The Hidden H i -Massive Luminous Infrared Galaxy HIZOA J0836-43: Inside-Out Galaxy Formation

HIZOA J0836-43 is an extreme gas-rich ($M_{ m{HI}}$=7.5$ imes10^{10} M_{sun}$) disk galaxy which lies hidden behind the strongly obscuring Vela region of the Milky Way. Utilizing observations from the {it Spitzer Space Telescope}, we have found it to be a luminous infrared starburst galaxy with a star formation rate of $sim 21 M_{sun} m{yr^{-1}}$, arising from exceptionally strong molecular PAH emission ($L_{7.7micron} = 1.50 imes 10^{9} L_{odot}$) and far-infrared emission from cold dust. The galaxy exhibits a weak mid-infrared continuum compared to other starforming galaxies and U/LIRGs. This relative lack of emission from small grains suggests atypical interstellar medium conditions compared to other starbursts. We do not detect significant $[$Ne {sc v}$]$ or $[$O {sc iv}$]$, which implies an absent or very weak AGN. The galaxy possesses a prominent bulge of evolved stars and a stellar mass of 4.4($pm$1.4)$ imes10^{10} M_{sun}$. With its plentiful gas supply and current star formation rate, a doubling of stellar mass would occur on a timescale of $sim$2 Gyr. Compared to local galaxies, HIZOA J0836-43 appears to be a "scaled-up" spiral undergoing inside-out formation, possibly resembling stellar disk building processes at intermediate redshifts.Comment: 5 pages, 3 figures, 1 table; Accepted for publication in ApJL: August 25 2008. A version with full resolution figures is available at http://spider.ipac.caltech.edu/staff/jarrett/Cluver_ApJL.pd