The most massive galaxies with large depleted cores: structural parameter relations and black hole masses

Abstract

Luminous spheroids (M_(v) ≲ −21.50 ± 0.75 mag) contain partially depleted cores with sizes (R_(b)) typically 0.02–0.5 kpc. However, galaxies with R_(b) > 0.5 kpc are rare and poorly understood. Here, we perform detailed decompositions of the composite surface brightness profiles, extracted from archival Hubble Space Telescope and ground-based images, of 12 extremely luminous "large-core" galaxies that have R_(b) > 0.5 kpc and M_(v) ≲ −23.50 ± 0.10 mag, fitting a core-Sérsic model to the galaxy spheroids. Using 28 "normal-core" (i.e., R_(b) < 0.5 kpc) galaxies and one "large-core" (i.e., R_(b) > 0.5 kpc) galaxy from the literature, we constructed a final sample of 41 core-Sérsic galaxies. We find that large-core spheroids (with stellar masses M_(*) ≳ 10^(12)M☉) are not simple high-mass extensions of the less luminous normal-core spheroids having M_(*) ∼ 8 × 10^(10)–10^(12)M☉. While the two types follow the same strong relations between the spheroid luminosity L_(v) and R_(b) (R_(b)∝ L_(v)^(1.38±0.13), and the spheroid half-light radius R_(e) (R_(e) ∝ L_(v)^(1.8±0.13), for ellipticals plus Brightest Cluster Galaxies), we discover a break in the core-Sérsic σ–L_(v) relation occurring at M_(v) ∼ −23.50 ± 0.10 mag. Furthermore, we find a strong log-linear R_(b)–M_(BH) relation for the 11 galaxies in the sample with directly determined supermassive black hole (SMBH) masses M_(BH)—3/11 galaxies are large-core galaxies—such that R_(b)∝ M_(BH)^( 0.83 ± 0.10) . However, for the large-core galaxies the SMBH masses estimated from the M_(BH)–σ and core-Sérsic M_(BH)–L relations are undermassive, by up to a factor of 40, relative to expectations from their large Rb values, confirming earlier results. Our findings suggest that large-core galaxies harbor overmassive SMBHs (M_(BH) ≳ 10^(10) M☉), considerably (∼3.7–15.6σ and ∼0.6–1.7σ) larger than expectations from the spheroid σ and L, respectively. We suggest that the R_(b) –M_(BH) relation can be used to estimate SMBH masses in the most massive galaxies.