A study of extreme C iii]1908 & [O iii]88/[C ii]157 emission in Pox 186: Implications for JWST+ALMA (FUV+FIR) studies of distant galaxies

Abstract

Carbon spectral features are ubiquitous in the ultraviolet (UV) and far-infrared (FIR) spectra of the reionization-era galaxies. We probe the ionized carbon content of a dwarf galaxy Pox 186 using the UV, optical, mid-infrared and FIR data taken with Hubble, Gemini, Spitzer and Herschel, respectively. This local (z∼0.0040705) galaxy is likely an analogue of reionization-era galaxies, as revealed by its extreme FIR emission line ratio, [O iii] 88μm/[C ii] 157μm (>10). The UV spectra reveal extreme C iii]  λλ 1907, 1909 emission with the strongest equivalent width (EW) = 35.85 ± 0.73 Å detected so far in the local (z∼0) Universe, a relatively strong C iv  λλ 1548, 1550 emission with EW = 7.95 ±0.45 Å, but no He ii λ 1640 detection. Several scenarios are explored to explain the high EW of carbon lines, including high effective temperature, high carbon-to-oxygen ratio, slope and upper mass of top-heavy initial mass function, hard ionizing radiation and in-homogeneous dust distribution. Both C iii] and C iv line profiles are broadened with respect to the O iii] λ 1660 emission line. Each emission line of C iv  λλ 1548, 1550 shows the most distinct double-peak structure ever detected, which we model via two scenarios, firstly a double-peaked profile that might emerge from resonant scattering and secondly, a single nebular emission line along with a weaker interstellar absorption. The study demonstrates that galaxies with extreme FIR properties may also show extreme UV properties, hence paving a promising avenue of using FIR+UV in the local (via Hubble+Herschel/SOFIA) and distant (via JWST+ALMA) Universe for unveiling the mysteries of the reionization-era.