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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 Lodot and total masses in the 106-7 Modot 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

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

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.

The amount of gas brought into the Milky Way from the accretion of satellite galaxy material can be estimated by presuming that all of its currently gas-less satellites had their fuel assimilated into the Galactic disk. Known stellar streams around the Milky Way can also be included, though obviously we cannot account for the dwarfs that have also had their stellar component completely assimilated in the disk. The gas-less dwarf galaxies in Figure 1 most likely once had a similar gas mass to the dwarf galaxies with gas, or ~ 105-7 Modot. The gas-less dwarf galaxies and 4 other destroyed dwarfs traced by stellar streams (Newberg et al. 2007; Belokurov et al. 2007) would therefore have contributed ~ 106-8 Modot of HI to fuel the Galaxy. This value would increase with the inclusion of the satellites' molecular and ionized gas, but it is unlikely to increase by more than a factor of two (e.g., Leroy et al. 2007). Chemical evolution models of the Galactic disk typically require an average infall rate of 1 Modot/yr over the past 5-7 Gyr (Chiappini et al. 2001), and therefore the dwarf galaxies appear to provide sufficient fuel for only ~ 108 years. It should be noted that the 1 Modot/yr is an average rate of infall, and it should be closer to half that value today (Chiappini, this proceedings). In any case, to accomplish the average of 1 Modot/yr from only the accretion of gas from small dwarf galaxies would require an average of 300 accretion events per Gyr. This is extremely high compared to the predictions of LambdaCDM models (Zentner et al. 2005), and the number of satellites we currently see in the halo.

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 Modot 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 Modot. Back.

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