Deep spectroscopy of the low-metallicity blue compact dwarf galaxy SBS 0335-052

The results of deep long-slit spectroscopy of the extremely low-metallicity blue compact dwarf (BCD) galaxy SBS 0335-052 are presented. Down to intens ity levels of 10(-3)...(-4) of H beta, unprecedented for spectroscopy of ex tra-galactic giant H II regions, we detect numerous weak permitted and forb idden nebular lines in the brightest part of the galaxy. With varying degre es of confidence, the detections include lines of high-ionization ions like Fe4+-Fe6+, implying very hard ionizing radiation. Two broad emission featu res, possibly from Wolf-Rayet stars, and stellar He II lambda 4200 absorpti on are seen in the same region. The large spatial extent of He II lambda 46 86 emission (implying the presence of sufficient ionizing photons with ener gies above 54 eV) and the spatial distribution of the electron temperature suggest that at least some part of the hard radiation is associated with sh ocks. Extended H alpha emission is detected over similar to6-8 kpc, a much larger area than in previous studies, suggesting that hot ionized gas is sp read out far away from the central ionizing clusters. This shows that nebul ar line and continuous emission can significantly modify the colours of the se extended regions and must be taken into account in studies of the underl ying stellar population.