We have searched for HI 21cm absorption toward the two brightest radio AGN at high redshift , J0924–2201 at z = 5.20 , and J0913+5919 at z = 5.11 , using the Giant Meter Wave Radio Telescope ( GMRT ) .
These data set a 3 \sigma upper limit to absorption of < 30 \% at 40 km s ^ { -1 } resolution for the 30 mJy source J0913+5919 , and < 3 \% for the 0.55 Jy source J0924–2201 at 20 km s ^ { -1 } resolution .
For J0924–2201 , limits to broader lines at the few percent level are set by residual spectral baseline structure .
For J0924–2201 the column density limit per 20 km s ^ { -1 } channel is : N ( HI ) < 2.2 \times 10 ^ { 18 } T _ { s } cm ^ { -2 } over a velocity range of -700 km s ^ { -1 } to +1180 km s ^ { -1 } centered on the galaxy redshift determined through CO emission , assuming a covering factor of one .
For J0913+5919 the column density limit per 40 km s ^ { -1 } channel is : N ( HI ) < 2.2 \times 10 ^ { 19 } T _ { s } cm ^ { -2 } within \pm 2400 km s ^ { -1 } of the optical redshift .
These data rule out any cool , high column density HI clouds within roughly \pm 1000 km s ^ { -1 } of the galaxies , as are often seen in Compact Steep Spectrum radio AGN , or clouds that might correspond to residual gas left over from cosmic reionization .