Core shapes and orientations of core-Sersic galaxies

TL;DR

This study measures the shapes and orientations of core-Sersic galaxies' inner and outer regions, revealing correlations with merger histories and black hole influence, using high-resolution HST data.

Contribution

It provides the first analysis of the relationship between galaxy shapes, black hole mass ratios, and merger history in core-Sersic galaxies.

Findings

Inner ellipticity median of 0.13, rounder than outer 0.20.

Weak trend linking larger black hole mass ratios to rounder shapes.

Shapes are robust against HST seeing effects.

Abstract

The inner and outer shapes and orientations of core-Sersic galaxies may hold important clues to their formation and evolution. We have therefore measured the central and outer ellipticities and position angles for a sample of 24 core-Sersic galaxies using archival HST images and data. By selecting galaxies with core-Sersic break radii R_b---a measure of the size of their partially depleted core---that are > 0".2, we find that the ellipticities and position angles are quite robust against HST seeing. For the bulk of the galaxies, there is a good agreement between the ellipticities and position angles at the break radii and the average outer ellipticities and position angles determined over R_e/2 < R < R_e, where R_e is the spheroids' effective half light radius. However there are some interesting differences. We find a median "inner" ellipticity at R_b of e_med = 0.13 +- 0.01, rounder than the median ellipticity of the "outer" regions e_med = 0.20 +- 0.01, which is thought to reflect the influence of the central supermassive black hole at small radii. In addition, for the first time we find a trend, albeit weak (2 sigma significance), such that galaxies with larger (stellar deficit)-to- (supermassive black hole) mass ratios---thought to be a measure of the number of major dry merger events---tend to have rounder inner and outer isophotes, suggesting a connection between the galaxy shapes and their merger histories. We show that this finding is not simply reflecting the well known result that more luminous galaxies are rounder, but it is no doubt related.