Nicholas J. McConnell

REVISITING THE SCALING RELATIONS OF BLACK HOLE MASSES AND HOST GALAXY PROPERTIES

New kinematic data and modeling efforts in the past few years have substantially expanded and revised dynamical measurements of black hole masses (M ?) at the centers of nearby galaxies. Here we compile an updated sample of 72 black holes and their host galaxies, and present revised scaling relations between M ? and stellar velocity dispersion (?), V-band luminosity (L), and bulge stellar mass (M bulge), for different galaxy subsamples. Our best-fitting power-law relations for the full galaxy sample are log10(M ?) = 8.32 + 5.64log10(?/200 km s?1), log10(M ?) = 9.23 + 1.11log10(L/1011 L ?), and log10(M ?) = 8.46 + 1.05log10(M bulge/1011 M ?). A log-quadratic fit to the M ?-? relation with an additional term of ?2 [log10(?/200 km s?1)]2 gives ?2 = 1.68 ? 1.82 and does not decrease the intrinsic scatter in M ?. Including 92 additional upper limits on M ? does not change the slope of the M ?-? relation. When the early- and late-type galaxies are fit separately, we obtain similar slopes of 5.20 and 5.06 for the M ?-? relation but significantly different intercepts?M ? in early-type galaxies are about two times higher than in late types at a given sigma. Within early-type galaxies, our fits to M ?(?) give M ? that is about two times higher in galaxies with central core profiles than those with central power-law profiles. Our M ?-L and M ?-M bulge relations for early-type galaxies are similar to those from earlier compilations, and core and power-law galaxies yield similar L- and M bulge-based predictions for M ?. When the conventional quadrature method is used to determine the intrinsic scatter in M ?, our data set shows weak evidence for increased scatter at M bulge < 1011 M ? or LV < 1010.3 L ?, while the scatter stays constant for 1011 < M bulge < 1012.3 M ? and 1010.3 < LV < 1011.5 L ?. A Bayesian analysis indicates that a larger sample of M ? measurements would be needed to detect any statistically significant trend in the scatter with galaxy properties.

Two ten-billion-solar-mass black holes at the centres of giant elliptical galaxies

Observational work conducted over the past few decades indicates that all massive galaxies have supermassive black holes at their centres. Although the luminosities and brightness fluctuations of quasars in the early Universe suggest that some were powered by black holes with masses greater than 10 billion solar masses, the remnants of these objects have not been found in the nearby Universe. The giant elliptical galaxy Messier 87 hosts the hitherto most massive known black hole, which has a mass of 6.3 billion solar masses. Here we report that NGC 3842, the brightest galaxy in a cluster at a distance from Earth of 98 megaparsecs, has a central black hole with a mass of 9.7 billion solar masses, and that a black hole of comparable or greater mass is present in NGC 4889, the brightest galaxy in the Coma cluster (at a distance of 103 megaparsecs). These two black holes are significantly more massive than predicted by linearly extrapolating the widely used correlations between black-hole mass and the stellar velocity dispersion or bulge luminosity of the host galaxy. Although these correlations remain useful for predicting black-hole masses in less massive elliptical galaxies, our measurements suggest that different evolutionary processes influence the growth of the largest galaxies and their black holes.

THE BLACK HOLE MASS IN THE BRIGHTEST CLUSTER GALAXY NGC 6086

We present the first direct measurement of the central black hole mass, M ?, in NGC?6086, the Brightest Cluster Galaxy (BCG) in A2162. Our investigation demonstrates for the first time that stellar-dynamical measurements of M ? in BCGs are possible beyond the nearest few galaxy clusters. We observed NGC?6086 with laser guide star adaptive optics and the integral-field spectrograph (IFS) OSIRIS at the W. M. Keck Observatory and with the seeing-limited IFS GMOS-N at Gemini Observatory North. We combined the IFS data sets with existing major-axis kinematics and used axisymmetric stellar orbit models to determine M ? and the R-band stellar mass-to-light ratio, M /LR . We find M ? = 3.6+1.7 ?1.1 ? 109 M ? and M /L R = 4.6+0.3 ?0.7 M ? L ? ?1 (68% confidence) from models using the most massive dark matter halo allowed within the gravitational potential of the host cluster. Models fitting only IFS data confirm M ? ~ 3 ? 109 M ? and M /LR ~ 4 M ? L ? ?1, with weak dependence on the assumed dark matter halo structure. When data out to 19 kpc are included, the unrealistic omission of dark matter causes the best-fit black hole mass to decrease dramatically, to 0.6 ? 109 M ?, and the best-fit stellar mass-to-light ratio to increase to 6.7 M ? L ?1 ?,R . The latter value is at further odds with stellar population studies favoring M /LR ~ 2 M ? L ?1 ?. Biases from dark matter omission could extend to dynamical models of other galaxies with stellar cores, and revised measurements of M ? could steepen the empirical scaling relationships between black holes and their host galaxies.

A 17-billion-solar-mass black hole in a group galaxy with a diffuse core

Quasars are associated with and powered by the accretion of material onto massive black holes; the detection of highly luminous quasars with redshifts greater than z = 6 suggests that black holes of up to ten billion solar masses already existed 13 billion years ago. Two possible present-day ‘dormant’ descendants of this population of ‘active’ black holes have been found in the galaxies NGC 3842 and NGC 4889 at the centres of the Leo and Coma galaxy clusters, which together form the central region of the Great Wall—the largest local structure of galaxies. The most luminous quasars, however, are not confined to such high-density regions of the early Universe; yet dormant black holes of this high mass have not yet been found outside of modern-day rich clusters. Here we report observations of the stellar velocity distribution in the galaxy NGC 1600—a relatively isolated elliptical galaxy near the centre of a galaxy group at a distance of 64 megaparsecs from Earth. We use orbit superposition models to determine that the black hole at the centre of NGC 1600 has a mass of 17 billion solar masses. The spatial distribution of stars near the centre of NGC 1600 is rather diffuse. We find that the region of depleted stellar density in the cores of massive elliptical galaxies extends over the same radius as the gravitational sphere of influence of the central black holes, and interpret this as the dynamical imprint of the black holes.

The VAST Survey - II. Orbital motion monitoring of A-type star multiples

As a part of our ongoing Volume-limited A-Star (VAST) adaptive optics survey, we have obtained observations of 26 binary systems with projected separations <100 AU, 13 of which have sufficient historical measurements to allow for refinem ent of their orbital elements. For each system with an estimated orbit, the dynamical system mass obtained was compared with the system mass estimated from mass-magnitude relations. Discrepancies between the dynamical and theoretical system mass can be explained by the presence of a previously unresolved spectroscopic component, or by a non-solar metallicity of the system. Using this approach to infer the presence of additional companions, a lower limit to the fraction of binaries, triples, and quadruples can be estimated as 39, 46, and 15 per cent, for systems with at least one companion within 100 AU. The fraction of multiple systems with three or more components shows a relative increase compared to the fraction for Solar-type primaries resolved in previous volume-limited surveys. The observations have also revealed a pair of potentially young (<100 Myr) M-dwarf companions, which would make an ideal benchmark for the theoretical models during the pre-Main Sequence contraction phase for M-dwarfs. In addition to those systems with orbit fits, we report 13 systems for which furthe r orbital monitoring observations are required, 11 of which are newly resolved as a part of the VAST survey.

The Volume-limited A-Star (VAST) survey - II. Orbital motion monitoring of A-type star multiples

As a part of our ongoing Volume-limited A-Star (VAST) adaptive optics survey, we have obtained observations of 26 binary systems with projected separations <100 AU, 13 of which have sufficient historical measurements to allow for refinem ent of their orbital elements. For each system with an estimated orbit, the dynamical system mass obtained was compared with the system mass estimated from mass-magnitude relations. Discrepancies between the dynamical and theoretical system mass can be explained by the presence of a previously unresolved spectroscopic component, or by a non-solar metallicity of the system. Using this approach to infer the presence of additional companions, a lower limit to the fraction of binaries, triples, and quadruples can be estimated as 39, 46, and 15 per cent, for systems with at least one companion within 100 AU. The fraction of multiple systems with three or more components shows a relative increase compared to the fraction for Solar-type primaries resolved in previous volume-limited surveys. The observations have also revealed a pair of potentially young (<100 Myr) M-dwarf companions, which would make an ideal benchmark for the theoretical models during the pre-Main Sequence contraction phase for M-dwarfs. In addition to those systems with orbit fits, we report 13 systems for which furthe r orbital monitoring observations are required, 11 of which are newly resolved as a part of the VAST survey.

THE EFFECT OF SPATIAL GRADIENTS IN STELLAR MASS-TO-LIGHT RATIO ON BLACK HOLE MASS MEASUREMENTS

We have tested the effect of spatial gradients in stellar mass-to-light ratio () on measurements of black hole masses (M •) derived from stellar orbit superposition models. Such models construct a static gravitational potential for a galaxy and its central black hole, but typically assume spatially uniform . We have modeled three giant elliptical galaxies with gradients α ≡ dlog ()/dlog (r) from –0.2 to +0.1. Color and line strength gradients suggest mildly negative α in these galaxies. Introducing a negative (positive) gradient in increases (decreases) the enclosed stellar mass near the center of the galaxy and leads to systematically smaller (larger) M • measurements. For models with α = –0.2, the best-fit values of M • are 28%, 27%, and 17% lower than the constant- case, in NGC 3842, NGC 6086, and NGC 7768, respectively. For α = +0.1, M • are 14%, 22%, and 17% higher than the constant- case for the three respective galaxies. For NGC 3842 and NGC 6086, this bias is comparable to the statistical errors from individual modeling trials. At larger radii, negative (positive) gradients in cause the total stellar mass to decrease (increase) and the dark matter fraction within one effective radius to increase (decrease).

Black Hole Safari: Tracking Populations and Hunting Big Game

Understanding the physical connection, or lack thereof, between the growth of galaxies and supermassive black holes is a key challenge in extragalactic astronomy. Dynamical studies of nearby galaxies are building a census of black hole masses across a broad range of galaxy types and uncovering statistical correlations between galaxy bulge properties and black hole masses. These local correlations provide a baseline for studying galaxies and black holes at higher redshifts. Recent measurements have probed the extremes of the supermassive black hole population and introduced surprises that challenge simple models of black hole and galaxy co-evolution. Future advances in the quality and quantity of dynamical black hole mass measurements will shed light upon the growth of massive galaxies and black holes in different cosmic environments.