701 research outputs found
The Effect of Fluctuations on the Helium-Ionizing Background
Interpretation of He II Ly{\alpha} absorption spectra after the epoch of He
II reionization requires knowledge of the He II ionizing background. While past
work has modelled the evolution of the average background, the standard
cosmological radiative transfer technique assumes a uniform radiation field
despite the discrete nature of the (rare) bright quasars that dominate the
background. We implement a cosmological radiative transfer model that includes
the most recent constraints on the ionizing spectra and luminosity function of
quasars and the distribution of IGM absorbers. We also estimate, for the first
time, the effects of fluctuations on the evolving continuum opacity in two
ways: by incorporating the complete distribution of ionizing background
amplitudes into the standard approach, and by explicitly treating the quasars
as discrete -- but isolated -- sources. Our model results in a He II ionization
rate that evolves steeply with redshift, increasing by a factor ~2 from z=3.0
to z=2.5. This causes rapid evolution in the mean He II Ly{\alpha} optical
depth -- as recently observed -- without appealing to the reionization of He
II. The observed behaviour could instead result from rapid evolution in the
mean free path of ionizing photons as the helium in higher H I column density
absorbers becomes fully ionized.Comment: 14 pages, 9 figures. Accepted by MNRAS; significantly modified from
previous versio
Determining the Nature of Late Gunn-Peterson Troughs with Galaxy Surveys
Recent observations have discovered long (up to ~110 Mpc/h), opaque
Gunn-Peterson troughs in the z ~ 5.5 Lyman-alpha forest, which are challenging
to explain with conventional models of the post-reionization intergalactic
medium. Here we demonstrate that observations of the galaxy populations in the
vicinity of the deepest troughs can distinguish two competing models for these
features: deep voids where the ionizing background is weak due to fluctuations
in the mean free path of ionizing photons would show a deficit of galaxies,
while residual temperature variations from extended, inhomogeneous reionization
would show an overdensity of galaxies. We use large (~550 Mpc/h) semi-numerical
simulations of these competing explanations to predict the galaxy populations
in the largest of the known troughs at z ~ 5.7. We quantify the strong
correlation of Lyman-alpha effective optical depth and galaxy surface density
in both models and estimate the degree to which realistic surveys can measure
such a correlation. While a spectroscopic galaxy survey is ideal, we also show
that a relatively inexpensive narrowband survey of Lyman-alpha-emitting
galaxies is ~90% likely to distinguish between the competing models.Comment: 12 pages, 16 figures. Submitted to Ap
The Opacity of the Intergalactic Medium Measured Along Quasar Sightlines at
We publicly release a new sample of medium resolution quasar spectra at
observed with the Echellette Spectrograph and
Imager (ESI) on the Keck telescope. This quasar sample represents an ideal
laboratory to study the intergalactic medium (IGM) during the end stages of the
epoch of reionization, and constrain the timing and morphology of the phase
transition. For a subset of of our highest signal-to-noise ratio spectra
(S/N, per pixel), we present a new measurement of
the Lyman- (Ly) forest opacity spanning the redshift range
. We carefully eliminate spectral regions that could
be causing biases in our measurements due to additional transmitted flux in the
proximity zone of the quasars, or extra absorption caused by strong intervening
absorption systems along the line of sight. We compare the observed evolution
of the IGM opacity with redshift to predictions from a hydrodynamical
simulation with uniform ultraviolet background (UVB) radiation, as well as two
semi-numerical patchy reionization models, one with a fluctuating UVB and
another with a fluctuating temperature field. Our measurements show a steep
rise in opacity at and an increased scatter and thus support the
picture of a spatially inhomogeneous reionization process, consistent with
previous work. However, we measure significantly higher optical depths at
than previous studies, which reduces the contrast
between the highest opacity Gunn-Peterson troughs and the average opacity trend
of the IGM, which may relieve some of the previously noted tension between
these measurements and reionization models.Comment: accepted for publication at Ap
Large Fluctuations in the High-Redshift Metagalactic Ionizing Background
Recent observations have shown that the scatter in opacities among coeval
segments of the Lyman-alpha forest increases rapidly at z > 5. In this paper,
we assess whether the large scatter can be explained by fluctuations in the
ionizing background in the post-reionization intergalactic medium. We find that
matching the observed scatter at z ~ 5.5 requires a short spatially averaged
mean free path of 3 shorter than direct
measurements at z ~ 5.2. We argue that such rapid evolution in the mean free
path is difficult to reconcile with our measurements of the global H I
photoionization rate, which stay approximately constant over the interval z ~
4.8 - 5.5. However, we also show that measurements of the mean free path at z >
5 are likely biased towards higher values by the quasar proximity effect. This
bias can reconcile the short values of the mean free path that are required to
explain the large scatter in opacities. We discuss the implications of this
scenario for cosmological reionization. Finally, we investigate whether other
statistics applied to the z > 5 Lyman-alpha forest can shed light on the origin
of the scatter. Compared to a model with a uniform ionizing background, models
that successfully account for the scatter lead to enhanced power in the
line-of-sight flux power spectrum on scales k < 0.1 h/Mpc. We find tentative
evidence for this enhancement in observations of the high-redshift Lyman-alpha
forest.Comment: Matches version published by MNRAS with clarifications and expanded
discussio
Modeling the HeII Transverse Proximity Effect: Constraints on Quasar Lifetime and Obscuration
The HeII transverse proximity effect - enhanced HeII Ly{\alpha} transmission
in a background sightline caused by the ionizing radiation of a foreground
quasar - offers a unique opportunity to probe the emission properties of
quasars, in particular the emission geometry (obscuration, beaming) and the
quasar lifetime. Building on the foreground quasar survey published in
Schmidt+2017, we present a detailed model of the HeII transverse proximity
effect, specifically designed to include light travel time effects, finite
quasar ages, and quasar obscuration. We post-process outputs from a
cosmological hydrodynamical simulation with a fluctuating HeII UV background
model, plus the added effect of the radiation from a single bright foreground
quasar. We vary the age and obscured sky fractions
of the foreground quasar, and explore the resulting
effect on the HeII transverse proximity effect signal. Fluctuations in IGM
density and the UV background, as well as the unknown orientation of the
foreground quasar, result in a large variance of the HeII Ly{\alpha}
transmission along the background sightline. We develop a fully Bayesian
statistical formalism to compare far UV HeII Ly{\alpha} transmission spectra of
the background quasars to our models, and extract joint constraints on
and for the six Schmidt+2017 foreground
quasars with the highest implied HeII photoionization rates. Our analysis
suggests a bimodal distribution of quasar emission properties, whereby one
foreground quasar, associated with a strong HeII transmission spike, is
relatively old and unobscured ,
whereas three others are either younger than or highly
obscured .Comment: 19 pages, 6 figures, submitted to Ap
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