An infrared search for extinguished supernovae in starburst galaxies

IR and radio-band observations of heavily extinguished regions in starburst galaxies suggest a high supernova (SN) rate associated with such regions. Optically measured SN rates may therefore underestimate the total SN rate b y factors of up to 10, as a result of the very high extinction (A(B) simila r to 10-20 mag) to core-collapse SNe in starburst regions. The IR/radio SN rates come from a variety of indirect means, however, which suffer from mod el dependence and other problems. We describe a direct measurement of the S N rate from a regular patrol of starburst galaxies done with K'-band imagin g to minimize the effects of extinction. A collection of K'-band measuremen ts of core-collapse SNe near maximum light is presented. Such measurements (excluding 1987A) are not well reported in the literature. Results of a pre liminary K'-band search, using the MIRC camera at the Wyoming Infrared Obse rvatory and an improved search strategy using the new ORCA optics, are desc ribed. A monthly patrol of a sample of IRAS bright (mostly starburst) galax ies within 25 Mpc should yield 1-6 SNe yr,corresponding to the range of est imated SN rates. Our initial MIRC search with low resolution (2." 2 pixels) failed to find extinguished SNe in the IRAS galaxies, limiting the SN rate outside the nucleus (at greater than 15 " radius) to less than 3.8 far-IR SN rate units (SNe per century per 10(10) L-circle dot measured at 60 and 1 00 mu m, or FIRSRU) at 90% confidence. The MIRC camera had insufficient res olution to search nuclear starburst regions, where starburst and SN activit y is concentrated; therefore, we were unable to rigorously test the hypothe sis of high SN rates in heavily obscured star-forming regions. We conclude that high-resolution nuclear SN searches in starburst galaxies with small f ields are more productive than low-resolution, large-held searches, even fo r our sample of large (often several arcminutes) galaxies. With our ORCA hi gh-resolution optics, we could limit the total SN rate to less than 1.3 FIR SRU at 90% confidence in 3 years of observations, lower than most estimates .