The Argo Simulation: I. Quenching of Massive Galaxies at High Redshift
  as a Result of Cosmological Starvation
  Observations show a prevalence of high redshift galaxies with large stellar
masses and predominantly passive stellar populations. A variety of processes
have been suggested that could reduce the star formation in such galaxies to
observed levels, including quasar mode feedback, virial shock heating, or
galactic winds driven by stellar feedback. However, the main quenching
mechanisms have yet to be identified. Here we study the origin of star
formation quenching using Argo, a cosmological, hydrodynamical zoom-in
simulation that follows the evolution of a massive galaxy at $z\geq{}2$. This
simulation adopts the same sub-grid recipes of the Eris simulations, which have
been shown to form realistic disk galaxies, and, in one version, adopts also a
mass and spatial resolution identical to Eris. The resulting galaxy has
properties consistent with those of observed, massive (M_* ~ 1e11 M_sun)
galaxies at z~2 and with abundance matching predictions. Our models do not
include AGN feedback indicating that supermassive black holes likely play a
subordinate role in determining masses and sizes of massive galaxies at high z.
The specific star formation rate (sSFR) of the simulated galaxy matches the
observed M_* - sSFR relation at early times. This period of smooth stellar mass
growth comes to a sudden halt at z=3.5 when the sSFR drops by almost an order
of magnitude within a few hundred Myr. The suppression is initiated by a
leveling off and a subsequent reduction of the cool gas accretion rate onto the
galaxy, and not by feedback processes. This "cosmological starvation" occurs as
the parent dark matter halo switches from a fast collapsing mode to a slow
accretion mode. Additional mechanisms, such as perhaps radio mode feedback from
an AGN, are needed to quench any residual star formation of the galaxy and to
maintain a low sSFR until the present time.
