Intense C iv and He ii Emission in z ∼ 0 Galaxies: Probing High-energy Ionizing Photons^∗

Abstract

In the last few years, prominent high-ionization nebular emission lines (i.e., O III], C III], C IV, He II) have been observed in the deep UV spectra of z ∼ 5 − 7 galaxies, indicating that extreme radiation fields characterize reionization-era systems. These lines have been linked to the leakage of Lyman continuum photons (necessary for reionization) both theoretically and observationally. Consequently, high-ionization UV emission lines present our best probe to detect and characterize the most distant galaxies that we will observe in the coming years, and are key to understanding the sources of reionization, yet the physics governing their production is poorly understood. Here we present recent high-resolution Hubble Space Telescope spectra of two nearby extreme UV emission-line galaxies, J104457 and J141851. We report the first observations of intense nebular He II and double-peaked, resonantly-scattered C IV emission, a combination that suggests these galaxies both produce and transmit a significant number of very high-energy ionizing photons (E > 47.89 eV) through relatively low column densities of high-ionization gas. This suggests that, in addition to photons at the Hionizing edge, the very hard ionizing photons that escape from these galaxies may provide a secondary source of ionization that is currently unconstrained observationally. Simultaneous radiative transfer models of Lyα and C IV are needed to understand how ionizing radiation is transmitted through both low-and high-ionization gas. Future rest-frame FUV observations of galaxies within the epoch of reionization using the James Webb Space Telescope (JWST) or extremely large telescopes (ELTs) will allow us to constrain the escape of helium-ionizing photons and provide an estimate for their contribution to the reionization budget.

Stars and gas in the most metal-poor galaxies – I. COS and MUSE observations of SBS 0335−052E

“…1548, 1551 EW which has been reported for a nearby starburst galaxy is ∼ 10 Å. This is for galaxy J104457, which has 12+log(O/H)=7.45 (Berg et al 2019b) and is at a very similar redshift (0.013) compared to SBS 0335-052E. Senchyna & Stark (2019) present HST COS UV observations of six star-forming XMPs with 12+log(O/H) between 7.4 and 7.7, i.e., lower than in the work of Senchyna et al (2017).…”

Deciphering the Lyman α blob 1 with deep MUSE observations

“…Nevertheless, the C iv emissivity also peaks in a region of slightly lower density (n H ∼ 2−5 cm −3 ) compared to He ii (n H ∼ 5−20 cm −3 ), thus C iv could potentially also be more extended (Cabot et al 2016). This effect could be further enhanced by the resonant nature of C iv (see also Berg et al 2019). On the other hand, if photo-ionisation from an AGN produces C 4+ (ionisation potential 64.4 eV) that emits C iv as a result of recombinations, the C 4+ zone would be confined within the He 2+ zone.…”

Ultraviolet spectra of extreme nearby star-forming regions: Evidence for an overabundance of very massive stars

“…Along with C iv and He ii emission which become prominent at yet lower metallicities (12 + log O/H < 7.7; e.g. Senchyna et al 2019;Berg et al 2019b), these strong UV nebular lines may be a powerful probe of early chemical evolution in the JWST era when detection of [O iii] λ4363 is intractable due to faintness or when the optical lines are entirely inaccessible at the highest redshifts.…”