We report the results of our Chandra observations of six QSOs at z ∼ 3 from the Palomer Transit Grism Survey. Our primary goal is to investigate the possible systematic change of α ox between z > 4 and z ∼ 3, between which a rapid rise of luminous QSO number density with cosmic time is observed. The summed spectrum showed a power-law spectrum with a photon index of F ≈ 1.9, which is similar to other unabsorbed active galactic nuclei. Combining our z ∼ 3 QSOs with X-ray observations of QSOs at z > 4 from literature or archives, we find a correlation of α ox with optical luminosity. This is consistent with the fact that the luminosity function slope of the luminous end of the X-ray-selected QSOs is steeper than that of optically selected QSOs. We discuss an upper limit to the redshift dependence of α ox using a Monte Carlo simulation. Within the current statistical errors, including the derived limits on the redshift dependence of α ox, we found that the behaviors of the X-ray and optically selected QSO number densities are consistent with each other.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science