Introduction

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 $\sim$ 1/50 (O/H) $_{\odot}$ , 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) $_{\odot}$ (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 $M_{\odot}$ ) 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 $Z_{\odot}$ ) (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) < 3 $\times$ 10^-5(O/H) $_{\odot}$ , 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) $_{\odot}$ 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) $_{\odot}$ . This is one of the nearest, and at the same time the dimmest candidate young galaxy known.

**Figure:** R-band image of HS 0822+3542. North is to the top, east is to the left. Low-contrast filaments NW of the main bright body indicate gas structures typical of high-velocity ejecta. At the adopted distance of 12.5 Mpc, 1 $^{\prime \prime }$ = 60.6 pc. Surface brightness isophotes (white contours) are superposed on the central bright region to show the morphology of supergiant H II region.
$\begin{figure}\psfig{figure=HS0822+3542R_direct.ps,angle=0,width=8.5cm,bbllx=70pt,bblly=300pt,bburx=520pt,bbury=750pt} \end{figure}$

**Table 1:** Main parameters of the young galaxy candidates
Parameter	0822+3542¹	IZw18	0335-052E
$B_{\rm tot}$	17.92 $\pm$ 0.07	16.21²	17.00 $\pm$ 0.02³
$(B-V)_{\rm tot}$	0.32 $\pm$ 0.08	0.18²	0.31 $\pm$ 0.05³
$(V-R)_{\rm tot}$	0.17 $\pm$ 0.09	0.41⁴	0.15 $\pm$ 0.05³
$V_{\rm Hel}$ (km s^-1)	732 $\pm$ 6	751 $\pm$ 2⁵	4043 $\pm$ 5⁶
$D_{\rm Vir}$ (Mpc)	12.5	14.2¹	52.8⁶
E(B-V)	0.047¹¹	0.032¹¹	0.047¹¹
M_B $^{\dag }$	-12.7	-14.64	-16.8³
Angular size () $^{\ddag }$	14.8 $\times$ 7.4	22 $\times$ 15¹	14 $\times$ 10³
Optical size (kpc)	0.90 $\times$ 0.45	1.5 $\times$ 1.0	3.7 $\times$ 2.6³
12+log(O/H)	7.35	7.16⁷	7.29⁸
$T_{\rm e}$ (O III) (K)	20,350	19,600⁷	19,300⁸
H I flux^*	0.68 $\pm$ 0.07	2.97⁹	2.46⁶
W₅₀ (km s^-1)	42 $\pm$ 5	49⁹	83⁵
W₂₀ (km s^-1)	58 $\pm$ 8	84⁹	105⁵
M(H I) (10⁸ $M_{\odot}$ )	0.24	1.41⁹	16.2⁶
M(H I)/L_B^**	1.40	1.40	2.3
SFR ( $M_{\odot}$ /year)	0.007	0.04¹⁰	0.4¹⁰
$B_{\rm tot}$ - total blue magnitude; M_B - absolute blue mag.
L_B - total blue luminosity. ^* Units of Jy km s^-1;
^** In units of (M/L_B) $_{\odot}$ ; $^{\dag }$ With the Galaxy extinction
A_B = 0.20, 0.14, 0.20, respectively,
corresponding to E(B-V) in the previous line;
$^{\ddag }$ $a \times b$ at surface brightness $\mu_{B}$ =25 mag arcsec^-2.
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])