Reading the tea leaves in the $M_{\rm bh}$-$M_{\rm *,sph}$ and $M_{\rm bh}$-$R_{\rm e,sph}$ diagrams: dry and gaseous mergers with remnant angular momentum

TL;DR

This paper explores how galaxy mergers, involving varying amounts of angular momentum and gas, influence the relationships between black hole mass, spheroid mass, and size, revealing new evolutionary pathways and galaxy types.

Contribution

It introduces a nuanced understanding of galaxy merger effects on black hole scaling relations, including new galaxy classifications and a merger-built $M_{bh}$-$M_{*,sph}$ relation.

Findings

Merger dynamics explain the positions of S0 galaxies in scaling diagrams.

Different merger types lead to distinct evolutionary pathways.

A new merger-built $M_{bh}$-$M_{*,sph}$ relation is proposed.

Abstract

We recently revealed that bulges and elliptical galaxies broadly define distinct, super-linear relations in the $M_{\rm bh}$-$M_{\rm *,sph}$ diagram, with the order-of-magnitude lower $M_{\rm bh}/M_{\rm *,sph}$ 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 $M_{\rm bh}$-$M_{\rm *,sph}$ and $M_{\rm bh}$-$R_{\rm e,sph}$ diagrams. We show how this explains the (previously excluded merger remnant) Sersic S0 galaxies near the bottom of the elliptical sequence and core-Sersic 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 $M_{\rm bh}$-$M_{\rm *,sph}$ diagram, and present a new merger-built $M_{\rm bh}$-$M_{\rm *,sph}$ 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 $M_{\rm bh}$-$M_{\rm *,sph}$ `red sequence' patched together with select bulges and elliptical galaxies.