Showing papers by "Ortwin Gerhard published in 2001"

Dynamical Family Properties and Dark Halo Scaling Relations of Giant Elliptical Galaxies

Abstract: Based on a uniform dynamical analysis of the line-pro—le shapes of 21 mostly luminous, slowly rotating, and nearly round elliptical galaxies, we have investigated the dynamical family relations and dark halo properties of ellipticals. Our results include: (i) The circular velocity curves (CVCs) of elliptical galaxies are —at to within ^10% for (ii) Most ellipticals are moderately radially anisotropic; R Z 0.2R e . their dynamical structure is surprisingly uniform. (iii) Elliptical galaxies follow a Tully-Fisher (TF) relation with marginally shallower slope than spiral galaxies, and km s~1 for an galaxy. At

The Galactic Center HE I Stars: Remains of a Dissolved Young Cluster?

Abstract: A massive young star cluster, initially embedded in its parent molecular cloud, will spiral into the Galactic center from 30m pc during the lifetime of its most massive stars, if the combined total mass is ~106m6 M?. On its way inward, the system loses most of its mass to the strong tidal field, until the dense cluster core of high-mass stars is finally disrupted near the central black hole. A simple model is presented to argue that this scenario may, under plausible conditions, explain the observed location and rotation of the Galactic center He I stars. Accretion of star clusters into the Galactic center could be recurrent and could play an important role in regulating the activity of Sagittarius A*.

Detection, photometry and slitless radial velocities of 535 planetary nebulae in the flattened elliptical galaxy ngc 4697

Abstract: We have detected 535 planetary nebulae (PNs) in the flattened elliptical galaxy NGC 4697 using the classic on-band, off-band filter technique with the Focal Reducer and Spectrograph at the Cassegrain focus of the first 8 m telescope unit of the ESO Very Large Telescope. From our photometry, we have built the [O III] ?5007 planetary nebula luminosity function (PNLF) of NGC 4697. It indicates a distance of 10.5 ? 1 Mpc to this galaxy, in good agreement with the distance obtained from surface brightness fluctuations and substantially smaller than a previous estimate of 24 Mpc used in earlier dynamical studies. The PNLF also provides an estimate of the specific PN formation rate: (6 ? 2) ? 10-12 PNs yr-1 L?-1. Combining the information from on-band images with PN positions on dispersed, slitless grism images, we have obtained radial velocities for 531 of the 535 PNs. We describe the slitless velocity method and the calibration procedures that we have followed. The radial velocities have errors of about 40 km s-1 and provide kinematic information up to a distance of almost three effective radii from the nucleus. Some rotation is detected in the outer regions, but the rotation curve of this galaxy appears to drop beyond one effective radius. Assuming an isotropic velocity distribution, the velocity dispersion profile is consistent with no dark matter within three effective radii of the nucleus (however, some dark matter can be present if the velocity distribution is anisotropic). We obtain a blue mass-to-light ratio of 11. Earlier M/L ratios for NGC 4697 were too small because of the too large distance used for their derivation.

Intracluster Planetary Nebulae in Virgo: Photometric selection, spectroscopic validation and cluster depth

Abstract: We have imaged an empty area of 34'x34' one and a half degree north of the Virgo cluster core to survey for intracluster planetary nebula candidates. We have implemented and tested a fully automatic procedure for the selection of emission line objects in wide-field images, based on the on-off technique from Ciardullo and Jacoby. Freeman et al. have spectroscopically confirmed a sample of intracluster planetary nebulae in one Virgo field. We use the photometric and morphological properties of this sample to test our selection procedure. In our newly surveyed Virgo field, 75 objects were identified as best candidates for intracluster PNe. The luminosity function of the spectroscopically confirmed PNe shows a brighter cut-off than the planetary nebula luminosity function for the inner regions of M87. Such a brighter cut-off is also observed in the newly surveyed field and indicates a smaller distance modulus, implying that the front end of the Virgo cluster is closer to us by a significant amount: 14% closer (2.1 Mpc) than M87 for the spectroscopic field, using the PN luminosity function distance of 14.9 Mpc to M87, and 19% closer (2.8 Mpc) than M87 for the newly surveyed field. Independent distance indicators (Tully-Fisher relation for Virgo spirals and surface brightness fluctuations for Virgo ellipticals) agree with these findings. From these two Virgo cluster fields there is no evidence that the surface luminosity density for the diffuse stellar component in the cluster decreases with radius. The luminosity surface density of the diffuse stellar population is comparable to that of the galaxies.

The dark matter problem in disc galaxies

Abstract: In the generic CDM cosmogony, dark-matter haloes emerge too lumpy and centrally concentrated to host observed galactic discs. Moreover, discs are predicted to be smaller than those observed. We argue that the resolution of these problems may lie with a combination of the effects of protogalactic discs, which would have had a mass comparable to that of the inner dark halo and be plausibly non-axisymmetric, and of massive galactic winds, which at early times may have carried off as many baryons as a galaxy now contains. A host of observational phenomena, from quasar absorption lines and intracluster gas through the G-dwarf problem, point to the existence of such winds. Dynamical interactions will homogenize and smooth the inner halo, and the observed disc will be the relic of a massive outflow. The inner halo expanded after absorbing energy and angular momentum from the ejected material. Observed discs formed at the very end of the galaxy formation process, after the halo had been reduced to a minor contributor to the central mass budget and strong radial streaming of the gas had died down.

Spiral Arms, Bar Shape and Bulge Microlensing in the Milky Way

Abstract: A new model for the luminosity distribution in the inner Milky Way is found, using a non-parametric penalized maximum-likelihood algorithm to deproject a dereddened COBE/ DIRBE L-band map of the inner Galaxy The model isalso constrained by the apparent magnitude (line-of-sight) distributions of clump giant stars in certain bulge fields An important new feature is the inclusion of a spiral arm model in the disc Spiral arms make the model appear broader on the sky; thus our bar is more elongated than in previous eight-fold symmetric models They also lead to a smoother disc model interior to the Sun The bar length is 35 kpc, and its axis ratios are 1: (03-04): 03, independent of whether the spiral arm model is four-armed or two-armed The larger elongation in the plane makes it possible to reproduce the observed clump giant distributions as well With only the surface brightness data, a small model degeneracy is found even for fixed orientation of the bar, amounting to about ′01 uncertainty in the in-plane axial ratio Including the clump giant data removes most of this degeneracy and also places additional constraints on the orientation angle of the bar We estimate 15° ≤ φ b a r ≤ 30°, with the best models obtained for 20° ≤ φ b a r ≤ 25° We use our reference model to predict a microlensing optical depth map towards the bulge, normalizing its mass by the observed terminal velocity curve For clump giant sources at (l,b) = (3°9, -3°8) we find τ - 6 ≡ τ/10 - 6 = 127, within 18σ of the new MACHO measurement given by Popowski et al The value for all sources at (l, b) = (2°68, -3°35) is τ-6 = 11, still >3σ away from the published MACHO DIA value The dispersion of these τ - 6 values within our models is ≃ 10 per cent Because the distribution of sources is well fitted by the near-infrared model, increasing the predicted optical depths by >20 per cent will be difficult Thus the high value of the measured clump giant optical depth argues for a near-maximal disc in the Milky Way

Detection, photometry and slitless radial velocities of 535 planetary nebulae in the flattened elliptical galaxy NGC 4697

Abstract: We have detected 535 planetary nebulae (PNs) in NGC 4697, using the classic on-band, off-band filter technique with the Focal Reducer and Spectrograph (FORS) at the Cassegrain focus of the first 8-meter telescope unit of the ESO Very Large Telescope. From our photometry we have built the [O III] 5007 planetary nebula luminosity function (PNLF) of NGC 4697. It indicates a distance of 10.5 Mpc, substantially smaller than a previous estimate of 24 Mpc used in earlier dynamical studies. The PNLF also provides an estimate of the specific PN formation rate. Combining the information from on-band images with PN positions on dispersed, slitless grism images, we have obtained radial velocities for 531 of the 535 PNs. They provide kinematic information up to a distance of almost three effective radii from the nucleus. Some rotation is detected in the outer regions, but the rotation curve of this galaxy appears to drop beyond one effective radius. Assuming an isotropic velocity distribution, the velocity dispersion profile is consistent with no dark matter within three effective radii of the nucleus (however, some dark matter can be present if the velocity distribution is anisotropic). We obtain a blue mass-to-light ratio of 11. Earlier M/L ratios for NGC 4697 were too small, because of the too large distance used for their derivation.

Element Abundance Patterns of Metal-Poor Halo Stars

Abstract: We describe a stochastic halo formation model to compute the early chemical enrichment of the interstellar medium (ISM) of the halo. Local inhomogeneities caused by single supernovae lead to different element abundance patterns in very metal-poor stars. These can be seen as scatter in the abundance ratios of metal-poor halo stars. The early chemical evolution of the halo proceeds in different enrichment phases: At first, the halo ISM is unmixed and dominated by local inhomogeneities. The mixing then gradually increases, leading finally to a chemically homogeneous ISM with an IMF averaged element abundance pattern. For some elements (Si, Ca), the scatter in the element-to-iron ratio [El/Fe] of metal-poor halo stars can be reproduced. Stellar yields of other elements predict a scatter which is too large (O, Mg) or too small (Ni). This result does not depend on the details of the galactic model but is solely determined by theoretical SN yields. This demonstrates the need for revised, self-consistent nucleosynthesis models. A more detailed discussion of the model and its results can be found in Argast et al. (2000).

Structure and Mass Distribution of the Milky Way Bulge and Disk

Abstract: This article summarizes the structural parameters of the Galactic bulge and disk, and discusses the interpretation of the bulge microlensing observations and the determination of the Milky Way's luminous mass from the terminal velocity curve and the Oort limit. The bulge is a rotating bar with corotation radius around 4 kpc. The NIR disk has a short scale-length. The measured surface density of the local disk is in good agreement with the prediction of a maximum NIR disk model for the Milky Way. The preliminary new value for the microlensing optical depth of clump giant sources is within $1.7\sigma$ of the prediction by the maximum NIR disk model, while the optical depth for all sources is still significantly higher. These results imply that cold dark matter cannot dominate inside the solar radius.

Spiral arms, bar shape and bulge microlensing in the Milky Way

Abstract: A new model for the luminosity distribution in the inner Milky Way is found, using a non-parametric penalized maximum-likelihood algorithm to deproject a dereddened COBE/DIRBE L-band map of the inner Galaxy. The model is also constrained by the apparent magnitude (line-of-sight) distributions of clump giant stars in certain bulge fields. An important new feature is the inclusion of a spiral arm model in the disk. Spiral arms make the model appear broader on the sky, thus our bar is more elongated than in previous eight-fold symmetric models. They also lead to a smoother disk model interior to the Sun. The bar length is approx. 3.5kpc and its axis ratios are 1:(0.3-0.4):0.3, independent of whether the spiral arm model is 4-armed or 2-armed. The larger elongation in the plane makes it possible to reproduce the observed clump giant distributions as well. For the bar angle we estimate 15^o 3*sigma away from the published MACHO DIA value. The dispersion of these tau_{-6} values within our models is 10%. Because the distribution of sources is well-fit by the NIR model, increasing the predicted optical depths by >20% will be difficult. Thus the high value of the measured clump giant optical depth argues for a near-maximal disk in the Milky Way. (abridged)