  We report spectroscopic confirmation and high-resolution infrared imaging of
a z=2.79 triply-imaged galaxy behind the Bullet Cluster. This source, a
Spitzer-selected luminous infrared galaxy (LIRG), is confirmed via polycyclic
aromatic hydrocarbon (PAH) features using the Spitzer Infrared Spectrograph
(IRS) and resolved with HST WFC3 imaging. In this galaxy, which with a stellar
mass of M*=4e9 Msun is one of the two least massive ones studied with IRS at
z>2, we also detect H_2 S(4) and H_2 S(5) pure rotational lines (at 3.1 sigma
and 2.1 sigma) - the first detection of these molecular hydrogen lines in a
high-redshift galaxy. From the molecular hydrogen lines we infer an excitation
temperature T=377+68-84 K. The detection of these lines indicates that the warm
molecular gas mass is 6(+36-4)% of the stellar mass and implies the likely
existence of a substantial reservoir of cold molecular gas in the galaxy.
Future spectral observations at longer wavelengths with facilities like the
Herschel Space Observatory, the Large Millimeter Telescope, and the Atacama
Pathfinder EXperiment (APEX) thus hold the promise of precisely determining the
total molecular gas mass. Given the redshift, and using refined astrometric
positions from the high resolution imaging, we also update the magnification
estimate and derived fundamental physical properties of this system. The
previously published values for total infrared luminosity, star formation rate,
and dust temperature are confirmed modulo the revised magnification; however we
find that PAH emission is roughly a factor of five stronger than would be
predicted by the relations between the total infrared and PAH luminosity
reported for SMGs and starbursts in Pope et al. (2008).
