Westerbork H I observations of high-velocity clouds near M 31 and M 33

Free access article

Issue		A&A Volume 436, Number 1, June II 2005


Page(s)		101 - 115
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20052867

A&A 436, 101-115 (2005)
DOI: 10.1051/0004-6361:20052867

Westerbork H I observations of high-velocity clouds near M 31 and M 33

T. Westmeier^{1, 2}, R. Braun² and D. Thilker³

¹ Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: twestmei@astro.uni-bonn.de
² ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
³ Center for Astrophysical Sciences, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA

(Received 11 February 2005 / Accepted 28 February 2005)

Abstract
We have undertaken high-resolution follow-up of a sample of high velocity $\ion{H}{i}$ clouds apparently associated with M 31 . Our sample was chosen from the population of high-velocity clouds (HVCs) detected out to 50 kpc projected radius of the Andromeda Galaxy by Thilker et al. (2004, ApJ, 601, L39) with the Green Bank Telescope. Nine pointings were observed with the Westerbork Synthesis Radio Telescope to determine the physical parameters of these objects and to find clues to their origin. One additional pointing was directed at a similar object near M 33 .

At 2' resolution we detect 16 individual HVCs around M 31 and 1 HVC near M 33 with typical $\ion{H}{i}$ masses of a few times $10^5 \, M_{\odot}$ and sizes of the order of 1 kpc. Estimates of the dynamical and virial masses of some of the HVCs indicate that they are likely gravitationally dominated by additional mass components such as dark matter or ionised gas. Twelve of the clouds are concentrated in an area of only $1^{\circ} \times 1^{\circ}$ at a projected separation of less than 15 kpc from the disk of M 31 . This HVC complex has a rather complicated morphological and kinematical structure and partly overlaps with the giant stellar stream of M 31 , suggesting a tidal origin. Another detected feature is in close proximity, in both position and velocity, with NGC 205 , perhaps also indicative of tidal processes. Other HVCs in our survey are isolated and might represent primordial, dark-matter dominated clouds.

Key words: ISM: clouds -- galaxies: local group -- galaxies: individual: M 31 -- galaxies: individual: M 33 -- galaxies: evolution -- cosmology: dark matter

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