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.

OUTSKIRTS OF NEARBY DISK GALAXIES: STAR FORMATION AND STELLAR POPULATIONS

Bruce G. Elmegreen 1 and Deidre A. Hunter 2


1 IBM Research Division, T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598 USA
2 Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, Arizona 86001, USA


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

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