HI associated with Sculptor and Phoenix

Figure 2: HI surface densities of Sculptor, primary beam corrected, superposed on an STScI Digitized Sky Survey optical image. The contours are 0.2, 0.6, 1.0, 1.4, 1.8 & 2.2

x 10¹⁹ cm^-2. The moment maps were derived for the velocity range [72.3, 151.7] kms^-1.

The recent ATCA observation of Sculptor (Carignan et al. 1998) is the best case, so far, of HI probably associated with a bona-fide dSph galaxy. As can be seen in Fig.2, not only is the detected gas symmetrically distributed on each side of the optical, but it is also at the same systemic velocity than the optical. However, we know that the of HI is only a lower limit since most of the detected HI lies outside the HPBW of the ATCA antennae. Moreover, the size of the clouds ( = 0.5 kpc) corresponds to the largest scale that could be seen with the 375 m. configuration used (see section 6).

Better observations are now available for the dIrr/dSph Phoenix (Carignan, St-Germain, & Oosterloo 1998). It consists of a mosaic of 9 pointings obtained with the ATCA. Fig. 3 shows that the component around -23kms^-1now clearly overlaps the optical image. Moreover, this is the only velocity component that shows well ordered motion, with clear rotation kms^-1, while the other components only show random motions. However, optical velocities are clearly needed to make sure that this gas is truly associated with Phoenix.

Figure 3: HI surface densities for the component at kms^-1 in the field of Phoenix, superposed on an STScI Digitized Sky Survey optical image, for a mosaic of 9 pointings obtained with the ATCA. The contours are 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 & 4.0

x 10¹⁹ cm^-2. The moment maps were derived for the velocity range [-43.4, -7.1] kms^-1.

Already, some conclusions can be derived from those observations. First, the HI distribution, seen in Sculptor, with a concentration in two clouds just outside the optical is very similar to what is seen in the dIrr galaxy Sextans A (Skillman et al. 1988). Second, the slight offset of the HI distribution seen in Phoenix is more reminiscent of what is seen in the dE galaxies NGC 185 & NGC 205 (Young & Lo 1997a). Finally, the HI content around 10⁴ - 10⁵ compares to what is seen in dE's but is a factor 10² - 10³ less than what is seen in dIrr galaxies. Those similarities suggest possible evolutionary links between those three types of dwarfs.

© Copyright Astronomical Society of Australia 1997