Showing papers by "Ortwin Gerhard published in 1998"

The Stellar Dynamics and Mass of NGC 1316 Using the Radial Velocities of Planetary Nebulae

Abstract: We present a study of the kinematics of the outer regions of the early-type galaxy NGC 1316, based on radial velocity measurements of 43 planetary nebulae as well as deep integrated light absorption line spectra. The smoothed velocity field of NGC 1316 indicates fast rotation at a distance of 16 kpc, possibly associated with an elongated feature orthogonal to the inner dust lanes. The mean square stellar velocity is approximately independent of radius, and the estimated total mass of the system is 2.9 × 1011 M☉ within a radius of 16 kpc, implying an integrated mass-to-light ratio of M/LB 8.

Breaking the degeneracy between anisotropy and mass: the dark halo of the E0 galaxy NGC 6703

Abstract: We have measured line-of-sight velocity profiles (VPs) in the E0 galaxy NGC 6703 out to 2.6Re. Comparing these with the VPs predicted from spherical distribution functions (DFs), we constrain the mass distribution and the anisotropy of the stellar orbits in this galaxy. We have developed a non-parametric technique to determine the DF f(E, L2) directly from the kinematic data. We test this technique on Monte Carlo simulated data with the spatial extent, sampling, and error bars of the NGC 6703 data. We find that smooth underlying DFs can be recovered to an rms accuracy of 12 per cent inside three times the radius of the last kinematic data point, and the anisotropy parameter β(r) can be recovered to an accuracy of 0.1, in a known potential. These uncertainties can be reduced with improved data. By comparing such best-estimate, regularized models in different potentials, we can derive constraints on the mass distribution and anisotropy. Tests show that, with presently available data, an asymptotically constant halo circular velocity v0 can be determined with an accuracy of A±≲50 km s−1. This formal range often includes high-v0 models with implausibly large gradients across the data boundary. However, even with extremely high quality data some uncertainty on the detailed shape of the underlying circular velocity curve remains. In the case of NGC 6703, we thus determine the true circular velocity at 2.6Re to be 250±40 km s−1 at 95 per cent confidence, corresponding to a total mass in NGC 6703 inside 78 arcsec (13.5 h−150 kpc, where h50≡H0/50 km s−1 Mpc−1) of 1.6–2.6A–1011h−150 M⊙. No model without dark matter will fit the data; however, a maximum stellar mass model in which the luminous component provides nearly all the mass in the centre will. In such a model, the total luminous mass inside 78 arcsec is 9A–1010 M⊙ and the integrated B-band mass-to-light ratio out to this radius is ΥB=5.3–10, corresponding to a rise from the centre by at least a factor of 1.6. The anisotropy of the stellar distribution function in NGC 6703 changes from near-isotropic at the centre to slightly radially anisotropic (β=0.3–0.4 at 30 arcsec, β=0.2–0.4 at 60 arcsec) and is not well-constrained at the outer edge of the data, where β=−0.5 to +0.4, depending on variations of the potential in the allowed range. Our results suggest that also elliptical galaxies begin to be dominated by dark matter at radii of ∼10 kpc.

Intracluster Planetary Nebulae in the Virgo Cluster

Abstract: We briefly describe the properties of the confirmed spectroscopic sample of intracluster planetary nebulae recently discovered in the Virgo cluster. We find 23 bonafide intracluster planetary nebulae and 8 high redshift (z ~ 3.1) Lyalpha emitters identified by their broad asymmetric emission line.

The Galactic Bar

Abstract: We summarize recent work on the structure and dynamics of the Galactic bar and inner disk. Current work focusses on constructing a quantitative model which integrates NIR photometry, source count observations, gas kinematics, stellar dynamical observations, and microlensing. Some avenues for future research are discussed.

Gas Dynamics and Large-Scale Morphology of the Milky Way Galaxy

Abstract: We present a new model for the gas dynamics in the galactic disk inside the Sun's orbit. Quasi-equilibrium flow solutions are determined in the gravitational potential of the deprojected COBE NIR bar and disk, complemented by a central cusp and, in some models, an outer halo. These models generically lead to four-armed spiral structure between corotation of the bar and the solar circle; their large-scale morphology is not sensitive to the precise value of the bar's pattern speed, to the orientation of the bar with respect to the observer, and to whether or not the spiral arms carry mass. Our best model provides a coherent interpretation of many observed gas dynamical features. Its four-armed spiral structure outside corotation reproduces quantitatively the directions to the five main spiral arm tangents at |l|<=60deg observed in a variety of tracers. The 3-kpc-arm is identified with one of the model arms emanating from the ends of the bar, extending into the corotation region. The model features an inner gas disk with a cusped orbit shock transition to an x_2 orbit disk of radius R~150pc. The bar's corotation radius is fairly well--constrained at R_c=3.5 +/- 0.5 kpc. The best value for the orientation angle of the bar is probably 20-25deg, but the uncertainty is large since no detailed quantitative fit to all features in the observed lv-diagrams is yet possible. The Galactic terminal velocity curve from HI and CO observations out to l=+/-45deg (=5 kpc) is approximately described by a maximal disk model with constant mass-to-light ratio for the NIR bulge and disk.

Dynamical analysis of elliptical galaxy halos

Abstract: We describe a non-parametric method for determining both the anisotropy and mass profiles of elliptical galaxies from velocity dispersions and velocity profile shapes along the line of sight. In this way, mass-to-light ratios out to 2-3 effective radii can be derived from stellar absorption line spectroscopy. We discuss two illustrative cases, NGC 1399 and NGC 6703. So far, all ellipticals which have been analysed show evidence for dark matter dominating in the outer parts. The circular velocity curves are nearly flat, as in spiral galaxies.