Hst Observations of the Blue Compact Dwarf Sbs 0335–052: a Probable Young Galaxy

We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the spectral regions around Lyα and OI λ1302 of the extremely metal-deficient ( Z ∼ Z⊙/41 ) blue compact dwarf (BCD) galaxy SBS 0335–052. All the star formation in the BCD occurs in six superstar clusters (SSC) with ages ≤ 3-4 Myr. As there is no evident sign of tidal interaction, star formation in the BCD is probably triggered by stochastic cloud collisions in the HI envelope. Dust is clearly present and mixed spatially with the SSCs. There is a supershell of radius ∼ 380 pc, delineating a large supernova cavity. The instantaneous star formation rate is ∼ 0.4 M⊙ yr −1 . Strong narrow Lyα emission is not observed. Rather there is low intensity broad (FWZI = 20 Å) Lyα emission superposed on even broader Lyα absorption by the HI envelope. This broad low-intensity emission is caused by resonant scattering of Lyα photons. The absence of strong Lyα emission may be due partly to dust absorption, but mainly to multiple scattering which removes Lyα photons from the small HST aperture. As the HI cloud is seen nearly edge-on, geometrical effects may play also a role as photons escape more easily in a direction perpendicular to the plane than along it. The BCD appears to be a young galaxy, undergoing its very first burst of star formation. This conclusion is based on the following evidence: 1) the underlying extended low-surfacebrightness component is very irregular and filamentary, suggesting that a significant part of the emission comes from ionized gas; 2) it has very blue colors (–0.34 ≤ (V–I)0 ≤ 0.16), consistent with gaseous emission colors; 3) the OI λ1302 line is not detected in absorption in the GHRS spectrum, setting an upper limit for N(O)/N(H) in the HI envelope of the BCD of more than 3000 times smaller than the value in Orion. Subject headings: galaxies – young: interstellar matter – nebulae: HII regions