Authors: Paolo Bonfini & Alister W. Graham
Abstract:Partially-depleted cores are practically ubiquitous in luminous early-type galaxies (MB < -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 Rb,cS = 0.55 kpc, three times smaller than the published value. We use this galaxy to caution that other reportedly large cores 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 Rb,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 in-falling perturber" core-formation scenario, in which this galaxy's core could have been excavated by the action of an infalling massive perturber.