Active Disk Building in a Local H I-massive LIRG: The Synergy Between Gas, Dust, and Star Formation

HIZOA J0836-43 is the most H I-massive (M _{H I} = 7.5 × 10¹⁰ M _sun) galaxy detected in the HIPASS volume (δ = -90° to + 25°, v<12,700 km s^-1) and lies optically hidden behind the Milky Way. Markedly different from other extreme H I disks in the local universe, it is a luminous infrared galaxy (LIRG) with an actively star-forming disk (>50 kpc), central to its ~130 kpc gas disk, with a total star formation rate (SFR) of ~20.5 M _sun yr^-1. Spitzer spectroscopy reveals an unusual combination of powerful polycyclic aromatic hydrocarbon (PAH) emission coupled to a relatively weak warm dust continuum, suggesting photodissociation-region-dominated emission. Compared to a typical LIRG with similar total infrared luminosity (L _TIR = 10¹¹ L _sun), the PAHs in HIZOA J0836-43 are more than twice as strong, whereas the warm dust continuum (λ>20 μm) is best fit by a star-forming galaxy with L _TIR = 10¹⁰ L _sun. Mopra CO observations suggest an extended molecular gas component (H₂ + He>3.7 × 10⁹ M _sun) and a lower limit of ~64% for the gas-mass fraction; this is above average compared to local disk systems, but similar to that of z ~ 1.5 BzK galaxies (~57%). However, the star formation efficiency (SFE = L _IR/L'_CO) for HIZOA J0836-43 of 140 L _sun (K km s^-1 pc²)^-1 is similar to that of local spirals and other disk galaxies at high redshift, in strong contrast to the increased SFE seen in merging and strongly interacting systems. HIZOA J0836-43 is actively forming stars and building a massive stellar disk. Its evolutionary phase of star formation (M _stellar, SFR, and gas fraction) compared to more distant systems suggests that it would be considered typical at redshift z ~ 1. This galaxy provides a rare opportunity in the nearby universe for studying (at z ~ 0.036) how disks were building and galaxies evolving at z ~ 1, when similarly large gas fractions were likely more common.