297 research outputs found
ART^2 : Coupling Lyman-alpha Line and Multi-wavelength Continuum Radiative Transfer
Narrow-band Lya line and broad-band continuum have played important roles in
the discovery of high-redshift galaxies in recent years. Hence, it is crucial
to study the radiative transfer of both Lya and continuum photons in the
context of galaxy formation and evolution in order to understand the nature of
distant galaxies. Here, we present a three-dimensional Monte Carlo radiative
transfer code, All-wavelength Radiative Transfer with Adaptive Refinement Tree
(ART^2), which couples Lya line and multi-wavelength continuum, for the study
of panchromatic properties of galaxies and interstellar medium. This code is
based on the original version of Li et al., and features three essential
modules: continuum emission from X-ray to radio, Lya emission from both
recombination and collisional excitation, and ionization of neutral hydrogen.
The coupling of these three modules, together with an adaptive refinement grid,
enables a self-consistent and accurate calculation of the Lya properties. As an
example, we apply ART^2 to a cosmological simulation that includes both star
formation and black hole growth, and study in detail a sample of massive
galaxies at redshifts z=3.1 - 10.2. We find that these galaxies are Lya
emitters (LAEs), whose Lya emission traces the dense gas region, and that their
Lya lines show a shape characteristic of gas inflow. Furthermore, the Lya
properties, including photon escape fraction, emergent luminosity, and
equivalent width, change with time and environment. Our results suggest that
LAEs evolve with redshift, and that early LAEs such as the most distant one
detected at z ~ 8.6 may be dwarf galaxies with a high star formation rate
fueled by infall of cold gas, and a low Lya escape fraction.Comment: 20 pages, 16 figures, accepted for publication in MNRA
Reionization and Cosmic Dawn: theory and simulations
We highlight recent progress in the sophistication and diversification of
cosmic dawn and reionization simulations. The application of these modeling
tools to current observations has allowed us narrow down the timing of
reionization, which we now know to within dz ~ 1 for the bulk of reionization.
The strongest constraints come from the optical depth to the CMB measured with
the {\it Planck} satellite and the first detection of ongoing reionization from
the spectra of the z=7.1 QSOs ULASJ1120+0641. However, we still know virtually
nothing about the astrophysical sources during the first billion years. The
revolution in our understanding will be led by upcoming interferometric
observations of the cosmic 21-cm signal. The properties of the sources and
sinks of UV and X-ray photons are encoded in the 3D patterns of the signal. The
development of Bayesian parameter recovery techniques, which tap into the
wealth of the 21-cm signal, will soon usher in an era of precision
astrophysical cosmology.Comment: Invited review for the IAU Symposium 333 "Peering towards Cosmic
Dawn", Dubrovnik, October 2-6, 2017; to appear in the proceedings, eds. Vibor
Jelic and Thijs van der Hulst [8 pages, 3 figures
seurat: SPH scheme extended with ultraviolet line radiative transfer
We present a novel Lyman alpha (Ly α) radiative transfer code, seurat (SPH scheme Extended with Ultraviolet line RAdiative Transfer), where line scatterings are solved adaptively with the resolution of the smoothed particle hydrodynamics (SPH). The radiative transfer method implemented in seurat is based on a Monte Carlo algorithm in which the scattering and absorption by dust are also incorporated. We perform standard test calculations to verify the validity of the code; (i) emergent spectra from a static uniform sphere, (ii) emergent spectra from an expanding uniform sphere, and (iii) escape fraction from a dusty slab. Thereby, we demonstrate that our code solves the Lyα radiative transfer with sufficient accuracy. We emphasize that seurat can treat the transfer of Lyαphotons even in highly complex systems that have significantly inhomogeneous density fields. The high adaptivity of seurat is desirable to solve the propagation of Lyα photons in the interstellar medium of young star-forming galaxies likeLyα emitters (LAEs). Thus, seurat provides a powerful tool to model the emergent spectra of Lyα emission, which can be compared to the observations of LAEs.Lyα radiative transfer with sufficient accuracy. We emphasize that seurat can treat the transfer of Lyα photons even in highly complex systems that have significantly inhomogeneous density fields. The high adaptivity of seurat is desirable to solve the propagation of Lyα photons in the interstellar medium of young star-forming galaxies like Lyα emitters (LAEs). Thus, seurat provides a powerful tool to model the emergent spectra of Lyα emission, which can be compared to the observations of LAEs
Signatures of X-rays in the early Universe
[abridged] With their long mean free paths and efficient heating of the
intergalactic medium (IGM), X-rays could have a dramatic impact on the thermal
and ionization history of the Universe. We explore this in various signals: (i)
Reionization history: including X-rays results in an earlier, more extended
reionization. Efficient thermal feedback from X-ray heating could yield an
extended, ~10% ionized epoch. (ii) Reionization morphology: a sizable (~10%)
contribution of X-rays to reionization results in a more uniform morphology,
though the impact is modest when compared at the same global neutral fraction,
xH. However, changes in morphology cannot be countered by increasing the bias
of the ionizing sources, making them a robust signature. (iii) The kinetic
Sunyaev-Zel'dovich (kSZ) effect: at a fixed reionization history, X-rays
decrease the kSZ power at l=3000 by ~0.5 microK^2. Our extreme model in which
X-rays dominate reionization is the only one that is marginally consistent with
upper limits from the South Pole Telescope, assuming no thermal
Sunyaev-Zel'dovich (tSZ) - dusty galaxy correlation. Since this extreme model
is unlikely, we conclude that there should be a sizable tSZ-dusty galaxy
signal. (iv) The cosmic 21cm signal: the impact of X-rays on the 21cm power
spectrum during the advanced stages of reionization (xH<0.7) is modest, except
in extreme, X-ray dominated models. The largest impact of X-rays is to govern
IGM heating. In fact, unless thermal feedback is efficient, the epoch of X-ray
heating likely overlaps with the beginning of reionization (xH>0.9). This
results in a 21cm power spectrum which is ~ 10-100 times higher than obtained
from naive estimates ignoring this overlap. However, if thermal feedback is
efficient, the resulting extended epoch between X-ray heating and reionization
could provide a clean probe of the matter power spectrum in emission.Comment: 17 pages, 12 figures, MNRAS in-pres
Computer Simulations of Cosmic Reionization
The cosmic reionization of hydrogen was the last major phase transition in
the evolution of the universe, which drastically changed the ionization and
thermal conditions in the cosmic gas. To the best of our knowledge today, this
process was driven by the ultra-violet radiation from young, star-forming
galaxies and from first quasars. We review the current observational
constraints on cosmic reionization, as well as the dominant physical effects
that control the ionization of intergalactic gas. We then focus on numerical
modeling of this process with computer simulations. Over the past decade,
significant progress has been made in solving the radiative transfer of
ionizing photons from many sources through the highly inhomogeneous
distribution of cosmic gas in the expanding universe. With modern simulations,
we have finally converged on a general picture for the reionization process,
but many unsolved problems still remain in this young and exciting field of
numerical cosmology.Comment: Invited Review to appear on Advanced Science Letters (ASL), Special
Issue on Computational Astrophysics, edited by Lucio Maye
Galactic Outflows and Photoionization Heating in the Reionization Epoch
We carry out a new suite of cosmological radiation hydrodynamic simulations
and explore the relative impacts on reionization-epoch star formation of
galactic outflows and photoionization heating. By itself, an extragalactic
ultraviolet background (EUVB) suppresses the luminosity function by less than
50% at z=6, overproducing the observed galaxy abundance by a factor of 3-5.
Galactic outflows restore agreement with observations without preventing
Population II star formation from reionizing the Universe by z=6. The resulting
EUVB suppresses star formation in halos with virial temperatures below 10^5K
but has a weaker impact in more massive halos. Nonetheless, the low-mass halos
contribute up to 50% of all ionizing photons owing to the EUVB's inhomogeneity.
Overall, star formation rate scales as halo mass M_h to the 1.3-1.4 in halos
with M_h=10^{8.2--10.2}\msun. This is a steeper dependence than is often
assumed in reionization models, boosting the expected power spectrum of 21
centimeter fluctuations on large scales. The luminosity function rises steeply
to at least M_1600=-13, indicating that reionization was driven by faint
galaxies (M_1600 >= -15) that have not yet been observed. Our models cannot
simultaneously explain observations of galaxies, the cosmic microwave
background, and the intergalactic medium. Increased dynamic range will
alleviate the existing discrepancies, but observations may still require
additional physics such as a variable ionizing escape fraction (abridged).Comment: 23 pages, 15 Figures, accepted to Ap
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