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Дата изменения: Sat Mar 11 12:16:53 2000
Дата индексирования: Sat Sep 11 21:00:19 2010
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A candidate young galaxy?

The properties of HS 0822+3542 presented above (extremely low abundance of heavy elements, very blue colour of underlying nebulosity, and extremely small mass ratio of stars to neutral gas) suggest that this could be the nearest candidate young dwarf galaxy forming its first stellar generation. However, on the basis of the present data, we cannot exclude the presence of an underlying older stellar populations originating from earlier SF episodes.

Similar studies of the two previously known young galaxy candidates I Zw 18 and the pair SBS 0335-052E/0335-052W (Izotov et al. [1997a]; Thuan & Izotov [1997]; Pustilnik et al. [1997]; Lipovetsky et al. [1999]) have not been conclusive whether such young systems really do exist.

While the chemical properties (see Table 3) of these candidates seem to follow a general trend and behave quite homogeneously, other global properties have still to be understood. From Table 1 it appears that they cover a broad range of neutral hydrogen masses and blue luminosities (a factor of $\sim$ 40-60). The same is true for their current SFRs and for the mass of stars formed in a single star formation episode.

We suggest that a simple linear scaling between several important parameters holds for forming galaxies, at least in the range of baryon masses of (0.3-20)$\times$10⁸ $M_{\odot}$ (see Table 1, and assuming that most of baryons in these BCGs are in atomic hydrogen and helium). This is important both for the analysis of conditions capable of maintaining pristine gas clouds stable for $\sim$ a Hubble time, and for the planning of further searches for such objects. In particular, such low-mass objects can represent a significant fraction of the Ly$\alpha$ absorbers at high redshifts.

One more indirect argument for the possible youth of the three BCGs considered here comes from the calculations of mass and heavy elements loss in dwarfs with active SF (Mac Low & Ferrara [1999]). These show that the rate of metal loss is strongly dependent on the baryon mass in the range of 10⁷ to 10⁹ $M_{\odot}$. Therefore, if these extremely metal-poor galaxies with masses ranging from 3$\times$10⁷ to 2.5$\times$10⁹ $M_{\odot}$ were not in their first SF episode, we should expect significant differences in their observational properties. For the three BCGs discussed here we do not find these differences.

From the surface density of already known objects with extremely low metallicity and small radial velocity one can expect to find at least ten more such galaxies within 15 Mpc, if the search will be extended to the entire sky.