Early Science with the Large Millimeter Telescope: CO and [C II]
  Emission in the z=4.3 AzTEC J095942.9+022938 (COSMOS AzTEC-1)
  Measuring redshifted CO line emission is an unambiguous method for obtaining
an accurate redshift and total cold gas content of optically faint, dusty
starburst systems. Here, we report the first successful spectroscopic redshift
determination of AzTEC J095942.9+022938 ("COSMOS AzTEC-1"), the brightest 1.1mm
continuum source found in the AzTEC/JCMT survey (Scott et al. 2008), through a
clear detection of the redshifted CO (4-3) and CO (5-4) lines using the
Redshift Search Receiver on the Large Millimeter Telescope. The CO redshift of
$z=4.3420\pm0.0004$ is confirmed by the detection of the redshifted 158 micron
[C II] line using the Submillimeter Array. The new redshift and Herschel
photometry yield $L_{FIR}=(1.1\pm0.1)\times 10^{13} L_\odot$ and $SFR = 1300\,
M_\odot$ yr$^{-1}$. Its molecular gas mass derived using the ULIRG conversion
factor is $1.4\pm0.2 \times 10^{11} M_\odot$ while the total ISM mass derived
from the 1.1mm dust continuum is $3.7\pm0.7 \times 10^{11} M_\odot$ assuming
dust temperature of 35 K. Our dynamical mass analysis suggests that the compact
gas disk ($r\approx 1.1$ kpc, inferred from dust continuum and SED analysis)
has to be nearly face-on, providing a natural explanation for the uncommonly
bright, compact stellar light seen by the HST. The [C II] line luminosity
$L_{[C~II]} = 7.8\pm1.1 \times 10^9 L_\odot$ is remarkably high, but it is only
0.04 per cent of the total IR luminosity. AzTEC COSMOS-1 and other high
redshift sources with a spatially resolved size extend the tight trend seen
between [C II]/FIR ratio and $\Sigma_{FIR}$ among IR-bright galaxies reported
by Diaz-Santos et al. (2013) by more than an order of magnitude, supporting the
explanation that the higher intensity of the IR radiation field is responsible
for the "[C II] deficiency" seen among luminous starburst galaxies.
