Published in "Outskirts of Galaxies", Astrophysics and Space Science Library, Volume 434. ISBN 978-3-319-56569-9. Springer International Publishing AG, 2017, p. 115.
https://arxiv.org/abs/1612.05615vi

For a PDF version of the article, click here.

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 Hα 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 Hα photons.

Table of Contents

• INTRODUCTION
• OUTER DISK STRUCTURE FROM COLLAPSE MODELS OF GALAXY FORMATION
• OUTER DISK STRUCTURE: THREE EXPONENTIAL TYPES
• OUTER DISK STELLAR POPULATIONS: COLOUR AND AGE GRADIENTS
• MONO-AGE STRUCTURE OF STELLAR POPULATIONS
• OUTER DISK STRUCTURE: ENVIRONMENTAL EFFECTS AND THE ROLE OF BULGES AND BARS
• OUTER DISK STRUCTURE: STAR FORMATION MODELS
• THE DISKS OF DWARF IRREGULAR GALAXIES
• Radial Trends
• The Gas Disk
• The Stellar Disk
• Star Formation in Dwarfs
• The Hα / FUV Ratio
• Breaks in Radial Profiles in dIrr Galaxies
• SUMMARY
• REFERENCES