2010ApJ...725.1550C

HIZOA J0836-43 is the most H I-massive (M_ H I_= 7.5x10¹⁰ M_☉) galaxy detected in the HIPASS volume (δ = -90° to + 25°, v<12,700 km/s) 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. 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_☉), 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_☉. Mopra CO observations suggest an extended molecular gas component (H₂+ He>3.7x10⁹ 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/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.