The inner structure of very massive elliptical galaxies: implications for the inside-out formation mechanism of z∼ 2 galaxies

We analyse a sample of 23 supermassive elliptical galaxies (central velocity dispersion larger than 330 km s−1) drawn from the Sloan Digital Sky Survey. For each object, we estimate the dynamical mass from the light profile and central velocity dispersion, and compare it with the stellar mass derived from stellar population models. We show that these galaxies are dominated by luminous matter within the radius for which the velocity dispersion is measured. We find that the sizes and stellar masses are tightly correlated, with Re∝M1.1*, making the mean density within the de Vaucouleurs radius a steeply declining function of M*: ρe∝M−2.2*. These scalings are easily derived from the virial theorem if one recalls that this sample has essentially fixed (but large) σ0. In contrast, the mean density within 1 kpc is almost independent of M*, at a value that is in good agreement with recent studies of z∼ 2 galaxies. The fact that the mass within 1 kpc has remained approximately unchanged suggests assembly histories that were dominated by minor mergers – but we discuss why this is not the unique way to achieve this. Moreover, the total stellar mass of the objects in our sample is typically a factor of ∼5 larger than that in the high-redshift (z∼ 2) sample, an amount which seems difficult to achieve. If our galaxies are the evolved objects of the recent high-redshift studies, then we suggest that major mergers are required at z≳ 1.5 and that minor mergers become the dominant growth mechanism for massive galaxies at z≲ 1.5.