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 × 10 10 M ☉ ) 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 ☉ 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 = 10 11 L ☉ ), 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 = 10 10 L ☉ . Mopra CO observations suggest an extended molecular gas component (H 2 + He>3.7 × 10 9 M ☉ ) 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 ☉ (K km s −1 pc 2 ) −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.