The Arecibo Galaxy Environment Survey X: The Structure of Halo Gas Around M33

TL;DR

This study maps the halo gas around M33, discovering 22 HI clouds, including a peculiar one without optical counterparts, and discusses their implications for galaxy formation and interactions in the Local Group.

Contribution

First detailed HI survey of M33's environment revealing new HI clouds and analyzing their properties in relation to galaxy formation models.

Findings

Identified 22 HI clouds, 11 of which are new detections.

The velocity dispersion of clouds aligns with galaxy formation expectations.

One cloud, AGESM33-31, resembles a galaxy disc but lacks an optical counterpart.

Abstract

As part of the HI Arecibo Galaxy Environments Survey (AGES) we have observed 5$\times$4 degrees of sky centred on M33, reaching a limiting column density of $\sim 1.5 \times 10^{17}$ 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 virialised 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 $1.22 \times 10^{7}$ M$_{\odot}$ and a diameter of 18 kpc if at the distance of M33 ($D_{M33}=840$ 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.