Lyman continuum escape fraction and mean free path of hydrogen ionizing photons for brightz∼ 4 QSOs from SDSS DR14

Abstract

Context. One of the major challenges in observational cosmology is related to the redshift evolution of the average hydrogen (HI) ionization in the Universe, as evidenced by the changing in the ionization level of the intergalactic medium (IGM) through cosmic time. In particular, starting from the first cosmic reionization, the rapid evolution of the IGM physical properties poses severe constraints for the identification of the sources responsible for keeping its high level of ionization up to lower redshifts. Aims. In order to probe the ionization level of the IGM and the ionization capabilities of bright QSOs at z = 4, we have selected a sample of 2508 QSOs drawn from the SDSS survey (DR14) in the redshift interval 3.6 ≤ z ≤ 4.6 and absolute magnitude range −29.0 M 1450 −26.0. Particularly, we focus on the estimate of the escape fraction of HI ionizing photons and their mean free path (MFP), which are fundamental in the characterization of the surrounding intergalactic medium. Methods. Starting from UV/optical rest-frame spectra of the whole QSO sample from the SDSS survey, we estimate the escape fraction and free path individually for each of them. We calculate the Lyman Continuum escape fraction as the flux ratio blue-ward (∼ 900 Å rest-frame) and red-ward (∼ 930 Å rest-frame) of the Lyman limit (LL). We then obtain the probability distribution function (PDF) of the individual free paths of the QSOs in the sample and study its evolution in luminosity and redshift, comparing our results with the ones found in literature so far. Results. We find a lower limit to the mean Lyman Continuum escape fraction of 0.49, in agreement with the values obtained for both brighter and fainter sources at the same redshift. We show that the free paths of ionizing photons are characterized by a skewed distribution function peaked at low values, with an average of ∼ 49 − 59 proper Mpc at z ∼ 4, after excluding possible associated absorbers (AAs). This value is larger than the one obtained at the same redshift by many authors in the literature using different techniques. Moreover, the probability distribution function of free path gives a complementary information w.r.t. the mean free path derived through the stack technique. Finally, we also find that the redshift evolution of this parameter results to be possibly milder than previously thought. Conclusions. Our new determination of the mean free path at z ∼ 4 implies that previous estimates of the HI photo-ionization rate Γ HI available in the literature should be corrected by a factor of 1.2-1.7. These results have important implications if extrapolated at the epoch of reionization.

The space density of ultra-luminous QSOs at the end of reionization epoch by the QUBRICS Survey and the AGN contribution to the hydrogen ionizing background

“…This enhancement in the ionization level of the IGM due to the ionizing QSO flux extends to several proper Mpc (∼ 6 − 10 pMpc) as observed in bright QSOs (M 1450 −27; Fan et al 2006;Calverley et al 2011;Eilers et al 2017). This implies that their escape fraction is in general relatively large (∼ 70%) and appears to keep values > 50% in fainter AGNs (M 1450 −23; Cristiani et al 2016;Grazian et al 2018;Romano et al 2019). Thus, the relevance of AGNs in the reionization process mainly depends on the space density evolution of AGNs with M 1450 < −22 in the redshift interval 3.5 < z < 6.5.…”

The mean free path of ionizing photons at 5 &lt; z &lt; 6: evidence for rapid evolution near reionization

“…While a skewed distribution of free paths along individual lines of sight is expected (e.g. Romano et al 2019), and while this particular QSO is the brightest (and lowest redshift) one in our ESI + X-Shooter sample, a larger sample at z ∼ 6 would help to characterize the spatial variations in ionizing opacity near the end of reionization. Given the rapid increase in λ mfp between z = 6 to 5, and the different evolutions predicted by the models over this redshift range (e.g.…”

“…They find values that are ∼10-20 per cent higher than those of Prochaska et al (2009) and Worseck et al (2014) over the same redshifts. In trials using the two lower redshift GGG composites from Worseck et al (2014) we found that this discrepancy is well explained by the lack of foreground Lyman series absorption in the Romano et al (2019) analysis. Worseck et al (2014) fit a power law of the form λ mfp (z) ∝ (1 + z) −5.4 over 2.44 < z < 5.16 (dotted line in Fig.…”

“…Lusso et al (2018) find somewhat lower values of λ mfp at z ∼ 2 towards QSO pairs, potentially due to an increased incidence of optically thick absorbers in pair environments. We note that Romano et al (2019) measured the mean free path towards QSOs at z ∼ 4. They find values that are ∼10-20 per cent higher than those of Prochaska et al (2009) and Worseck et al (2014) over the same redshifts.…”

“…The latter approach has arguably produced the most precise estimates of the mean free path at high redshifts, with results now spanning 2 ࣠ z ࣠ 5 (Prochaska et al 2009;Fumagalli et al 2013;O'Meara et al 2013;Worseck et al 2014;Lusso et al 2018). One can also infer the mean free path from the average of free paths along individual QSO lines of sight (Romano et al 2019).…”