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

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-Sérsic model, and found that the former galaxy has a core-Sérsic 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 Sérsic) 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.