Australia Telescope National Facility

Search for HI in Dwarf Spheroidal Galaxies
Claude Carignan, PASA, 16 (1), in press.

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Discussion & conclusions

What is the origin of that detected gas? Where is the missing gas in dSph galaxies? As discussed for the case of Sculptor in Carignan et al. (1998), the origin of the gas could be internal as well as external. For the internal case, it can be shown that stellar mass loss in normal giants could easily account for the observed neutral gas. As summarized by Mould et al (1990), the total mass loss rate expected from normal evolution is about 0.015 ${\cal M}$ $_\odot$ yr^-1 per 10⁹ $\L$ $_\odot$ _,B. For Sculptor ( $L_B \sim 10^7$ $\L$ $_\odot$ ), this implies a total return of

1.5 x 10⁵ ${\cal M}$ $_\odot$ per Gyr. Since most of the SF seems to have taken place between 8 and 10 Gyr in Sculptor (Da Costa 1984), it would have produced a gas reservoir of

$\sim 3.0 \times 10^5$ ${\cal M}$ $_\odot$ . So, only 10% of this need be retained in its neutral form to account for the HI detected by the present observations.

On the other hand, there is circumstantial evidence supporting the notion that dSph galaxies may be accreting gas of external origin. In Carina, Smecker-Hane et al (1994) argue for a large age spread yet a surprisingly small spread in abundance. This is in striking contrast to Leo I which has a complex SF history (Lee et al 1993) and a broad giant branch indicative of a large abundance spread. This behavior makes somewhat more sense if the gas that formed distinct generations of stars in these galaxies was accreted or captured from distinct clouds with their individual - and therefore random - mean abundances. Wakker and van Woerden (1997) review the observations of HVC in the halo; their summary of past surveys suggest that many small, low-column density, and distant clouds could still be hidden throughout the Galactic halo.

As for the missing gas, the reason why HI was not detected before, especially with single dish observations having typically $\sim 15'$ beams, is probably because we were not looking at the right place, as suggested by Puche & Westpfahl (1994). This is why, in the coming months, we intend to search large areas around the LG dSph galaxies (

$\delta \le 0^{\rm o}$ ) observed by the HIPASS (HI Parkes All-Sky Survey), namely: Sculptor, Phoenix, Fornax, Carina, Antlia, Sextans, DDO 210, and Tucana.

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