Keck Cosmic Web Imager Observations of He II Emission in I Zw 18

TL;DR

This study uses the Keck Cosmic Web Imager to observe He II emission in I Zw 18, revealing two new nebular regions and exploring their potential connection to an ultraluminous X-ray source, with implications for understanding ionization sources in low-metallicity galaxies.

Contribution

First detailed spatially resolved observations of He II emission in I Zw 18, linking nebular regions to possible X-ray or stellar ionization sources.

Findings

Two new He II emission regions identified in I Zw 18.

He II/Hβ ratios greater than 2% in all observed regions.

No evidence of shocks; possible beamed X-ray or stellar ionization.

Abstract

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.