1 Introduction

Since its discovery by Izotov et al. (1990) as a very metal-poor galaxy the blue compact dwarf (BCD) galaxy SBS 0335-052 (SBS - the Second Byurakan Survey) has been studied extensively. The oxygen abundance in SBS 0335-052 is 12 + log O/H = 7.30 (Melnick et al. 1992; Izotov et al. 1997, 1999) and places it after I Zw 18 as the second most metal-deficient BCD. The properties of SBS 0335-052 as a probable young galaxy are of great interest for cosmology (e.g., Izotov et al. 1997). Therefore, detailed studies of this galaxy can shed light on the formation and the properties of the high-redshift primeval galaxies. Using deep spectroscopic observations we concentrate in the present paper on two problems: (1) the origin of very hard radiation at wavelengths shorter 228 Å. This radiation is indicated by the presence of the strong He II $\lambda$4686 emission line (Izotov et al. 1997) and implies that some other weak emission lines of high-ionization species could be present in the spectrum of SBS 0335-052. (2) the analysis of the properties of extended emission around SBS 0335-052. This problem is of great importance in understanding the evolutionary status of SBS 0335-052. In particular, we aim to understand how important gaseous emission is in SBS 0335-052 and how far it extends from the ionizing clusters.

Figure 1: The spectrum of the brightest region with line identifications. On each panel a continuum is drawn by hand to guide the eye. 2$\sigma$ error in the continuum placement is shown in the upper panel.

Figure 2: a) The spatial distributions along the slit of the relative fluxes of the emission lines and continuum near H$\beta$. The fluxes of the H$\beta$ emission line and continuum are normalized to their maximum values, while the fluxes of [O III] $\lambda$4363 and He II $\lambda$4686 are relative to the flux of H$\beta$. The slit is oriented at position angle PA = $-30^\circ$. Origin is set at the maximum flux of the H$\beta$ emission line. b) The spatial distribution of the electron temperature $T_{\rm e}$ and oxygen abundance 12 + log O/H.