The hidden HI-massive LIRG HIZOA J0836-43: Inside-out galaxy formation

Abstract

HIZOA J0836-43 is an extreme gas-rich ($M_{\rm{HI}}$=7.5\times10^{10} M_{\sun}) disk galaxy which lies hidden behind the strongly obscuring Vela region of the Milky Way. Utilizing observations from the Spitzer Space Telescope, we have found it to be a luminous infrared starburst galaxy with a star formation rate of \sim 21 M_{\sun} \rm{yr^{-1}}, arising from exceptionally strong molecular PAH emission (L_{7.7\micron} = 1.50 \times 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)\times10^{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.