The Inner Workings of Early-Type Galaxies: Cores, Nuclei and Supermassive Black Holes (Including a Critical Comparison of Nuker and core-Sersic/Sersic models)

TL;DR

This paper reviews the structure of early-type galaxy cores, highlighting the roles of supermassive black holes and stellar nuclei, and compares different models describing their surface brightness profiles, emphasizing the prevalence of nuclei and black holes.

Contribution

It provides a critical comparison of Nuker and core-Sersic models and clarifies the relationship between galaxy core features, supermassive black holes, and stellar nuclei based on recent survey data.

Findings

Surface brightness profiles are better described by the Sersic model than the Nuker model.

Bright galaxies tend to have luminosity deficits and host supermassive black holes.

Faint galaxies often show luminosity excesses with compact stellar nuclei.

Abstract

Recent years have seen dramatic progress in the study of the core and nuclear properties of galaxies. The structure of the cores has been shown to vary methodically with global and nuclear properties, as cores respond to the mechanisms by which galaxies form/evolve. The dynamical centers of galaxies have been found capable of hosting two seemingly disparate objects: supermassive black holes (SBHs) and compact stellar nuclei. In a drastic departure from previous beliefs, it has been discovered that both structures are common: galaxies lacking SBHs and/or stellar nuclei are the exception, rather than the norm. This review explores the connection between cores, SBHs and stellar nuclei in early-type galaxies, as revealed by the ACS Virgo Cluster Survey. We find that, from parsec to kiloparsec scales, the surface brightness profiles of early-type galaxies are well described by a Sersic model -- a parametrization that is shown to be preferable to the so called `Nuker' model. Relative to the inward extrapolation of the best fitting Sersic model, the profiles within the inner few tens of parsecs transition from luminosity `deficits', in the brightest (M_B < ~-20) galaxies, to luminosity `excesses' in the faintest. The compact stellar nuclei responsible for such luminosity excesses comprise approximatively 0.2% of the total galactic mass. Remarkably, this is the same fraction believed to reside in the SBHs, which are predominantly found in the brightest galaxies showing luminosity deficits.