The Space Density of Extended Ultraviolet (Xuv) Disks in the Local Universe and Implications for Gas Accretion Onto Galaxies

Abstract

We present results of the first unbiased search for extended ultraviolet (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 Galaxy Evolution Explorer (GALEX) deep imaging (exposure time >1.5 × 10 4 s) and Sloan Digital Sky Survey 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%-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) × 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) × 10 −3 M 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.

Star formation sustained by gas accretion

“…Surveys of extended UV disks around both early-and latetype galaxies actually suggest on-going gas accretion and star formation in the outer parts (Lemonias et al 2011;Moffett et al 2012). These ideas are consistent with cosmological numerical simulations, where disk galaxies acquire their spin together with their mass through cold gas accretion.…”

High Angular Momentum Halo Gas: A Feedback and Code-independent Prediction of LCDM

“…Additionally, Figure 6 demonstrates that the codirectional mass fraction of cold gas in the halo stayed consistently high over cosmic time (at least until z=1). We speculate that it may be possible that this high angular momentum accretion helps to explain observations of extended XUV disks (e.g., Thilker et al 2005Thilker et al , 2007Lemonias et al 2011;Holwerda et al 2012), local extended H I disks (e.g., García-Ruiz et al 2002;Oosterloo et al 2007;Christlein & Zaritsky 2008;Sancisi et al 2008;Walter et al 2008;Wang et al 2013;Huang et al 2014;Courtois et al 2015), and corotating cold halo gas around local Milky Way analogs (e.g., Diamond-Stanic et al 2016).…”

Resolved H I Imaging of a Population of Massive H I-Rich Galaxies With Suppressed Star Formation

“…1 we show that GLSBs and the VLA sample lie in the same region of the HI gas fraction -stellar mass plane. The prevalence of XUV-disks (Thilker et al 2007;Lemonias et al 2011) confirms that inefficient star formation in extended disks is common. Galaxies in the VLA sample are probably not XUV-disks (except for 45664) because their UV images do not show evidence of recent star formation within the extended HI, but galaxies in the VLA sample and XUV-disks might represent different stages of the life of an extended HI disk.…”