Ultramassive dense early-type galaxies: velocity dispersions and number
  density evolution since z=1.6
  In this paper we investigate the mass assembly history of ultramassive (Mstar
> 10^11Msun) dense (Sigma = Mstar/(2*pi*Re^2) > 2500 Msun/pc^2) early-type
galaxies (ETGs) over the last 9 Gyr. We have traced the evolution of the number
density rho of ultramassive dense ETGs and have compared their structural
(effective radius Re and stellar mass Mstar) and dynamical (velocity dispersion
sigma_e) parameters over the redshift range 0 < z < 1.6. We have derived the
number density at 1.6 < z < 1 from the MUNICS and GOODS-South surveys, while we
have used the COSMOS and SDSS spectroscopic surveys to probe the intermediate
and local redshift range. For the comparison of the dynamical and structural
parameters, we have collected the ultramassive dense ETGs at 1.2 < z < 1.6 for
which velocity dispersion measurements are available (11 ETGs). For 4 of them
we present unpublished estimates of sigma_e. We probe the intermediate redshift
range, and the local universe using the samples of ETGs by Saglia et al.
(2010), Zahid et al. (2015), and by Thomas et al. (2010). We find that the
number density of ultramassive dense ETGs evolves as rho(z) = K*(1 +
z)^(0.3\pm0.8) implying a decrease of ~ 25% of the population since z = 1.6. By
comparing the values of Re, Mstar, and sigma_e of ultramassive dense ETGs over
the range 0 < z < 1.6 we find that all the high-z ETGs have a counterpart in
the local universe. This implies either that the majority (~70%) of
ultramassive dense ETGs has already completed its assembly and its shaping at
<z> = 1.4, or that, if a significant fraction of them evolves in size, new
ultramassive dense ETGs must form at z < 1.5 to maintain their number density
almost constant. The difficulty into identify good progenitors for these new
dense ETGs at z < 1.5, and the stellar populations properties of local
ultramassive dense ETGs point toward the first hypothesis.
