Column Density Profiles of Multi-Phase Gaseous Halos
  We analyze circumgalactic medium (CGM) in a suite of high-resolution
cosmological re-simulations of a Milky-Way size galaxy and show that CGM
properties are quite sensitive to details of star formation--feedback loop
modelling. The simulation that produces a realistic late-type galaxy, fails to
reproduce existing observations of the CGM. In contrast, simulation that does
not produce a realistic galaxy has the predicted CGM in better agreement with
observations. This illustrates that properties of galaxies and properties of
their CGM provide strong ${\it complementary}$ constraints on the processes
governing galaxy formation. Our simulations predict that column density
profiles of ions are well described by an exponential function of projected
distance $d$: $N \propto e^{-d/h_s}$. Simulations thus indicate that the sharp
drop in absorber detections at larger distances in observations does not
correspond to a "boundary" of an ion, but reflects the underlying steep
exponential column density profile. Furthermore, we find that ionization energy
of ions is tightly correlated with the scale height $h_s$: $h_s \propto E_{\rm
ion}^{0.74}$. At $z \approx 0$, warm gas traced by low-ionization species
(e.g., Mg II and C IV) has $ h_s \approx 0.03-0.07 R_{\rm vir}$, while higher
ionization species (O VI and Ne VIII) have $h_s \approx 0.32-0.45R_{\rm vir}$.
Finally, the scale heights of ions in our simulations evolve slower than the
virial radius for $z\leq 2$, but similarly to the halo scale radius, $r_s$.
Thus, we suggest that the column density profiles of galaxies at different
redshifts should be scaled by $r_s$ rather than the halo virial radius.
