Roeland P. van der Marel

A new method for the identification of non-Gaussian line profiles in elliptical galaxies

A new parameterization for the line profiles of elliptical galaxies, the Gauss-Hermite series, is proposed. This approach expands the line profile as a sum of orthogonal functions which minimizes the correlations between the errors in the parameters of the fit. This method also make use of the fact that Gaussians provide good low-order fits to observed line profiles. The method yields measurements of the line strength, mean radial velocity, and the velocity dispersion as well as two extra parameters, h3 and h4, that measure asymmetric and symmetric deviations of the line profiles from a Gaussian, respectively. The new method was used to derive profiles for three elliptical galaxies which all have asymmetric line profiles on the major axis with symmetric deviations from a Gaussian. Results confirm that elliptical galaxies have complex structures due to their complex formation history.

ARE THE MAGELLANIC CLOUDS ON THEIR FIRST PASSAGE ABOUT THE MILKY WAY

Recent proper-motion measurements of the Large and Small Magellanic Clouds (LMC and SMC, respectively) by Kallivayalil and coworkers suggest that the 3D velocities of the Clouds are substantially higher (~100 km s-1) than previously estimated and now approach the escape velocity of the Milky Way (MW). Previous studies have also assumed that the Milky Way can be adequately modeled as an isothermal sphere to large distances. Here we reexamine the orbital history of the Clouds using the new velocities and a ΛCDM-motivated MW model with virial mass Mvir = 1012 M☉ (e.g., Klypin and coworkers). We conclude that the LMC and SMC are either currently on their first passage about the MW or, if the MW can be accurately modeled by an isothermal sphere to distances 200 kpc (i.e., Mvir > 2 × 1012 M☉), that their orbital period and apogalacticon distance must be a factor of 2 larger than previously estimated, increasing to 3 Gyr and 200 kpc, respectively. A first passage scenario is consistent with the fact that the LMC and SMC appear to be outliers when compared to other satellite galaxies of the MW: they are irregular in appearance and are moving faster. We discuss the implications of this orbital analysis for our understanding of the star formation history, the nature of the warp in the MW disk and the origin of the Magellanic Stream (MS), a band of H I gas trailing the LMC and SMC that extends ~100° across the sky. Specifically, as a consequence of the new orbital history of the Clouds, the origin of the MS may not be explainable by current tidal and ram pressure stripping models.

The proper motion of the large magellanic cloud using HST

The authors present a measurement of the systemic proper motion of the Large Magellanic Cloud (LMC) from astrometry with the High Resolution Camera (HRC) of the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). They observed LMC fields centered on 21 background QSOs that were discovered from their optical variability in the MACHO database. The QSOs are distributed homogeneously behind the central few degrees of the LMC. With 2 epochs of HRC data and a {approx} 2 year baseline they determine the proper motion of the LMC to better than 5% accuracy: {mu}{sub W} = -2.03 {+-} 0.08 mas yr{sup -1}, {mu}{sub N} = 0.44 {+-} 0.05 mas yr{sup -1}. This is the most accurate proper motion measurement for any Milky Way satellite thus far. When combined with HI data from the Magellanic Stream this should provide new constraints on both the mass distribution of the Galactic Halo and models of the Stream.

Is the SMC bound to the LMC? The Hubble space telescope proper motion of the SMC

We present a measurement of the systemic proper motion of the Small Magellanic Cloud (SMC) made using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). We tracked the SMCs motion relative to four background QSOs over a baseline of approximately 2 yr. The measured proper motion is μW = -1.16 ± 0.18 mas yr-1, μN = -1.17 ± 0.18 mas yr-1. This is the best measurement yet of the SMCs proper motion. We combine this new result with our prior estimate of the proper motion of the Large Magellanic Cloud (LMC) from the same observing program to investigate the orbital evolution of both Clouds over the past 9 Gyr. The current relative velocity between the Clouds is 105 ± 42 km s-1. Our investigations of the past orbital motions of the Clouds in a simple model for the dark halo of the Milky Way imply that the Clouds could be unbound from each other. However, our data are also consistent with orbits in which the Clouds have been bound to each other for approximately a Hubble time. Smaller proper-motion errors and better understanding of the LMC and SMC masses would be required to constrain their past orbital history and their bound versus unbound nature unambiguously. The new proper-motion measurements should be sufficient to allow the construction of improved models for the origin and properties of the Magellanic Stream. In turn, this will provide new constraints on the properties of the Milky Way dark halo.

Magellanic Cloud Structure from Near-Infrared Surveys. I. The Viewing Angles of the Large Magellanic Cloud

We present a detailed study of the viewing angles of the LMC disk plane. We find that our viewing direction differs considerably from the commonly accepted values, which has important implications for the structure of the LMC. The discussion is based on an analysis of spatial variations in the apparent magnitude of features in the near-IR color-magnitude diagrams extracted from the Deep Near-Infrared Southern Sky Survey (DENIS) and Two Micron All-Sky Survey (2MASS). Sinusoidal brightness variations with a peak-to-peak amplitude of ~0.25 mag are detected as a function of position angle. The same variations are detected for asymptotic giant branch stars (using the mode of their luminosity function) and for red giant branch stars (using the tip of their luminosity function), and these variations are seen consistently in all of the near-IR photometric bands in both DENIS and 2MASS data. The observed spatial brightness variations are naturally interpreted as the result of distance variations because of one side of the LMC plane being closer to us than the opposite side. There is no evidence that any complicating effects, such as possible spatial variations in dust absorption or the age/metallicity of the stellar population, cause large-scale brightness variations in the near-IR at a level that exceeds the formal errors (~0.03 mag). The best-fitting geometric model of an inclined plane yields an inclination angle i = 347 ? 62 and line-of-nodes position angle ? = 1225 ? 83. The quoted errors are conservative estimates that take into account the possible influence of systematic errors; the formal errors are much smaller, 07 and 16, respectively. There is tentative evidence for variations of ~10? in the viewing angles with distance from the LMC center, suggesting that the LMC disk plane may be warped. Traditional methods to estimate the position angle of the line of nodes have used either the major-axis position angle ?maj of the spatial distribution of tracers on the sky or the position angle ?max of the line of maximum gradient in the velocity field, given that for a circular disk ?maj = ?max = ?. The present study does not rely on the assumption of circular symmetry and is considerably more accurate than previous studies of its kind. We find that the actual position angle of the line of nodes differs considerably from both ?maj and ?max, for which measurements have fallen in the range 140??190?. This indicates that the intrinsic shape of the LMC disk is not circular but elliptical. Paper II of this series explores the implications of this result through a detailed study of the shape and structure of the LMC. The inclination angle inferred here is consistent with previous estimates, but this is to some extent a coincidence, given that also for the inclination angle most previous estimates were based on the incorrect assumption of circular symmetry.

Improved Evidence for a Black Hole in M32 from HST/FOS Spectra. II. Axisymmetric Dynamical Models

Axisymmetric dynamical models are constructed for the E3 galaxy M32 to interpret high spatial resolution stellar kinematical data obtained with the Hubble Space Telescope (HST). Models are studied with two-integral, f(E, Lz), phase-space distribution functions and with fully general three-integral distribution functions. The latter are built using an extension of Schwarzschilds approach: individual orbits in the axisymmetric potential are calculated numerically, and populated using nonnegative least-squares fitting so as to reproduce all available kinematical data, including line-of-sight velocity profile shapes. The details of this method are described in companion papers by Rix et al. and Cretton et al. Models are constructed for inclinations i = 90° (edge on) and i = 55°. No model without a nuclear dark object can fit the combined ground-based and HST data, independent of the dynamical structure of M32. Models with a nuclear dark object of mass M• = 3.4 × 106 M☉ (with 1 σ and 3 σ error bars of 0.7 × 106 M☉ and 1.6 × 106 M☉, respectively) do provide an excellent fit. The inclined models provide the best fit, but the inferred M• does not depend sensitively on the assumed inclination. The models that best fit the data are not two-integral models, but like two-integral models they are azimuthally anisotropic. Two-integral models therefore provide useful low-order approximations to the dynamical structure of M32. We use them to show that an extended dark object can fit the data only if its half-mass radius is rh 008 (=0.26 pc), implying a central dark matter density exceeding 1 × 108 M☉ pc-3. The inferred M• is consistent with that suggested previously by ground-based kinematical data. However, radially anisotropic axisymmetric constant mass-to-light ratio models are now ruled out for the first time, and the limit on the dark matter density implied by the HST data is now stringent enough to rule out most plausible alternatives to a massive black hole. The dynamically inferred M• is identical to that suggested by existing models for HST photometry of M32 that assume adiabatic growth (over a timescale exceeding 106 yr) of a black hole into a preexisting core. The low activity of the nucleus of M32 implies either that only a very small fraction of the gas that is shed by evolving stars is accreted onto the black hole or, alternatively, that accretion proceeds at very low efficiency, e.g., in an advection-dominated mode.

A Hubble Space Telescope Census of Nuclear Star Clusters in Late-Type Spiral Galaxies. II. Cluster Sizes and Structural Parameter Correlations

We investigate the structural properties of nuclear star clusters in late-type spiral galaxies. More specifically, we fit analytical models to Hubble Space Telescope images of 39 nuclear clusters in order to determine their effective radii after correction for the instrumental point-spread function. We use the results of this analysis to compare the luminosities and sizes of nuclear star clusters to those of other ellipsoidal stellar systems, in particular the Milky Way globular clusters. Our nuclear clusters have a median effective radius of e = 3.5 pc, with 50% of the sample falling in the range 2.4 pc ≤ re ≤ 5.0 pc. This narrow size distribution is statistically indistinguishable from that of Galactic globular clusters, even though the nuclear clusters are, on average, 4 mag brighter than the old globular clusters. We discuss some possible interpretations of this result. From a comparison of nuclear cluster luminosities with various properties of their host galaxies, we confirm that more luminous galaxies harbor more luminous nuclear clusters. It remains unclear whether this correlation mainly reflects the influence of galaxy size, mass, and/or star formation rate. Since the brighter galaxies in our sample typically have stellar disks with a higher central surface brightness, nuclear cluster luminosity also correlates with this property of their hosts. On the other hand, we find no evidence for a correlation between the presence of a nuclear star cluster and the presence of a large-scale stellar bar.

NEW LIMITS ON AN INTERMEDIATE-MASS BLACK HOLE IN OMEGA CENTAURI. II. DYNAMICAL MODELS*

We present a detailed dynamical analysis of the projected density and kinematical data available for the globular cluster ω Centauri. We solve the spherical anisotropic Jeans equation for a given density profile to predict the projected profiles of the rms velocity , in each of the three orthogonal coordinate directions (line of sight, proper motion radial, and proper motion tangential). The models allow for the presence of a central dark mass, such as a possible intermediate-mass black hole (IMBH). We fit the models to new Hubble Space Telescope star count and proper motion data near the cluster center presented in the companion paper, which is Paper I in this series, combined with existing ground-based measurements at larger radii. The projected density profile is consistent with being flat near the center, with an upper limit γ 0.07 on the central logarithmic slope. The rms proper motion profile is also consistent with being flat near the center. The velocity anisotropy profile, distance, and stellar mass-to-light ratio are all tightly constrained by the data and found to be in good agreement with previous determinations by van de Ven et al. To fit the kinematics, we consider anisotropic models with either a flat core (γ = 0) or a shallow cusp (γ = 0.05). Core models provide a good fit to the data with M BH = 0; cusp models require a dark mass. If the dark mass in cusp models is an IMBH, then M BH = (8.7 ± 2.9) × 103 M ☉; if it is a dark cluster, then its extent must be 0.16 pc. Isotropic models do not fit the observed proper motion anisotropy and yield spuriously high values for any central dark mass. These models do provide a good fit to the Gauss-Hermite moments of the observed proper motion distributions (h 4 = –0.023 ± 0.004, h 6 = 0.001 ± 0.004). There are no unusually fast-moving stars observed in the wings of the proper motion distribution, but we show that this does not strongly constrain the mass of any possible IMBH. The overall end result of the modeling is an upper limit to the mass of any possible IMBH in ω Centauri: M BH 1.2 × 104 M ☉ at ~1σ confidence (or 1.8 × 104 M ☉ at ~3σ confidence). The 1σ limit corresponds to M BH/M tot 0.43%. We combine this with results for other clusters and discuss the implications for globular cluster IMBH demographics. Tighter limits will be needed to rule out or establish whether globular clusters follow the same black hole demographics correlations as galaxies. The arguments put forward by Noyola et al. to suspect an IMBH in ω Centauri are not confirmed by our study; the value of M BH they suggested is firmly ruled out.

Hubble Space Telescope STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble Type*

We study the nuclear star clusters (NCs) in spiral galaxies of various Hubble types using spectra obtained with the STIS on board the Hubble Space Telescope (HST). We observed the nuclear clusters in 40 galaxies, selected from two previous HST WFPC2 imaging surveys. At a spatial resolution of ~02 the spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra. Approximately half of the spectra have a sufficiently high signal-to-noise ratio for detailed stellar population analysis. For the other half we only measure the continuum slope, as quantified by the B - V color. To infer the star formation history, metallicity, and dust extinction, we fit weighted superpositions of single-age stellar population templates to the high signal-to-noise ratio spectra. We use the results to determine the luminosity-weighted age, mass-to-light ratio, and masses of the clusters. Approximately half of the sample clusters contain a population younger than 1 Gyr. The luminosity-weighted ages range from 10 Myr to 10 Gyr. The stellar populations of NCs are generally best fit as a mixture of populations of different ages. This indicates that NCs did not form in a single event, but that instead they had additional star formation long after the oldest stars formed. On average, the sample clusters in late-type spirals have a younger luminosity-weighted mean age than those in early-type spirals ( = 8.37 ± 0.25 vs. 9.23 ± 0.21). The average mass-weighted ages are older by ~0.7 dex, indicating that there often is an underlying older population that does not contribute much light but does contain most of the mass. The average cluster masses are smaller in late-type spirals than in early-type spirals ( = 6.25 ± 0.21 vs. 7.63 ± 0.24) and exceed the masses typical of globular clusters. The cluster mass correlates loosely with total galaxy luminosity. It correlates more strongly with both the Hubble type of the host galaxy and the luminosity of its bulge. The latter correlation has the same slope as the well-known correlation between supermassive black hole mass and bulge luminosity. The properties of both nuclear clusters and black holes in the centers of spiral galaxies are therefore intimately connected to the properties of the host galaxy, and in particular its bulge component. Plausible formation scenarios have to account for this. We discuss various possible selection biases in our results, but conclude that none of them can explain the differences seen between clusters in early- and late-type spirals. The inability to infer spectroscopically the populations of faint clusters does introduce a bias toward younger ages, but not necessarily toward higher masses.

THE M31 VELOCITY VECTOR. II. RADIAL ORBIT TOWARD THE MILKY WAY AND IMPLIED LOCAL GROUP MASS

We determine the velocity vector of M31 with respect to the Milky Way and use this to constrain the mass of the Local Group, based on Hubble Space Telescope proper-motion measurements of three fields presented in Paper I. We construct N-body models for M31 to correct the measurements for the contributions from stellar motions internal to M31. This yields an unbiased estimate for the M31 center-of-mass motion. We also estimate the center-of-mass motion independently, using the kinematics of satellite galaxies of M31 and the Local Group, following previous work but with an expanded satellite sample. All estimates are mutually consistent, and imply a weighted average M31 heliocentric transverse velocity of (vW , vN ) = (– 125.2 ± 30.8, –73.8 ± 28.4) km s–1. We correct for the reflex motion of the Sun using the most recent insights into the solar motion within the Milky Way, which imply a larger azimuthal velocity than previously believed. This implies a radial velocity of M31 with respect to the Milky Way of V rad, M31 = –109.3 ± 4.4 km s–1, and a tangential velocity of V tan, M31 = 17.0 km s–1, with a 1σ confidence region of V tan, M31 ≤ 34.3 km s–1. Hence, the velocity vector of M31 is statistically consistent with a radial (head-on collision) orbit toward the Milky Way. We revise prior estimates for the Local Group timing mass, including corrections for cosmic bias and scatter, and obtain M LG ≡ M MW, vir + M M31, vir = (4.93 ± 1.63) × 1012 M ☉. Summing known estimates for the individual masses of M31 and the Milky Way obtained from other dynamical methods yields smaller uncertainties. Bayesian combination of the different estimates demonstrates that the timing argument has too much (cosmic) scatter to help much in reducing uncertainties on the Local Group mass, but its inclusion does tend to increase other estimates by ~10%. We derive a final estimate for the Local Group mass from literature and new considerations of M LG = (3.17 ± 0.57) × 1012 M ☉. The velocity and mass results at 95% confidence imply that M33 is bound to M31, consistent with expectation from observed tidal deformations.