Ionizing radiation from AGNs at z>3.3 with the Subaru Hyper Suprime-Cam Survey and the CFHT Large Area U-band Deep Survey (CLAUDS)

TL;DR

This study estimates the ionizing radiation escape fraction from high-redshift AGNs using deep imaging data, finding it generally low and suggesting a minor role for AGNs in cosmic reionization at z>3.3.

Contribution

It provides new measurements of LyC escape fractions from AGNs at z>3.3 using combined deep imaging surveys, highlighting their limited contribution to ionizing background.

Findings

LyC escape fraction is typically below unity for AGNs at z>3.3.

Average LyC to UV flux ratio is 0.182, corresponding to an escape fraction of 0.303.

AGN contribution to cosmic LyC emissivity at this epoch is minor.

Abstract

We use deep and wide imaging data from the CFHT Large Area U-band Deep Survey (CLAUDS) and the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) to constrain the ionizing radiation (Lyman Continuum; LyC) escape fraction from AGNs at $z \sim 3 - 4$. For 94 AGNs with spectroscopic redshifts at $3.3 < z < 4.0$, we use their U-band / i-band flux ratios to estimate LyC transmission of individual AGNs. The distribution of their LyC transmission shows values lower than the range of LyC transmission values for IGM of the same redshift range, which suggests that LyC escape fraction of AGNs at $z>3.3$ is considerably lower than unity in most cases. We do not find any trend in LyC transmission values depending on their UV luminosities. Based on the photometry of stacked images we find the average flux ratio of LyC and non-ionizing UV photons escaping from the objects $(f_{LyC}/f_{UV})^{out} = 0.182 \pm 0.043$ for AGNs at $3.3<z<3.6$, which corresponds to LyC escape fraction $f_{esc} = 0.303 \pm 0.072$ if we assume a fiducial intrinsic SED of AGN. Based on the estimated LyC escape fraction and the UV luminosity function of AGNs, we argue that UV-selected AGNs' contribution to the LyC emissivity at the epoch is minor, although the size of their contribution largely depends on the shape of the UV luminosity function.