HUBBLE-SPACE-TELESCOPE 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 t he spectral regions around Ly alpha and O I lambda 1302 of the extreme ly metal-deficient (Z similar to Z./41) blue compact dwarf galaxy (BCD ) SBS 0335-052. All the star formation in the BCD occurs in six super- star clusters (SSCs) with ages less than or equal to 25 Myr, within a region of similar to 2'' or 520 pc in size. Dust is clearly present an d mixed spatially with the SSCs. The SSCs are roughly aligned in the s outheast-northwest direction, and there is a systematic increase in re ddening of the clusters away from the brightest one. The observed colo r dependence on position may be the combined effects of differential e xtinction by dust and color evolution with time due to sequential prop agating star formation. There is a supershell of radius similar to 380 pc, delineating a large supernova cavity. The instantaneous star form ation rate is similar to 0.4 M. yr(-1). Strong narrow Ly alpha emissio n is not observed. Rather there is low-intensity broad (FWZI = 20 Angs trom) Ly alpha emission superposed on even broader Ly alpha absorption by the H I envelope. This broad low-intensity emission is caused by r esonant scattering of Ly alpha photons. The absence of strong Ly alpha emission may be due partly to dust absorption, but is due mainly to m ultiple scattering that removes Ly alpha photons from the small HST ap erture. As the H I cloud is seen nearly edge-on, geometrical effects m ay also play a role as photons escape more easily in a direction perpe ndicular to the plane than along it. The BCD appears to be a young gal axy, undergoing one of its very first bursts of star formation. This c onclusion is based on the following evidence: (1) The underlying exten ded low surface brightness component is irregular and filamentary, sug gesting that a significant part of the emission comes from ionized gas . Any underlying stellar population must be younger than similar to 10 (8) yr. (2) The underlying component has very blue colors [-0.34 less than or equal to (V-I)(0) less than or equal to 0.16], consistent with gaseous emission colors. (3) The O I lambda 1302 line is not detected in absorption in the Goddard High Resolution Spectrograph spectrum, s etting an upper limit for N(O)/N(H) in the H I envelope of the BCD of more than 3000 times smaller than the value in Orion.