H I observations of an Ultra-Compact High-Velocity Cloud

Free access article

Issue		A&A Volume 426, Number 1, October IV 2004


Page(s)		L9 - L13
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:200400068

A&A 426, L9-L13 (2004)
DOI: 10.1051/0004-6361:200400068

Letter

H I observations of an Ultra-Compact High-Velocity Cloud

C. Brüns and T. Westmeier

Radioastronomisches Institut, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: cbruens@astro.uni-bonn.de

(Received 29 July 2004 / Accepted 6 September 2004)

Abstract
We present $\ion{H}{i}$ 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 $\Delta v_{\it FWHM} = 4.9$ km s ^-1 providing an upper limit to the kinetic temperature of the $\ion{H}{i}$ gas of $T_{\rm k} \le 532$ K. The total observed flux indicates an $\ion{H}{i}$ mass of M( $\ion{H}{i}$ ) = $5.66\times10^{4}$ $M_\odot$ $\left[d/150\,{\rm kpc}\right]^2$ . Follow-up $\ion{H}{i}$ observations using the Australia Telescope Compact Array (ATCA) resolve HVC289+33+251 into 5 condensations that are embedded in a common $\ion{H}{i}$ envelope. The HVC shows a faint tail, indicating an ongoing ram-pressure interaction with an ambient low-density medium. A FWHM diameter of $\vartheta$ = 4 $\farcm$ 4 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 $\Lambda$ CDM 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.

Key words: Galaxy: halo -- ISM: clouds -- ISM: individual objects: high-velocity clouds -- dark matter

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