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/Advanced Camera for Surveys imaging and Gemini/NIFS [O Pi]3727 imaging spectroscopy, and obtain a tentative detection at the 4.3 sigma level with a flux of 0.104 +/- 0.024 Jy km s(-1). From the CO line luminosity and assuming a CO-to-H-2 conversion factor alpha = 2, we derive a gas mass M-gas similar to 1(-0.6)(+1) x 10(9) M-circle dot. Combined with the existing data, we derive a gas fraction M-gas/(M-gas + 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.