4.4. Differences in Gas Content
The H I in dIrrs is generally more extended than the oldest stellar populations and shows a clumpy distribution. Gas and stars in a number of low-mass dIrrs exhibit distinct spatial distributions and different kinematic properties. Shell-like structures, central H I holes, or off-centered gas may be driven by recent star formation episodes (Young & Lo 1996; 1997a, b). H I shells, however, do not always expand, which may argue against their formation through propagating star formation (Points et al. 1999, de Blok & Walter 2000).
Ongoing gas accretion appears to be feeding the starburst in the dIrr IC 10 (Wilcots & Miller 1998). An infalling or interacting H I complex is observed in the dIrr NGC 6822 (de Blok & Walter 2000).
DEs in the Local Group contain low amounts of gas (a few 105 M; Sage, Welch, & Mitchell 1998) or none (NGC 147). The apparent lack of gas in dSphs (e.g., Young 2000) continues to be hard to understand, in particular when considering that some dSphs show evidence for recent (Fornax: ~ 200 Myr, Grebel & Stetson 1999) or pronounced intermediate-age star formation episodes (e.g., Carina: 3 Gyr; Hurley-Keller, Mateo, & Nemec 1998; Leo I: 2 Gyr; Gallart et al. 1999). Gas concentrated in two extended lobes along the direction of motion of the Sculptor was detected beyond the tidal radius of this galaxy (Carignan et al. 1998). This gas may be moving inwards or away from Sculptor. Its amount is consistent with the expected mass loss from red giants, though that does not explain its location along the probable orbital direction of Sculptor. Blitz & Robishaw (2000) suggested the existence of similar gas concentrations with matching radial velocities in the surroundings of several other dSphs. Simulations by Mac Low & Ferrara (1999) suggest that total gas loss through star formation events can only occur in galaxies with masses of less than a few 10-6 M. Blitz & Robishaw discuss tidal effects as the most likely agent for the displacement of the gas. However, the absence of gas in Cetus and Tucana, two isolated dSphs in the Local Group, requires a different mechanism.