Orbiting Circum-galactic Gas as a Signature of Cosmological Accretion

TL;DR

This study uses cosmological simulations to identify kinematic signatures of cold gas accretion onto galaxy halos, predicting observable features that distinguish accretion from outflows, and linking these to observed galaxy phenomena.

Contribution

It introduces a novel simulation-based prediction of high angular momentum cold flow disks as signatures of cosmological accretion, observable via absorption line studies.

Findings

Accreted gas orbits with high angular momentum, producing observable velocity offsets.

Cold flow disks co-rotate with galaxy disks and show large one-sided velocity offsets.

The fraction of galaxies with cold flow signatures decreases at low redshift.

Abstract

We use cosmological SPH simulations to study the kinematic signatures of cool gas accretion onto a pair of well-resolved galaxy halos. Cold-flow streams and gas-rich mergers produce a circum-galactic component of cool gas that generally orbits with high angular momentum about the galaxy halo before falling in to build the disk. This signature of cosmological accretion should be observable using background-object absorption line studies as features that are offset from the galaxy's systemic velocity by ~100 km/s. Accreted gas typically co-rotates with the central disk in the form of a warped, extended cold flow disk, such that the observed velocity offset is in the same direction as galaxy rotation, appearing in sight lines that avoid the galactic poles. This prediction provides a means to observationally distinguish accreted gas from outflow gas: the accreted gas will show large one-sided velocity offsets in absorption line studies while radial/bi-conical outflows will not (except possibly in special polar projections). This rotation signature has already been seen in studies of intermediate redshift galaxy-absorber pairs; we suggest that these observations may be among the first to provide indirect observational evidence for cold accretion onto galactic halos. Cold mode halo gas typically has ~3-5 times more specific angular momentum than the dark matter. The associated cold mode disk configurations are likely related to extended HI/XUV disks seen around galaxies in the local universe. The fraction of galaxies with extended cold flow disks and associated offset absorption-line gas should decrease around bright galaxies at low redshift, as cold mode accretion dies out.