The Evolution of Stellar Populations in the Outer Disks of Spiral Galaxies

TL;DR

This study examines the ages and properties of stellar populations in the outer disks of five nearby spiral galaxies, revealing that most associations are around 100 million years old, which explains the lack of certain emission lines.

Contribution

It provides the first detailed age and mass estimates of stellar complexes in the extended ultraviolet disks of multiple galaxies using spectral energy distribution fitting.

Findings

Median age of stellar associations is ~100 Myr.

Large age dispersion spans from 1 Myr to 1 Gyr.

Age dispersion is robust against variations in extinction and IMF slope.

Abstract

We investigate recent star formation in the extended ultraviolet (XUV) disks of five nearby galaxies (NGC 0628, NGC 2090, NGC 2841, NGC 3621, and NGC 5055) using a long wavelength baseline comprised of ultraviolet and mid-infrared imaging from the Galaxy Evolution Explorer and the Spitzer Infrared Array Camera. We identify 229 unresolved stellar complexes across targeted portions of their XUV disks and utilize spectral energy distribution fitting to measure their stellar ages and masses through comparison with Starburst99 population synthesis models of instantaneous burst populations. We find that the median age of outer disk associations in our sample is ~100 Myr with a large dispersion that spans the entire range of our models (1 Myr-1 Gyr). This relatively evolved state for most associations addresses the observed dearth of Halpha emission in some outer disks, as Halpha can only be observed in star forming regions younger than ~10 Myr. The large age dispersion is robust against variations in extinction (in the range E(B-V)=0-0.3 mag) and variations in the upper end of the stellar Initial Mass Function (IMF). In particular, we demonstrate that the age dispersion is insensitive to steepening of the IMF, up to extreme slopes.