Extended UV (XUV) Emission in Nearby Galaxy Disks

TL;DR

This paper reviews the properties of extended UV emission in nearby galaxy disks, identifying two classes of XUV disks and discussing their characteristics, formation, and ongoing research efforts.

Contribution

It classifies XUV disks into two types and provides detailed properties, highlighting their role in galaxy growth and the need for further multi-wavelength studies.

Findings

Type 1 XUV disks have structured UV features beyond star-formation thresholds.

Type 2 XUV disks are large, blue, and indicative of inside-out galaxy growth.

XUV disks are rich in HI but poor in molecular gas, with young stellar populations.

Abstract

We summarize the main properties of the extended UV (XUV) emission found in roughly 30% of the nearby spiral galaxies observed by the GALEX satellite. Two different classes of XUV disks are identified, the Type 1 XUV disks where significant, structured UV-bright features are found beyond the "classical" azimuthally-averaged star-formation threshold, and the Type 2 XUV disks, which are characterized by very extended (seven times the area where most of the stellar mass is found), blue [(FUV-K)<5mag] outer disks. These latter disks are extreme examples of galaxies growing inside-out. The few XUV disks studied in detail to date are rich in HI but relatively poor in molecular gas, have stellar populations with luminosity-weighted ages of ~1 Gyr, and ionized-gas metal abundances of ~Zsun/10. As part of the CAHA-XUV project we are in the process of obtaining deep multi-wavelength imaging and spectroscopy of 65 XUV-disk galaxies so to determine whether or not these properties are common among XUV disks.