The quest for the largest depleted galaxy core: supermassive black hole binaries and stalled in-falling satellites

TL;DR

This paper investigates the largest known galaxy cores, analyzing their sizes and formation mechanisms, and finds evidence supporting the stalled infalling perturber scenario over the traditional black hole binary scouring model.

Contribution

It provides refined measurements of galaxy core sizes and challenges previous estimates, proposing an alternative core formation scenario involving infalling perturbers.

Findings

2MASX J09194427+5622012 has a smaller core radius than previously reported.

2MASX J17222717+3207571 has the largest known partially-depleted core.

Support found for the stalled infalling perturber scenario in core formation.

Abstract

Partially-depleted cores are practically ubiquitous in luminous early-type galaxies (M$_{B}\lesssim-$20.5 mag), and typically smaller than 1 kpc. In one popular scenario, supermassive black hole binaries --- established during dry (i.e. gas-poor) galaxy mergers --- kick out the stars from a galaxy's central region via three-body interactions. Here, this "binary black hole scouring scenario" is probed at its extremes by investigating the two galaxies reported to have the largest partially-depleted cores found to date: 2MASX~J09194427+5622012 and 2MASX~J17222717+3207571 (the brightest galaxy in Abell~2261). We have fit these galaxy's two-dimensional light distribution using the core-S\'{e}rsic model, and found that the former galaxy has a core-S\'{e}rsic break radius $R_{b,cS}=0.55$~kpc, three times smaller than the published value. We use this galaxy to caution that other reportedly large break radii may too have been over-estimated if they were derived using the "sharp-transition" (inner core)-to-(outer S\'{e}rsic) model. In the case of 2MASX~J17222717+3207571, we obtain $R_{b,cS}=3.6$~kpc. While we confirm that this is the biggest known partially-depleted core of any galaxy, we stress that it is larger than expected from the evolution of supermassive black hole binaries --- unless one invokes substantial gravitational-wave-induced (black hole)-recoil events. Given the presence of multiple nuclei located (in projection) within the core radius of this galaxy, we explored and found support for the alternative "stalled infalling perturber" core-formation scenario, in which this galaxy's core could have been excavated by the action of an infalling massive perturber.