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Дата изменения: Sat Mar 11 12:16:53 2000 Дата индексирования: Sat Sep 11 20:54:41 2010 Кодировка: Поисковые слова: diurnal motion |
Since the Searle & Sargent (1972) paper identifying blue compact galaxies
(BCGs), that is, low-mass galaxies showing emission line spectra
characteristic of H II regions, intense star formation (SF), and oxygen
abundances of 1/50 - 1/3 solar, such objects
have been considered as young galaxies undergoing
one of their first star formation bursts.
I Zw 18, a BCG with the lowest known oxygen abundance among the
galaxies (O/H
1/50 (O/H)
,
Searle & Sargent [1972];
Izotov & Thuan [1999]), has been suggested as a candidate to be a
truly-local young galaxy, experiencing
its first short SF episode.
The second candidate young galaxy, SBS 0335-052E, with an oxygen abundance of
1/41 (O/H)
(Melnick et al. [1992]; Izotov et al.
[1997a]; Lipovetsky et al. [1999]) was discovered
18 years later by Izotov et al. ([1990]).
With only two probable examples, we must be extremely lucky to be witnessing
local galaxy formation. The proximity of these probable
young galaxies allows one to study their properties in detail
and to set important constraints on models of galaxy formation.
Such studies are important for understanding the nature
of very faint and compact probable primeval galaxies at high redshifts.
Most of such galaxies at z = 3-5 were discovered only recently
(e.g. Steidel et al. [1996]; Dey et al. [1998]),
and it seems that the majority of them are already rather evolved systems.
Moreover, the local candidate young galaxies are at least one order of
magnitude less massive than the faintest candidate young galaxies
at high redshifts, and represent the range of baryon mass
(108-109
)
within which possibly most of primeval
galaxies have formed (e.g. Rees [1988]).
Evidence for the existence of old low-mass stellar populations was obtained in the last 25 years for most of the studied BCGs (Thuan [1983]; Loose & Thuan [1986]). Moreover, no conclusive answer has been reached yet about the youth of the few most metal-poor BCG. However, some observational data have been collected lately, which apparently support young ages for these BCGs. Among them we point out:
a) Extremely low abundances of heavy elements in H II regions
surrounding young
clusters, consistent with theoretical expectations of ``metal'' yield
during a first SF event (Z < 1/20 )
(e.g., Pilyugin
[1993]);
b) Very blue colours outside the location of the current SF burst, consistent with a lack of stars older than 100 Myr (Hunter & Thronson [1995]; Papaderos et al. [1998]). While the recent analysis of HST data for I Zw 18 by Aloisi et al. ([1999]) suggests an age of 1 Gyr for the underlying stellar population of the galaxy, Izotov et al. ([2000]) argue that a self-consistent treatment of all data favours a significantly larger distance to I Zw 18 then adopted by Aloisi et al., and a 100 Myr stellar population;
c) A large amount of neutral gas, making up 99% of all baryonic (luminous) mass (van Zee et al. [1998]; Pustilnik et al. [2000]);
d) Practically zero metallicity for this H I gas,
e.g., (O/H) < 310-5(O/H)
,
as reported for
SBS 0335-052E (Thuan & Izotov [1997]).
This emphasizes either an extremely slow evolution on these systems,
or a very recent onset of metal production.
The latter suggests that the neutral gas clouds in these galaxies are composed
of pregalactic material not yet polluted by stellar nucleosynthesis products.
It was suggested recently by Izotov & Thuan ([1999]),
from the analysis of carbon and nitrogen abundances, that several
BCGs with O/H < 1/20 (O/H)
in H II regions
are good candidate galaxies with a recent first
SF episode. Until now, less than ten such galaxies with good abundance
determinations are known. Even though the point on the existence of
truly young
local galaxies is debatable (see, e.g., Kunth & Östlin [1999]),
the importance of studies of extremely metal-poor galaxies is undoubtful,
since
they best approximate the properties of primeval galaxies at
large redshifts.
In this paper we describe the data obtained for the third most
metal-deficient galaxy, HS 0822+3542 with O/H = 1/36 (O/H).
This is one of the nearest, and at the same time the
dimmest candidate young galaxy known.
![]() |
Parameter | 0822+35421 | IZw18 | 0335-052E |
|
17.92![]() |
16.212 | 17.00![]() |
![]() |
0.32![]() |
0.182 | 0.31![]() |
![]() |
0.17![]() |
0.414 | 0.15![]() |
![]() |
732![]() |
751![]() |
4043![]() |
![]() |
12.5 | 14.21 | 52.86 |
E(B-V) | 0.04711 | 0.03211 | 0.04711 |
MB![]() |
-12.7 | -14.64 | -16.83 |
Angular size ()![]() |
14.8![]() |
22![]() |
14![]() |
Optical size (kpc) | 0.90![]() |
1.5![]() |
3.7![]() |
12+log(O/H) | 7.35 | 7.167 | 7.298 |
![]() |
20,350 | 19,6007 | 19,3008 |
H I flux* | 0.68![]() |
2.979 | 2.466 |
W50 (km s-1) | 42![]() |
499 | 835 |
W20 (km s-1) | 58![]() |
849 | 1055 |
M(H I) (108![]() |
0.24 | 1.419 | 16.26 |
M(H I)/LB** | 1.40 | 1.40 | 2.3 |
SFR (![]() |
0.007 | 0.0410 | 0.410 |
![]() |
|||
LB - total blue luminosity. * Units of Jy km s-1; | |||
** In units of (M/LB)![]() ![]() |
|||
AB = 0.20, 0.14, 0.20, respectively, | |||
corresponding to E(B-V) in the previous line; | |||
![]() ![]() ![]() |
|||
References: 1This paper; 2Mazzarella & Boroson ([1993]); | |||
3Papaderos et al. ([1998]); 4Huchra ([1977]); 5Thuan et al. | |||
([1999a]); 6Pustilnik et al. ([2000]); 7Izotov & Thuan ([1998]); | |||
8Izotov & Thuan ([1999]); 9van Zee et al. ([1998]); | |||
10 Thuan et al. ([1997]); 11 Schlegel et al. ([1998]) |