A Search for Hi21 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 athigh redshift, J0924--2201 at $z = 5.20$, and J0913+5919 at $z=5.11$, using theGiant Meter Wave Radio Telescope (GMRT). These data set a 3$\sigma$ upper limitto absorption of $< 30%$ at 40 km s$^{-1}$ resolution for the 30 mJy sourceJ0913+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 levelare set by residual spectral baseline structure. For J0924--2201 the columndensity limit per 20 km s $^{-1}$ channel is: N(HI) $< 2.2\times 10^{18} \rmT_s$ 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 acovering factor of one. For J0913+5919 the column density limit per 40 kms$^{-1}$ channel is: N(HI) $< 2.2\times 10^{19} \rm T_s$ cm$^{-2}$ within $\pm2400$ km s$^{-1}$ of the optical redshift. These data rule out any cool, highcolumn 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 mightcorrespond to residual gas left over from cosmic reionization.Comment: accepted to the Astronomical Journal; 7 figures; 9 pages aastex forma