Matthew G. Walker

Velocity dispersion profiles of seven dwarf spheroidal galaxies

We present stellar velocity dispersion profiles for seven Milky Way dwarf spheroidal (dSph) satellite galaxies. We have measured 8394 line-of-sight velocities (±2.5 km s-1) for 6804 stars from high-resolution spectra obtained at the Magellan and MMT telescopes. We combine these new data with previously published velocities to obtain the largest available kinematic samples, which include more than 5500 dSph members. All the measured dSphs have stellar velocity dispersion of order 10 km s-1 that remains approximately constant with distance from the dSph center, out to and in some cases beyond the radius at which the mean surface brightness falls to the background level. Assuming dSphs reside within dark matter halos characterized by the NFW density profile, we obtain reasonable fits to the empirical velocity dispersion profiles. These fits imply that, among the seven dSphs, Mvir ~ 108-109 M☉. The mass enclosed at a radius of 600 pc, the region common to all data sets, lies in the range (2-7) × 107 M☉.

Internal Kinematics of the Fornax Dwarf Spheroidal Galaxy

We present new radial velocity results for 176 stars in the Fornax dwarf spheroidal galaxy, of which at least 156 are probable Fornax members. We combine with previously published data to obtain a radial velocity sample with 206 stars, of which at least 176 are probable Fornax members. We detect the hint of rotation about an axis near Fornaxs morphological minor axis, although the significance of the rotation signal in the galactic rest frame is sensitive to the adopted value of Fornaxs proper motion. Regardless, the observed stellar kinematics is dominated by random motions, and we do not find kinematic evidence of tidal disruption. The projected velocity dispersion profile of the binned data set remains flat over the sampled region, which reaches a maximum angular radius of 65. Single-component King models in which mass follows light fail to reproduce the observed flatness of the velocity dispersion profile. Two-component (luminous plus dark matter) models can reproduce the data, provided that the dark component extends sufficiently beyond the luminous component and the central dark matter density is of the same order as the central luminous density. These requirements suggest a more massive, darker Fornax than standard core-fitting analyses have previously concluded, with M/LV over the sampled region reaching 10-40 times the M/LV of the luminous component. We also apply a nonparametric mass estimation technique, introduced in a companion paper. Although it is designed to operate on data sets containing velocities for >1000 stars, the estimation yields preliminary results suggesting M/LV ~ 15 inside r < 1.5 kpc.

A reverberation-based mass for the central black hole in NGC 4151

We have undertaken a new ground-based monitoring campaign to improve the estimates of the mass of the central black hole in NGC 4151. We measure the lag time of the broad H? line response compared to the optical continuum at 5100 ? and find a lag of 6.6 days. We combine our data with the recent reanalysis of UV emission lines by Metzroth and coworkers to calculate a weighted mean of the black hole mass, MBH = (4.57) ? 107 M?. The absolute calibration of the black hole mass is based on normalization of the AGN black hole mass-stellar velocity dispersion (MBH-?*) relationship to that of quiescent galaxies by Onken and coworkers. The scatter in the MBH-?* relationship suggests that reverberation-mapping-based mass measurements are typically uncertain by a factor of 3-4.

The Velocity Dispersion Profile of the Remote Dwarf Spheroidal Galaxy Leo I: A Tidal Hit and Run?

We present new kinematic results for 387 stars near the Milky Way satellite dwarf spheroidal galaxy Leo I. Spectra were obtained with the Hectochelle multiobject echelle spectrograph on the MMT, centered in the optical near 5200 A. From 297 repeat measurements of 108 stars, we estimate the mean velocity error (1 σ) of our sample to be 2.4 km s−1, with a systematic precision of ≤1 km s−1. The final sample of 328 Leo I members gives a mean heliocentric velocity of -->282.9 ± 0.5 km s−1 and a dispersion of -->9.2 ± 0.4 km s−1. The dispersion profile of Leo I is flat to beyond its classical tidal radius. We fit the profile to various equilibrium dynamical models. We strongly rule out all models where mass follows light. Anisotropic Sersic+NFW models fit the dispersion profile well, but isotropic models are ruled out at a 95% confidence level. Inside a projected radius of ~1040 pc, the mass and V-band mass-to-light ratio of Leo I from equilibrium models are in the ranges -->(5–7) × 107 M☉ and 9-14 (solar units), respectively. Leo I members outside a break radius of -->Rb ~ 400 (500 pc) exhibit significant velocity anisotropy, whereas stars interior to this radius are consistent with an isotropic velocity distribution. We interpret the break radius as the tidal radius of Leo I at perigalacticon some 1-2 Gyr ago. This interpretation accounts for the complex star formation history of Leo I, population segregation within the galaxy, and Leo Is large outward galactocentric velocity. The lack of evident tidal arms in Leo I suggests that the galaxy may have been injected into its present highly elliptical orbit by a third body a few Gyr before its last perigalacticon. This scenario is plausible within current hierarchical structure formation models.

STELLAR VELOCITIES IN THE CARINA, FORNAX, SCULPTOR, AND SEXTANS dSph GALAXIES: DATA FROM THE MAGELLAN/MMFS SURVEY ∗

We present spectroscopic data for individual stars observed from 2004 March through 2008 August as part of our Michigan/MIKE Fiber System (MMFS) survey of four dwarf spheroidal (dSph) galaxies: Carina, Fornax, Sculptor, and Sextans. Using MMFS at the Magellan/Clay Telescope at Las Campanas Observatory, we have acquired 8855 spectra from 7103 red-giant candidates in these Galactic satellites. We list measurements of each stars line-of-sight velocity (median error ±2.1 km s–1) and spectral line indices for iron and magnesium absorption features. We use globular cluster spectra to calibrate the indices onto standard [Fe/H] metallicity scales, but comparison of the resulting metallicities with published values suggests that the MMFS indices are best used as indicators of relative, not absolute, metallicity. The empirical distributions of velocity and spectral indices also allow us to quantify the amount of contamination by foreground stars. In a companion paper, we develop an algorithm that evaluates the membership probability for each star, showing that the present MMFS sample contains more than 5000 dSph members, including 774 Carina members, 2483 Fornax members, 1365 Sculptor members, and 441 Sextans members.

The Mass of the Black Hole in the Seyfert 1 Galaxy NGC 4593 from Reverberation Mapping

We present new observations leading to an improved black hole mass estimate for the Seyfert 1 galaxy NGC 4593 as part of a reverberation-mapping campaign conducted at the MDM Observatory. Cross-correlation analysis of the Hβ emission-line light curve with the optical continuum light curve reveals an emission-line time delay of τcent = 3.73 ± 0.75 days. By combining this time delay with the Hβ line width, we derive a central black hole mass of MBH = (9.8 ± 2.1) × 106 M☉, an improvement in precision of a factor of several over past results.

NGC 5548 in a Low-Luminosity State: Implications for the Broad-Line Region

We describe results from a new ground-based monitoring campaign on NGC 5548, the best-studied reverberation-mapped AGN. We find that it was in the lowest luminosity state yet recorded during a monitoring program, namely L5100 = 4.7 × 1042 ergs s-1. We determine a rest-frame time lag between flux variations in the continuum and the Hβ line of 6.3 days. Combining our measurements with those of previous campaigns, we determine a weighted black hole mass of MBH = 6.54 × 107 M☉ based on all broad emission lines with suitable variability data. We confirm the previously discovered virial relationship between the time lag of emission lines relative to the continuum and the width of the emission lines in NGC 5548, which is the expected signature of a gravity-dominated broad-line region. Using this lowest luminosity state, we extend the range of the relationship between the luminosity and the time lag in NGC 5548 and measure a slope that is consistent with α = 0.5, the naive expectation for the broad-line region for an assumed form of r ∝ Lα. This value is also consistent with the slope recently determined by Bentz et al. for the population of reverberation-mapped AGNs as a whole.

CLEAN KINEMATIC SAMPLES IN DWARF SPHEROIDALS: AN ALGORITHM FOR EVALUATING MEMBERSHIP AND ESTIMATING DISTRIBUTION PARAMETERS WHEN CONTAMINATION IS PRESENT

We develop an algorithm for estimating parameters of a distribution sampled with contamination. We employ a statistical technique known as expectation maximization (EM). Given models for both member and contaminant populations, the EM algorithm iteratively evaluates the membership probability of each discrete data point, then uses those probabilities to update parameter estimates for member and contaminant distributions. The EM approach has wide applicability to the analysis of astronomical data. Here we tailor an EM algorithm to operate on spectroscopic samples obtained with the Michigan-MIKE Fiber System (MMFS) as part of our Magellan survey of stellar radial velocities in nearby dwarf spheroidal (dSph) galaxies. These samples, to be presented in a companion paper, contain discrete measurements of line-of-sight velocity, projected position, and pseudo-equivalent width of the Mg-triplet feature, for ~1000-2500 stars per dSph, including some fraction of contamination by foreground Milky Way stars. The EM algorithm uses all of the available data to quantify dSph and contaminant distributions. For distributions (e.g., velocity and Mg-index of dSph stars) assumed to be Gaussian, the EM algorithm returns maximum-likelihood estimates of the mean and variance, as well as the probability that each star is a dSph member. These probabilities can serve as weights in subsequent analyses. Applied to our MMFS data, the EM algorithm identifies more than 5000 stars as probable dSph members. We test the performance of the EM algorithm on simulated data sets that represent a range of sample size, level of contamination, and amount of overlap between dSph and contaminant velocity distributions. The simulations establish that for samples ranging from large (N ~ 3000, characteristic of the MMFS samples) to small (N ~ 30), resembling new samples for extremely faint dSphs), the EM algorithm distinguishes members from contaminants and returns accurate parameter estimates much more reliably than conventional methods of contaminant removal (e.g., sigma clipping).

Systemic proper motions of milky way satellites from stellar redshifts: The carina, fornax, sculptor, and sextans dwarf spheroidals

The transverse motions of nearby dwarf spheroidal (dSph) galaxies contribute line-of-sight components that increase with angular distance from the dSph centers, inducing detectable gradients in stellar redshift. In the absence of an intrinsic velocity gradient (e.g., due to rotation or streaming), an observed gradient in the heliocentric rest frame (HRF) relates simply to a dSphs systemic proper motion (PM). Kinematic samples for the Milky Ways brightest dSph satellites are now sufficiently large that we can use stellar redshifts to constrain systemic PMs independently of astrometric data. Data from our Michigan/MIKE Fiber System (MMFS) Survey reveal significant HRF velocity gradients in Carina, Fornax, and Sculptor, and no significant gradient in Sextans. Assuming there are no intrinsic gradients, the data provide a relatively tight constraint on the PM of Fornax, (μαHRF, μδHRF) = (+48 ± 15, –25 ± 14) mas century−1, that agrees with published HST astrometric measurements. Smaller data sets yield weaker constraints in the remaining galaxies, but our Carina measurement, (μαHRF, μδHRF) = (+25 ± 36, +16 ± 43) mas century−1, agrees with the published astrometric value. The disagreement of our Sculptor measurement, (μHRFα, μδHRF) = (–40 ± 29, –69 ± 47) mas century−1, with astrometric measurements is expected if Sculptor has a rotational component as reported by Battaglia and coworkers. For Sextans, which at present lacks an astrometric measurement, we measure (μαHRF, μδHRF) = (–26 ± 41, +10 ± 44) mas century−1.

On Kinematic Substructure in the Sextans Dwarf Spheroidal Galaxy

We present multifiber echelle radial velocity results for 551 stars in the Sextans dwarf spheroidal galaxy and identify 294 stars as probable Sextans members. The projected velocity dispersion profile of the binned data remains flat to a maximum angular radius of 30. We introduce a nonparametric technique for estimating the projected velocity dispersion surface and use this to search for kinematic substructure. Our data do not confirm previous reports of a kinematically distinct stellar population at the Sextans center. Instead we detect a region near the Sextans core radius that is kinematically colder than the overall Sextans sample with 95% confidence. Subject headings: galaxies: dwarf — galaxies: individual (Sextans) — galaxies: kinematics and dynamics — Local Group