Observational limits on the gas mass of a z=4.9 galaxy
  We present the results of a search for molecular gas emission from a
star-forming galaxy at z = 4.9. The galaxy benefits from magnification of 22
+/- 5x due to strong gravitational lensing by the foreground cluster MS1358+62.
We target the CO(5-4) emission at a known position and redshift from existing
Hubble Space Telescope/ACS imaging and Gemini/NIFS [OII]3727 imaging
spectroscopy, and obtain a tentative detection at the 4.3sigma level with a
flux of 0.104 +/- 0.024Jkm/s. From the CO line luminosity and assuming a
CO-to-H2 conversion factor alpha=2, we derive a gas mass M_gas ~ 1^{+1}_{-0.6}
x 10^9 M_sun. Combined with the existing data, we derive a gas fraction
Mgas/(Mgas + M*) = 0.59^{+0.11}_{-0.06}. The faint line flux of this galaxy
highlights the difficulty of observing molecular gas in representative galaxies
at this epoch, and suggests that routine detections of similar galaxies in the
absence of gravitational lensing will remain challenging even with ALMA in full
science operations.
