Reading the tea leaves in the Mbh–M*,sph and Mbh–Re,sph diagrams: dry and gaseous mergers with remnant angular momentum

Abstract

ABSTRACT We recently revealed that bulges and elliptical galaxies broadly define distinct, superlinear relations in the (black hole mass, Mbh)–(spheroid stellar mass, M*,sph) diagram or Mbh−M*,sph diagram, with the order-of-magnitude lower Mbh/M*,sph ratios in the elliptical galaxies due to major (disc-destroying, elliptical-building) dry mergers. Here, we present a more nuanced picture. Galaxy mergers, in which the net orbital angular momentum does not cancel, can lead to systems with a rotating disc. This situation can occur with either wet (gas-rich) mergers involving one or two spiral galaxies, e.g. NGC 5128, or dry (relatively gas-poor) collisions involving one or two lenticular galaxies, e.g. NGC 5813. The spheroid and disc masses of the progenitor galaxies and merger remnant dictate the shift in the Mbh−M*,sph and Mbh−Re,sph diagrams. We show how this explains the (previously excluded merger remnant) Sérsic S0 galaxies near the bottom of the elliptical sequence and core-Sérsic S0 galaxies at the top of the bulge sequence, neither of which are faded spiral galaxies. Different evolutionary pathways in the scaling diagrams are discussed. We also introduce two ellicular (ES) galaxy types, explore the location of brightest cluster galaxies and stripped ‘compact elliptical’ galaxies in the Mbh−M*,sph diagram, and present a new merger-built Mbh−M*,sph relation which may prove helpful for studies of nanohertz gravitational waves. This work effectively brings into the fold many systems previously considered outliers with either overly massive or undermassive black holes relative to the near-linear Mbh−M*,sph ‘red sequence’ patched together with select bulges and elliptical galaxies.