8 research outputs found
The need for obscured supermassive black hole growth to explain quasar proximity zones in the epoch of reionization
Proximity zones of quasars with redshifts are unique probes of
the growth of supermassive black holes. But simultaneously explaining proximity
zone sizes and black hole masses at this redshift has proved to be challenging
because of the very short quasar lifetimes implied by the proximity zones. We
study the robustness of some of the assumptions that are usually made to infer
quasar lifetimes from proximity zone sizes. We show that thanks to the short
equilibration time of gas inside the proximity zones, small proximity zones can
be readily explained by quasars that vary in brightness with a short duty cycle
of and short bright periods of yr, even for long lifetimes. We further show that reconciling this with
black hole mass estimates requires the black hole to continue to grow and
accrete during its obscured phase. The consequent obscured fractions of
0.7 or higher are consistent with low-redshift measurements and
models of black hole accretion. Further, the large dynamic range of our
simulation, and its calibration to the Lyman- forest, allows us to
investigate the influence of the large-scale topology of reionization and the
quasar's host halo mass on proximity zones. We find that incomplete
reionization can impede the growth of proximity zones and make them smaller up
to 30%, but the quasar host halo mass only affects proximity zones weakly and
indirectly. Our work suggests that high-redshift proximity zones can be an
effective tool to study quasar variability and black hole growth.Comment: 19 pages, 17 figures, submitted to MNRAS, comments welcom
Robustness of direct measurements of the mean free path of ionizing photons in the epoch of reionization
Measurements of the mean free path of Lyman-continuum photons in the
intergalactic medium during the epoch of reionization can help constrain the
nature of the sources as well as sinks of hydrogen-ionizing radiation. A recent
approach to this measurement has been to utilize composite spectra of multiple
quasars at , and infer the mean free path after correcting the spectra
for the presence of quasar proximity zones. This has revealed not only a steep
drop in the mean free path from to , but also potentially a mild
tension with reionization simulations. We critically examine such direct
measurements of the mean free path for biases due to quasar environment,
incomplete reionization, and quasar proximity zones. Using cosmological
radiative transfer simulations of reionization combined with one-dimensional
radiative transfer calculations of quasar proximity zones, we find that the
bias in the mean free path due to overdensities around quasars is minimal at
. Patchiness of reionization at this redshift also does not affect the
measurements significantly. Fitting our model to the data results in a mean
free path of pMpc at , which
is consistent with the recent measurements in the literature, indicating
robustness with respect to the modelling of quasar proximity zones. We also
compare various ways in which the mean free path has been defined in
simulations before the end of reionization. Overall, our finding is that recent
measurements of the mean free path appear to be robust relative to several
sources of potential bias.Comment: 10 pages, 9 figures, submitted to MNRA
Chronicling the reionization history at with emergent quasar damping wings
The spectra of high-redshift () quasars contain valuable
information on the progression of the Epoch of Reionization (EoR). At redshifts
, the observed Lyman-series forest shows that the intergalactic medium
(IGM) is nearly ionized, while at the observed quasar damping wings
indicate high neutral gas fractions. However, there remains a gap in neutral
gas fraction constraints at where the Lyman series
forest becomes saturated but damping wings have yet to fully emerge. In this
work, we use a sample of 18 quasar spectra at redshifts to close
this gap. We apply neural networks to reconstruct the quasars' continuum
emission around the partially absorbed Lyman line to normalize their
spectra, and stack these continuum-normalized spectra in three redshift bins.
To increase the robustness of our results, we compare the stacks to a grid of
models from two hydrodynamical simulations, ATON and CROC, and we measure the
volume-averaged neutral gas fraction, , while jointly fitting
for the mean quasar lifetime, , for each stacked spectrum. We
chronicle the evolution of neutral gas fraction using the ATON (CROC) models as
follows: () at , () at , and () at . At the same time we
constrain the average quasar lifetime to be
across all redshift bins, in good agreement with previous studies.Comment: 28 pages, 14 figure
New quasar proximity zone size measurements at using the enlarged XQR-30 sample
Proximity zones of high-redshift quasars are unique probes of their central
supermassive black holes as well as the intergalactic medium in the last stages
of reionization. We present 22 new measurements of proximity zones of quasars
with redshifts between 5.8 and 6.6, using the enlarged XQR-30 sample of
high-resolution, high-SNR quasar spectra. The quasars in our sample have UV
magnitudes of and black hole masses of
\unicode{x2013} M. Our inferred proximity zone sizes
are 2\unicode{x2013}7 physical Mpc, with a typical uncertainty of less than
0.5 physical Mpc, which, for the first time, also includes uncertainty in the
quasar continuum. We find that the correlation between proximity zone sizes and
the quasar redshift, luminosity, or black hole mass, indicates a large
diversity of quasar lifetimes. Two of our proximity zone sizes are
exceptionally small. The spectrum of one of these quasars, with ,
displays, unusually for this redshift, damping wing absorption without any
detectable metal lines, which could potentially originate from the IGM. The
other quasar has a high-ionization absorber 0.5 pMpc from the edge of the
proximity zone. This work increases the number of proximity zone measurements
available in the last stages of cosmic reionization to 87. This data will lead
to better constraints on quasar lifetimes and obscuration fractions at high
redshift, which in turn will help probe the seed mass and formation redshift of
supermassive black holes.Comment: 16 pages, 9 figures, Accepted in MNRA
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Robustness of direct measurements of the mean free path of ionizing photons in the epoch of reionization
Abstract
Measurements of the mean free path of Lyman-continuum photons in the intergalactic medium during the epoch of reionization can help constrain the nature of the sources as well as sinks of hydrogen-ionizing radiation. A recent approach to this measurement has been to utilize composite spectra of multiple quasars at z ⌠6, and infer the mean free path after correcting the spectra for the presence of quasar proximity zones. This has revealed not only a steep drop in the mean free path from z = 5 to z = 6, but also potentially a mild tension with reionization simulations. We critically examine such direct measurements of the mean free path for biases due to quasar environment, incomplete reionization, and quasar proximity zones. Using cosmological radiative transfer simulations of reionization combined with one-dimensional radiative transfer calculations of quasar proximity zones, we find that the bias in the mean free path due to overdensities around quasars is minimal at z ⌠6. Patchiness of reionization at this redshift also does not affect the measurements significantly. Fitting our model to the data results in a mean free path of  pMpc at z = 6, which is consistent with the recent measurements in the literature, indicating robustness with respect to the modelling of quasar proximity zones. We also compare various ways in which the mean free path has been defined in simulations before the end of reionization. Overall, our finding is that recent measurements of the mean free path appear to be robust relative to several sources of potential bias.</jats:p
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Chronicling the Reionization History at 6 âČ z âČ 7 with Emergent Quasar Damping Wings
Abstract
The spectra of high-redshift (z âł 6) quasars contain valuable information on the progression of the Epoch of Reionization. At redshifts z < 6, the observed Lyman-series forest shows that the intergalactic medium is nearly ionized, while at z > 7 the observed quasar damping wings indicate high neutral gas fractions. However, there remains a gap in neutral gas fraction constraints at 6 âČ z âČ 7 where the Lyman-series forest becomes saturated but damping wings have yet to fully emerge. In this work, we use a sample of 18 quasar spectra at redshifts 6.0 < z < 7.1 to close this gap. We apply neural networks to reconstruct the quasarsâ continuum emission around the partially absorbed Lyα line to normalize their spectra, and stack these continuum-normalized spectra in three redshift bins. To increase the robustness of our results, we compare the stacks to a grid of models from two hydrodynamical simulations, ATON and CROC, and we measure the volume-averaged neutral gas fraction,
x
ÂŻ
HI
, while jointly fitting for the mean quasar lifetime, t
Q, for each stacked spectrum. We chronicle the evolution of neutral gas fraction using the ATON (CROC) models as follows:
x
ÂŻ
HI
=
0.21
â
0.07
+
0.17
(
x
ÂŻ
HI
=
0.10
<
10
â
4
+
0.73
) at ăză = 6.10,
x
ÂŻ
HI
=
0.21
â
0.07
+
0.33
(
x
ÂŻ
HI
=
0.57
â
0.47
+
0.26
) at ăză = 6.46, and
x
ÂŻ
HI
=
0.37
â
0.17
+
0.17
(
x
ÂŻ
HI
=
0.57
â
0.21
+
0.26
) at ăză = 6.87. At the same time, we constrain the average quasar lifetime to be t
Q âČ 7 Myr across all redshift bins, in good agreement with previous studies.</jats:p
Probing Ultralate Reionization: Direct Measurements of the Mean Free Path over 5 < z < 6
The mean free path of ionizing photons, λ _mfp , is a critical parameter for modeling the intergalactic medium (IGM) both during and after reionization. We present direct measurements of λ _mfp from QSO spectra over the redshift range 5 < z < 6, including the first measurements at z â 5.3 and 5.6. Our sample includes data from the XQR-30 VLT large program, as well as new Keck/ESI observations of QSOs near z ⌠5.5, for which we also acquire new [C ii ] 158 ÎŒ m redshifts with ALMA. By measuring the Lyman continuum transmission profile in stacked QSO spectra, we find , , , and pMpc at z = 5.08, 5.31, 5.65, and 5.93, respectively. Our results demonstrate that λ _mfp increases steadily and rapidly with time over 5 < z < 6. Notably, we find that λ _mfp deviates significantly from predictions based on a fully ionized and relaxed IGM as late as z = 5.3. By comparing our results to model predictions and indirect λ _mfp constraints based on IGM Ly α opacity, we find that the evolution of λ _mfp is consistent with scenarios wherein the IGM is still undergoing reionization and/or retains large fluctuations in the ionizing UV background well below redshift 6