7 A search for HI 21 cm absorption toward the highest redshift ( z ∼ 5 . 2 ) radio loud objects

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 3σ upper limit to absorption of < 30% at 40 km s resolution for the 30 mJy source J0913+5919, and < 3% for the 0.55 Jy source J0924–2201 at 20 km s 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 × 1018Ts cm −2 over a velocity range of −700 km s to +1180 km s 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 channel is: N(HI) < 2.2× 1019Ts cm within ±2400 km s of the optical redshift. These data rule out any cool, high column density HI clouds within roughly ±1000 km s 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. Subject headings: cosmology; radio lines; galaxies high redshift