Keck Cosmic Web Imager Observations of He II Emission in I Zw 18
  With a metallicity of 12 + Log(O/H) $\approx$ 7.1-7.2, I Zw 18 is a canonical
low-metallicity blue compact dwarf (BCD) galaxy. A growing number of BCDs,
including I Zw 18, have been found to host strong, narrow-lined, nebular He II
($\lambda$4686) emission with enhanced intensities compared to H$\beta$ (e.g.,
He II($\lambda$4686)/H$\beta$ > 1%). We present new observations of I Zw 18
using the Keck Cosmic Web Imager. These observations reveal two nebular He II
emission regions (or He III regions) northwest and southeast of the He III
region in the galaxy's main body investigated in previous studies. All regions
exhibit He II($\lambda4686$)/Hbeta greater than 2%. The two newly resolved He
III regions lie along an axis that intercepts the position of I Zw 18's
ultraluminous X-ray (ULX) source. We explore whether the ULX could power the
two He III regions via shock activity and/or beamed X-ray emission. We find no
evidence of shocks from the gas kinematics. If the ULX powers the two regions,
the X-ray emission would need to be beamed. Another potential explanation is
that a class of early-type nitrogen-rich Wolf-Rayet stars with low winds could
power the two He III regions, in which case the alignment with the ULX would be
coincidental.
