Central Stellar Mass Deficits in the Bulges of Local Lenticular Galaxies, and the Connection with Compact z ~ 1.5 Galaxies

TL;DR

This study analyzes the central stellar deficits in local lenticular galaxy bulges, linking them to the properties of compact galaxies at z ~ 1.5, and suggests a formation history involving dry mergers and subsequent disk growth.

Contribution

It provides detailed measurements of stellar mass deficits in lenticular bulges and connects these to the evolution of compact high-redshift galaxies.

Findings

Central stellar deficits are 0.5 to 2 times the black hole mass.

Bulges have Sersic index ~3 and radii < 2 kpc.

These bulges are likely descendants of z ~ 1.5 compact galaxies.

Abstract

We have used the full radial extent of images from the Hubble Space Telescope's Advanced Camera for Surveys and Wide Field Planetary Camera 2 to extract surface brightness profiles from a sample of six, local lenticular galaxy candidates. We have modelled these profiles using a core-Sersic bulge plus an exponential disk model. Our lenticular disk galaxies with bulge magnitudes M_V < ~ -21.30 mag have central stellar deficits, suggesting that these bulges may have formed from `dry' merger events involving supermassive black holes while their surrounding disk was subsequently built up, perhaps via cold gas accretion scenarios. The central stellar mass deficits M_def are roughly 0.5 to 2 M_BH (black hole mass), rather than ~ 10 to 20 M_BH as claimed from some past studies, which is in accord with core-Sersic model mass deficit measurements in elliptical galaxies. Furthermore, these bulges have Sersic indices n ~ 3, half light radii R_e < 2 kpc and masses > 10^11 M_sun, and therefore appear to be descendants of the compact galaxies reported at z ~ 1.5 to 2. Past studies which have searched for these local counterparts by using single-component galaxy models to provide the z ~ 0 size comparisons have over-looked these dense, compact and massive bulges in today's early-type disk galaxies. This evolutionary scenario not only accounts for what are today generally old bulges---which must be present in z ~ 1.5 images---residing in what are generally young disks, but it eliminates the uncomfortable suggestion of a factor of 3 to 5 growth in size for the compact, z ~ 1.5 galaxies that are known to possess infant disks.