[Paper Review] The Black Hole Mass of Abell 1836-BCG and Abell 3565-BCG

E. Dalla Bontà, Laura Ferrarese+4 more|ArXiv.org|Jun 13, 2007

Astronomy and Astrophysical Research4 citations

TL;DR

This study uses Hubble Space Telescope ACS and STIS data to dynamically measure the supermassive black hole masses in two brightest cluster galaxies, Abell 1836-BCG and Abell 3565-BCG, by modeling ionized gas kinematics. It finds black hole masses of $4.8^{+0.8}_{-0.7} \times 10^9\,M_\odot$ and $1.3^{+0.3}_{-0.4} \times 10^9\,M_\odot$ respectively, with Abell 1836-BCG hosting the most massive dynamically measured black hole to date.

ABSTRACT

Two brightest cluster galaxies (BCGs), namely Abell 1836-BCG and Abell 3565-BCG, were observed with the Advanced Camera for Surveys (ACS) and the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. By modeling the available photometric and kinematic data, it resulted that the mass of Abell 1836-BCG and Abell 3565-BCG are M_bh=4.8(+0.8,-0.7)x10^9 M_sun and M_bh=1.3(+0.3,-0.4)x10^9 M_sun at 1 sigma confidence level, respectively.

Motivation & Objective

• To determine the supermassive black hole masses in two brightest cluster galaxies (BCGs), Abell 1836-BCG and Abell 3565-BCG, using high-resolution Hubble Space Telescope data.
• To test whether the most massive black holes in the local universe, expected in BCGs, follow established scaling relations such as $M_\bullet - \sigma$ and $M_\bullet - L_{\text{bulge}}$.
• To assess whether BCGs deviate from standard black hole scaling relations due to their unique formation history and high merger activity.
• To provide precise dynamical mass measurements using ionized gas kinematics to resolve discrepancies in indirect mass estimates from photometric and velocity dispersion data.

Proposed method

• High-resolution imaging with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in three filters to map stellar light and dust distribution.
• Spectroscopy with the Space Telescope Imaging Spectrograph (STIS) using a 0.′′1 wide slit to measure ionized gas kinematics along the major axis and offset positions.
• Modeling of ionized gas kinematics assuming circular orbits in a thin disk under the combined gravitational potential of stars and a central black hole.
• Synthetic velocity fields were projected and degraded to match observational conditions, including PSF, charge bleeding, and slit geometry.
• A three-dimensional grid of models was explored, varying black hole mass $M_\bullet$, stellar mass-to-light ratio $(M/L)_\star$, and disk inclination $i$, with $\chi^2$ minimization to find the best fit.
• The velocity dispersion profile was modeled using a radial function $\sigma(r) = \sigma_0 + \sigma_1 e^{-r/r_\sigma}$, with parameters adjusted to match observations.

Experimental results

Research questions

• RQ1 What is the dynamically measured mass of the supermassive black hole in Abell 1836-BCG, and does it exceed the predictions of the $M_\bullet - L_{\text{bulge}}$ relation?
• RQ2 How does the black hole mass in Abell 3565-BCG compare to predictions from the $M_\bullet - \sigma$ and $M_\bullet - L_{\text{bulge}}$ scaling relations?
• RQ3 Do the most massive black holes in BCGs follow the same scaling relations as less massive galaxies, or do they represent a distinct population?
• RQ4 Is the observed discrepancy in Abell 1836-BCG's mass due to a deviation in the $M_\bullet - L_{\text{bulge}}$ relation, or is it consistent with measurement uncertainty and environmental effects?

Key findings

• The black hole mass in Abell 1836-BCG is measured as $4.8^{+0.8}_{-0.7} \times 10^9\,M_\odot$ at 1σ confidence, making it the most massive dynamically measured black hole to date.
• The black hole mass in Abell 3565-BCG is $1.3^{+0.3}_{-0.4} \times 10^9\,M_\odot$ at 1σ confidence, consistent with the $M_\bullet - L_{\text{bulge}}$ relation.
• Abell 1836-BCG's black hole mass lies significantly above the $M_\bullet - L_{\text{bulge}}$ relation predicted by Marconi & Hunt (2003), with a predicted value of $8.9 \times 10^8\,M_\odot$, indicating a possible deviation at the high-mass end.
• The inclination of the ionized gas disk in Abell 1836-BCG is $76 \pm 1^\circ$, and the stellar mass-to-light ratio is constrained to $\leq 4.0\,(M/L)_\odot$, consistent with a low-mass-to-light ratio stellar population.
• For Abell 3565-BCG, the best-fit model yields an inclination of $50 \pm 1^\circ$ and a stellar mass-to-light ratio of $9.0 \pm 0.8\,(M/L)_\odot$, indicating a more massive stellar component.
• The results suggest that while most galaxies with $M_\bullet > 10^9\,M_\odot$ lie on the $M_\bullet - L_{\text{bulge}}$ relation, Abell 1836-BCG is an outlier, raising questions about the universality of scaling relations at the high-mass end.