arXiv subjects

T. Westmeier

Publications and source records attributed to T. Westmeier.

57 records · Page 4Linked to original sources

arXiv2005

Effelsberg HI observations of compact high-velocity clouds

We have mapped 11 compact high-velocity clouds (CHVCs) in the 21-cm line emission of neutral, atomic hydrogen, using the Effelsberg 100-m radio telescope. The aim of our observations was to study the overall distribution of the warm neutral medium of CHVCs with high sensitivity. The achieved baseline rms of about 50 mK at the original 2.6 km/s velocity resolution allows us to search for evidence of ram-pressure interaction with the ambient medium. In addition, we have obtained spectra along an appropriate axis across each CHVC with longer integration times and denser angular sampling. These deep slices with a spectral baseline rms of about 25...35 mK allow us to determine the column density profile in greater detail as well as the velocity and line width gradient across each cloud. The most outstanding result of our observations is the complexity of the HI column density distribution and the line profiles of the investigated CHVCs. We have found only one cloud with a spherically-symmetric appearance. Among the remaining clouds we observe head-tail structures, bow-shock shapes, and objects with irregular shapes. These complex morphologies in combination with the obtained physical parameters suggest that ram-pressure interactions with an ambient medium may play a significant role in shaping some of the CHVCs from our sample. These results are consistent with a circumgalactic distribution of CHVCs with typical distances of the order of 100 kpc. The pressure of the ambient medium might also stabilise CHVCs in addition to their own gravitational potential.

astro-ph↗

arXiv2004

HI Observations of an Ultra-Compact High-Velocity Cloud

We present HI observations of the compact high-velocity cloud HVC289+33+251 that was discovered by Putman et al. (2002). Observations with the 100-m Effelsberg telescope demonstrate that this cloud is still unresolved by the 9' beam of the Effelsberg telescope. The cloud shows a small line width of Delta v_FWHM = 4.9 km/s providing an upper limit to the kinetic temperature of the HI gas of T_k < 532 K. The total observed flux indicates an HI mass of M(HI) = 5.66 10^4 M_sun [d/150kpc]^2. Follow-up HI observations using the Australia Telescope Compact Array (ATCA) resolve HVC289+33+251 into 5 condensations that are embedded in a common HI envelope. The HVC shows a faint tail, indicating an ongoing ram-pressure interaction with an ambient low-density medium. A FWHM diameter of theta = 4.4 arcmin makes this HVC the by far most compact HVC known till now. The observed parameters suggest that pressure stabilization by an ambient medium is rather unlikely. At a distance of 150 kpc, the virial mass is by a factor of 5.6 higher than the observed gas mass -- consistent with HVC289+33+251 being one of the "missing" dark matter mini halos that were predicted by cosmological LCDM simulations (e.g. Klypin et al. 1999; Moore et al. 1999). Comparable clouds in other groups of galaxies or even around the Milky Way are not detectable with the resolution and sensitivity of present surveys.

astro-ph↗

arXiv2003

Effelsberg HI Survey of Compact High-Velocity Clouds

We studied 11 compact high-velocity clouds (CHVCs) in the 21-cm line emission of neutral hydrogen with the 100-m telescope in Effelsberg. We find that most of our CHVCs are not spherically-symmetric as we would expect in case of a non-interacting, intergalactic population. Instead, many CHVCs reveal a complex morphology suggesting that they are disturbed by ram-pressure interaction with an ambient medium. Thus, CHVCs are presumably located in the neighborhood of the Milky Way instead of being spread across the entire Local Group.

astro-ph↗