Active disk building in a local Hi-massive LIRG: The synergy between gas, dust, and star formation

Abstract

HIZOA J0836-43 is the most Hi-massive (MHi = 7.5 × 10 10M⊙) 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 Hi 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 ∼20.5M⊙ 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 (LTIR = 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 LTIR = 1010 L ⊙.Mopra CO observations suggest an extended molecular gas component (H2+He > 3.7×109 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 = LIR/L ′ CO) for HIZOA J0836-43 of 140 L⊙ (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 (Mstellar, 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. © 2010. The American Astronomical Society. All rights reserved.