Stars and Stellar Black Holes in the Low-metallicity Galaxy I Zw 18

I Zw 18 is a star-forming dwarf galaxy having a very low metal content, O/H similar to 1/50 solar (Skillman & Kennicutt 1993). While galaxies with such low metallicity are rare in the low-redshift universe, they are likely to be common in galaxies at cosmic dawn. Thus, I Zw 18 is a "living" template for z > 6 galaxies. We have obtained HST/COS far-UV spectra of the northwest star cluster in I Zw 18 and have compared them to SYNSPEC model spectra by Lanz & Hubeny (2003, 2007) in order to determine the properties of the stellar population. We have also compared the observed spectra of I Zw 18-NW to the CLOUDSPEC models (Hubeny et al. 2000) of the stellar cluster with an embedded ultra-luminous X-ray source (ULX). This comparison reveals feedback of the stellar black hole in the form of photoionization and heating. Such models can be used as starting points to explore the physical conditions in which stars and black holes form and evolve in an extremely low-metallicity environment at high redshift.