Joris Gerssen

The shape of the velocity ellipsoid in ngc 488

Theories of stellar orbit diffusion in disc galaxies predict different rates of increase of the velocity dispersions parallel and perpendicular to the disc plane, and it is therefore of interest to measure the different velocity dispersion components in galactic discs of different types. We show that it is possible to extract the three components of the velocity ellipsoid in an intermediate-inclination disc galaxy from measured line-of-sight velocity dispersions on the major and minor axes. On applying the method to observations of the Sb galaxy NGC 488, we find evidence for a higher ratio of vertical-to-radial dispersion in NGC 488 than in the solar neighbourhood of the Milky Way (the only other place where this quantity has ever been measured). The difference is qualitatively consistent with the notion that spiral structure has been relatively less important in the dynamical evolution of the disc of NGC 488 than molecular clouds.

p3d: a general data-reduction tool for fiber-fed integral-field spectrographs

The reduction of integral-field spectrograph (IFS) data is demanding work. Many repetitive operations are required to convert raw data into, typically, a large number of spectra. This effort can be markedly simplified through the use of a tool or pipeline, which is designed to complete many of the repetitive operations without human interaction. Here we present our semi-automatic data-reduction tool p3d, which is designed to be used with fiber-fed IFSs. Important components of p3d include a novel algorithm for automatic finding and tracing of spectra on the detector and two methods of optimal spectrum extraction in addition to standard aperture extraction. p3d also provides tools to combine several images, perform wavelength calibration and flat field data. p3d is at the moment configured for four IFSs. To evaluate its performance, we tested the different components of the tool. For these tests we used both simulated and observational data. We demonstrate that a correction for so-called cross-talk due to overlapping spectra on the detector is required for three of the IFSs. Without such a correction, spectra will be inaccurate, in particular if there is a significant intensity gradient across the object. Our tests showed that p3d is able to produce accurate results. p3d is a highly general and freely available tool. It is easily extended to include improved algorithms, new visualization tools, and support for additional instruments. The program code can be downloaded from the p3d-project web site http://p3d.sourceforge.net.

Addendum: Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15. II. Kinematic Analysis and Dynamical Modeling [Astron. J. 124, 3270 (2002)]

It has been reported that there is an error in the figure in Dull et al. (1997, D97) that shows the radial M/L profile in Fokker-Planck models of M15. We discuss how this modifies the interpretation of our kinematical data. These imply the existence of a dark and compact mass component near the center of M15, either a single black hole (BH) or a collection of dark remnants that have sunk to the cluster center due to mass segregation. We previously showed that the latter interpretation is in conflict with the D97 M/L profile, which supported the BH interpretation. We repeat our analysis here with the corrected D97 profile. Models without a BH are now found to be statistically acceptable (within 1-sigma), although inclusion of a BH still provides a marginally better fit. It does not necessarily follow that dark remnants are now the preferred interpretation of the data. The D97 models, as well as N-body models by Baumgardt et al. (2002), assume that all neutron stars are retained during cluster evolution. This conflicts with predictions of the neutron-star retention rate (typically below 10 per cent) based on pulsar kick velocities. The presence of a single BH therefore continues to be a viable interpretation of the data. The best fit BH mass with the corrected D97 M/L profile is M_BH = 1.7^{+2.7}_{-1.7} x 10^3 solar masses, and with a constant M/L it is M_BH = 3.2^{+2.2}_{-2.2} x 10^3 solar masses. A model that includes both neutron star escape and mass segregation would probably yield a value between these numbers. This agrees with the correlation between velocity dispersion and BH mass inferred for galaxies. However, with the presently available models and data it is neither uniquely implied nor ruled out that M15 has an intermediate-mass BH.

Is There Really a Black Hole at the Center of NGC 4041? Constraints from Gas Kinematics

We present Space Telescope Imaging Spectrograph spectra of the Sbc spiral galaxy NGC 4041, which were used to map the velocity field of the gas in its nuclear region. We detect the presence of a compact (r 04 40 pc), high surface brightness, rotating nuclear disk cospatial with a nuclear star cluster. The disk is characterized by a rotation curve with a peak-to-peak amplitude of ~40 km s-1 and is systematically blueshifted by ~10-20 km s-1 with respect to the galaxy systemic velocity. With the standard assumption of constant mass-to-light ratio and with the nuclear disk inclination taken from the outer disk, we find that a dark point mass of (1) × 107 M☉ is needed to reproduce the observed rotation curve. However, the observed blueshift suggests the possibility that the nuclear disk could be dynamically decoupled. Following this line of reasoning, we relax the standard assumptions and find that the kinematical data can be accounted for by the stellar mass provided that either the central mass-to-light ratio is increased by a factor of ~2 or the inclination is allowed to vary. This model results in a 3 σ upper limit of 6 × 106 M☉ on the mass of any nuclear black hole (BH). Overall, our analysis only allows us to set an upper limit of 2 × 107 M☉ on the mass of the nuclear BH. If this upper limit is taken in conjunction with an estimated bulge B magnitude of -17.7 and with a central stellar velocity dispersion of 95 km s-1, then these results are not inconsistent with both the MBH-Lsph and the MBH-σ* correlations. Constraints on BH masses in spiral galaxies of types as late as Sbc are still very scarce; therefore, the present result adds an important new data point to our understanding of BH demography.

Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15. I. STIS Spectroscopy and WFPC2 Photometry*

In this series of two papers, we describe a project with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) to measure the line-of-sight velocities of stars in the central few arcseconds of the dense globular cluster M15. The main goal of this project is to search for the possible presence of an intermediate-mass central black hole. This first paper focuses on the observations and reduction of the data. We scanned the central region of M15 spectroscopically by consecutively placing the 01 HST/STIS slit at 18 adjacent positions. The spectral pixel size exceeds the velocity dispersion of M15. This puts the project at the limit of what is feasible with STIS, and exceedingly careful and complicated data reduction and analysis were required. We applied corrections for the following effects: drifts in the STIS wavelength scale during an HST orbit; the orbital velocity component of HST along the line of sight to the cluster, and its variations during the HST orbit; and the apparent wavelength shift that is perceived for a star that is not centered in the slit. The latter correction is particularly complicated and requires many pieces of information: (1) the positions and magnitudes of all the stars near the center of M15; (2) accurate positionings of the STIS slits during the observations; (3) and the HST/STIS point-spread function (PSF) and line-spread function (LSF). To address the first issue we created a stellar catalog of M15 from the existing HST/WFPC2 data discussed previously by Guhathakurta et al., but with an improved astrometric and photometric calibration. The catalog is distributed electronically as part of this paper. It contains 31,983 stars with their positions and U, B, and V magnitudes. To address the second issue, we model the observed intensity profiles along the STIS slits to determine the slit positionings to 0007 accuracy in each coordinate. To address the third issue, we obtained observations of a bright field star to which we fitted multi-Gaussian PSF and LSF models. Upon reduction of the M15 spectroscopy we ultimately obtain 19,200 one-dimensional STIS spectra, each for a different aperture position in M15, with a velocity scale accurate to better than 2.5 km s-1. We develop an algorithm that co-adds the spectra for individual apertures and use it to extract spectra of individual stars with minimum blending and maximum signal-to-noise ratio. In Paper II we use these spectra to extract reliable line-of-sight velocities for 64 stars, half of which reside within R = 24 from the cluster center. These velocities constrain the central structure, dynamics, and mass distribution of the cluster.

Model-independent measurements of bar pattern speeds

The pattern speed is one of the fundamental parameters that determines the structure of barred galaxies. This quantity is usually derived from indirect methods or by employing model assumptions. The number of bar pattern speeds derived using the model-independent Tremaine & Weinberg technique is still very limited. We present the results of model-independent measurements of the bar pattern speed in four galaxies ranging in Hubble type from SB0 to SBbc. Three of the four galaxies in our sample are consistent with bars being fast rotators. The lack of slow bars is consistent with previous observations and suggests that barred galaxies do not have centrally concentrated dark matter haloes. This contradicts simulations of cosmological structure formation and observations of the central mass concentration in nonbarred galaxies.

Hubble Space Telescope observations of NGC 6240: A Case study of an ultraluminous infrared galaxy with obscured activity

We present results from a Hubble Space Telescope (HST) study of the morphology and kinematics of NGC 6240. This merging galaxy with a double nucleus is one of the nearest and best-studied ultraluminous infrared galaxies. HST resolves both nuclei into separate components. The distance between the northern and southern optical/near-infrared components is greater than that observed in radio and X-ray studies, arguing that even in K band we may not be seeing all the way through the dust to the true nuclei. The ionized gas does not display rotation around either of the nuclei, or equilibrium motion in general. There is a strong velocity gradient between the nuclei, similar to what is seen in CO data. There is no such gradient in our stellar kinematics. The velocity dispersion of the gas is larger than expected for a cold disk. We also map and model the emission-line velocity field at an off-nuclear position where a steep velocity gradient was previously detected in ground-based data. Overall, the data indicate that line-of-sight projection effects, dust absorption, nonequilibrium merger dynamics, and the possible influence of a wind may be playing an important role in the observed kinematics. Chandra observations of hard X-rays have shown that both of the nuclei contain an active galactic nucleus (AGN). The HST data show no clear sign of the two AGNs: neither continuum nor narrowband imaging shows evidence for unresolved components in the nuclei, and there are no increased emission line widths or rapid rotation near the nuclei. This underscores the importance of X-ray data for identifying AGNs in highly dust-enshrouded environments.

The central dynamics of M3, M13, and M92: stringent limits on the masses of intermediate-mass black holes

We used the PMAS integral field spectrograph to obtain large sets of radial velocities in the central regions of three northern Galactic globular clusters: M3, M13, and M92. By applying the novel technique of crowded field 3D spectroscopy, we measured radial velocities for about 80 stars within the central ~ 10 arcsec of each cluster. These are by far the largest spectroscopic datasets obtained in the innermost parts of these clusters up to now. To obtain kinematical data across the whole extent of the clusters, we complement our data with measurements available in the literature. We combine our velocity measurements with surface brightness profiles to analyse the internal dynamics of each cluster using spherical Jeans models, and investigate whether our data provide evidence for an intermediate-mass black hole in any of the clusters. The surface brightness profiles reveal that all three clusters are consistent with a core profile, although shallow cusps cannot be excluded. We find that spherical Jeans models with a constant mass-to-light ratio provide a good overall representation of the kinematical data. A massive black hole is required in none of the three clusters to explain the observed kinematics. Our 1sigma (3sigma) upper limits are 5300 M_sun (12000 M_sun) for M3, 8600 M_sun (13000 M_sun) for M13, and 980 M_sun (2700 M_sun) for M92. A puzzling circumstance is the existence of several potential high velocity stars in M3 and M13, as their presence can account for the majority of the discrepancies that we find in our mass limits compared to M92.

Nuclear Properties of a Sample of Nearby Spiral Galaxies from Hubble Space Telescope STIS Imaging

ABSTRACTWe present surface photometry for the central regions of a sample of 48 spiral galaxies (mostly unbarred andbarred of type Sbc or Sc) observed with the Space Telescope Imaging Spectrograph on board theHubble SpaceTelescope. Surface brightness profiles (SBPs) were derived and modeled with a Nuker law. We also analyzedarchival Wide Field Planetary Camera 2 images with a larger field of view, which are available for 18 galaxies inour sample. We modeled the extracted bulge SBPs with an exponential, anr 1/4 ,oranr n profile. In agreementwith previous studies, we find that bulges of Sbc galaxies fall into two categories: bulges well described by anexponential profile and those well described by anr 1/4 profile. Only one galaxy requires the use of a more generalSe´rsic profile to properly describe the bulge. Nuclear photometrically distinct components are found in 55% ofthe galaxies. For those that we classify as star clusters on the basis of their resolved extent, we find absolutemagnitudes that are brighter on average than those previously identified in spiral galaxies. This might be due to abias in our sample toward star-forming galaxies, combined with a trend for star-forming galaxies to host brightercentral clusters.Key words: galaxies: bulges — galaxies: nuclei — galaxies: spiral — galaxies: structure1. INTRODUCTIONThis article is part of a series of papers presenting the resultsof our Hubble Space Telescope(HST) program GO-8228 (PI:D. Axon), executed with the Space Telescope Imaging Spec-trograph (STIS). The goal of the program is to study the blackhole (BH) mass distribution in spiral galaxies.Studies of the centers of nearby early-type galaxies (ellip-tical and lenticular galaxies) have revealed that most containsupermassive black holes (see Kormendy & Gebhardt 2001 fora recent review). These studies also revealed a strong corre-lation between the mass of the BH (MWe present surface photometry for the central regions of a sample of 48 spiral galaxies (mostly unbarred and barred of types Sbc or Sc) observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Surface brightness profiles were derived and modeled with a Nuker law. We also analyzed archival Wide Field Planetary Camera 2 images with a larger field of view, available for 18 galaxies in our sample. We modeled the extracted bulge surface brightness profiles with an exponential, an r1/4, or an rn profile. In agreement with previous studies, we find that bulges of Sbc galaxies fall into two categories: bulges well described by an exponential profile and those well described by an r1/4 profile. Only one galaxy requires the use of a more general Sersic profile to properly describe the bulge. Nuclear photometrically distinct components are found in ∼ 55% of the galaxies. For those that we classify as star clusters based on their resolved extent we find absolute magnitudes that are brighter on average than those previously identified in spiral galaxies. This might be due to a bias in our sample toward star forming galaxies, combined with a trend for star forming galaxies to host brighter central clusters. Department of Astronomy, Univeristà degli Studi di Padova, Padova, Italy Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield, Hertfordshire, AL10 9AB, UK Department of Physics, RIT, 84 Lomb Memorial Dr., Rochester, NY 14623-5603 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 Oxford University, Theoretical Physics, Keble Road, Oxford, OX1 3NP, UK INAF-Osservatorio Astronomico di Torino, I-10025 Pino Torinese, Italy Physics Department, ETH, Hoenggerberg HPF G4.3, CH–8092 Zurich, Switzerland INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy School of Physics and Astronomy, University of Nottingham, NG7 2RD, UK Center for Astrophysical Sciences, Johns Hopkins University, 239 Bloomberg Center for Physics & Astronomy, 3400 North Charles Street, Baltimore, MD 21218

The effect of dust on Tremaine-Weinberg measurements

We investigate the effect of dust on the observed rotation rate of a stellar bar. The only direct way to measure this quantity relies on the Tremaine & Weinberg (TW) method which requires that the tracer satisfies the continuity equation. Thus, it has been applied largely to early-type barred galaxies. We show using numerical simulations of barred galaxies that dust attenuation factors typically found in these systems change the observed bar pattern speed by 20‐ 40 per cent. We also address the effect of star formation on the TW method and find that it does not change the results significantly. The results presented here suggest that applications of the TW method can be extended to include barred galaxies covering the full range of Hubble type.