The Arecibo Galaxy Environment Survey - X. The structure of halo gas around M33

Abstract

As part of the HI Arecibo Galaxy Environment Survey (AGES), we have observed 5 × 4 deg of sky centred on M33, reaching a limiting column density of ~1.5 × 1017 cm-2 (line width of 10 km s-1 and resolution 3.5 arcmin). We particularly investigate the absence of optically detected dwarf galaxies around M33, something that is contrary to galaxy formation models. We identify 22 discrete HI clouds, 11 of which are new detections. The number of objects detected and their internal velocity dispersion distribution is consistent with expectations from standard galaxy formation models. However, the issue remains open as to whether the observed velocity dispersions can be used as a measure of the HI clouds total mass, i.e. are the velocities indicative of virialized structures or have they been influenced by tidal interactions with other structures in the Local Group? We identify one particularly interesting HI cloud, AGESM33-31, that has many of the characteristics of HI distributed in the disc of a galaxy, yet there is no known optical counterpart associated with it. This object has a total HI mass of 1.22 × 107 M⊙ and a diameter of 18 kpc if at the distance of M33 (DM33 = 840 kpc). However, we also find that there are numerous other HI clouds in this region of sky that have very similar velocities and so it is plausible that all these clouds are actually associated with debris from the Magellanic stream.