Broad-line emission in low-metallicity blue compact dwarf galaxies: Evidence for stellar wind, supernova, and possible AGN activity

We present spectra of a large sample of low-metallicity blue compact dwarf galaxies which exhibit broad components in their strong emission lines, mainly in H beta, [OIII] lambda lambda 4959, 5007, and H alpha. Twenty-three spectra have been obtained with the MMT, 14 of which show broad emission. The remaining 21 spectra with broad emission have been selected from Data Release 5 of the Sloan Digital Sky Survey. The most plausible origin of broad-line emission is the evolution of massive stars and their interaction with the circumstellar and interstellar medium. The broad emission with the lowest H alpha luminosities (10(36)-10(39) ergs s(-1)) is likely produced in circumstellar envelopes around hot Ofp/WN9 and/or LBV stars. The broad emission with the highest H alpha luminosities (10(40)-10(42) ergs s(-1)) probably arises from Type IIp or IIn supernovae (SNe). It can also come from active galactic nuclei (AGNs) containing intermediate-mass black holes, although we find no strong evidence for hard nonthermal radiation in our sample galaxies. The oxygen abundance in the host galaxies with SN candidates is low and varies in the range 12 + log O/H = 7.36-8.31. However, Type IIn SN/AGN candidates are found only in galaxies with 12 + log O/H less than or similar to 7.99. Spectroscopic monitoring of these Type IIn SN/AGN candidates over a timescale of several years is necessary to distinguish between the two possibilities.