The JWST and ALMA view of local dwarf Pox 186: Evidence of stripped binaries driving a hard radiation field and little cold molecular gas resulting in high star-formation efficiency

Abstract: We present a JWST mid-infrared (MIR) view of Pox 186, one of the best local analogs of reionization-era galaxies based on its extreme properties in the ultraviolet, optical, and far-infrared (FIR). We extracted a high signal-to-noise spectrum of Pox 186 from the spatially resolved MIR data taken with the Medium Resolution Spectrometer (MRS) on the MIR Instrument (MIRI), and compared its MIR properties with those of the other dwarf galaxies in the Herschel Dwarf Galaxy Survey. We probe the radiation hardness of Pox 186 using the so-called softness diagram, based on the MIR line ratios [NeIII]/[NeII] and [SIV]/[SIII], which suggest that Pox 186 hosts ionization sources with a hard radiation field. We probe the dominant ionization source in Pox 186 using diagnostic diagrams based on MIR line ratios. Our photoionization models provide evidence that stripped binary stars are the main cause of the extreme MIR line ratios. Similarly, a young starburst is needed to reproduce the MIR dust continuum. We probe the warm and cold molecular gas content using the MIR H$_2$ rotational lines (S(1)-S(7)) from JWST, and the archival sub-mm CO(2-1) line from ALMA, which indicate a high star-formation efficiency (SFE; $&gt;$ 15 $\times$ 10$<sup>{-9}$ yr$<sup>{-1}$) and low gas depletion time comparable to that of the reionization-era galaxies. This analysis strongly suggests that star formation rather than active galactic nuclei drives the hard radiation field observed in Pox 186, and cautions against relying solely on the UV diagnostics to establish the nature of high-z galaxies.