The Local Group contains two wonderful examples of H I without stars, both associated with the Magellanic Clouds: the Magellanic Stream (MS) and the Leading Arm (LA). These are the rather enormous streams of gas (containing about 10% the mass of the Milky Way's ISM) that are currently being lost from the Magellanic Clouds and are entering the Milky Way's CGM. A distance of 55 kpc is assumed for both objects, though this has a large uncertainty; for a recent review see D'Onghia & Fox, 2016. The relevant H I masses are 3 × 108 M⊙, and 3 × 107 M⊙, for the Stream and Leading Arm, respectively (Brüns et al., 2005). The total mass of the Stream is ∼ 109 M⊙, and is dominated by ionized gas (Fox et al., 2014). The Magellanic Clouds are the only stellar systems in the Local Group that are observed to be losing their gas. The Magellanic Stream is important for our purposes because it gives us a model for how we might interpret observations of other systems. Although the combined MS + LA is 200∘ long (Putman et al., 2003; Nidever et al., 2010) I estimate that no more than half this length would appear with NHI > 1019 cm−2 at a few Mpc distance, and thus be detectable in 21cm H I emission in another galaxy group. The detectable part of the MS would have a length ∼ 100 kpc and for the Leading Arm, about 60 kpc.
Although we can see that the Stream and the Leading Arm are anchored in the stellar systems of the Magellanic Clouds, there are no stars associated with the gas of either object (D'Onghia & Fox, 2016). The absence of stars in the gas mirrors the fact that none of the stellar streams around the Milky Way or M31 have been found to have a convincing association with any neutral gas (Lewis et al., 2013).
The Milky Way's CGM seems quite resistant to the passage of neutral gas clouds even when they are attached to galaxies. Dwarf spheroidals within a few 100 kpc of the Milky Way seem utterly devoid of H I: the limits on some systems with L* ≈ 2.5 × 105 L⊙ are smaller than 100 M⊙ (Spekkens et al., 2014). A similar deficit is found for the dwarf satellites of M31 (Beaton, private communication). Because dwarf galaxies > 400 kpc from the Milky Way seem to have retained their gas, the deficit for nearer dwarfs presumably reflects their stripping by the CGM (Blitz & Robishaw, 2000, Grcevich & Putman, 2009). Distant dwarfs have MHI / L* ≈ 1 (Spekkens et al., 2014), so we can estimate the H I mass that has been deposited in the CGM through this stripping. For the dwarf galaxies currently known around the Milky Way, it amounts to MHI ≈ 3 × 107 M⊙, an value almost identical to the estimated total H I mass of HVCs in each of the HVC systems of the Milky Way, M31, and M33 (Putman et al., 2012; Westmeier et al., 2008; Keenan et al., 2016). It is tempting to dismiss this as a coincidence, especially because the inferred mass of the “stripped” H I for the Milky Way is dominated by the most massive galaxy, the Sagittarius dSph (Spekkens et al., 2014). But we do not have a good understanding of the inflow rate and location of the stripped material, which likely eventually make its way to the disk.