Ultramassive Black Holes in the Most Massive Galaxies: M BH–σ versus M BH–R b

We investigate the nature of the relations between black hole (BH) mass (M_(BH)) and the central velocity dispersion (σ) and, for core-S´ersic galaxies, the size of the depleted core (R_(b)). Our sample of 144 galaxies with dynamically determined M_(BH) encompasses 24 core-Sersic galaxies, thought to be products of gas-poor mergers, and reliably identified based on high-resolution HST imaging. For core-S´ersic galaxies—i.e., combining normal-core (R_(b) < 0.5 kpc) and large-core galaxies (R_(b) ˃ 0.5 kpc), we find that M_(BH) correlates remarkably well with R_(b) such that M_(BH) ∝ R^(1.20±0.14)_(b) (rms scatter in log M_(BH) of ∆_(rms) ∼ 0.29 dex), confirming previous works on the same galaxies except three new ones. Separating the sample into S´ersic, normal-core and large-core galaxies, we find that S´ersic and normal-core galaxies jointly define a single log-linear M_(BH) − σ relation M_(BH) ∝ σ^(4.88±0.29) with ∆_(rms) ∼ 0.47 dex, however, at the high-mass end large-core galaxies (four with measured M_(BH)) are offset upward from this relation by (2.5 − 4) × σ_(s), explaining the previously reported steepening of the M_(BH) − σ relation for massive galaxies. Large-core spheroids have magnitudes M_(V) ˂ −23.50 mag, half-light radii R_(e) ˃ 10 kpc and are extremely massive M * ˃ 10^(12)Mꙩ. Furthermore, these spheroids tend to host ultramassive BHs (M_(BH) ˃ 10^(10)Mꙩ) tightly connected with their R_(b) rather than σ. The less popular M_(BH) – R_(b) relation exhibits ∼ 62% less scatter in log M_(BH) than the M_(BH – σ) relations. Our findings suggest that large-core spheroids form via multiple major ‘dry’ merger events involving super/ultramassive BHs, consistent with the flattening of the σ – L_(V) relation observed at M_(V) ˂ −23.5 mag.