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

TL;DR

This study uses galaxy merger simulations to demonstrate that the largest galactic cores can form through a combination of binary black hole scouring and gravitational wave recoil, explaining observed features in massive ellipticals like A2261-BCG.

Contribution

It introduces a comprehensive simulation approach combining binary scouring and GW recoil to explain massive galactic core formation, including features previously unexplained.

Findings

Large cores explained by combined binary scouring and GW recoil.

Offset SMBH with bound star cluster predicted.

Bright knots are likely minor merger perturbers or ejected SMBHs.

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 ~3kpc 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 ~1kpc 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. 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.