NIHAO VIII: Circum-galactic medium and outflows - The puzzles of HI and OVI gas distributions

TL;DR

This study uses the NIHAO simulation suite to analyze the distribution and properties of hot and cold circum-galactic medium in galaxies, comparing results with observations and making predictions about outflow shapes and gas enrichment.

Contribution

It provides detailed simulation-based insights into the CGM's composition, structure, and outflow morphology across a wide range of galaxy masses, with observable predictions and empirical relations.

Findings

HI covering fraction matches observations well

OVI column densities increase with halo mass but are lower than observed

Simulated CGM remains remarkably spherical across masses

Abstract

We study the hot and cold circum-galactic medium (CGM) of 86 galaxies of the cosmological, hydrodynamical simulation suite NIHAO. NIHAO allows a study of how the $z=0$ CGM varies across 5 orders of magnitude of stellar mass using OVI and HI as proxies for hot and cold gas. The cool HI covering fraction and column density profiles match observations well, particularly in the inner CGM. OVI shows increasing column densities with mass, a trend seemingly echoed in the observations. As in multiple previous simulations, the OVI column densities in simulations are lower than observed and optically thick HI does not extend as far out as in observations. We take a look at the collisional ionisation fraction of OVI as a function of halo mass. We make observable predictions of the bipolarity of outflows and their effect on the general shape of the CGM. Bipolar outflows can be seen out to around 40 kpc in intermediate and low mass halos ($M_{\mathrm{halo}}<10^{11}M_{\mathrm{sun}}$), but outside that radius, the CGM is too well mixed to detect an elongated shape. Larger halos have extended gas discs beyond the stellar disc that dominate the shape of the inner CGM. The simulated CGM is remarkably spherical even in low mass simulations. The chemical enrichment of both halo and disc gas follow expected increasing trends as a function of halo mass that are well fit with power laws. These relations can be used in non-hydrodynamic models, such as semi-analytic models.