We have used the upgraded Giant Metrewave Radio Telescope to carry out a deep ( 117 on-source hours ) L-band observation of the Extended Groth Strip , to measure the average neutral hydrogen ( H i ) mass and median star formation rate ( SFR ) of star-forming galaxies , as well as the cosmic H i mass density , at 0.2 < z < 0.4 .
This was done by stacking the H i 21 cm emission and the rest-frame 1.4 GHz radio continuum from 445 blue star-forming galaxies with M _ { B } \leq - 17 at z _ { mean } \approx 0.34 .
The stacked H i 21 cm emission signal is detected at \approx 7 \sigma significance , implying an average H i mass of \langle M _ { HI } \rangle = ( 4.93 \pm 0.70 ) \times 10 ^ { 9 } \ > M _ { \odot } .
We also stacked the rest-frame 1.4 GHz radio continuum emission of the same galaxies , to obtain a median SFR of ( 0.54 \pm 0.06 ) \ > { M } _ { \odot } yr ^ { -1 } ; this implies an average atomic gas depletion time scale of \langle \Delta t _ { HI } \rangle \approx 9 Gyr , consistent with values in star-forming galaxies in the local Universe .
This indicates that the star-formation efficiency does not change significantly over the redshift range 0 - 0.4 .
We used the detection of the stacked H i 21 cm emission signal to infer the normalized cosmic H i mass density ( \rho _ { HI } / \rho _ { c, 0 } ) in star-forming galaxies at z \approx 0.34 .
Assuming the local relation between H i mass and absolute B-magnitude , we obtain \rho _ { HI } / \rho _ { c, 0 } = ( 4.81 \pm 0.75 ) \times 10 ^ { -4 } , implying no significant evolution in \rho _ { HI } / \rho _ { c, 0 } from z \approx 0.4 to the present epoch .