Compact high-velocity clouds
High-velocity clouds (HVCs) are gas clouds which are characterised by high radial velocities incompatible with a participation in the rotation of our Galaxy. They were discovered by Muller, Oort & Raimond (1963) in the 21-cm line emission of neutral, atomic hydrogen. HVCs can be found all across the sky in the form of compact, isolated clouds as well as extended complexes, some of which span tens of degrees across the sky. Among the countless hypotheses about their origin, Blitz et al. (1999) suggested that HVCs might represent the gaseous counterparts of the primordial dark-matter mini-halos predicted by ΛCDM structure formation scenarios. This suggestion was taken up by Braun & Burton (1999) who identified 66 compact high-velocity clouds (CHVCs) with positional and kinematic characteristics consistent with a distribution across the entire Local Group.
More recent investigations have casted doubt on a Local Group population of CHVCs. Zwaan (2000) investigated five galaxy groups with the 300-m Arecibo radio telescope, but was unable to detect any intra-group gas. A similar survey by Pisano et al. (2004) with the 64-m Parkes radio telescope also produced a negative result, suggesting an upper limit of 160 kpc for the distance of CHVCs from the Galaxy. In addition, Sternberg et al. (2002) performed hydrostatic simulations of CHVCs and found their circum-galactic model with typical CHVC distances of about 150 kpc to be more consistent with the observed properties of CHVCs. Similar results were presented by Kravtsov, Gnedin & Klypin (2004).
Effelsberg observations of CHVCs
To investigate the properties of CHVCs in more detail, we observed a sample of 11 CHVCs in H Ⅰ with the 100-m radio telescope at Effelsberg. The most outstanding result from our observations is the apparent complexity of both the H Ⅰ column density distribution and the line profiles of the investigated CHVCs. We found only one cloud with a spherically-symmetric appearance. Among the remaining clouds we discovered head-tail structures, bow-shock shapes and objects with an entirely irregular morphology. These complex morphologies of the clouds in combination with their physical properties suggest that ram-pressure stripping by an ambient medium could have affected 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 where the gas of the extended Galactic corona might provide the necessary particle densities to account for the observed deformations. The pressure of the ambient medium could also help to stabilise CHVCs in addition to their own gravitational potential.
Publications