Early formation of massive, compact, spheroidal galaxies with classical profiles by violent disc instability or mergers

Understand

We address the formation of massive stellar spheroids between redshifts $z=4$ and 1 using a suite of AMR hydro-cosmological simulations.

• The spheroids form as bulges, and the spheroid mass growth is partly driven by violent disc instability (VDI) and partly by mergers.
• A kinematic decomposition to disc and spheroid yields that the mass fraction in the spheroid is between 50\% and 90\% and is roughly constant in time, consistent with a cosmological steady state of VDI discs that are continuously fed from the cosmic web.
• The density profile of the spheroid is typically "classical", with a Sersic index $n = 4.5\pm 1$, independent of whether it grew by mergers or VDI and independent of the feedback strength.