A Gunn-Peterson test with a QSO at z=6.4

TL;DR

This study uses a high-redshift quasar spectrum to measure hydrogen absorption, providing insights into cosmic re-ionization and indicating a significant increase in optical depth at redshifts above 5.7.

Contribution

It presents the highest redshift hydrogen absorption measurement to date using a faint quasar, extending the understanding of re-ionization at z=6.4.

Findings

Average transmitted flux is consistent with zero, indicating high hydrogen absorption.

Optical depth lower limit is tau_eff>4.9, showing strong evolution at z>5.7.

Results support rapid change in the intergalactic medium's ionization state at high redshift.

Abstract

Understanding the cosmic re-ionization is one of the key goals of the modern observational cosmology. High redshift QSO spectra can be used as background light sources to measure absorption by intervening neutral hydrogen. We investigate neutral hydrogen absorption in a deep, moderate-resolution Keck/Deimos spectrum of QSO CFHQSJ2329-0301 at z=6.4. This QSO is one of the highest redshift QSOs presently known at z=6.4 but is 2.5 mag fainter than a previously well-studied QSO SDSSJ1148+5251 at z=6.4. Therefore, it has a smaller Stromgren sphere, and allows us to probe the highest redshift hydrogen absorption to date. The average transmitted flux at 5.915<z_abs<6.365 (200 comoving Mpc) is consistent with zero, in Ly_alpha, Ly_beta, and Ly_gamma absorption measurements. This corresponds to the lower limit of optical depth, tau_eff>4.9. These results are consistent with strong evolution of the optical depth at z>5.7.