2010ApJ...721.1755S

In order to investigate the structure and dynamics of the recently discovered massive (M_*≳ 10¹¹ M_☉) compact z ∼ 2 galaxies, cosmological hydrodynamical/N-body simulations of a ∼50,000 Mpc³ comoving (Lagrangian), proto-cluster region have been undertaken. At z = 2, the highest resolution simulation contains ∼5800 resolved galaxies, of which 509, 27, and 5 have M_*>10¹⁰ M_☉, M_*>10¹¹ M_☉, and M_*>4x10¹¹ M_☉, respectively. Total stellar masses, effective radii, and characteristic stellar densities have been determined for all galaxies. At z = 2, for the definitely well-resolved mass range of M_*≳ 10¹¹ M_☉, we fit the relation R_eff= R_eff,12 M ^1/3_*,12to the data, where M_*,12is the total stellar mass in units of 10¹² M_☉. This yields R_eff,12= (1.20 ±0.04) kpc, in line with observational findings for compact z ∼ 2 galaxies, though somewhat more compact than the observed average. The only line-of-sight velocity dispersion measured for a z ∼ 2 compact galaxy is very large, σ_*,p = 510⁺¹⁶⁵_–95 km/s. This value can be matched at about the 1σ level, although a somewhat larger mass than the estimated M_*=~ 2 x10¹¹ M_☉ is indicated. For the above mass range, the galaxies have an average axial ratio 〈b/a〉 = 0.64±0.02 with a dispersion of 0.1, and an average rotation to one-dimensional velocity-dispersion ratio 〈v/σ〉 = 0.46±0.06 with a dispersion of 0.3, and a maximum value of v/σ =~ 1.1. Both rotation and velocity anisotropy contribute significantly in flattening the compact galaxies. Some of the observed compact galaxies appear flatter than any of the simulated galaxies. Finally, it is found that the massive compact galaxies are strongly baryon dominated in their inner parts, with typical dark matter mass fractions of order only 20% inside of r = 2 R_eff.