Column Density Profiles of Multi-Phase Gaseous Halos

TL;DR

This study investigates the circumgalactic medium (CGM) around Milky-Way-like galaxies using high-resolution simulations, revealing that CGM properties are highly sensitive to feedback models and that ion column density profiles follow an exponential decline with scale heights linked to ionization energy.

Contribution

It demonstrates that galaxy and CGM properties together constrain galaxy formation processes and introduces a model where ion column densities follow exponential profiles with ionization energy-dependent scale heights.

Findings

Column density profiles are well described by exponential functions.

Higher ionization energy ions have larger scale heights.

Simulations suggest scaling profiles by halo scale radius $r_s$ over virial radius.

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.