1,291 research outputs found
CRASH3: cosmological radiative transfer through metals
Here we introduce CRASH3, the latest release of the 3D radiative transfer
code CRASH. In its current implementation CRASH3 integrates into the reference
algorithm the code Cloudy to evaluate the ionisation states of metals,
self-consistently with the radiative transfer through H and He. The feedback of
the heavy elements on the calculation of the gas temperature is also taken into
account, making of CRASH3 the first 3D code for cosmological applications which
treats self-consistently the radiative transfer through an inhomogeneous
distribution of metal enriched gas with an arbitrary number of point sources
and/or a background radiation. The code has been tested in idealized
configurations, as well as in a more realistic case of multiple sources
embedded in a polluted cosmic web. Through these validation tests the new
method has been proven to be numerically stable and convergent. We have studied
the dependence of the results on a number of physical quantities such as the
source characteristics (spectral range and shape, intensity), the metal
composition, the gas number density and metallicity.Comment: accepted for publication in MNRA
CRASH: a Radiative Transfer Scheme
We present a largely improved version of CRASH, a 3-D radiative transfer code
that treats the effects of ionizing radiation propagating through a given
inhomogeneous H/He cosmological density field, on the physical conditions of
the gas. The code, based on a Monte Carlo technique, self-consistently
calculates the time evolution of gas temperature and ionization fractions due
to an arbitrary number of point/extended sources and/or diffuse background
radiation with given spectra. In addition, the effects of diffuse ionizing
radiation following recombinations of ionized atoms have been included. After a
complete description of the numerical scheme, to demonstrate the performances,
accuracy, convergency and robustness of the code we present four different test
cases designed to investigate specific aspects of radiative transfer: (i) pure
hydrogen isothermal Stromgren sphere; (ii) realistic Stromgren spheres; (iii)
multiple overlapping point sources, and (iv) shadowing of background radiation
by an intervening optically thick layer. When possible, detailed quantitative
comparison of the results against either analytical solutions or 1-D standard
photoionization codes has been made showing a good level of agreement. For more
complicated tests the code yields physically plausible results, which could be
eventually checked only by comparison with other similar codes. Finally, we
briefly discuss future possible developments and cosmological applications of
the code.Comment: 17 pages, 14 figures, accepted for pubblication in MNRAS, high res
figures available at
http://www.arcetri.astro.it/science/cosmology/IGM/radtrans.htm
X-ray ionization of the intergalactic medium by quasars
We investigate the impact of quasars on the ionization of the surrounding
intergalactic medium (IGM) with the radiative transfer code \texttt{CRASH4},
now accounting for X-rays and secondary electrons. After comparing with
analytic solutions, we post-process a cosmic volume (
Mpc) containing a ULAS J1120+0641-like quasar (QSO) hosted by a dark matter (DM) halo. We find that: (i)
the average HII region (~pMpc in a lifetime ~yrs) is
mainly set by UV flux, in agreement with semi-analytic scaling relations; (ii)
a largely neutral (), warm (~K) tail
extends up to few Mpc beyond the ionization front, as a result of the X-ray
flux; (iii) LyC-opaque inhomogeneities induce a line of sight (LOS) scatter in
as high as few physical Mpc, consistent with the DLA scenario proposed to
explain the anomalous size of the ULAS J1120+0641 ionized region. On the other
hand, with an ionization rate ~s, the
assumed DLA clustering and gas opacity, only one LOS shows an HII region
compatible with the observed one. We deduce that either the ionization rate of
the QSO is at least one order of magnitude lower or the ULAS J1120+0641 bright
phase is shorter than ~yrs.Comment: Accepted for publication in MNRAS Main Journal, Accepted 2018 May 2
X-ray background and its correlation with the 21 cm signal
We use high resolution hydrodynamical simulations to study the contribution
to the X-ray background from high- energetic sources, such as X-ray
binaries, accreting nuclear black holes and shock heated interstellar medium.
Adopting the model discussed in Eide et al. (2018), we find that these X-ray
sources during the Epoch of Reionization (EoR) contribute less than a few
percent of the unresolved X-ray background. The same sources contribute to less
than 2\% of the measured angular power spectrum of the fluctuations of
the X-ray background. The outputs of radiative transfer simulations modeling
the EoR are used to evaluate the cross-correlations of X-ray background with
the 21~cm signal from neutral hydrogen. Such correlation could be used to
confirm the origin of the 21 cm signal, as well as give information on the
properties of the X-ray sources during the EoR. We find that the correlations
are positive during the early stages of reionization when most of the hydrogen
is neutral, while they become negative when the intergalactic medium gets
highly ionized, with the transition from positive to negative depending on both
the X-ray model and the scale under consideration. With {\tt SKA} as the
reference instrument for the 21~cm experiment, the predicted S/N for such
correlations is if the corresponding X-ray survey is only able to resolve
and remove X-ray sources with observed flux , while the cumulative S/N from to at
is if sources with observed flux are detected.Comment: 9 pages, 8 figure
PopIII signatures in the spectra of PopII/I GRBs
We investigate signatures of population III (PopIII) stars in the
metal-enriched environment of GRBs originating from population II-I (PopII/I)
stars by using abundance ratios derived from numerical simulations that follow
stellar evolution and chemical enrichment. We find that at more than
of PopII/I GRBs explode in a medium previously enriched by PopIII stars
(we refer to them as GRBIIIII). Although the formation of
GRBIIIII is more frequent than that of pristine PopIII GRBs
(GRBIIIs), we find that the expected GRBIIIII observed rate is
comparable to that of GRBIIIs, due to the usually larger luminosities of these
latter. GRBIIIII events take place preferentially in small
proto-galaxies with stellar masses , star formation rates
and metallicities . On the other hand,
galaxies with are dominated by metal enrichment
from PopIII stars and should preferentially host GRBIIIII. Hence,
measured GRB metal content below this limit could represent a strong evidence
of enrichment by pristine stellar populations. We discuss how to discriminate
PopIII metal enrichment on the basis of various abundance ratios observable in
the spectra of GRBs' afterglows. By employing such analysis, we conclude that
the currently known candidates at redshift -- i.e. GRB 050904
\cite[][]{2006Natur.440..184K} and GRB 130606A \cite[][]{2013arXiv1312.5631C}
-- are likely not originated in environments pre-enriched by PopIII stars.Comment: 9 pages, 7 figures; MNRAS accepte
Detecting Unresolved Binaries in TESS Data with Speckle Imaging
The Transiting Exoplanet Survey Satellite (TESS) is conducting a two-year
wide-field survey searching for transiting exoplanets around nearby bright
stars that will be ideal for follow-up characterization. To facilitate studies
of planet compositions and atmospheric properties, accurate and precise
planetary radii need to be derived from the transit light curves. Since 40 -
50% of exoplanet host stars are in multiple star systems, however, the observed
transit depth may be diluted by the flux of a companion star, causing the
radius of the planet to be underestimated. High angular resolution imaging can
detect companion stars that are not resolved in the TESS Input Catalog, or by
seeing-limited photometry, to validate exoplanet candidates and derive accurate
planetary radii. We examine the population of stellar companions that will be
detectable around TESS planet candidate host stars, and those that will remain
undetected, by applying the detection limits of speckle imaging to the
simulated host star populations of Sullivan et al. (2015) and Barclay et al.
(2018). By detecting companions with contrasts of delta m < 7 - 9 and
separations of ~0.02 - 1.2'', speckle imaging can detect companion stars as
faint as early M stars around A - F stars and stars as faint as mid-M around G
- M stars, as well as up to 99% of the expected binary star distribution for
systems located within a few hundred parsecs.Comment: Accepted for publication in The Astronomical Journal; 16 pages, 8
figures, 2 table
Constraining the PopIII IMF with high-z GRBs
We study the possibility to detect and distinguish signatures of enrichment
from PopIII stars in observations of PopII GRBs (GRBIIs) at high redshift by
using numerical N-body/hydrodynamical simulations including atomic and
molecular cooling, star formation and metal spreading from stellar populations
with different initial mass functions (IMFs), yields and lifetimes. PopIII and
PopII star formation regimes are followed simultaneously and both a top-heavy
and a Salpeter-like IMF for pristine PopIII star formation are adopted. We find
that the fraction of GRBIIs hosted in a medium previously enriched by PopIII
stars (PopIII-dominated) is model independent. Typical abundance ratios, such
as [Si/O] vs [C/O] and [Fe/C] vs [Si/C], can help to disentangle enrichment
from massive and intermediate PopIII stars, while low-mass first stars are
degenerate with regular PopII generations. The properties of galaxies hosting
PopIII-dominated GRBIIs are not very sensitive to the particular assumption on
the mass of the first stars.Comment: 9 pages, 4 figure
Inhomogeneous Reionization Regulated by Radiative and Stellar Feedbacks
We study the inhomogeneous reionization in a critical density CDM universe
due to stellar sources, including Population III objects. The spatial
distribution of the sources is obtained from high resolution numerical N-body
simulations. We calculate the source properties taking into account a
self-consistent treatment of both radiative (ie ionizing and H2
-photodissociating photons) and stellar (ie SN explosions) feedbacks regulated
by massive stars. This allows us to describe the topology of the ionized and
dissociated regions at various cosmic epochs and derive the evolution of H, He,
and H2 filling factors, soft UV background, cosmic star formation rate and the
final fate of ionizing objects. The main results are: (i) galaxies reionize the
IGM by z~10 (with some uncertainty related to the gas clumping factor), whereas
H2 is completely dissociated already by z~25; (ii) reionization is mostly due
to the relatively massive objects which collapse via H line cooling, while
objects whose formation relies on H2 cooling alone are insufficient to this
aim; (iii) the diffuse soft UV background is the major source of radiative
feedback effects for z<15; at higher z direct flux from neighboring objects
dominates; (iv) the match of the calculated cosmic star formation history with
the one observed at lower redshifts suggests that the conversion efficiency of
baryons into stars is ~1%; (v) we find that a very large population of dark
objects which failed to form stars is present by z~8. We discuss and compare
our results with similar previous studies.Comment: 34 pages, emulateapj.sty, LaTeX, 13 figures. MNRAS, submitte
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