Michele Cappellari

Abstract:

We explore the nucleus of the nearby 109 M⊙ early-type galaxy, NGC 404, using Hubble Space Telescope (HST)/STIS spectroscopy and WFC3 imaging. We first present evidence for nuclear variability in UV, optical, and infrared filters over a time period of 15 years. This variability adds to the already substantial evidence for an accreting black hole at the center of NGC 404. We then redetermine the dynamical black hole mass in NGC 404 including modeling of the nuclear stellar populations. We combine HST/STIS spectroscopy with WFC3 images to create a local color-M/L relation derived from stellar population modeling of the STIS data. We then use this to create a mass model for the nuclear region. We use Jeans modeling to fit this mass model to adaptive optics stellar kinematic observations from Gemini/NIFS. From our stellar dynamical modeling, we find a 3σ upper limit on the black hole mass of 1.5 × 105 M⊙. Given the accretion evidence for a black hole, this upper limit makes NGC 404 the lowest mass central black hole with dynamical mass constraints. We find that the kinematics of H2 emission line gas show evidence for non-gravitational motions preventing the use of gas dynamical modeling to constrain the black hole mass. Our stellar population modeling also reveals that the central, counter-rotating region of the nuclear cluster is dominated by ∼1 Gyr old populations.

Abstract:

We study trends in the slope of the total mass profiles and dark matter fractions within the central half-light radius of 258 early-type galaxies, using data from the volume-limited ATLAS3D survey. We use three distinct sets of dynamical models, which vary in their assumptions and also allow for spatial variations in the stellar mass-to-light ratio, to test the robustness of our results. We confirm that the slopes of the total mass profiles are approximately isothermal, and investigate how the total mass slope depends on various galactic properties. The most statistically significant correlations we find are a function of either surface density, Σe, or velocity dispersion, σe. However there is evidence for a break in the latter relation, with a nearly universal logarithmic slope above log10[σe/(km s−1)] ∼ 2.1 and a steeper trend below this value. For the 142 galaxies above that critical σe value, the total mass–density logarithmic slopes have a mean value 〈γ΄〉 = −2.193 ± 0.016 (1σ error) with an observed rms scatter of only σγ′=0.168±0.015. Considering the observational errors, we estimate an intrinsic scatter of σγ′intr≈0.15. These values are broadly consistent with those found by strong lensing studies at similar radii and agree, within the tight errors, with values recently found at much larger radii via stellar dynamics or H I rotation curves (using significantly smaller samples than this work).

Abstract:

To ascertain whether photometric decompositions of galaxies into bulges and discs are astrophysically meaningful, we have developed a new technique to decompose spectral data cubes into separate bulge and disc components, subject only to the constraint that they reproduce the conventional photometric decomposition. These decompositions allow us to study the kinematic and stellar population properties of the individual components and how they vary with position, in order to assess their plausibility as discrete elements, and to start to reconstruct their distinct formation histories. An initial application of this method to Calar Alto Integral Field Area integral field unit observations of three isolated S0 galaxies confirms that in regions where both bulge and disc contribute significantly to the flux, they can be physically and robustly decomposed into a rotating dispersion-dominated bulge component and a rotating low-dispersion disc component. Analysis of the resulting stellar populations shows that the bulges of these galaxies have a range of ages relative to their discs, indicating that a variety of processes are necessary to describe their evolution. This simple test case indicates the broad potential for extracting from spectral data cubes the full spectral data of a wide variety of individual galaxy components, and for using such decompositions to understand the interplay between these various structures, and hence how such systems formed.