HI distribution in extreme dIrr's & dE's

Before looking at the ISM in dSph galaxies, it is instructive to look at what is known on the HI distribution in low luminosity dE and dIrr galaxies. For the dE's, the best study is the one of Young & Lo (1997a), where they looked at the HI content and distribution of NGC 185 and NGC 205. In both cases, they found HI masses , with the peak of the distribution having a slight offset ( S for NGC 205 and NE for NGC 185) from the optical center.

For the low-luminosity (M

) dIrr's, mainly two kinds of HI distributions are found: first, there are systems like GR8 (Carignan, Beaulieu & Freeman 1990) and Sextans A (Skillman et al. 1988) where most of the HI is in a few clouds just outside the optical and second, those such as Leo A (Young & Lo 1996), SagDig (Young & Lo 1997b) and M81dwA (Fig. 1) where the HI is distributed in a complete ring, again just outside the optical. In fact, it is possible that we may see the same distribution in GR8 and Sextans A when higher spatial resolution data become available. Those systems have HI masses around 10⁶ - 10⁸.

Figure 1: HI surface densities of M81dwA from VLA observations. (courtesy of E. Brinks & F. Walter.

Puche & Westpfahl (1994) gave a good summary of the situation. Large expanding cavities, surrounded by dense shells are found in the ISM of the dwarfs they observed at high resolution. Energy arguments suggest that they are created by stellar winds and supernova explosions. In the smallest dwarfs, one large, slowly expanding shell usually dominates the ISM. The expansion and contraction of the entire HI component is interpreted as being associated with bursts of SF. They speculated that in extremely low mass dwarfs, the initial burst of SF could deplete the galaxy of almost all its gas. This scenario suggests a search for HI emission outside the optical disk in galaxies which may have evolved in this way, such as dSph's. This is the program we started a few years ago.

б© Copyright Astronomical Society of Australia 1997