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. Key predictions of this scenario are an offset SMBH surrounded by a compact cluster of bound stars and a non-divergent central density profile. We show that the bright ‘knots’ observed in the core region of A2261-BCG are best explained as stalled perturbers resulting from minor mergers, though the brightest may also represent ejected SMBHs surrounded by a stellar cloak of bound stars.

Signatures of the Many Supermassive Black Hole Mergers in a Cosmologically Forming Massive Early-Type Galaxy

“…However, it is encouraging that the results obtained here agree well with simulations of similar systems run in isolation at higher spatial resolution (e.g. Rantala et al 2018;Nasim et al 2021).…”

“…This has been suggested to give rise to the observed offset active galactic nuclei (e.g. Comerford et al 2015), drive the formation of large galactic cores (Nasim et al 2021), and contribute to the scatter in the M • -σ relation (Blecha et al 2011).…”

Dynamical modelling of the twisted galaxy PGC 046832

“…In a triaxial galaxy, a massive black hole will spiral in rather quickly after such ejection, but may oscillate for extended time in the galaxy core (Gualandris & Merritt 2008). This mechanism has been used to explain the unusually large core of the BCG galaxy in Abell 2261 (Nasim et al 2021). A third explanation may be that the final black hole formed through a sequence of multiple mergers.…”

Galaxy Core Formation by Supermassive Black Hole Binaries: the Importance of Realistic Initial Conditions and Galaxy Morphology

“…On the theoretical side, the formation of cores during galaxy mergers has been studied by many authors (e.g., Milosavljević & Merritt 2001;Milosavljević et al 2002;Merritt 2006;Boylan-Kolchin & Ma 2007;Rantala et al 2018Rantala et al , 2019Nasim et al 2021). Merritt (2006) used Nbody simulations to follow the evolution of a spherical galaxy model containing a central SMBH binary.…”

“…Several alternative models for the formation of depleted cores are recently gaining attention due to their ability to produce even larger cores, which could explain the extremely large cores and hence, depleted masses, found in a handful of galaxies (i.e., core radius > 1Kpc; e.g., Laine et al 2003;Lauer et al 2007;Hyde et al 2008;Bonfini & Graham 2016;Dullo 2019). These models include the "recoiled SMBH" scenario (e,g, Redmount & Rees 1989;Boylan-Kolchin, Ma, & Quataert 2004;Merritt et al 2004;Gualandris & Merritt 2008;Nasim et al 2021); the "stalled perturber" scenario (Read et al 2006;Goerdt et al 2010); the "multiple-SMBH scouring" scenario (Kulkarni & Loeb 2012); and the com-bined "sinking SMBH-AGN feedback" scenario (Martizzi, Teyssier, & Moore 2012).…”