Bright Ultraviolet Regions and Star Formation Characteristics in Nearby Dwarf Galaxies

TL;DR

This study examines star formation in the inner and outer disks of nearby dwarf irregular galaxies using UV and optical data, revealing no radial gradients in region properties and insights into star formation thresholds.

Contribution

It provides a comparative analysis of star-forming regions in dwarf galaxies, highlighting the relationship between region mass, age, and gas density, and discusses implications for star formation processes.

Findings

No radial gradients in region masses and ages.

Outer regions lack Halpha emission due to faint nebular emission.

Star formation can occur at low gas surface densities.

Abstract

We compare star formation in the inner and outer disks of 11 dwarf Irregular galaxies (dIm) within 3.6 Mpc. The regions are identified on GALEX near-UV images, and modeled with UV, optical, and near-IR colors to determine masses and ages. A few galaxies have made 10^5-10^6 Msun complexes in a starburst phase, while others have not formed clusters in the last 50 Myrs. The maximum region mass correlates with the number of regions as expected from the size-of-sample effect. We find no radial gradients in region masses and ages, even beyond the realm of Halpha emission, although there is an exponential decrease in the luminosity density and number density of the regions with radius. Halpha is apparently lacking in the outer parts only because nebular emission around massive stars is too faint to see. The outermost regions for the 5 galaxies with HI data formed at average gas surface densities of 1.9-5.9 Msun/pc2. These low average densities imply either that local gas densities are high or sub-threshold star formation is possible. The distribution of regions on a log Mass - log age plot is is usually uniform along log age for equal intervals of log Mass. This uniformity results from either an individual region mass that varies as 1/age or a region disruption probability that varies as 1/age. A correlation between fading-corrected surface brightness and age suggests the former.