Formation of the largest galactic cores through binary scouring and gravitational wave recoil

Abstract

Massive elliptical galaxies are typically observed to have central cores in their projected radial light profiles. Such cores have long been thought to form through 'binary scouring' as supermassive black holes (SMBHs), brought in through mergers, form a hard binary and eject stars from the galactic centre. However, the most massive cores, like the ∼ 3 kpc core in A2261-BCG, remain challenging to explain in this way. In this paper, we run a suite of dry galaxy merger simulations to explore three different scenarios for central core formation in massive elliptical galaxies: 'binary scouring', 'tidal deposition' and 'gravitational wave (GW) induced recoil'. Using the griffin code, we self-consistently model the stars, dark matter and SMBHs in our merging galaxies, following the SMBH dynamics through to the formation of a hard binary. We find that we can only explain the large surface brightness core of A2261-BCG with a combination of a major merger that produces a small ∼ 1 kpc core through binary scouring, followed by the subsequent GW recoil of its SMBH that acts to grow the core size. We show that this same model can also explain the bright 'knots' observed in the core region of A2261-BCG. A key prediction of this scenario is an offset SMBH surrounded by a compact cluster of bound stars. We note that a core in the surface brightness profile does not imply a three-dimensional region of (nearly) constant density, but rather a shallow cusp in space density scaling as ∼ r −0.5 .

Ultramassive black holes in the most massive galaxies: $M_{\rm BH}-σ$ versus $M_{\rm BH}-R_{\rm b}$

“…The excess merger rates derived above for large-core galaxies are too large to be explained by additional core scouring mechanisms (e.g., repeated corepassage by a gravitational radiation kicked SMBH, Dullo & Graham 2014, their Sections 5.3 and 5.4) which have been suggested to generate large M def /M BH ∼ 5 by amplifying a pre-existing depleted core carved out by a binary SMBH (e.g., Redmount & Rees 1989;Merritt et al 2004;Gualandris & Merritt 2012), albeit see Nasim et al (2020). Numerical simulations of a Virgo-like galaxy cluster incorporating the effect of AGN feedback (Martizzi et al 2012) produce a BCG with a core that is extremely large in size (∼10 kpc).…”

“…As the inspiraling SMBH binary sinks to the center of the merger remnant, it transfers orbital angular momentum to the surrounding stars. The gravitation slingshot ejection of the inner stars by this decaying SMBH binary creates the central light deficit, i.e., the flattened core (e.g., Begelman et al 1980;Ebisuzaki et al 1991;Milosavljević & Merritt 2001;Merritt 2006;Gualandris & Merritt 2012;Khan et al 2013;Vasiliev et al 2015;Rantala et al 2018;Nasim et al 2020). The light profiles of core-Sérsic galaxies, which break from a steep outer Sérsic profile to a flattened core, are well described using the core-Sérsic model (Graham et al 2003).…”