Heart of Darkness: the influence of galactic dynamics on quenching star
  formation in galaxy spheroids
  Quenched galaxies are often observed to contain a strong bulge component. The
key question is whether this reflects a causal connection - can star formation
be quenched dynamically by bulges or the spheroids of early-type galaxies? We
systematically investigate the impact of these morphological components on star
formation, by performing a suite of hydrodynamical simulations of isolated
galaxies containing a spheroid. We vary the bulge mass and scale radius, while
the total initial stellar, halo and gas mass are kept constant, with a gas
fraction of 5 per cent. In addition, we consider two different sub-grid star
formation prescriptions. The first follows most simulations in the literature
by assuming a constant star formation efficiency per free-fall time, whereas in
the second model it depends on the gas virial parameter, following
high-resolution simulations of turbulent fragmentation. Across all simulations,
central spheroids increase the gas velocity dispersion towards the galactic
centre. This increases the gravitational stability of the gas disc, suppresses
fragmentation and star formation, and results in galaxies hosting extremely
smooth and quiescent gas discs that fall below the galaxy main sequence. These
effects amplify when using the more sophisticated, dynamics-dependent star
formation model. Finally, we discover a pronounced relation between the central
stellar surface density and star formation rate (SFR), such that the most
bulge-dominated galaxies show the strongest deviation from the main sequence.
We conclude that the SFR of galaxies is not only set by the balance between
accretion and feedback, but carries a (sometimes dominant) dependence on the
gravitational potential.
