arXiv subjects

Tom A. Oosterloo

Publications and source records attributed to Tom A. Oosterloo.

21 records · Page 2Linked to original sources

arXiv2007

The shape of the dark matter halo in the early-type galaxy NGC 2974

We present HI observations of the elliptical galaxy NGC 2974, obtained with the Very Large Array. These observations reveal that the previously detected HI disc in this galaxy (Kim et al. 1988) is in fact a ring. By studying the harmonic expansion of the velocity field along the ring, we constrain the elongation of the halo and find that the underlying gravitational potential is consistent with an axisymmetric shape. We construct mass models of NGC 2974 by combining the HI rotation curve with the central kinematics of the ionised gas, obtained with the integral-field spectrograph SAURON. We introduce a new way of correcting the observed velocities of the ionised gas for asymmetric drift, and hereby disentangle the random motions of the gas caused by gravitational interaction from those caused by turbulence. To reproduce the observed flat rotation curve of the HI gas, we need to include a dark halo in our mass models. A pseudo-isothermal sphere provides the best model to fit our data, but we also tested an NFW halo and Modified Newtonian Dynamics (MOND), which fit the data marginally worse. The mass-to-light ratio M/L_I increases in NGC 2974 from 4.3 (M/L_I)sun at one effective radius to 8.5 (M/L_I)sun at 5 Re. This increase of M/L already suggests the presence of dark matter: we find that within 5 Re at least 55 per cent of the total mass is dark.

astro-ph↗

arXiv2007

Kinematic modelling of disk galaxies I. A new method to fit tilted rings to data cubes

This is the first of a series of papers in which the kinematics of disk galaxies over a range of scales is scrutinised employing spectroscopy. A fundamental aspect of these studies is presented here: the new publicly available software tool TiRiFiC (http://www.astro.uni-bonn.de/~gjozsa/tirific.html) enables a direct fit of a ``tilted-ring model'' to spectroscopic data cubes. The algorithm generates model data cubes from the tilted-ring parametrisation of a rotating disk, which are automatically adjusted to reach an optimum fit via a chi-squared minimisation method to an observed data cube. The structure of the new software, the shortcomings of the previously available programs to produce a tilted-ring model, and the performance of TiRiFiC are discussed. Our method is less affected by the well-known problem of beam smearing that occurs when fitting to the velocity field. Since with our method we fit many data points in a data cube simultaneously, TiRiFiC is sensitive to very faint structures and can hence be used to derive tilted-ring models significantly extending in radius beyond those derived from a velocity field. The software is able to parametrise HI disks of galaxies that are intersected by the line-of-sight twice or more, i.e. if the disks are heavily warped, and/or with a significant shift of the projected centre of rotation, and/or if seen edge-on. Furthermore, our method delivers the surface-brightness profile of the examined galaxy in addition to the orientational parameters and the rotation curve. In order to derive kinematic and morphological models of disk galaxies, especially reliable rotation curves, a direct-fit method as implemented in our code should be the tool of choice.

astro-ph↗

arXiv1997

The Metallicity and Dust Content of HVC 287.5+22.5+240: Evidence for a Magellanic Clouds Origin

We estimate the abundances of S and Fe in the high velocity cloud HVC 287.5+22.5+240, which has a velocity of +240 km/s with respect to the local standard of rest and is in the Galactic direction l~287, b~23. The measurements are based on UV absorption lines of these elements in the Hubble Space Telescope spectrum of NGC 3783, a background Seyfert galaxy, as well as new H I 21-cm interferometric data taken with the Australia Telescope. We find S/H=0.25+/-0.07 and Fe/H=0.033+/-0.006 solar, with S/Fe=7.6+/-2.2 times the solar ratio. The S/H value provides an accurate measure of the chemical enrichment level in the HVC, while the super-solar S/Fe ratio clearly indicates the presence of dust, which depletes the gas-phase abundance of Fe. The metallicity and depletion information obtained here, coupled with the velocity and the position of the HVC in the sky, strongly suggest that the HVC originated from the Magellanic Clouds. It is likely (though not necessary) that the same process(es) that generated the Magellanic Stream is also responsible for HVC 287.5+22.5+240.

astro-ph↗