High-Velocity Clouds

High-Velocity Clouds (HVCs) can be considered more or less as primordial gas clouds. They are distributed across the whole sky and appear as coherent structures only in the HI 21-cm line emission. The term "high-velocity" denotes the fact that HVCs do not participate in the rotation of the Milky Way Galaxy. Their radial velocities deviate at least by 50 km s^-1 from the normally rotating interstellar medium.

Shortly after the discovery of HVCs in 1963 it was proposed that HVCs might represent halo clouds falling onto the Milky Way. Despite of the fact that HVCs are bright in HI 21-cm line emission, for more than 30 years all attempts failed to detect them in any other wavelength regime in emission until the 90th. Moreover, only a hand full of HVCs distances could be determined using interstellar absorption lines so far. Obviously the majority of the HVCs is located beyond the stellar distribution of the Milky Way.

Today HVCs are also studied in emission in hydrogen recombination lines and by our group in soft X-ray emission. Already in 1994 we proposed that some HVCs are interacting with the disk-halo interface of the Milky Way. By compressing the Milky Way gas the volume density increases and yields an excess of soft X-ray emission. The association of collisionally ionized gas with HVCs is now generally accepted and independently confirmed by far ultra-violet absorption lines measurements.

Further evidence for the interaction of HVCs with their environment is provided by the HI 21-cm line observation itself. Our group detected the so-called "velocity bridges" which connect HVC emission with the Milky Way HI 21-cm line emission in velocity space. Moreover, we could establish the existence of "head-tail structures" in all but one HVC complexes. These head-tail structures can be interpreted as tracers for the interaction of HVCs with their ambient medium.

Compact High-Velocity Clouds

Compact High-Velocity Clouds (CHVCs) are a sub-class of HVCs. While HVCs are considered to be located within the Milky Way Halo and interacting with the gaseous layer of the Galaxy, CHVCs are thought to represent the gaseous debris, residuals from the formation of the three spiral and many dwarf galaxies within the Local Group of galaxies. CHVCs were first identified in 1998 based on the Leiden/Dwingeloo HI survey. Today we know about 200 CHVCs. Their high number is of utmost interest for cold-dark-matter cosmological models. Because of their high frequency they might be the missing link between the theoretically expected high number of low-mass objects and the low number of known dwarf galaxies in the Local Group.

In case that CHVCs represent the low-mass end of dwarf galaxies, some of them might have formed stars. Using one of the Very Large Telescope 8-m class telescopes we searched for stars in several CHVCs without any success. This investigation clearly points out that CHVCs are most probably pure gas clouds without any stellar population.

Of actual scientific interest are the structure and the physical conditions within CHVCs. This information will allow to constrain the location of CHVCs, and also allow to differentiate between a circum-galactic and extragalactic location of CHVCs. This information is of major interest for present cold-dark-matter models.

For further information please do not hesitate to contact Tobias Westmeier, Christian Brüns or Jürgen Kerp.

The (C)HVC 125+41-207

The figure shows the Effelsberg HI 21-cm observations of the Compact High-Velocity Cloud CHVC 125+41-207. This CHVCs is the prime example for an interacting HVC with its ambient medium. Like a comet the CHVC consists of a dense core (head) and a trailing tail. The HI morphology of HVC 125+41-207 indicates that it is currently interacting with its ambient medium. The estimated distance of about 130 kpcs might prove the existence of a very extended gaseous halo of the Milky Way.

Selected Publications

Brüns C., Kerp J., Pagels A., 2001, "Deep HI observations of the compact high-velocity cloud HVC 125+41-207" A&A 370, L26 see astro-ph/0103119

Brüns C., Kerp J., Staveley-Smith L., et al. 2004 "The Parkes HI survey of the Magellanic System" Astronomy & Astrophysics, see astro-ph/0411453

Brüns C., Kerp J., Kalberla P.M.W., Mebold U., 2000, "The head-tail structure of high-velocity clouds. A survey of the northern sky", A&A 357, 120

Brüns C., Westmeier T. 2004 "HI observations of an ultra-compact high-velocity cloud" Astronomy & Astrophysics Letters , see astro-ph/0409153

Hopp U., Schulte-Ladbeck R., Kerp J., 2003,"Searching for Stars in Compact High-Velocity Clouds I. First Results from VLT and 2MASS", MNRAS 339, 33 see astro-ph/0210287

Kerp J., Burton W.B., Egger R., Freyberg M.J., Hartmann Dap, Kalberla P.M.W., Mebold U., Pietz J., 1999, A Search for Soft X-ray Emission of Prominent High-Velocity-Cloud Complexes, A&A 342, 213

Kerp J., Lesch H., Mack K.-H., 1994, "The Impact of a High Velocity Cloud onto the Galactic Disk", A&A 286, L13

Kerp J., Mack K.-H., Egger R., Pietz J., Zimmer F., Mebold U., Burton W.B., Hartmann Dap, 1996, "High-Velocity Clouds impacting onto the Galactic Disk", A&A 312, 67

Konz C., Brüns C., Birk G.T., 2002,"The dynamical evolution of HVCs in the intergalactic medium", A&A 391, 713

Pietz J., Kerp J., Kalberla P.M.W., Mebold U., Burton W.B., Hartmann Dap, 1996, "HI Velocity Bridges: Signature of HVC Interaction with the Galactic Halo", A&A 308, L37

Westmeier T., Brüns C., Kerp J. 2004 "Compact high-velocity clouds around the Galaxy and M 31" in Proc. of the extraplanar gas conference, Dwingloo June 2004, see astro-ph/0409371

Co-Investigators

We like to mention our co-investigators: Dr. Ulrich Hopp (Universitäts Sternwarte München) Dr. Christian Konz (MPI für Plasmaphysik, München), Dr. Mary E. Puntman (University of Colorado, United States) and Prof. Dr. Regina Schulte-Ladbeck (University of Pittsburgh, United States).

Acknowledgements

We are grateful for the financial support by the Deutsche Forschungsgemeinschaft under grant No. Ke 757/4-1.

Verantwortlich: Jürgen Kerp (11/2004)