A GALEX Ultraviolet Imaging Survey of Galaxies in the Local Volume

TL;DR

This study uses GALEX ultraviolet imaging to analyze star formation and dust in 390 nearby galaxies, especially dwarf galaxies, revealing that UV-based star formation rates differ from previous H-alpha estimates and are more inclusive of star-forming dwarf galaxies.

Contribution

First comprehensive UV survey of local galaxies, providing new insights into star formation efficiency and detection of star-forming dwarf galaxies missed by H-alpha observations.

Findings

Dwarf galaxies may be less inefficient at forming stars than previously thought.

Most UV-bright, H-alpha-undetected dwarf irregulars are actively forming stars.

UV data reveals star formation in galaxies previously considered quiescent.

Abstract

We present results from a GALEX ultraviolet (UV) survey of a complete sample of 390 galaxies within ~11 Mpc of the Milky Way. The UV data are a key component of the composite Local Volume Legacy (LVL), an ultraviolet-to-infrared imaging program designed to provide an inventory of dust and star formation in nearby spiral and irregular galaxies. The ensemble dataset is an especially valuable resource for studying star formation in dwarf galaxies, which comprise over 80% of the sample. We describe the GALEX survey programs which obtained the data and provide a catalog of far-UV (~1500 Angstroms) and near-UV (~2200 Angstroms) integrated photometry. General UV properties of the sample are briefly discussed. We compute two measures of the global star formation efficiency, the SFR per unit HI gas mass and the SFR per unit stellar mass, to illustrate the significant differences that can arise in our understanding of dwarf galaxies when the FUV is used to measure the SFR instead of H-alpha. We find that dwarf galaxies may not be as drastically inefficient in coverting gas into stars as suggested by prior H-alpha studies. In this context, we also examine the UV properties of late-type dwarf galaxies that appear to be devoid of star formation because they were not detected in previous H-alpha narrowband observations. Nearly all such galaxies in our sample are detected in the FUV, and have FUV SFRs that fall below the limit where the H-alpha flux is robust to Poisson fluctuations in the formation of massive stars. The UV colors and star formation efficiencies of H-alpha-undetected, UV-bright dwarf irregulars appear to be relatively unremarkable with respect to those exhibited by the general population of star-forming galaxies.