Column Density Profiles of Multi-Phase Gaseous Halos

Abstract

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.