47 research outputs found
Self-Consistent Modeling of Reionization in Cosmological Hydrodynamical Simulations
The ultraviolet background (UVB) emitted by quasars and galaxies governs the
ionization and thermal state of the intergalactic medium (IGM), regulates the
formation of high-redshift galaxies, and is thus a key quantity for modeling
cosmic reionization. The vast majority of cosmological hydrodynamical
simulations implement the UVB via a set of spatially uniform photoionization
and photoheating rates derived from UVB synthesis models. We show that
simulations using canonical UVB rates reionize and, perhaps more importantly,
spuriously heat the IGM, much earlier z ~ 15 than they should. This problem
arises because at z > 6, where observational constraints are nonexistent, the
UVB amplitude is far too high. We introduce a new methodology to remedy this
issue, and we generate self-consistent photoionization and photoheating rates
to model any chosen reionization history. Following this approach, we run a
suite of hydrodynamical simulations of different reionization scenarios and
explore the impact of the timing of reionization and its concomitant heat
injection on the the thermal state of the IGM. We present a comprehensive study
of the pressure smoothing scale of IGM gas, illustrating its dependence on the
details of both hydrogen and helium reionization, and argue that it plays a
fundamental role in interpreting Lyman-alpha forest statistics and the thermal
evolution of the IGM. The premature IGM heating we have uncovered implies that
previous work has likely dramatically overestimated the impact of
photoionization feedback on galaxy formation, which sets the minimum halo mass
able to form stars at high redshifts. We make our new UVB photoionization and
photoheating rates publicly available for use in future simulations.Comment: 28 pages, 15 figures, 5 tables, matches version accepted in Ap
The High- Universe Confronts Warm Dark Matter: Galaxy Counts, Reionization and the Nature of Dark Matter
We use -body simulations to show that high-redshift galaxy counts provide
an interesting constraint on the nature of dark matter, specifically Warm Dark
Matter (WDM), owing to the lack of early structure formation these models. Our
simulations include three WDM models with thermal-production masses of 0.8 keV,
1.3 keV, and 2.6 keV, as well as CDM. Assuming a relationship between dark halo
mass and galaxy luminosity that is set by the observed luminosity function at
bright magnitudes, we find that 0.8 keV WDM is disfavored by direct galaxy
counts in the Hubble Ultra Deep Field at . Similarly, 1.3 keV
WDM is statistically inconsistent at . Future observations with JWST
(and possibly HST via the Frontier Fields) could rule out keV WDM at high
significance, and may be sensitive to WDM masses greater than 2.6 keV. We also
examine the ability of galaxies in these WDM models to reionize the universe,
and find that 0.8 keV and 1.3 keV WDM produce optical depths to the Cosmic
Microwave Background (CMB) that are inconsistent at 68% C.L. with current
Planck results, even with extremely high ionizing radiation escape fractions,
and 2.6 keV WDM requires an optimistic escape fraction to yield an optical
depth consistent with Planck data. Although CMB optical depth calculations are
model dependent, we find a strong challenge for stellar processes alone to
reionize the universe in a 0.8 keV and 1.3 keV WDM cosmology
Modelling the cosmological Lyman-Werner background radiation field in the Early Universe
The Lyman-Werner (LW) radiation field is a key ingredient in the
chemo-thermal evolution of gas in the Early Universe, as it dissociates H2
molecules, the primary cooling channel in an environment devoid of metals and
dust. Despite its important role, it is still not implemented in cosmological
simulations on a regular basis, in contrast to the ionising UV background. This
is in part due to uncertainty in the source modelling, their spectra and
abundance, as well as the detailed physics involved in the propagation of the
photons and their interactions with the molecules. The goal of this work is to
produce an accurate model of the LW radiation field at , by
post-processing the physics-rich high-resolution FiBY simulation. Our novelties
include updated cross sections for H, H and H chemical species,
IGM absorption by neutral Hydrogen and various spectral models for Population
III and Population II stars. With our fiducial set of parameters, we show that
the mean LW intensity steadily increases by three orders of magnitude from
to , while spatial inhomogeneities originate from massive
star-forming galaxies that dominate the photon budget up to a distance of
proper kpc. Our model can be easily applied to other simulations or
semi-analytical models as an external radiation field that regulates the
formation of stars and massive black hole seeds in high- low-mass halos.Comment: 20 pages, 16 figures, plus 4 figures in the appendices. Main result
in Figure 16. Published by MNRA
Inhomogeneous Reionization Models in Cosmological Hydrodynamical Simulations
In this work we present a new hybrid method to simulate the thermal effects
of the reionization in cosmological hydrodynamical simulations. The method
improves upon the standard approach used in simulations of the intergalactic
medium (IGM) and galaxy formation without a significant increase of the
computational cost allowing for efficient exploration of the parameter space.
The method uses a small set of phenomenological input parameters and combines a
semi-numerical reionization model to solve for the topology of reionization and
an approximate model of how reionization heats the IGM, with the massively
parallel \texttt{Nyx} hydrodynamics code, specifically designed to solve for
the structure of diffuse IGM gas. We have produced several large-scale high
resolution cosmological hydrodynamical simulations (, Mpc/h) with different instantaneous and inhomogeneous HI reionization
models that use this new methodology. We study the IGM thermal properties of
these models and find that large scale temperature fluctuations extend well
beyond the end of reionization. Analyzing the 1D flux power spectrum of these
models, we find up to differences in the large scale properties
(low modes, s/km) of the post-reionization power spectrum due
to the thermal fluctuations. We show that these differences could allow one to
distinguish between different reionization scenarios already with existing
Ly forest measurements. Finally, we explore the differences in the
small-scale cutoff of the power spectrum and we find that, for the same heat
input, models show very good agreement provided that the reionization redshift
of the instantaneous reionization model happens at the midpoint of the
inhomogeneous model.Comment: 24 pages, 16 figures. Accepted by MNRAS. Minor changes to match
published versio
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