The Accretion of Gas onto Galaxies as Traced by their Satellites

TL;DR

This study estimates gas accretion onto galaxies via satellites, finding it insufficient to sustain star formation but revealing strong correlations between satellite gas content and galaxy star formation activity, suggesting a large-scale gas reservoir.

Contribution

It introduces a method to estimate satellite gas content and accretion rates, highlighting their correlation with galaxy star formation and proposing a large-scale ionized gas reservoir as the fueling source.

Findings

Satellite gas accretion rates are much lower than needed for star formation.

Strong correlation between satellite gas mass and galaxy star formation rates.

Gas-rich satellites may trace a large-scale ionized gas reservoir.

Abstract

We have compiled a large sample of isolated central galaxies from the Sloan Digital Sky Survey, which do not have a neighbour of comparable brightness within a projected distance of 1 Mpc. We use the colours, luminosities and surface brightnesses of satellite galaxies in the vicinity of these objects to estimate their atomic gas content and to derive the average total mass of HI gas contained in satellites as a function of projected radius from the primary. Recent calibrations of merging timescales from N-body simulations are used to estimate the rate at which this gas will accrete onto the primaries. Our estimated accretion rates fall short of those needed to maintain the observed level of star formation in these systems by nearly two orders of magnitude. Nevertheless, there are strong correlations between the total mass of gas in satellites and the colours and specific star formation rates of central galaxies of all stellar masses. The correlations are much weaker if we consider the total stellar mass in the satellites, rather than their total gas mass. We ask why star formation in the central galaxies should be correlated with gas contained in satellites at projected separations of a Mpc or more, well outside the virial radius of the dark matter halos of these systems. We suggest that gas-rich satellites trace an underlying reservoir of ionized gas that is accreted continuously, and that provides the fuel for ongoing star formation in galaxies in the local Universe.