Molecular gas and dust in the highly magnified z = 2.8 galaxy behind the
  Bullet Cluster
  The gravitational magnification provided by massive galaxy clusters makes it
possible to probe the physical conditions in distant galaxies that are of lower
luminosity than those in blank fields and likely more representative of the
bulk of the high-redshift galaxy population. We aim to constrain the basic
properties of molecular gas in a strongly magnified submm galaxy located behind
the massive Bullet Cluster. This galaxy (SMM J0658) is split into three images,
with a total magnification factor of almost 100. We used the Australia
Telescope Compact Array (ATCA) to search for {12}CO(1--0) and {12}CO(3--2) line
emission from SMM J0658. We also used the SABOCA bolometer camera on the
Atacama Pathfinder EXperiment (APEX) telescope to measure the continuum
emission at 350 micron. CO(1--0) and CO(3--2) are detected at 6.8 sigma and 7.5
sigma significance when the spectra toward the two brightest images of the
galaxy are combined. From the CO(1-0) luminosity we derive a mass of cold
molecular gas of (1.8 \pm 0.3) * 10^9 Msun, using the CO to H_2 conversion
factor commonly used for luminous infrared galaxies. This is 45 \pm 25 % of the
stellar mass. From the width of the CO lines we derive a dynamical mass within
the CO-emitting region L of (1.3 \pm 0.4) * 10^10 (L/1 kpc) Msun. We refine the
redshift determination of SMM J0658 to z=2.7793 \pm 0.0003. Continuum emission
at 350 micron from SMM J0658 was detected with SABOCA at a signal-to-noise
ratio of 3.6. We study the spectral energy distribution of SMM J0658 and derive
a dust temperature of 33\pm 5 K and a dust mass of 1.1*10^7 Msun. SMM J0658 is
one of the least massive submm galaxies discovered so far. As a likely
representative of the bulk of the submm galaxy population, it is a prime target
for future observations.
