The number of satellites in the Local Group has increased substantially recently with the results from the Sloan Digital Sky Survey (SDSS; e.g., Willman et al. 2005; Zucker et al. 2006; Belokurov et al. 2006) and surveys of the M31 environment (e.g., Ibata et al. 2007). The new satellites have typical V-band luminosities of 103-5 L and total masses in the 106-7 M range (Simon & Geha 2007). They are found at a range of distances about both the Galaxy and M31. Gas from satellites is generally the first thing to be stripped as it passes through the diffuse halo medium of the larger galaxy. The stripped gas will fuel, or has fueled, the Galaxy and M31.
Figure 1 shows the distribution of Galactic and M31 satellites in distance and HI mass (Grcevich & Putman 2008). This plot is similar to Blitz & Robishaw (2000), but is updated to incorporate new HI data, optical velocities, and all of the known Local Group dwarf galaxies. The majority of the dwarf galaxies in the Local Group that still have neutral hydrogen are beyond 300 kpc from the Galaxy and M31 1. In general those galaxies still with gas lie towards the outskirts of the Local Group. The gaseous detection of the new SDSS galaxy Leo T (Irwin et al. 2007) with the Galactic Arecibo L-Band Feed Array (GALFA) is shown in Figure 2 (see also Ryan-Weber et al. 2008). The majority of the galaxies that do not have neutral hydrogen are within ~ 300 kpc, with the exception of Tucana and Cetus. There could be a SDSS sensitivity effect for why there are not more gas-less galaxies at large radii (Koposov et al. 2007), but the result of having all of the galaxies with HI beyond 300 kpc does not change. The galaxies without gas have most likely been subjected to ram pressure stripping (e.g., Mayer et al. 2006), but halo densities high enough to effectively strip dwarf galaxies are not expected at radii of 150-300 kpc (see Grcevich & Putman 2008 for details).
Figure 1. The galactocentric distance to dwarf satellites of the Galaxy or M31 verses the HI mass or limit on the HI mass of the satellite (Grcevich & Putman 2008). HI detections are shown by the open diamonds and the majority are beyond 300 kpc from the Galaxy or M31. The non-detections (or limits) are shown by downward arrows, and Sculptor and Fornax are labeled as ambiguous detections as there are clouds in the vicinity that may or may not be associated.
Figure 2. A position-velocity plot of GALFA HI data showing the detection of the new SDSS gas-rich dwarf galaxy Leo T at ~ 35 km s-1 and the emission from the Galaxy.
There is a source of gaseous fuel associated with Galactic satellites that has not yet been discussed, the Magellanic System. This system is currently at ~ 55 kpc and consists of the Small and Large Magellanic Clouds, and the Magellanic Stream and Leading Arm that are being stripped from the Clouds (Putman et al. 2003). The Magellanic System will eventually contribute ~ 109 M of fuel just in neutral hydrogen to the Galaxy. Galaxies such as the Magellanic Clouds and larger bodies may have contributed to fueling the Galaxy and M31 in the past. In this case, only an average of 1 accretion event per gigayear would be required; however the stability of the disk would most likely be an issue (Stewart et al. 2007).
1 Excluding the Magellanic Clouds which together have a total mass > 1010 M. Back.