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 1 "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, and the spheroid half-light radius R e (R e ∝ L 1.08±0.09 V , for ellipticals plus BCGs), 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 SMBH masses M BH -3/11 galaxies are large-core galaxies-such that R b ∝ M 0.83±0.10 BH . 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 R b values, confirming earlier results. Our findings suggest that large-core galaxies harbour 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.

Formation of the largest galactic cores through binary scouring and gravitational wave recoil

“…Large-core galaxies are found to obey the same scaling relations between break radius and surface brightness or spheroid luminosity as normal core galaxies, as well as a log-linear break radius-SMBH mass relation, when a measurement is available (Dullo 2019). However, directly measured SMBHs are overmassive with respect to predictions from the M SMBH −L and the M SMBH −σ relations, with a more significant offset for the latter relation.…”

“…Dullo & Graham 2012, 2014. Furthermore, it has been shown that adopting a multicomponent model rather than a single core-Sérsic model over the entire radial range provides a more reliable estimate of the core size (Dullo & Graham 2014;Dullo 2019). Measured core sizes for massive ellipticals -derived in this way -are typically tens to a few hundred parsecs (e.g.…”

“…A study by Lauer et al (2007) considered a large sample of brightest cluster galaxies (BCGs) and found that fewer than 10 systems had a core size of ∼ 1kpc or greater, with the largest cored system being NGC 6166 which has a core size of ∼ 1.5 kpc. More recently, Dullo (2019) considered the largest sample of 'largecore' galaxies to date, finding that only 13(7) galaxies have core sizes larger than 0.5(1) kpc.…”

“…This is generally interpreted as a result of an increased number of dry mergers for the most massive galaxies (e.g. Dullo 2019).…”

“…An extremely large surface brightness core has been found in the BCG of Abell 2261 (A2261-BCG hereafter) (Postman et al 2012). Assuming a single-component model, Bonfini & Graham (2016) infer a core-Sérsic break radius of 3.6 kpc, while assuming a multicomponent model Dullo (2019) derive a core radius of 2.7 kpc. With either estimate, this represents the second largest depleted core known to date (Dullo 2019).…”