Highly ionized gas in extreme starburst galaxies: high resolution He II and (the first) C IV emission imaging

Nebular emission lines in the far ultraviolet are rapidly drawing attention as quantitative diagnostics of nebular gas. In the coming decades, they will be among the few information-carriers regarding the ionized ISM of faint galaxies at the highest redshifts. In particular, the C IV 1550 AA and He II 1640 AA lines are very prevalent in low-metallicity dwarf starbursts. However when using them to estimate the conditions in the ISM, C IV and He II diagnostics provide degenerate solutions between photo- and shock-ionization scenarios.

Here we propose to obtain emission line images of two nearby starburst galaxies with the strongest emission in He II recombination line and C IV 1550 AA doublet. This will be the first C IV emission imaging ever performed. The targets are well-known low-metallicity starbursts, and analogs of galaxy building blocks at the highest redshifts. We will measure the C IV and He II light profile, and compute the total C IV (large-aperture) luminosity for the first time. Most importantly we will map line ratios involving C IV, He II, and H-alpha at ~20 pc resolution, while state-of-the-art modeling will derive the ionizing output of the star clusters. Hence we will directly couple stellar parameters to nebular diagnostics on the requisite spatial scales. Our results will determine the impact of photoionization and shocks to UV line diagnostics, which will be pivotal for understanding the ISM conditions of the earliest galaxies.