The role of galaxies and AGNs in reionizing the IGM – II. Metal-tracing the faint sources of reionization at 5 ≲ z ≲ 6

We present a new method to study the contribution of faint sources to the ultraviolet background using the 1D correlation of metal absorbers with the intergalactic medium transmission in a quasi-stellar object (QSO) sightline. We take advantage of a sample of 25 high signal-to-noise ratio QSO spectra to retrieve 150 triply-ionized carbon (C IV) absorbers at 4.5 ≲ z ≲ 6.2, of which 37 systems whose expected H I absorption lie in the Lyman-α forest. We derive improved constraints on the cosmic density of C IV at 4.3 < z < 6.2 and infer from abundance matching that C IV absorbers trace MUV ≲ −16 galaxies. Correlation with the Lyman-α forest of the QSOs indicates that these objects are surrounded by a highly opaque region at r ≲ 5 cMpc h−1 followed by an excess of transmission at r ≳ 10 cMpc h−1 detected at 2.7σ. This is in contrast to equivalent measurements at lower redshifts where only the opaque trough is detected. We interpret this excess as a statistical enhancement of the local photoionization rate due to clustered faint galaxies around the C IV absorbers. Using the analytical framework described in Paper I of this series, we derive a constraint on the average product of the escape fraction and the Lyman continuum photon production efficiency of the galaxy population clustered around the C IV absorbers, log⟨fescξion⟩/[erg−1Hz]=25.01+0.30−0.19⁠. This implies that faint galaxies beyond the reach of current facilities may have harder radiation fields and/or larger escape fractions than currently detected objects at the end of the reionization epoch