73 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
Impact of Self-shielding Minihalos on the Ly Forest at High Redshift
Dense gas in minihalos (MHs) with masses of can shield
themselves from reionization for about megayears after being exposed to
UV radiation. These self-shielded systems, often unresolved in cosmological
simulations, can introduce strong absorption in quasar spectra. This paper is
the first systematic study on the impact of these systems on the Ly
forest. We first derive the HI column density profile of photoevaporating MHs
by conducting 1-dimensional radiation-hydrodynamics simulations. We utilize
these results to estimate the Ly opacity from minihalos in a
large-scale simulation that cannot resolve the self-shielding process. When the
ionization rate of the background radiation is , as expected near the end of reionization at , we find that
the incidence rate of damped Ly absorbers, , increases by nearly
a factor of 2 - 4 compared to at . The Ly flux is, on average,
suppressed by of its mean due to absorption by MHs. The extended
absorption features contribute to a enhancement in the 1D power
spectrum at (),
which is comparable to the enhancement caused by inhomogeneous reionization.
Notably, the flux is particularly suppressed in the vicinity of large halos
along the line-of-sight direction at separations of up to
at . However, these effects become much
smaller for higher ionizing rates ( and )
expected in the post-reionization universe . Our findings
highlight the need to consider the absorption by MHs when interpreting the
Ly forest at . Moreover, the sensitivity of this effect to
the ionizing background intensity can be exploited to constrain the intensity
itself.Comment: 15 pages; 11 figures; Submitted to the Ap
HyPhy: Deep Generative Conditional Posterior Mapping of Hydrodynamical Physics
Generating large volume hydrodynamical simulations for cosmological
observables is a computationally demanding task necessary for next generation
observations. In this work, we construct a novel fully convolutional
variational auto-encoder (VAE) to synthesize hydrodynamic fields conditioned on
dark matter fields from N-body simulations. After training the model on a
single hydrodynamical simulation, we are able to probabilistically map new dark
matter only simulations to corresponding full hydrodynamical outputs. By
sampling over the latent space of our VAE, we can generate posterior samples
and study the variance of the mapping. We find that our reconstructed field
provides an accurate representation of the target hydrodynamical fields as well
as a reasonable variance estimates. This approach has promise for the rapid
generation of mocks as well as for implementation in a full Bayesian inverse
model of observed data.Comment: 13 pages, 11 figure
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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