MAJOR MERGING: THE WAY TO MAKE A MASSIVE, PASSIVE GALAXY

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 similar to 10(11) M-circle dot: 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 similar to 3-4 times higher than the threshold mass above which quiescent galaxies dominate in number over star-forming galaxies, which suggests that these mass scales are unrelated. At masses lower than similar to 10(11) M-circle dot, 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-circle dot), quiescent galaxies, as it inevitably results in spheroids.