Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://www.sao.ru/precise/Laboratory/Publications/2000/0822_AA_v0/node15.html
Дата изменения: Sat Mar 11 12:16:53 2000 Дата индексирования: Sat Sep 11 21:00:19 2010 Кодировка: Поисковые слова: annular solar eclipse |
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
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)108
(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
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
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 .
Therefore, if these
extremely metal-poor galaxies with masses ranging from 3
107 to
2.5
109
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.