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 \pm 5 \times 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 [ O ii ] 3727 imaging spectroscopy , and obtain a tentative detection at the 4.3 \sigma level with a flux of 0.104 \pm 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 } \sim 1 ^ { +1 } _ { -0.6 } \times 10 ^ { 9 } M _ { \sun } .
Combined with the existing data , we derive a gas fraction M _ { gas } / \left ( M _ { gas } + M _ { \ast } \right ) = 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 .