Double-Barred Galaxies

TL;DR

This review summarizes current knowledge on double-barred galaxies, focusing on their demographics, photometric properties, and dynamics, highlighting their prevalence, structure, and rotation independence.

Contribution

It provides a comprehensive overview of the demographics, sizes, structures, and rotation characteristics of double-barred galaxies, emphasizing their photometric properties and independence of bar rotation.

Findings

Approximately 20% of certain galaxy types are double-barred.

Inner bars are typically ~500 pc in radius.

Inner bars rotate independently of outer bars.

Abstract

I present a brief review of what is known about double-barred galaxies, where a small ("inner") bar is nested inside a larger ("outer") bar; the review is focused primarily on their demographics and photometric properties. Roughly 20% of S0--Sb galaxies are double-barred; they may be rarer in later Hubble types. Inner bars are typically ~ 500 pc in radius (~ 12% the size of outer bars), but sizes range from ~ 100 pc to > 1 kpc. The structure of at least some inner bars appears very similar to that of outer bars (and single large-scale bars). Direct and indirect evidence all support the hypothesis that inner bars rotate independently of outer bars, although actual pattern speeds for inner bars are poorly constrained. Finally, I note that inner bars do not appear to promote nuclear activity.