GALEX Ultraviolet Imaging of Dwarf Galaxies and Star Formation

TL;DR

This study uses GALEX ultraviolet imaging to analyze star formation in 44 dwarf galaxies, revealing extended UV emission beyond Halpha regions and insights into their star formation histories and gas content.

Contribution

It provides a comprehensive comparison of UV, Halpha, and V-band properties in dwarf galaxies, highlighting the extent of star formation and its relation to gas content and disk structure.

Findings

UV emission extends beyond Halpha in most galaxies.

No disk edges observed down to low surface densities.

Outer disk star formation correlates with HI content.

Abstract

We present ultraviolet integrated and azimuthally-averaged surface photometric properties of a sample of 44 dIm, BCD, and Sm galaxies measured from archival NUV and FUV images obtained with GALEX. We compare the UV to Halpha and V-band properties and convert FUV, Halpha, and V-band luminosities into star formation rates (SFRs). We also model the star formation history from colors and compare the integrated SFRs and SFR profiles with radius for these methods. In most galaxies, the UV photometry extends beyond Halpha in radius, providing a better measure of the star formation activity in the outer disks. The Halpha appears to be lacking in the outer disk because of faintness in low density gas. The FUV and V-band profiles are continuous with radius, although they sometimes have a kink from a double exponential disk. There is no obvious difference in star formation properties between the inner and outer disks. No disk edges have been observed, even to stellar surface densities as low as 0.1 Msun/pc2 and star formation rates as low as 10^{-4} Msun/yr/kpc2. Galaxies with low HI to luminosity ratios have relatively low FUV compared to V-band emission in the outer parts, suggesting a cessation of star formation there. Galaxies with relatively high HI apparently have fluctuating star formation with a Gyr timescale.