High–resolution imaging of compact high–velocity clouds

Free access article

Issue		A&A Volume 391, Number 1, August III 2002


Page(s)		67 - 81
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20020640

A&A 391, 67-81 (2002)
DOI: 10.1051/0004-6361:20020640

High-resolution imaging of compact high-velocity clouds

V. de Heij¹, R. Braun² and W. B. Burton^{1, 3}

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
² Netherlands Foundation for Research in Astronomy, PO Box 2, 7990 AA Dwingeloo, The Netherlands
³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22903, USA

(Received 14 March 2002 / Accepted 24 April 2002)

Abstract
We have imaged five compact high-velocity clouds in H I with arcmin angular resolution and km s ^-1 spectral resolution using the Westerbork Synthesis Radio Telescope. These CHVCs have a characteristic morphology, consisting of one or more quiescent, low-dispersion compact cores embedded in a diffuse warm halo. The compact cores can be unambiguously identified with the cool neutral medium of condensed atomic hydrogen, since their linewidths are significantly narrower than the thermal linewidth of the warm neutral medium. Because of the limited sensitivity to diffuse emission inherent to interferometric data, the warm medium is not directly detected in the WSRT observations. Supplementary total-power data, which is fully sensitive to both the cool and warm components of H I, is available for comparison for all the sources, albeit with angular resolutions that vary from 3 $^\prime$ to 36 $^\prime$ . The fractional H I flux in compact CNM components varies from 4% to 16% in our sample. All objects have at least one local peak in the CNM column density which exceeds about $10^{19}\rm\;cm^{-2}$ when observed with arcmin resolution. It is plausible that a peak column density of 1-2 $\,\times\,10^{19}\rm\;cm^{-2}$ is a prerequisite for the long-term survival of these sources. One object in our sample, CHVC 120-20-443 (Davies' cloud), lies in close projected proximity to the disk of M 31. This object is characterized by exceptionally broad linewidths in its CNM concentrations, more than 5 times greater than the median value found in the 13 CHVCs studied to date at comparable resolution. These CNM concentrations lie in an arc on the edge of the source facing the M 31 disk. The diffuse H I component of this source, seen in total-power data from the NRAO 140-foot telescope, has a positional offset in the direction of the M 31 disk. All of these attributes suggest that CHVC 120-20-443 is in a different evolutionary state than most of the other CHVCs which have been studied. Similarly broad CNM linewidths have only been detected in one other cloud, CHVC 110.6-07.0-466 (Wakker & Schwarz 1991) which also lies in the Local Group barycenter direction and has the most extreme radial velocity known. A distinct possibility for Davies' cloud seems to be physical interaction of some type with M 31. The most likely form of this interaction might be the ram-pressure or tidal-stripping by either one of M 31's visible dwarf companions, M 32 or NGC 205, or else by a dark companion with an associated H I condensation. The compact objects located in the direction of the Local Group barycenter have an important role to play in constraining the Local Group hypothesis for the deployment of CHVCs.

Key words: ISM: atoms -- ISM: clouds -- Galaxy: evolution -- Galaxy: formation -- galaxies: dwarf -- galaxies: Local Group

Offprint request: R. Braun, rbraun@astron.nl

SIMBAD Objects

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.