A search for HI 21cm absorption toward the highest redshift radio loud objects

Abstract

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) $<2.2\times 10^{18}{\mathrm{T}}_{\mathrm{s}}$ 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) $<2.2\times 10^{19}{\mathrm{T}}_{\mathrm{s}}$ 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.

accepted to the Astronomical Journal; 7 figures; 9 pages aastex format

References in corpus (2)

• Observational constraints on Cosmic Reionization
• The detectability of HI 21-cm absorption in damped Lyman-alpha systems

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• Prospects of a statistical detection of the 21-cm forest and its potential to constrain the thermal state of the neutral IGM during reionization
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