Massive present-day early-type (elliptical and lenticular) galaxies probably gained the bulk of their stellar mass and heavy elements through intense, dust-enshrouded starbursts - that is, increased rates of star formation - in the most massive dark
matter halos at early epochs. However, it remains unknown how soon after the Big Bang such massive starburst progenitors exist. The measured redshift distribution of dusty, massive starbursts has long been suspected to be biased low in redshift owing to selection effects, as confirmed by recent findings of systems out to redshift z~5. Here we report the identification of a massive starburst galaxy at redshift 6.34 through a submillimeter color-selection technique. We unambiguously determined the redshift from a suite of molecular and atomic fine structure cooling lines. These measurements reveal a hundred billion solar masses of highly excited, chemically evolved interstellar medium in this galaxy, which constitutes at least 40% of the baryonic mass. A maximum starburst converts the gas into stars at a rate more than 2,000 times that of the Milky Way, a rate among the highest observed at any epoch. Despite the overall downturn of cosmic star formation towards the highest redshifts, it seems that environments mature enough to form the most massive, intense starbursts existed at least as early as 880 million years after the Big Bang.
We present the rest-frame 200--320 mm spectrum of the z=3.91 quasar apm, obtained with Z-Spec at the Caltech Submillimeter Observatory. In addition to the jeight to jthirteen CO rotational transitions which dominate the CO cooling, we find six transi
tions of water originating at energy levels ranging up to 643 K. Most are first detections at high redshift, and we have confirmed one transition with CARMA. The CO cooling is well-described by our XDR model, assuming L$_{rm 1-100,keV}sim1times10^{46}rm,erg,s^{-1}$, and that the gas is distributed over a 550-pc sizescale, per the now-favored $mu$=4 lensing model. The total observed cooling in water corresponds to 6.5$times10^{9}$ ls, comparable to that of CO. We compare the water spectrum with that of Mrk 231, finding that the intensity ratios among the high-lying lines are similar, but with a total luminosity scaled up by a factor of $sim$50. Using this scaling, we estimate an average water abundance relative to hh of 1.4$times10^{-7}$, a good match to the prediction of the chemical network in the XDR model. As with Mrk 231, the high-lying water transitions are excited radiatively via absorption in the rest-frame far-infrared, and we show that the powerful dust continuum in apm is more than sufficient to pump this massive reservoir of warm water vapor.
