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