TY  - JOUR
A1  - Berg, Trystyn A. M.
A1  - Ellison, Sara L.
A1  - Sanchez-Ramirez, Ruben
A1  - Lopez, Sebastian
A1  - Becker, George D.
A1  - Christensen, Lise
A1  - Cupani, Guido
A1  - Denney, Kelly D.
A1  - Worseck, Gabor
T1  - Sub-damped Lyman alpha systems in the XQ-100 survey - I. Identification and contribution to the cosmological H I budget
JF  - Monthly notices of the Royal Astronomical Society
KW  - galaxies: high-redshift
KW  - galaxies: ISM
KW  - quasars: absorption lines
Y1  - 2019
U6  - https://doi.org/10.1093/mnras/stz2012
SN  - 0035-8711
SN  - 1365-2966
VL  - 488
IS  - 3
SP  - 4356
EP  - 4369
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Becker, George D.
A1  - D'Aloisio, Anson
A1  - Christenson, Holly M.
A1  - Zhu, Yongda
A1  - Worseck, Gábor
A1  - Bolton, James S.
T1  - The mean free path of ionizing photons at 5 < z < 6
BT  - evidence for rapid evolution near reionization
JF  - Monthly notices of the Royal Astronomical Society
N2  - The mean free path of ionizing photons, lambda(mfp), is a key factor in the photoionization of the intergalactic medium (IGM). At z greater than or similar to 5, however, lambda(mfp) may be short enough that measurements towards QSOs are biased by the QSO proximity effect. We present new direct measurements of lambda(mfp) that address this bias and extend up to z similar to 6 for the first time. Our measurements at z similar to 5 are based on data from the Giant Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At z similar to 6 we use QSO spectra from Keck ESI and VLT X-Shooter. We measure lambda(mfp) = 9.09(-1.28)(+1.62) proper Mpc and 0.75(-0.45)(+0.65) proper Mpc (68 percent confidence) at z = 5.1 and 6.0, respectively. The results at z = 5.1 are consistent with existing measurements, suggesting that bias from the proximity effect is minor at this redshift. At z = 6.0, however, we find that neglecting the proximity effect biases the result high by a factor of two or more. Our measurement at z = 6.0 falls well below extrapolations from lower redshifts, indicating rapid evolution in lambda(mfp) over 5 < z < 6. This evolution disfavours models in which reionization ended early enough that the IGM had time to fully relax hydrodynamically by z = 6, but is qualitatively consistent with models wherein reionization completed at z = 6 or even significantly later. Our mean free path results are most consistent with late reionization models wherein the IGM is still 20 percent neutral at z = 6, although our measurement at z = 6.0 is even lower than these models prefer.
KW  - intergalactic medium
KW  - quasars: absorption lines
KW  - cosmology: observations
KW  - dark ages
KW  - large-scale structure of Universe
KW  - reionization
KW  - first stars
Y1  - 2021
U6  - https://doi.org/10.1093/mnras/stab2696
SN  - 0035-8711
SN  - 1365-2966
VL  - 508
IS  - 2
SP  - 1853
EP  - 1869
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Becker, George D.
A1  - Pettini, Max
A1  - Rafelski, Marc
A1  - Boera, Elisa
A1  - Christensen, Lise
A1  - Cupani, Guido
A1  - Ellison, Sara L.
A1  - Farina, Emanuele Paolo
A1  - Fumagalli, Michele
A1  - Lopez, Sebastian
A1  - Neeleman, Marcel
A1  - Ryan-Weber, Emma
A1  - Worseck, Gabor
T1  - The Evolution of OI over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We present a survey for metal absorption systems traced by neutral oxygen over 3.2 < z < 6.5. Our survey uses Keck/ESI and VLT/X-Shooter spectra of 199 QSOs with redshifts up to 6.6. In total, we detect 74 OI absorbers, of which 57 are separated from the background QSO by more than 5000 km s(-1). We use a maximum likelihood approach to fit the distribution of OI lambda 1302 equivalent widths in bins of redshift and from this determine the evolution in number density of absorbers with W-1302 > 0.05 angstrom, of which there are 49 nonproximate systems in our sample. We find that the number density does not monotonically increase with decreasing redshift, as would naively be expected from the buildup of metal-enriched circumgalactic gas with time. The number density over 4.9 < z < 5.7 is a factor of 1.7-4.1 lower (68% confidence) than that over 5.7 < z < 6.5, with a lower value at z < 5.7 favored with 99% confidence. This decrease suggests that the fraction of metals in a low-ionization phase is larger at z similar to 6 than at lower redshifts. Absorption from highly ionized metals traced by CIV is also weaker in higher-redshift OI systems, supporting this picture. The evolution of OI absorbers implies that metal-enriched circumgalactic gas at z similar to 6 is undergoing an ionization transition driven by a strengthening ultraviolet background. This in turn suggests that the reionization of the diffuse intergalactic medium may still be ongoing at or only recently ended by this epoch.
Y1  - 2019
U6  - https://doi.org/10.3847/1538-4357/ab3eb5
SN  - 0004-637X
SN  - 1538-4357
VL  - 883
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  -