Circumnuclear supernova remnants and HII regions in NGC 253

摘要

Archival VLA data have been used to produce arcsecond-resolution 6 and 20 cm images of the region surrounding the nuclear 200 pc (similar to 15") starburst in NGC 253. Twenty-two discrete sources stronger than 0.4 mJy have been detected within similar to 2 kpc (similar to 3') of the galaxy nucleus; almost all these sources must be associated with the galaxy. None of the radio sources coincides with a detected X-ray binary, so they appear to be due to supernova remnants and H II regions. The region outside the central starburst has a derived radio supernova rate of less than or similar to 0.1 yr(-1) and may account for at least 20 of the recent star formation in NGC 253. Most of the newly identified sources have steep, nonthermal radio spectra, but several relatively strong thermal sources also exist, containing the equivalent of tens of O5 stars. These stars are spread over tens of parsecs and are embedded in regions having average ionized gas densities of similar to 20-200 cm(-3), much lower than in the most active nuclear star-forming regions in NGC 253 or in the super-star clusters seen in other galaxies. The strongest region of thermal emission coincides with a highly reddened area seen at near-infrared wavelengths, possibly containing optically obscured H II regions. References: 51