The Space Density of Extended Ultraviolet (XUV) Disks in the Local Universe and Implications for Gas Accretion on to Galaxies

TL;DR

This study investigates the prevalence of extended UV disks in local galaxies, revealing their frequency, properties, and implications for gas accretion and galaxy evolution, especially in transitional galaxy populations.

Contribution

It provides the first unbiased measurement of XUV-disk space density and links XUV emission to gas accretion and galaxy transition processes.

Findings

XUV-disk frequency up to 20% in nearby galaxies

XUV regions are bluer than galaxy centers

High XUV occurrence around red and green valley galaxies

Abstract

We present results of the first unbiased search for extended UV (XUV)-disk galaxies undertaken to determine the space density of such galaxies. Our sample contains 561 local (0.001 < z < 0.05) galaxies that lie in the intersection of available GALEX deep imaging (exposure time > 1.5 x 10^4 s) and SDSS DR7 footprints. We explore modifications to the standard classification scheme for our sample that includes both disk- and bulge-dominated galaxies. Visual classification of each galaxy in the sample reveals an XUV-disk frequency of up to 20% for the most nearby portion of our sample. On average over the entire sample (out to z=0.05) the frequency ranges from a hard limit of 4% to 14%. The GALEX imaging allows us to detect XUV-disks beyond 100 Mpc. The XUV regions around XUV-disk galaxies are consistently bluer than the main bodies. We find a surprisingly high frequency of XUV emission around luminous red (NUV-r > 5) and green valley (3 < NUV-r < 5) galaxies. The XUV-disk space density in the local universe is > 1.5-4.2 x 10^-3 Mpc^-3. Using the XUV emission as an indicator of recent gas accretion, we estimate that the cold gas accretion rate onto these galaxies is > 1.7-4.6 x 10^-3 Msun Mpc^-3 yr^-1. The number of XUV-disks in the green valley and the estimated accretion rate onto such galaxies points to the intriguing possibility that 7%-18% of galaxies in this population are transitioning away from the red sequence.