Showing papers by "Ortwin Gerhard published in 2006"

The radial velocity experiment (RAVE): First data release

Abstract: We present the first data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, and surface gravity) of up to one million stars using the Six Degree Field multiobject spectrograph on the 1.2 m UK Schmidt Telescope of the Anglo-Australian Observatory. The RAVE program started in 2003, obtaining medium-resolution spectra (median R 1⁄4 7500) in the Ca-triplet region (8410–8795 8) for southern hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogs, in the magnitude range 9 < I < 12. The first data release is described in this paper and contains radial velocities for 24,748 individual stars (25,274 measurements when including reobservations). Those data were obtained on 67 nights between 2003 April 11 and 2004 April 3. The total sky coverage within this data release is 4760 deg. The average signal-to-noise ratio of the observed spectra is 29.5, and 80% of the radial velocities have uncertainties better than 3.4 km s . Combining internal errors and zero-point errors, the mode is found to be 2 km s . Repeat observations are used to assess the stability of our radial velocity solution, resulting in a variance of 2.8 km s . We demonstrate that the radial velocities derived for the first data set do not show any systematic trend with color or signal-to-noise ratio. The RAVE radial velocities are complemented in the data release with proper motions from Starnet 2.0, Tycho-2, and SuperCOSMOS, in addition to photometric data from the major optical and infrared catalogs (Tycho-2, USNO-B, DENIS, and the TwoMicron All Sky Survey). The data release can be accessed via the RAVE Web site.

The Radial Velocity Experiment (RAVE): Fourth Data Release

Abstract: We present the stellar atmospheric parameters (effective temperature, surface gravity, overall metallicity), radial velocities, individual abundances, and distances determined for 425,561 stars, which constitute the fourth public data release of the RAdial Velocity Experiment (RAVE). The stellar atmospheric parameters are computed using a new pipeline, based on the algorithms of MATISSE and DEGAS. The spectral degeneracies and the Two Micron All Sky Survey photometric information are now better taken into consideration, improving the parameter determination compared to the previous RAVE data releases. The individual abundances for six elements (magnesium, aluminum, silicon, titanium, iron, and nickel) are also given, based on a special-purpose pipeline that is also improved compared to that available for the RAVE DR3 and Chemical DR1 data releases. Together with photometric information and proper motions, these data can be retrieved from the RAVE collaboration Web site and the Vizier database.

A deep kinematic survey of planetary nebulae in the Andromeda galaxy using the Planetary Nebula Spectrograph

Abstract: We present a catalogue of positions, magnitudes and velocities for 3300 emission-line objects found by the Planetary Nebula Spectrograph in a survey of the Andromeda galaxy, M31. Of these objects, 2615 are found likely to be planetary nebulae (PNe) associated with M31. The survey area covers the whole of M31's disc out to a radius of 1 degrees 5. Beyond this radius, observations have been made along the major and minor axes, and the Northern Spur and Southern Stream regions. The calibrated data have been checked for internal consistency and compared with other catalogues. With the exception of the very central, high surface brightness region of M31, this survey is complete to a magnitude limit of m(5007) similar to 23.75, 3.5 mag into the PN luminosity function. We have identified emission-line objects associated with M31's satellites and other background galaxies. We have examined the data from the region tentatively identified as a new satellite galaxy, Andromeda VIII, comparing it to data in the other quadrants of the galaxy. We find that the PNe in this region have velocities that appear to be consistent with membership of M31 itself. The luminosity function of the surveyed PNe is well matched to the usual smooth monotonic function. The only significant spatial variation in the luminosity function occurs in the vicinity of M31's molecular ring, where the luminosities of PNe on the near side of the galaxy are systematically similar to 0.2 mag fainter than those on the far side. This difference can be explained naturally by a modest amount of obscuration by the ring. The absence of any difference in luminosity function between bulge and disc suggests that the sample of PNe is not strongly populated by objects whose progenitors are more massive stars. This conclusion is reinforced by the excellent agreement between the number counts of PNe and the R-band light. The number counts of kinematically selected PNe also allow us to probe the stellar distribution in M31 down to very faint limits. There is no indication of a cut-off in M31's disc out to beyond four scalelengths, and no signs of a spheroidal halo population in excess of the bulge out to 10 effective bulge radii. We have also carried out a preliminary analysis of the kinematics of the surveyed PNe. The mean streaming velocity of the M31 disc PNe is found to show a significant asymmetric drift out to large radii. Their velocity dispersion, although initially declining with radius, flattens out to a constant value in the outer parts of the galaxy. There are no indications that the disc velocity dispersion varies with PN luminosity, once again implying that the progenitors of PNe of all magnitudes form a relatively homogeneous old population. The dispersion profile and asymmetric drift results are shown to be mutually consistent, but require that the disc flares with radius if the shape of its velocity ellipsoid remains invariant.

Kinematic Evidence for Different Planetary Nebula Populations in the Elliptical Galaxy NGC 4697

Abstract: We have analyzed the magnitudes, kinematics, and positions of a complete sample of 320 planetary nebulae (PNs) in the elliptical galaxy NGC 4697. We show the following. (1) The PNs in NGC 4697 do not constitute a single population that is a fair tracer of the distribution of all stars. The radial velocity distributions, mean velocities, and dispersions of bright and faint subsamples differ with high statistical confidence. (2) Using the combined data for PNs brighter than 26.2, we have identified a subpopulation of PNs that is azimuthally unmixed and kinematically peculiar, and thus neither traces the distribution of all stars nor can be in dynamical equilibrium with the galaxy potential. (3) The planetary nebula luminosity functions (PNLFs) of two kinematic subsamples in NGC 4697 differ with 99.7% confidence, ruling out a universal PNLF. We estimate that the inferred secondary PN population introduces an uncertainty in the bright cutoff magnitude of ~0.15 mag for this galaxy. We argue that this secondary PN distribution may be associated with a younger, 1 Gyr old stellar population, perhaps formed in tidal structures that have now fallen back onto the galaxy, as has previously been suggested for the X-ray point sources in this galaxy, or coming from a more recent merger/accretion with a red galaxy. The use of PNs for extragalactic distance determinations is not necessarily compromised, but their use as dynamical tracers of dark halos will require deep observations and careful analysis of large PN samples.

Diffuse light in Hickson compact groups: the dynamically young system HCG 44

Abstract: Context. Compact groups are associations of a few galaxies in which the environment plays an important role in galaxy evolution. The low group velocity dispersion favors tidal interactions and mergers, which may bring stars from galaxies to the diffuse intragroup light. Numerical simulations of galaxy clusters in hierarchical cosmologies show that the amount of the diffuse light increases with the dynamical evolution of the cluster. Aims. We search for diffuse light in the galaxy group HCG 44 in order to determine its luminosity and luminosity fraction. Combining with literature data, we aim to constrain the dynamical status of Hickson compact groups. Methods. We use Intra Group planetary nebulae (IGPNe) as tracers of diffuse light. These are detected by the so-called on band-off band technique. Results. We found 12 emission line objects in HCG 44, none of them associated with the galaxies of the group. The absence of PNe in the elliptical galaxy, NGC 3193, implies that this galaxy is located behind the group, leaving only three spiral galaxy members in HCG 44. 6/12 emission line objects are consistent with being IGPNe in HCG 44, but are also consistent with being Lyα background galaxies. Thus we derive an upper limit to the diffuse light fraction in HCG 44 of 4.7%, corresponding to 1.06 × 10 9 L� ,B and mean surface brightness of µB = 30.04 mag arcsec −2 . We find a correlation between the fraction of elliptical galaxies and the amount of diffuse light in Hickson compact groups. Those with large fraction of diffuse light are those with large fractions in number and luminosity of E/S0 galaxies. This indicates that the diffuse light is mainly created in dynamical processes during the formation of bright elliptical galaxies in major mergers. Conclusions. We propose an evolutionary sequence for Hickson compact groups in which the amount of diffuse light increases with the dynamical evolution of the group.

Stellar kinematics for the central spheroid in the polar disk galaxy NGC 4650A

Abstract: We have obtained high angular resolution, high signal-to-noise ratio spectra of the calcium triplet absorption lines on the photometric axes of the stellar spheroid in the polar disk galaxy NGC 4650A. Along the major axis, the observed rotation and velocity dispersion measurements show the presence of a kinematically decoupled nucleus and a flat velocity dispersion profile. The minor-axis kinematics is determined for the first time: along this direction some rotation is measured, and the velocity dispersion is nearly constant and slightly increases at larger distances from the center. The new high-resolution kinematic data suggest that the stellar component in NGC 4650A resembles a nearly exponential oblate spheroid supported by rotation. The main implications of these results on the previous mass models for NGC 4650A are discussed. Moreover, the new kinematic data set constraints on current models for the formation scenarios of polar ring galaxies, supporting a slow accretion rather than a secondary strong dissipative event.

Stellar kinematics for the central spheroid in the Polar Disk Galaxy NGC4650A

Abstract: We have obtained high angular resolution, high signal-to-noise spectra of the Calcium triplet absorption lines on the photometric axes of the stellar spheroid in the polar disk galaxy NGC4650A. Along the major axis, the observed rotation and velocity dispersion measurements show the presence of a kinematically decoupled nucleus, and a flat velocity dispersion profile. The minor axis kinematics is determined for the first time: along this direction some rotation is measured, and the velocity dispersion is nearly constant and slightly increases at larger distances from the center. The new high resolution kinematic data suggest that the stellar component in NGC4650A resembles a nearly-exponential oblate spheroid supported by rotation. The main implications of these results on the previous mass models for NGC4650A are discussed. Moreover, the new kinematic data set constraints on current models for the formation scenarios of Polar Ring Galaxies (PRGs), supporting a slow accretion rather then a secondary strong dissipative event.

Milky Way Gas Dynamics

Abstract: The Milky Way is made up of a central bar, a disk with embedded spiral arms, and a dark matter halo. Observational and theoretical constraints for the characteristic parameters of these components will be presented, with emphasis on the constraints from the dynamics of the Milky Way gas. In particular, the fraction of dark matter inside the solar radius, the location of the main resonances, and the evidence for multiple pattern speeds will be discussed.

A deep kinematic survey of planetary nebulae in the Andromeda Galaxy using the Planetary Nebula Spectrograph

Abstract: We present a catalogue of positions, magnitudes and velocities for 3300 emission-line objects found by the Planetary Nebula Spectrograph in a survey of the Andromeda Galaxy, M31. Of these objects, 2615 are found likely to be planetary nebulae (PNe) associated with M31. Initial results from this survey include: the likely non-existence of Andromeda VIII; a universal PN luminosity function, with the exception of a small amount of obscuration, and a small offset in normalization between bulge and disk components; very faint kinematically-selected photometry implying no cut-off in the disk to beyond 4 scalelengths and no halo population in excess of the bulge out to 10 effective bulge radii; disk kinematics that show significant dispersion and asymmetric drift out to large radii, consistent with a warm flaring disk; and no sign of any variation in kinematics with PN luminosity, suggesting that PNe arise from a fairly uniform population of old stars.

Mass distribution and bulge formation in the Milky Way galaxy

Abstract: In its first part, this paper summarizes recent work on the mass and shape of the Galactic dark halo. The second part presents a review of the large-scale structure of the Milky Way, and of the evidence that the inner Galaxy is dominated by baryonic matter. This is briefly compared with the predictions of Λ CDM and MOND. Finally, a summary is given of bulge formation from clumpy, gas-rich disks, a process which may give rise to old, disk-like, α -rich bulges similar to the Galactic bulge.

Dynamics of Rotating Elliptical Galaxies

Abstract: We present preliminary results from an ongoing study of rotation in triaxial elliptical galaxies. Using a newly developed adaptive particle method, we first construct a model of a rotating triaxial galaxy. We then attempt to build a non-rotating model with the same density and projected velocity and dispersion maps. Finally we compare the projected kinematic data to see whether we can distinguish between the rotating and non-rotating models.

Kinematic Substructures in the Coma Cluster Core as traced by Intracluster Planetary Nebulae

Abstract: The Coma cluster is the richest and most compact of the nearby clusters, yet there is growing evidence that its formation is still on-going. A sensitive probe of this evolution is the dynamics of intracluster stars, which are unbound from galaxies while the cluster forms, according to cosmological simulations. With a new multi-slit imaging spectroscopy technique pioneered at the 8.2 m Subaru telescope and FOCAS, we can now detect and measure the line-of-sight velocities of the intracluster planetary nebulae which are associated with the diffuse stellar population of stars, at 100 Mpc distance. We detect significant velocity substructures within a 6 arcmin diameter field, centred on the Coma X-ray cluster emission. One substructure is present at $\sim $ 5000 km s $^{-1}$ , probably from infall of a galaxy group, while the main intracluster stellar component moves at $\sim $ 6500 km s $^{-1}$ . Hence the ICPNs associated with the diffuse light at the position of the MSIS field are not bound to the nearby cD galaxy NGC 4874, whose radial velocity is $\sim $ 700 km s $^{-1}$ higher.

Milky Way Gas Dynamics

Abstract: The Milky Way is made up of a central bar, a disk with embedded spiral arms, and a dark matter halo. Observational and theoretical constraints for the characteristic parameters of these components will be presented, with emphasis on the constraints from the dynamics of the Milky Way gas. In particular, the fraction of dark matter inside the solar radius, the location of the main resonances, and the evidence for multiple pattern speeds will be discussed.

Planetary Nebulae as Dynamical Tracers: Mass-to-Light-Ratio Gradients in Early-Type Galaxies

Abstract: Planetary Nebulae (PNe) have enabled mass-to-light ratios (M/L) in early-type galaxies to be constrained to unprecedented distance from the center, showing in some cases clear evidence of increasing M/L, in other cases fairly constant M/L. We combine the information obtained from PN kinematics with radially extended long-slit spectroscopy data in order to constrain the M/L trends in a heterogeneous sample of early-type galaxies. We discuss whether these trends are expected in the Lambda CDM framework of galaxy formation.

Modelling Kinematics and Dark Matter: The Halos of Elliptical Galaxies

Abstract: This review is focussed on the outer halos of elliptical galaxies. Its emphasis is on: (i) planetary nebulae as test particles to trace the stellar kinematics at large radii; (ii) the observed angular momentum in elliptical galaxy halos and its theoretical relevance; (iii) dynamical modelling of stellar-kinematic data; and (iv) a discussion of the evidence for dark matter halos in ellipticals from a wide range of measurements.

NMAGIC: fast parallel implementation of a χ²-made-to-measure algorithm for modeling observational data

Abstract: We describe a made-to-measure algorithm (χ2M2M) for constructing N-particle models of stellar systems from observational data (De Lorenzi Debattista, Gerhard, et al. (2007)), extending earlier ideas by Syer & Tremaine (1996). The algorithm properly accounts for observational errors. We implemented this algorithm in a parallel code NMAGIC and carried out a sequence of tests to illustrate its power and performance: (i) We reconstructed an isotropic Hernquist (1990) model from density moments and projected kinematics including higher-order Gauss-Hermite moments (Gerhard (1993), van der Marel & Franx (1993)). We gave NMAGIC two initial models, made from distribution function (Debattista & Sellwood (2000)), with different density distributions to start with. While both recovered the correct differential energy distribution and intrinsic kinematics, that with density closer to the density of the final model had smaller final deviations from the target observables, and a narrower distribution of weights. (ii) We built a self-consistent oblate three-integral maximum rotator model and compared how the distribution function is recovered from integral field and slit kinematic data. In these experiments we gave the algorithm a difficult problem to solve. Since the target system was maximally rotating, the weights of all counter-rotating particles were zero. Using density observables and either slit or integral field kinematics, NMAGIC was asked to recover this maximally rotating model starting from an isotropic spherical system. A good fit to the kinematic constraint data was achieved. These experiments also showed the advantage of integral field data over slit data for constraining the model. The different applications show that the χ2M2M algorithm is practical, reliable and can be applied to various systems. High quality dynamical models of galaxies can be achieved which match targets to ~1σ for plausible uncertainties in the observables, and without symmetry restrictions. We conclude that χ2M2M holds great promise for unraveling the nature of galaxies.

Self-gravitating warped disks around nuclear black holes

Abstract: Many galactic nuclei contain disks of gas and possibly stars surrounding a supermassive black hole. These disks may play a key role in the evolution of galactic centers. Here we address the problem of finding stable warped equilibrium configurations for such disks, considering the attraction by the black hole and the disk self-gravity as the only acting forces. We model these disks as a collection of concentric, circular rings. We find the equilibria of such systems of rings, and determine how they scale with the ring parameters and the mass of the central black hole. We show that in some cases these disk equilibria may be highly warped. We then analyze the stability of these disks, using both direct time integration and linear stability analysis. This shows that the warped disks are stable for a range of disk-to-black hole mass ratios, when the rings extend over a limited range of radii.

New kinematics for the central spheroid in polar disk galaxy NGC4650A

Abstract: We have obtained high angular resolution spectra on the photometric axes of the stellar spheroid in the polar disk galaxy NGC4650A. We discuss the main implications by the new kinematics on the previous mass models for NGC4650A and on current formation scenarios of Polar Ring Galaxies (PRGs).

Mapping the Stellar Dynamics of M31

Abstract: Using the Planetary Nebula Spectrograph, we have observed and measured the velocities for 2764 PNe in the disk and halo of the Andromeda galaxy. Preliminary analysis using a basic ring model shows a rotation curve in good agreement with that obtained fromHi data out to∼ 20 kpc. Some substructure has also been detected within the velocity field, which can be modeled as the continuation of the tidal–remnant known as the Southern Stream, as it passes through Andromeda’s disk.

Planetary Nebulae Surveys Beyond the Local Group

Abstract: Distant planetary nebulae (PNe) are used to measure distances through the PN luminosity function, as kinematic tracers in determining the mass distribution in elliptical galaxies, and most recently, for measuring the kinematics of the diffuse stellar population in galaxy clusters. This article reviews the photometric and spectroscopic survey techniques that have been used to detect PNe beyond the Local Group, out to the Coma cluster at 100 Mpc distance. Contaminations by other emission sources and ways to overcome them will be discussed as well as some science highlights and future perspectives.

Dynamics of the Elliptical Galaxy NGC 4697 from Integrated Light and PNe

Abstract: We present results from our ongoing calculations pertaining to the dynamics of the E6 galaxy NGC 4697. We construct a (self-consistent) N-body model of the galaxy by employing the deprojected photometric data and absorption line kinematics. Assuming that the PNe in the vicinity of NGC 4697 are drawn from the stellar distribution, we can use them as kinematic probes that trace the underlying mass distribution particularly in the outer regions of the galaxy. We construct a smoothed, non-parametric velocity field from the PNe radial velocities, and compare with the kinematic predictions of our model.