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 by factors of up to 10, as a result of the very high extinction (A(B) similar 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 model dependence and other problems. We describe a direct measurement of the SN rate from a regular patrol of starburst galaxies done with K'-band imaging to minimize the effects of extinction. A collection of K'-band measurements of core-collapse SNe near maximum light is presented. Such measurements (excluding 1987A) are not well reported in the literature. Results of a preliminary K'-band search, using the MIRC camera at the Wyoming Infrared Observatory and an improved search strategy using the new ORCA optics, are described. A monthly patrol of a sample of IRAS bright (mostly starburst) galaxies within 25 Mpc should yield 1-6 SNe yr,corresponding to the range of estimated 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 100 mu m, or FIRSRU) at 90% confidence. The MIRC camera had insufficient resolution to search nuclear starburst regions, where starburst and SN activity is concentrated; therefore, we were unable to rigorously test the hypothesis of high SN rates in heavily obscured star-forming regions. We conclude that high-resolution nuclear SN searches in starburst galaxies with small fields are more productive than low-resolution, large-held searches, even for our sample of large (often several arcminutes) galaxies. With our ORCA high-resolution optics, we could limit the total SN rate to less than 1.3 FIRSRU at 90% confidence in 3 years of observations, lower than most estimates.