Keck Cosmic Web Imager Observations of He II Emission in the Blue Compact Dwarf I Zw 18

With a metallicity of 12+Log(O/H) ~ 7.1, IZw18 is a canonical low metallicity blue compact dwarf (BCD) galaxy. A growing number of BCDs, including IZw18, have been found to host strong HeII emission of uncertain origin and with enhanced intensities compared to Beta (e.g. HeII/Hbeta >1%). Conventional sources capable of doubly ionizing Helium are Wolf-Rayet and O stars. However, Wolf-Rayet stars are not detected in all HeII emitting galaxies, and modeling of the most massive O stars underpredicts HeII/Hbeta by an order of magnitude. Thus far, HeII in IZw18 has been spatially resolved to arc-second scales (Kehrig et al. 2015). To better characterize the morphology of this emission, we observed IZw18 to higher sensitivity/resolution using the Keck Cosmic Web Imager. These observations reveal, for the first time, two additional HeII emitting regions. The two new regions lie along an axis which intercepts the position of IZw18’s Ultra-luminous X-ray (ULX) source. Each region exhibits HeII/Hbeta greater than 2%. Some of the largest HeII/Hbeta enhancement occurs in a region with no detectable stellar population. These observations suggest that HeII emission in IZw18 is consistent with X-ray or shock ionized nebulae powered by IZw18’s ULX source, potentially with evidence of a jet and/or ionization cone.