An X-ray, optical, and radio search for supernova remnants in the nearby sculptor group Sd galaxy NGC 300

We have conducted a multiwavelength (X-ray, optical, and radio) search for supernova remnants (SNRs) in the nearby Sculptor Group Sd galaxy NGC 300. O ur Very Large Array (VLA) radio observations at 6 cm and 20 cm have been co mbined with previously published optical results, our own optical image, an d archived ROSAT X-ray data to search for new SNR candidates. Of the 28 opt ically identified SNRs found by Blair & Long, three exhibit some combinatio n of X-ray and radio emission, but in: general X-ray and radio emission fro m the optically identified SNRs is undetected. A radio-selected sample of SNR candidates is constructed by searching for p ositional coincidences between nonthermal radio continuum sources and sourc es of Ha emission with the intent of finding SNRs: that are typically overl ooked using the [S II]/H alpha criterion, i.e., those SNRs that are either confused by Ha emission or which are Balmer-dominated. This search has yiel ded 17 SNR candidates, of which 14 are new and three were already known fro m the optically selected sample. Four of the radio-selected candidates also possess detectable X-ray emission. A complementary analysis of the X-ray data has yielded an X-ray-selected sa mple of candidates consisting of six soft-spectrum sources (kT < 1 keV) coi ncident with regions of Her emission. Two of these candidate X-ray SNRs are common to the radio-selected sample and two more are common to the optical ly selected sample. Thus, the X-ray selection has yielded two additional ca ndidates. Through a multiwavelength campaign, we have added sixteen new candidate SNR s to the 28 previously known SNRs for a total of 44 SNR candidates in NGC 3 00. The fact that these new candidates were missed in the optical surveys s uggests a possible selection effect. SNRs identified through optical method s may represent only the SNRs located in regions with relatively low confus ion from H<alpha> emission, well away from star-forming regions. The SNRs i n or near star-forming regions are more likely to be confused by emission f rom H II regions and are therefore more likely to be missed in optical surv eys. The radio-selection process, on the other hand, suffers from a separat e selection effect: it can only identify candidates if they are associated with H II regions, such that our sample of radio-selected SNRs is biased to ward star-forming regions. Finally, the X-ray candidates are selected on th e basis of having soft spectra and association with H II regions, so that t he sample is biased against X-ray-emitting SNRs with hard spectra and no op tical counterparts. The fact that the optically selected, radio-selected, a nd X-ray-selected data sets of SNR candidates have limited overlap is consi stent with these opposing selection effects. We present a simulation to inv estigate the effects of optical confusion on a hypothetical optical survey for SNRs in a galaxy, using "artificial" SNRs that have been placed in a ra nge of confused environments. We find that the detection of SNRs becomes in creasingly difficult with distance, and that the most deeply embedded SNRs are not identified even in the nearest distance investigated. We conclude;t hat a multifrequency search for SNRs is required to uniformly sample the SN R population of a galaxy.