SPECTROPHOTOMETRY OF BLUE COMPACT GALAXIES WITH BROAD EMISSION-LINES - EVIDENCE FOR HIGH-VELOCITY GAS MOTION

We present spectrophotometric observations of four low-metallicity blu e compact galaxies obtained with the Blue Channel Spectrograph on the Multiple Mirror Telescope. Low-intensity broad emission in strong nebu lar lines with a spatial extent of several kiloparsec was detected in the spectra of three of them, suggesting rapid motion of the ionized g as with velocities up to 2000-3000 km s(-1). In 1256+351 = NGC 4861 an d 1408+551A, the detection of the broad WR emission lines N III lambda 4640, He II lambda 4686, and C IV lambda 5808 implies the existence o f a large population of WR stars. The ratio of nitrogen WR stars to O stars and the relative number of carbon stars WC/WR suggest that an in stantaneous burst of star formation may have occurred in these galaxie s. The most plausible explanation of the origin of the observed low-in tensity, large-scale broad emission is that it arises from the collect ive influence on the interstellar medium of a large number of massive stars through supernova explosions. However, we find differences in th e appearance of low-intensity broad emission lines in galaxies with di fferent metallicities. While broad emission in the [O III] lambda lamb da 4959, 5007 emission lines has been detected in three of the galaxie s studied, broad hydrogen emission was detected in only 1851+695, the galaxy with the lowest metallicity in our sample. On the other hand, b road hydrogen emission is not observed in 1256+351 or in 1408+551A, wh ose spectra show strong evidence for WR emission lines. We argue that these differences are related to differences in evolution of massive s tars with different metallicities. If broad emission originates in hig h-velocity gas driven by stellar winds and/or in fast-moving supernova remnants then the absence of hydrogen emission in galaxies with highe r metallicity could result simply from a hydrogen deficiency in massiv e stars at final stages of their evolution. Conversely, in galaxies wi th low metallicity the efficiency of stellar winds is lower and massiv e stars retain their hydrogen-rich envelopes before any explosive evol ution. We confirm the previous findings by Roy et al. (1992) that whil e superbubble blowout is the only mechanism so far proposed capable of producing large-scale high-velocity gas in giant H II regions, the su rface brightness distribution in emission lines in our sample of four galaxies cannot be explained by conventional models of superbubble evo lution.