PROBING STAR FORMATION AT LOW METALLICITY: THE RADIO EMISSION OF SUPER STAR CLUSTERSIN SBS 0335-052

摘要

We present high-resolution radio-continuum observations of the nascent starburst in the metal-poor galaxySBS 0335-052. These radio data were taken with the Very Large Array and include observations at 0.7 cm,1.3 cm, 2 cm, 3.6 cm, and 6 cm. These observations enable us to probe the thermal radio nebulae associated with theextremely young star-forming regions in this galaxy. Two discrete and luminous star-forming regions are detectedin the south of the galaxy that appear to be associated with massive star clusters previously identified at opticalwavelengths. However, the remaining optically identified massive star clusters are not clearly associated with radioemission (either thermal or nonthermal) down to the sensitivity limits of these radio data. The spectral energy distri-butions (SEDs) of the two radio-detected clusters are consistent with being purely thermal, and the entire region hasan inferred ionizing flux' of ～1.2 x 10~53s~-1, which is equivalent to～12,000 "typical" O-type stars (O7.5 V).The observations presented here have resolved out a significant contribution from diffuse nonthermal emissiondetected previously, implying a previous episode of significant star formation. The current star formation rate (SFR)for this southern region alone is ～1.3 M_⊙yr~-1,or～23 M_⊙1kpc~-2, which is nearing the maximum starburstintensity limit. This SFR derived from thermal radio emission also suggests that previous optical recombinationline studies do not detect a significant fraction of the current star formation in SBS 0335-052. From model fits tothe radio SED, we infer a global mean density in the two youngest clusters of n_e>10~3-4cm~-3.In addition, acomparison between the compact and diffuse radio emission indicates that up to -50 of the ionizing flux could beleaking out of the compact H II regions; this in is agreement with previous work, which suggests that the interstellarmedium surrounding the natal clusters in SBS 0335-052 is porous and clumpy.