Ultramassive Black Holes in the Most Massive Galaxies: M_BH-σ versus M_BH-R_b

Abstract

We investigate the nature of the relations between black hole (BH) mass (M_BH) and the central velocity dispersion (σ) and, for core-Sérsic galaxies, the size of the depleted core (R_b). Our sample of 144 galaxies with dynamically determined M_BH encompasses 24 core-Sérsic galaxies, thought to be products of gas-poor mergers, and reliably identified based on high-resolution Hubble Space Telescope imaging. For core-Sérsic 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 ${M}_{\mathrm{BH}}\propto {R}_{{\rm{b}}}^{1.20\pm 0.14}$ (rms scatter in log M_BH of Δ_rms ∼ 0.29 dex), confirming previous works on the same galaxies apart from three new ones. Separating the sample into Sérsic, normal-core and large-core galaxies, we find that Sérsic and normal-core galaxies jointly define a single log-linear M_BH-σ relation M_BH ∝ σ^4.88±0.29 with Δ_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_* ≳ 10¹² M_⊙. Furthermore, these spheroids tend to host ultramassive BHs (M_BH ≳ 10¹⁰ M_⊙) 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.