The properties of "dark" {\Lambda}CDM halos in the Local Group

TL;DR

This study uses cosmological simulations to analyze low-mass {cd}CDM halos in the Local Group, predicting properties of gas-rich, star-free halos called RELHICs and their potential observational signatures.

Contribution

The paper provides detailed predictions of RELHIC properties, linking them to observable HI features and offering insights into small-scale dark matter clustering.

Findings

RELHICs inhabit low-density regions and are in hydrostatic and thermal equilibrium.

Most RELHICs are nearly fully ionized with neutral cores detectable by HI surveys.

Local Group RELHICs are likely beyond 500 kpc from major galaxies and have specific velocity and size characteristics.

Abstract

We examine the baryon content of low-mass {\Lambda}CDM halos $(10^8<M_{200}/{\rm M_\odot}<5\times 10^{9})$ using the APOSTLE cosmological hydrodynamical simulations. Most of these systems are free of stars and have a gaseous content set by the combined effects of cosmic reionization, which imposes a mass-dependent upper limit, and of ram pressure stripping, which reduces it further in high-density regions. Halos mainly affected by reionization (RELHICs; REionization-Limited HI Clouds) inhabit preferentially low-density regions and make up a population where the gas is in hydrostatic equilibrium with the dark matter potential and in thermal equilibrium with the ionizing UV background. Their thermodynamic properties are well specified, and their gas density and temperature profiles may be predicted in detail. Gas in RELHICs is nearly fully ionized but with neutral cores that span a large range of HI masses and column densities and have negligible non-thermal broadening. We present predictions for their characteristic sizes and central column densities: the massive tail of the distribution should be within reach of future blind HI surveys. Local Group RELHICs (LGRs) have some properties consistent with observed Ultra Compact High Velocity Clouds (UCHVCs) but the sheer number of the latter suggests that most UCHVCs are not RELHICs. Our results suggest that LGRs (i) should typically be beyond 500 kpc from the Milky Way or M31; (ii) have positive Galactocentric radial velocities; (iii) HI sizes not exceeding 1 kpc, and (iv) should be nearly round. The detection and characterization of RELHICs would offer a unique probe of the small-scale clustering of cold dark matter.