Probing the CGM of Low-redshift Dwarf Galaxies Using FIRE Simulations
  Observations of UV metal absorption lines have provided insight into the
structure and composition of the circumgalactic medium (CGM) around galaxies.
We compare these observations with the low-redshift ($z \leq 0.3$) CGM around
dwarf galaxies in high-resolution cosmological zoom-in runs in the FIRE-2
simulation suite. We select simulated galaxies that match the halo mass,
stellar mass, and redshift of the observed samples. We produce absorption
measurements using Trident for UV transitions of C IV, O VI, Mg II and Si III.
The FIRE equivalent width (EW) distributions and covering fractions for the C
IV ion are broadly consistent with observations inside $0.5 R_{vir}$, but are
under-predicted for O VI, Mg II, and Si III. The absorption strengths of the
ions in the CGM are moderately correlated with the masses and star formation
activity of the galaxies. The correlation strengths increase with the
ionization potential of the ions. The structure and composition of the gas from
the simulations exhibit three zones around dwarf galaxies characterized by
distinct ion column densities: the disky ISM, the inner CGM (the wind-dominated
regime), and the outer CGM (the IGM accretion-dominated regime). We find that
the outer CGM in the simulations is nearly but not quite supported by thermal
pressure, so it is not in hydrostatic equilibrium (HSE), resulting in
halo-scale bulk inflow and outflow motions. The net gas inflow rates are
comparable to the SFR of the galaxy, but the bulk inflow and outflow rates are
greater by an order of magnitude, with velocities comparable to the virial
velocity of the halo. These roughly virial velocities (${\sim} 100 km s^{-1}$)
produce large EWs in the simulations. This supports a picture for dwarf
galaxies in which the dynamics of the CGM at large scales are coupled to the
small-scale star formation activity near the centre of their halos.
