A Search for H i 21 cm Absorption toward the Highest Redshift ( z ~ 5.2) Radio-loud Objects

Abstract

We have searched for H I 21 cm absorption toward the two brightest radio AGNs at high redshift, J0924-2201 at z=5.20 and J0913+5919 at z=5.11, using the Giant Metrewave Radio Telescope. These data set a 3 σ 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(H I)<2.2×1018Ts cm-2 over a velocity range of -700 to +1180 km s-1 centered on the galaxy redshift determined through CO emission, assuming a covering factor of 1. For J0913+5919 the column density limit per 40 km s-1 channel is N(H I)<2.2×1019Ts cm-2 within +/-2400 km s-1 of the optical redshift. These data rule out any cool, high column density H I clouds within roughly +/-1000 km s-1 of the galaxies, as are often seen in compact steep-spectrum radio AGNs, or clouds that might correspond to residual gas left over from cosmic reionization.