TY  - JOUR
A1  - Worseck, Gabor
A1  - Davies, Frederick B.
A1  - Hennawi, Joseph F.
A1  - Prochaska, J. Xavier
T1  - The Evolution of the He II-ionizing Background at Redshifts 2.3 < z < 3.8 Inferred from a Statistical Sample of 24 HST/COS He II Lyα Absorption Spectra
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We present measurements of the large-scale (≈40 comoving Mpc) effective optical depth of He ii Lyα absorption, ${\tau }_{\mathrm{eff}}$, at 2.54 < z < 3.86 toward 16 He ii-transparent quasars observed with the Cosmic Origins Spectrograph on the Hubble Space Telescope, to characterize the ionization state of helium in the intergalactic medium (IGM). We provide the first statistical sample of ${\tau }_{\mathrm{eff}}$ measurements in six signal-to-noise ratio gsim3 He ii sightlines at z > 3.5, and study the redshift evolution and sightline-to-sightline variance of ${\tau }_{\mathrm{eff}}$ in 24 He ii sightlines. We confirm an increase of the median ${\tau }_{\mathrm{eff}}$ from sime2 at z = 2.7 to ${\tau }_{\mathrm{eff}}\gtrsim 5$ at z > 3, and a scatter in ${\tau }_{\mathrm{eff}}$ that increases with redshift. The z > 3.5 He ii absorption is predominantly saturated, but isolated narrow (Δv < 650 km s−1) transmission spikes indicate patches of reionized helium. We compare our measurements to predictions for a range of UV background models applied to outputs of a large-volume (146 comoving Mpc)3 hydrodynamical simulation by forward-modeling our sample's quality and size. At z > 2.74, the variance in ${\tau }_{\mathrm{eff}}$ significantly exceeds expectations for a spatially uniform UV background, but is consistent with a fluctuating radiation field sourced by variations in the quasar number density and the mean free path in the post-reionization IGM. We develop a method to infer the approximate median He ii photoionization rate ${{\rm{\Gamma }}}_{\mathrm{He}{\rm{II}}}$ of a fluctuating UV background from the median ${\tau }_{\mathrm{eff}}$, finding a factor sime5 decrease in ${{\rm{\Gamma }}}_{\mathrm{He}{\rm{II}}}$ between z sime 2.6 and z sime 3.1. At z sime 3.1, ${{\rm{\Gamma }}}_{\mathrm{He}{\rm{II}}}=\left[{9.1}_{-1.2}^{+1.1}\,(\mathrm{stat}.){\,}_{-3.4}^{+2.4}\,(\mathrm{sys}.)\right]\times {10}^{-16}$ s−1 corresponds to a median He ii fraction of sime2.5%, indicating that our data probe the tail end of He ii reionization.
KW  - dark ages, reionization, first stars
KW  - diffuse radiation
KW  - intergalactic medium
KW  - quasars: absorption lines
Y1  - 2019
U6  - https://doi.org/10.3847/1538-4357/ab0fa1
SN  - 0004-637X
SN  - 1538-4357
VL  - 875
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Crighton, Neil H. M.
A1  - Prochaska, J. Xavier
A1  - Murphy, Michael T.
A1  - Worseck, Gabor
A1  - Smith, Britton D.
T1  - Imprints of the first billion years
BT  - Lyman limit systems at z similar to 5
JF  - Monthly notices of the Royal Astronomical Society
N2  - Lyman limit systems (LLSs) trace the low-density circumgalactic medium and the most dense regions of the intergalactic medium, so their number density and evolution at high-redshift, just after reionization, are important to constrain. We present a survey for LLSs at high redshifts, z(LLS) = 3.5-5.4, in the homogeneous data set of 153 optical quasar spectra at z similar to 5 from the Giant Gemini GMOS survey. Our analysis includes detailed investigation of survey biases using mock spectra which provide important corrections to the raw measurements. We estimate the incidence of LLSs per unit redshift at z approximate to 4.4 to be l(z) = 2.6 +/- 0.4. Combining our results with previous surveys at z(LLS) < 4, the best-fit power-law evolution is l(z) = l(*)[(1 + z)/4](alpha) with l* = 1.46 +/- 0.11 and alpha = 1.70 +/- 0.22 (68 per cent confidence intervals). Despite hints in previous z(LLS) < 4 results, there is no indication for a deviation from this single power-law soon after reionization. Finally, we integrate our new results with previous surveys of the intergalactic and circumgalactic media to constrain the hydrogen column density distribution function, f(N-HI, X), over 10 orders ofmagnitude. The data at z similar to 5 are not well-described by the f(N-HI, X) model previously reported for z similar to 2-3 (after re-scaling) and a 7-pivot model fitting the full z similar to 2-5 data set is statistically unacceptable. We conclude that there is significant evolution in the shape of f(N-HI, X) over this similar to 2-billion-year period.
KW  - quasars: absorption lines
KW  - cosmological parameters
KW  - cosmology: observations
Y1  - 2019
U6  - https://doi.org/10.1093/mnras/sty2762
SN  - 0035-8711
SN  - 1365-2966
VL  - 482
IS  - 2
SP  - 1456
EP  - 1470
PB  - Oxford Univ. Press
CY  - Oxford
ER  -