X-ray emission from the ultramassive black hole candidate NGC 1277: implications and speculations on its origin

Abstract

Abstract We study the X-ray emission from NGC 1277, a galaxy in the core of the Perseus cluster, for which van den Bosch et al. have recently claimed the presence of an ultramassive black hole (UMBH) of mass $1.7\times 10^{10}{\,\mathrm{M}_{{\odot }}}$, unless the initial mass function of the stars in the stellar bulge is extremely bottom heavy. The X-rays originate in a power-law component of luminosity $1.3\times 10^{40}{\,{\rm erg}\,{\rm s}^{-1}\,}$ embedded in a 1 keV thermal minicorona which has a half-light radius of about 1.3 kpc, typical of many early-type galaxies in rich clusters of galaxies. If Bondi accretion operated on to the UMBH from the minicorona with a radiative efficiency of 10 per cent, then the object would appear as a quasar with luminosity $10^{46}{\,{\rm erg}\,{\rm s}^{-1}\,}$, a factor of almost 106 times higher than observed. The accretion flow must be highly radiatively inefficient, similar to past results on M87 and NGC 3115. The UMBH in NGC 1277 is definitely not undergoing any significant growth at the present epoch. We note that there are three UMBH candidates in the Perseus cluster and that the inferred present mean mass density in UMBHs could be $10^5{\,\mathrm{M}_{{\odot }}}\,{\rm Mpc^{-3}}$, which is 20–30 per cent of the estimated mean mass density of all BHs. We speculate on the implied growth of UMBHs and their hosts, and discuss the possibility that extreme active galactic nucleus feedback could make all UMBH host galaxies have low stellar masses at redshifts around 3. Only those which end up at the centres of groups and clusters later accrete large stellar envelopes and become brightest cluster galaxies. NGC 1277 and the other Perseus core UMBH, NGC 1270, have not, however, been able to gather more stars or gas owing to their rapid orbital motion in the cluster core.