_sol: at higher masses there are hardly any galaxies with b/a<0.6, implying that essentially none of them have disk-like intrinsic shapes and must be spheroidal. This transition mass is ~3-4 times higher than the threshold mass above which quiescent galaxies dominate in number over star-forming galaxies, which suggests these mass scales are unrelated. At masses lower than ~10^{11} M_sol, quiescent galaxies show a large range in axial ratios, implying a mix of bulge- and disk-dominated galaxies. Our result strongly suggests that major merging is the most important, and perhaps only relevant, evolutionary channel to produce massive (>10^{11} M_sol), quiescent galaxies, as it inevitably results in spheroids.
@article{arxiv.0910.4018,
title = {Major Merging: The Way to Make a Massive, Passive Galaxy},
author = {Arjen van der Wel and Hans-Walter Rix and Bradford P. Holden and Eric F. Bell and Aday R. Robaina},
journal= {arXiv preprint arXiv:0910.4018},
year = {2014}
}
Comments
Minor changes to match published version in ApJ Letters