Context : Probing both star formation history and evolution of distant cluster galaxies is essential to evaluate the effect of dense environment on shaping the galaxy properties we observe today .

Aims : We investigate the effect of cluster environment on the processing of the molecular gas in distant cluster galaxies .
We study the molecular gas properties of two star-forming galaxies separated by 6 kpc in the projected space and belonging to a galaxy cluster selected from the Irac Shallow Cluster Survey , at a redshift z = 1.2 , that is , \sim 2 Gyr after the cosmic star formation density peak .
This work describes the first CO detection from 1 < z < 1.4 star-forming cluster galaxies with no clear reported evidence of active galactic nuclei .

Methods : We exploit observations taken with the NOEMA interferometer at \sim 3 mm to detect CO ( 2-1 ) line emission from the two selected galaxies , unresolved by our observations .

Results : Based on the CO ( 2-1 ) spectrum , we estimate a total molecular gas mass M ( { H _ { 2 } } ) = ( 2.2 ^ { +0.5 } _ { -0.4 } ) \times 10 ^ { 10 } M _ { \odot } , where fully excited gas is assumed , and a dust mass M _ { dust } < 4.2 \times 10 ^ { 8 } ~ { } M _ { \odot } for the two blended sources .
The two galaxies have similar stellar masses and H \alpha -based star formation rates ( SFRs ) found in previous work , as well as a large relative velocity of \sim 400 km/s estimated from the CO ( 2-1 ) line width .
These findings tend to privilege a scenario where both sources contribute to the observed CO ( 2-1 ) .
Using the archival Spitzer MIPS flux at 24 \mu m we estimate an { SFR ( 24 \mu m ) } = ( 28 ^ { +12 } _ { -8 } ) ~ { } M _ { \odot } /yr for each of the two galaxies .
Assuming that the two sources contribute equally to the observed CO ( 2-1 ) , our analysis yields a depletion timescale of { \tau _ { dep } = ( 3.9 ^ { +1.4 } _ { -1.8 } ) \times 10 ^ { 8 } } yr , and a molecular gas to stellar mass ratio of { 0.17 \pm 0.13 } for each of two sources , separately .
We also provide a new , more precise measurement of an unknown weighted mean of the redshifts of the two galaxies , z = 1.163 \pm 0.001 .

Conclusions : Our results are in overall agreement with those of other distant cluster galaxies and with model predictions for main sequence ( MS ) field galaxies at similar redshifts .
The two target galaxies have molecular gas mass and depletion times that are marginally compatible with , but smaller than those of MS field galaxies , suggesting that the molecular gas has not been sufficiently refueled .
We speculate that the cluster environment might have played a role in preventing the refueling via environmental mechanisms such as galaxy harassment , strangulation , ram-pressure , or tidal stripping .
Higher-resolution and higher-frequency observations will enable us to spatially resolve the two sources and possibly distinguish between different gas processing mechanisms .