On the 4640 Å feature in Wolf-Rayet galaxies

Abstract

Several Wolf-Rayet galaxies exhibit broad emission features around 4640 Å in their optical spectra. These features are usually identified as N III and C III/C IV emission lines from WR stars. In a few WR galaxies the flux in this feature has been measured to be as large or larger than that in the corresponding He II λ4686 stellar emission line. We demonstrate that a 4640/4686 flux ratio larger than unity cannot be produced by any known population of WR stars. In particular, we show that the enhanced ratio cannot be explained by the presence of carbon lines from WC stars. We examine the possible origins of the broad λ4640 feature and offer several possible explanations for the enhanced strength in this emission feature. The most plausible explanations involve the presence of large numbers of Of stars in the starburst regions and/or the contamination of the stellar lines by nebular emission features. We discuss the implications that both possibilities have for the interpretation of the star formation histories in WR galaxies, as derived from their massive star content. We find that the instantaneous burst scenario cannot be correct for any metal-rich region whose optical spectrum exhibits both an enhanced 4640/4686 flux ratio and C IV 5808 emission from WC stars. These regions must have experienced a “multiple-burst” star-formation event, composed of several instantaneous bursts separated by short time intervals (a few Myr).