Session 4
|
Thomas Fritz1, Ulrich Mebold1,
Stéphanie Côté2, and John Dickey3
tfritz@astro.uni-bonn.de
1RAIUB, Bonn, Germany
2ESO, Garching, Germany
3ATNF, Sydney, Australia
We present observations in the 21cm line of five dwarf galaxies in the Centaurus group of galaxies (NGC 5237, NGC 5264, NGC 5408, ESO 324-G24, ESO 383-G87) obtained with the Australia Telescope Compact Array. The extended region (d = 400 kpc) of X-ray emission around Cen A indicates the existence of a diffuse hot intra-group medium (IGM) in this nearby (3.5 Mpc) and loose group. Combined with optical data and the sample of dwarf irregular galaxies in this group observed by Côté et al. (1997), the data is used to investigate the effects of tidal perturbations and possible interaction with the IGM of the weakly bound HI halos of low mass dwarf galaxies. We find that one of the galaxies shows signs of ram pressure stripping. Most of the galaxies have extended HI halos and are dominated by rotation. The data is furthermore analysed with respect to the dark matter amount of the individual galaxies and the overall properties of the group.
Victoria V. Morozova
vvm@sao.ru
Special Astrophysical Observatory, Nizhnij Arkhyz, Russia
These are results of the computation of the linear diameters, density and mass spectra for about 5000 HI clouds, which were found by their emission at 21 cm. For our investigation we took 29 cross-sections from RATAN-600 HI Survey in the region in Galactic coordinates 180 < l < 260 degrees and -15 < b < +15 degrees (the 3-d Galactic quadrant). In all cross-sections we filtered out the details narrower then 0.5 degrees. Observed parameters of others details were determined by Gauss-analysis method. After correction for antenna effects and calculation of cloud distances from the Sun we receieved desired cloud characteristics, such as diameters, densities and masses and examined their statistics. The spectra were corrected for selection effects. Also we estimated rotation parameters of clouds in this region (were our cross-sections pass almost perpendicular Galactic plane), and for comparison in 2-d Galactic quadrant (cross-section pass along Galactic plane). Spectra shapes, correlation dependences and comparing with results of other observers are discussed.
Robert Rohde, Detlef Elstner, Günther Rüdiger
rohde@dynamo.aip.de
Astrophysicalisches Institut Potsdam, Germany
We present simulations of the 3D nonlinear induction equation for dynamos in spiral galaxies. Our model includes differential rotation, ambipolar diffusion and, based on small scale turbulence, eddy diffusivity and the tensorial alpha - effect with magnetic feed-back. The nonaxisymmetric radial-azimuthal spiral pattern and the vertical stratification of the galaxy are represented in its density and turbulence profile.
The resulting magnetic fields are concentrated in the galactic plane. For small correlation times taucorr of turbulent velocity we find axisymmetric steady solutions of even parity showing concentration between the spiral arms. The pitch angles are relatively large and increase with enlarged correlation time. Large correlation times are linked with oscillating BSS type solutions of again even parity, but they are clearly concentrated in the spiral arms.
Linda Schmidtobreick and Wolfhard Schlosser
linda@astro.ruhr-uni-bochum.de
Astronomisches Institut der Ruhr-Universität Bochum, Germany
As part of the D2-Space-Shuttle-Mission in 1993, the GAUSS-Camera has obtained photographic images of the Milky Way in various passbands in the Ultraviolet. Each film plate covers an area of the sky of about 140 deg. The calibration has been done using the catalogued intensity of stars and computing it into surface brightness. The images have been transformed into maps of the Milky Way in galactic coordinates l, b, covering the Milky Way between Cygnus and Vela (460 deg. >= l >= 270 deg., -40 deg. <= b <= 40 deg.) The image of the Milky Way at 217 nm and 280 nm is heavily dominated by interstellar extinction, leading to high intensity gradients all over the galactic plane. Previous photometries taken in U, B, V, and R have been taken for comparison. The resulting five-dimensional colour phase room provides important information about the scattering-properties of interstellar dust in different clouds. Simulations of Colour-Diagrams have been computed and fitted to the data to derive albedo a and phase factor g of the dust in the ultraviolet ranges. We show, that Colour-Diagrams provide a perfect mean to classify the regions of the Milky Way and select those of interest for further investigation of the molecular gas distribution in our Galaxy
Bülent Uyaniker
uyaniker@mpifr-bonn.mpg.de
Max-Planck Institut für Radioastronomie, Germany
The knowledge of the magnetic field in the Galactic halo is still rather poor because of the lack of observations of the linear polarization at the proper angular resolution and sensitivity. The existing observations are limited to the Galactic plane or suffer from angular resolution. To fill this gap new observations at 1.4 GHz are being carried out to cover the medium Galactic latitudes (|b| < 20 deg.) visible at Effelsberg. This survey is planned to reach a sensitivity at total power close to the confusion limit and a high sensitivity at linear polarization.
The interaction between the Galactic magnetic field and interstellar clouds leads to a variety of radio emitting structures, shells, filaments, and loops. Many of these features are linearly polarized. They extend up to large Galactic latitudes. They are very faint, an order of magnitude fainter than the large scale diffuse Galactic emission. They deform and compress the Galactic magnetic field. We present polarization and depolarization structures detected at medium latitudes with no corresponding observable structure in total power emission. Existence of such kind of polarization structures imply that the emission, due to the Faraday modulation of the Galactic synchrotron foreground, occurs in thin sheetlike regions. This phenomenon is a sign for fluctuating local magnetic field.
Fabian Walter1,3 and Elias Brinks2,3
walter@astro.uni-bonn.de
1Radioastronomisches Institut, Bonn, Germany
2Departamento de Astronomía, Guanajuato, Mexico
3National Radio Astronomy Observatory, Socorro, NM, USA
We present VLA HI-synthesis observations of the Violent Interstellar Medium of the nearby dwarf galaxy IC 2574 (a member of the M81 group of galaxies) at high spatial and velocity resolution. The HI-observations show a stunning amount of detail in the form of HI shells and holes in the neutral interstellar medium of IC 2574, ranging in size from 100 to 1500 pc. The most likely explanation, as has been proposed by previous studies, is combined effects of stellar winds and supernova--explosions of the most massive stars, blowing holes and shells into the interstellar medium. This picture is confirmed by a striking correlation between H-alpha emission and HI-shells: the smaller holes tend to be filled with H-alpha emission whereas for the larger HI holes the H-alpha seems to be restricted to the edges. HII-regions along the edges of larger shells are believed to correspond to secondary sites of star formation. As is the case in Holmberg II, a dwarf galaxy in the same group, HI holes are found to be much larger than those found in more massive spirals, which can be explained in terms of their gravitational potential.