Implications of z ∼ 6 Quasar Proximity Zones for the Epoch of Reionization and Quasar Lifetimes

Abstract

We study quasar proximity zones in the redshift range 5.77 ≤ z ≤ 6.54 by homogeneously analyzing 34 medium resolution spectra, encompassing both archival and newly obtained data, and exploiting recently updated systemic redshift and magnitude measurements. Whereas previous studies found strong evolution of proximity zone sizes with redshift, and argued that this provides evidence for a rapidly evolving intergalactic medium (IGM) neutral fraction during reionization, we measure a much shallower trend ∝ (1 + z) −1.44 . We compare our measured proximity zone sizes to predictions from hydrodynamical simulations post-processed with one-dimensional radiative transfer, and find good agreement between observations and theory irrespective of the ionization state of the ambient IGM. This insensitivity to IGM ionization state has been previously noted, and results from the fact that the definition of proximity zone size as the first drop of the smoothed quasar spectrum below the 10% flux transmission level probes locations where the ionizing radiation from the quasar is an order of magnitude larger than the expected ultraviolet ionizing background that sets the neutral fraction of the IGM. Our analysis also uncovered three objects with exceptionally small proximity zones (two have R p < 1 proper Mpc), which constitute outliers from the observed distribution and are challenging to explain with our radiative transfer simulations. We consider various explanations for their origin, such as strong absorption line systems associated with the quasar or patchy reionization, but find that the most compelling scenario is that these quasars have been shining for 10 5 yr.

Physical Properties of 15 Quasars at z ≳ 6.5

“…We show in Figure 16 an example of PCA for one of the quasars in our sample. Also, in this way we provide an analysis of the nearzone sizes consistent with Eilers et al (2017), making it possible to coherently compare the results obtained from the two data sets.…”

“…We then obtain the best coefficients that reproduce the entire spectrum by using the projection matrix presented by Pâris et al (2011). For further details on this modeling procedure, see Eilers et al (2017). We show in Figure 16 an example of PCA for one of the quasars in our sample.…”

“…Several studies provide estimates of near-zone radii for samples of z 5 > quasars and investigate its evolution as a function of redshift, in order to investigate the IGM evolution (Fan et al 2006;Carilli et al 2010;Venemans et al 2015a;Eilers et al 2017).…”

“…We here model the quasar emission at 1215.16 rf l < Å using a principal component analysis (PCA) approach: the total spectrum, q l ( ), is represented as the sum of a mean spectrum, m l ( ), and n N 1 ,.., = principal component spectra (PCS), n x l ( ), each weighted by a coefficient w n : Suzuki (2006) apply the PCA to a collection of 78 z 3 and 50 z 1  quasars from SDSS, respectively. In our study, we follow the approach by Eilers et al (2017), mainly referring to Pâris et al (2011), who provide PCS functions within the rest-frame wavelength window 1020 2000…”

“…However, the modest-sized and nonhomogeneous quasar samples at hand, large errors due to uncertain redshifts, and the limited theoretical models available have inhibited our understanding of these measurements to date, i.e., do they trace the evolution of the ionized gas fraction, or are they dominated by degeneracies (e.g., quasar lifetimes)? Recently, Eilers et al (2017) addressed some of these caveats, deriving near-zone sizes of 34 quasars at z 5.77 6.54  …”