This survey of H I in the Local Group has ranged from the disk-halo clouds, which are certainly confined to the stellar disks, through the high velocity clouds, which seem preferentially located around the disks of galaxies, to material that is not incorporated into galaxies: the Magellanic Stream and Leading Arm, the H I streams intersecting IC 10, and finally the M31-M33 clouds. The first two, at least, are associated with stellar systems, while the M31-M33 clouds are not. Their properties are summarized in Table 1. How common are objects like this in other galaxies and other groups?
| Object | MHI (M⊙) | Size (kpc) | Reference | Notes |
| Disk-halo clouds | 7 × 102 | 0.06 | Ford et al. (2010) | |
| Individual HVCs | ∼ 106 | 2-15 | Putman et al. (2012) | |
| All HVCs in MW, M31 or M33 | 2−5 × 107 | Westmeier et al. (2008); Putman et al. (2012); Keenan et al. (2016) | ||
| H I stripped from MW dwarfs | 3 × 107 | Spekkens et al. (2014) | ||
| IC 10 northern stream | 6 × 105 | 7 | Nidever et al. (2013); Ashley et al. (2014) | |
| IC 10 southern stream | ∼ 107 | 7 | Nidever et al. (2013); Ashley et al. (2014) | |
| Magellanic Stream | 3 × 108 | ∼ 100 | Brüns et al. (2005); Nidever et al. (2010) | a |
| Leading Arm | 3 × 107 | ∼ 60 | Putman et al. (2003) | a |
| M31-M33 clouds | ∼ 105 | 2 | Wolfe et al. (2016) | b |
| M31-M33 clouds all | > 1.6 × 106 | Wolfe et al. (2016) | b | |
| a. Assumed distance 55 kpc. b. Assumed distance 800 kpc. | ||||
The disk-halo clouds and most HVCs would be blended together in current H I observations of galaxies outside the Local Group, but similar material appears to be present in many other galaxies (see section 2). In very sensitive H I observations many, if not most spiral galaxies show H I streams or features extending from the disk. This suggests that we are witnessing interaction, even though in many cases an accreting stellar companion cannot be identified (Sancisi et al., 2008). True starless H I clouds not associated with a galaxy seem rare, existing perhaps at the level of 2% of the H I objects detected in deep surveys, and even so, many of these seem to be located near large galaxies or be associated with tidal debris (Kovač et al., 2009, Haynes et al., 2011). Some of the most interesting objects, like the M31-M33 clouds and the plumes of IC 10, could be detected only out to a few Mpc from the Milky Way with current instrumentation.
The amount of H I in starless gas in Local Group galaxies is rather modest compared to the enormous structures produced by galaxy interactions in other groups. The classic example is the M81 group (Yun et al., 1994; Chynoweth et al., 2008), but evidence for interactions like this in other galaxy groups abounds. The data are too numerous to summarize here, but as merely one example, recent observations (Leisman et al., 2016) have detected extended H I features 600 kpc in length without stellar counterparts around the NGC 3190 and NGC 3227 groups (Fig. 9).
 |
Figure 9. Map of the H I around the NGC 3190 and NGC 3227 groups, showing extended tails and intragroup gas probably resulting from interactions between galaxies (Leisman et al., 2016). |
If our Local Group is any example, the best place to detect starless neutral gas clouds is relatively close to galaxies. As deep H I surveys of the Local Group cover more area, we will obtain better insight into the origin of starless gas, and determine if it is always located near galaxies, or is more widespread.