A search for H I 21 cm absorption toward the highest redshift (z ∼ 5.2) radio-loud objects

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 × 10¹⁸T_scm^-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 × 10 ¹⁹T_s 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.