Outskirts of Nearby Disk Galaxies: Star Formation and Stellar Populations

TL;DR

This paper reviews star formation and stellar populations in the outer disks of nearby galaxies, highlighting their structure, age, and gas content, and discussing implications for galaxy evolution.

Contribution

It provides a comprehensive synthesis of observational and theoretical insights into the properties and processes in the far-outer disks of spiral and dwarf irregular galaxies.

Findings

Outer disks are older, redder, and thicker than inner disks.

Gas dominates the outer regions, with non-exponential radial profiles.

The H-alpha to UV flux ratio decreases with lower surface brightness.

Abstract

The properties and star formation processes in the far-outer disks of nearby spiral and dwarf irregular galaxies are reviewed. The origin and structure of the generally exponential profiles in stellar disks is considered to result from cosmological infall combined with a non-linear star formation law and a history of stellar migration and scattering from spirals, bars, and random collisions with interstellar clouds. In both spirals and dwarfs, the far-outer disks tend to be older, redder and thicker than the inner disks, with the overall radial profiles suggesting inside-out star formation plus stellar scattering in spirals, and outside-in star formation with a possible contribution from scattering in dwarfs. Dwarf irregulars and the far-outer parts of spirals both tend to be gas dominated, and the gas radial profile is often non-exponential although still decreasing with radius. The ratio of H-alpha to far-UV flux tends to decrease with lower surface brightness in these regions, suggesting either a change in the initial stellar mass function or the sampling of that function, or a possible loss of H-alpha photons.