Orbiting Circumgalactic Gas as a Signature of Cosmological Accretion

Abstract

We use cosmological SPH simulations to study the kinematic signatures of cool gas accretion onto a pair of wellresolved 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 galaxys 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.

An Introduction to Gas Accretion onto Galaxies

“…How the gas is able to ultimately cool to below 10 4 K and feed the star formation in the disk may be related to density enhancements in the filaments and the mixing with satellite and feedback material (Joung et al, 2012b;Fraternali & Binney, 2008). The simulations also find that much of the ongoing IGM accretion occurs towards the edges of the galaxy to avoid the dominant feedback from the central regions (Stewart et al, 2011;Fernández et al, 2012).…”

Star formation sustained by gas accretion

“…The difference arises because the disks accrete gas directly from their filament and inherit its angular momentum, while the spheroids grow from the mixture of other galaxies coming in along the filament (i.e, grow from mergers). Stewart et al (2011b) found that cold streams in haloes contain higher angular momentum than dark matter. The reason for this is that the total angular momentum of the dark matter adds positive and negative contributions from the mixing of many sub-haloes, with a net value that is small.…”

On the cosmic evolution of the specific star formation rate

“…In such a picture, galaxies at z < ∼ 2 are living off of their gas-rich earlier phases of evolution (and significant mass return) where star formation is kept inefficient through strong feedback from intense star formation. The high angular momentum of the gas that is being subsequently accreted allows galaxies to preferentially grow their outer disks (Stewart et al 2011), but this is likely moderated by the low stellar mass surface densities of outer disks because as argued through the generalized Schmidt law, their star formation should be kept relatively inefficient (Blitz & Rosolowsky 2006). Such a picture naturally explains the lack of a significant increase in the central surface densities of star forming galaxies from z ∼ 1 (Barden et al 2005;.…”