6.1. Gas and Environment
When plotting galaxy HI masses versus distance from the closest massive galaxy in the Local Group and its immediate surroundings, we see a tendency for HI masses to increase with galactocentric distance (Fig. 4; Grebel et al. 2003, and references therein). Only fairly large galaxies, such as the Magellanic Clouds, IC10, and M33, with HI masses >> 107 M, seem to be able to retain their gas reservoirs when closer than ~ 250 kpc to giants (Grebel et al. 2003). Note that weak lensing measurements and dynamical modeling indicate typical dark matter halo scales for massive galaxies of 260 h-1 kpc (e.g., McKay et al. 2002). The bulk of the Local Group dIrrs and transition-type galaxies are located beyond ~ 250 kpc from M31 and the Milky Way. At these distances these gas-rich galaxies seem to be less prone to galaxy harassment (i.e., in this case loss of gas through interactions with spirals), although the details will depend on their (yet unknown) orbital parameters (Grebel et al. 2003). A similar trend for dIrrs and dIrr/dSphs is seen in the Sculptor group (Skillman, Côté, & Miller 2003a).
Figure 4. Dwarf galaxy HI mass versus distance to the nearest massive galaxy. Filled circles stand for dwarf spheroidals (dSphs), open circles for dwarf ellipticals, open diamonds for dwarf irregulars (dIrrs), and filled diamonds for dIrr/dSph transition-type galaxies. Lower or upper HI mass limits are indicated by arrows. There is a general trend for the HI masses to increase with increasing distance from massive galaxies. DSphs lie typically below 105 M in HI mass limits, while potential transition-type galaxies have HI masses of ~ 105 to 107 M. DIrr galaxies usually exceed 107 M. (Figure from Grebel et al. 2003.)
Skillman, Côté, & Miller (2003b) investigated correlations between HI mass fraction and metallicity (oxygen abundance) for Local Group dIrrs and dIrrs in the Sculptor group of galaxies. They found that the Local Group dIrrs deviate from model curves expected for closed-box evolution, whereas the Sculptor group dIrrs follow these curves rather closely. Indeed the dIrrs that exhibit the strongest deviations are those with very low metallicity and gas content. Considering that the Sculptor group is, in contrast to the Local Group, a very loose and diffuse group or cloud (Karachentsev et al. 2003b), this may indicate that closed-box evolution is more likely to occur in low-density environments with little harassment.