77 research outputs found
A study of simulated histories of reionization with merger trees of HII regions
We describe a new methodology to analyze the reionization process in
numerical simulations: the chronology and the geometry of reionization is
investigated by means of merger histories of individual HII regions. From the
merger tree of ionized patches, one can track the individual evolution of the
regions properties such as e.g. their size, or the intensity of the percolation
process by looking at the formation rate, the frequency of mergers and the
number of individual HII regions involved in the mergers. We apply the merger
tree technique to simulations of reionization with three different kinds of
ionizing source models and two resolutions. Two of them use star particles as
ionizing sources. In this case we confront two emissivity evolutions for the
sources in order to reach the reionization at z ~ 6. As an alternative we built
a semi-analytical model where the dark matter halos extracted from the density
fields are assumed as ionizing sources. We then show how this methodology is a
good candidate to quantify the impact of the adopted star formation on the
history of the observed reionization. The semi-analytical model shows a
homogeneous reionization history with 'local' hierarchical growth steps for
individual HII regions. On the other hand auto-consistent models for star
formation tend to present fewer regions with a dominant region in size which
governs the fusion process early in the reionization at the expense of the
'local' reionizations. The differences are attenuated when the resolution of
the simulation is increased.Comment: Accepted for publication in A&
Local reionizations histories with merger tree of HII regions
We constrain the initial stage of the reionization process around progenitors
of galaxies, such as the extent of the initial HII region before its fusion
with the UV background and the duration of its propagation. We use a set of
reionisation simulations with different resolutions and ionizing source
recipes. A catalog of the HII regions properties is built thanks to a merger
tree of HII regions. We draw local reionization histories as a function of time
and investigate variations according to the halo mass progenitors of the
regions. We then extrapolate the halo mass inside the region from high z to z=0
to make predictions about the reionization histories of z=0 galaxies. We found
that the later an HII region appears, the smaller will be its related lifetime
and volume before it sees the global UV background. Quantitatively the duration
and the extent of the initial growth of an HII region is strongly dependent on
the mass of the inner halo and can be as long as 50 % of the reionization
epoch. We found that the most massive is a halo today, the earlier it appears
and the larger are the extension and the duration of propagation of its HII
region. Quantitative predictions differ depending on the box size or the source
model: small simulated volumes are affected by proximity effects between HII
regions and halo-based source models predict smaller regions and slower I-front
expansion than in models using star particles as ionizing sources. Our results
suggests that Milky Way-type halos have a maximal extent of 1.1 Mpc/h for the
initial HII region that established itself in 150-200 Myrs. This is
consistent with prediction made using constrained Local Group simulation.
Considering halos with masses comparable to those of the Local Group (MW+M31),
our result suggests that statistically it has not been influenced by an
external front coming from a Virgo-like cluster.Comment: 16 pages, 9 figures, Accepted for publication in A&
Analysis of stellar populations with large empirical libraries at high spectral resolution
The stellar population models dramatically progressed with the arrival of
large and complete libraries, ELODIE, CFLIB (=Indo-US) and MILES at a
relatively high resolution. We show that the quality of the fits is not anymore
limited by the size of the stellar libraries in a large range of ages (0.1 to
10 Gyrs) and metallicities (-2 to +0.4 dex). The main limitations of the
empirical stellar libraries are (i) the coverage of the parameters space (lack
of hot stars of low metallicity), (ii) the precision and homogeneity of the
atmospheric parameters and (iii) the non-resolution of individual element
abundances (in particular [/Fe]). Detailed abundance measurements in
the large libraries, and usage of theoretical libraries are probably the next
steps, and we show that a combination between an empirical (ELODIE) and a
theoretical library (Coelho et al. 2005) immediately improves the modeling of
(-enhanced) globular clusters.Comment: 4 pages; proceedings of IAU Symposium No. 241, "Stellar Populations
as Building Blocks of Galaxies", editors A. Vazdekis and R. Peletie
Reionisation time fields reconstruction from 21 cm signal maps
During the Epoch of reionisation, the intergalactic medium is reionised by
the UV radiation from the first generation of stars and galaxies. One tracer of
the process is the 21 cm line of hydrogen that will be observed by the Square
Kilometre Array (SKA) at low frequencies, thus imaging the distribution of
ionised and neutral regions and their evolution. To prepare for these upcoming
observations, we investigate a deep learning method to predict from 21 cm maps
the reionisation time field (treion(r)), i.e. the time at which each location
has been reionised. treion(r) encodes the propagation of ionisation fronts in a
single field, gives access to times of local reionisation or to the extent of
the radiative reach of early sources. Moreover it gives access to the time
evolution of ionisation on the plane of sky, when such evolution is usually
probed along the line-of-sight direction. We trained a convolutional neural
network (CNN) using simulated 21 cm maps and reionisation times fields produced
by the simulation code 21cmFAST . We also investigate the performance of the
CNN when adding instrumental effects. Globally, we find that without
instrumental effects the 21 cm maps can be used to reconstruct the associated
reionisation times field in a satisfying manner: the quality of the
reconstruction is dependent on the redshift at which the 21 cm observation is
being made and in general it is found that small scale (<10cMpc/h) features are
smoothed in the reconstructed field, while larger scale features are well
recovered. When instrumental effects are included, the scale dependance of
reconstruction is even further pronounced, with significant smoothing on small
and intermediate scales
DUSTiER (DUST in the Epoch of Reionization): dusty galaxies in cosmological radiation-hydrodynamical simulations of the Epoch of Reionization with RAMSES-CUDATON
In recent years, interstellar dust has become a crucial topic in the study of
the high and very high redshift Universe. Evidence points to the existence of
high dust masses in massive star forming galaxies already during the Epoch of
Reionization, potentially affecting the escape of ionising photons into the
intergalactic medium. Moreover, correctly estimating dust extinction at UV
wavelengths is essential for precise ultra-violet luminosity function (UVLF)
prediction and interpretation. In this paper, we investigate the impact of dust
on the observed properties of high redshift galaxies, and cosmic reionization.
To this end, we couple a physical model for dust production to the fully
coupled radiation-hydrodynamics cosmological simulation code RAMSES-CUDATON,
and perform a , , simulation, that we call DUSTiER for DUST in
the Epoch of Reionization. It yields galaxies with dust masses and UV slopes
compatible with constraints at z . We find that extinction has a
dramatic impact on the bright end of the UVLF, even as early as , and
our dusty UVLFs are in better agreement with observations than dust-less UVLFs.
The fraction of obscured star formation rises up to 55% at , in
agreement with some of the latest results from ALMA. Finally, we find that dust
reduces the escape of ionising photons from galaxies more massive than (brighter than MAB1600) by >10%, and possibly up to
80-90% for our most massive galaxies. Nevertheless, we find that the ionising
escape fraction is first and foremost set by neutral Hydrogen in galaxies, as
the latter produces transmissions up to 100 times smaller than through dust
alone.Comment: submitted to MNRAS, 1st report received: under revision Have
partially addressed referee's concerns, namely that the model predicts high
dust masses and redder bright galaxies than expected, by discussing this
aspect around the relevant results. Work is being carried out to present a
clearer parameter exploration of the dust mode
Modelling and interpreting optical spectra of galaxies at R=10000
One way to extract more information from the integrated light of galaxies is
to improve the spectral resolution at which observations and analysis are
carried out. The population synthesis code currently providing the highest
spectral resolution is Pegase-HR, which was made available by D. Le Borgne et
al. in 2004. Based on an empirical stellar library, it provides synthetic
spectra between 4000 and 6800 A at lambda/d(lambda)=10000 for any star
formation history, with or without chemical evolution. Such a resolution is
particularly useful for the study of low mass galaxies, massive star clusters,
or other galaxy regions with low internal velocity dispersions.
After a summary of the main features of Pegase-HR and comparisons with other
population synthesis codes, this paper focuses on the inversion of optical
galaxy spectra. We explore the limits of what information can or can not be
recovered, based on theoretical principles and extensive simulations. First
applications to extragalactic objects are shown.Comment: Inv. talk in "The Spectral Energy Distribution of Gas-Rich Galaxies:
Confronting Models with Data", Heidelberg, 4-8 Oct. 2004, eds. C.C. Popescu
and R.J. Tuffs, AIP Conf. Ser., in pres
Cosmic Dawn (CoDa): the first radiation-hydrodynamics simulation of reionization and galaxy formation in the Local Universe
Cosmic reionization by starlight from early galaxies affected their
evolution, thereby impacting reionization, itself. Star formation suppression,
for example, may explain the observed underabundance of Local Group dwarfs
relative to N-body predictions for Cold Dark Matter. Reionization modelling
requires simulating volumes large enough [~(100Mpc)^3] to sample reionization
"patchiness", while resolving millions of galaxy sources above ~10^8 Msun ,
combining gravitational and gas dynamics with radiative transfer. Modelling the
Local Group requires initial cosmological density fluctuations pre-selected to
form the well-known structures of the local universe today. Cosmic Dawn
("CoDa") is the first such fully-coupled, radiation-hydrodynamics simulation of
reionization of the local universe. Our new hybrid CPU-GPU code,
RAMSES-CUDATON, performs hundreds of radiative transfer and ionization
rate-solver timesteps on the GPUs for each hydro-gravity timestep on the CPUs.
CoDa simulated (91Mpc)^3 with 4096^3 particles and cells, to redshift 4.23, on
ORNL supercomputer Titan, utilizing 8192 cores and 8192 GPUs. Global
reionization ended slightly later than observed. However, a simple temporal
rescaling which brings the evolution of ionized fraction into agreement with
observations also reconciles ionizing flux density, cosmic star formation
history, CMB electron scattering optical depth and galaxy UV luminosity
function with their observed values. Photoionization heating suppressed the
star formation of haloes below ~2 x 10^9 Msun , decreasing the abun- dance of
faint galaxies around MAB_1600 = [-10,-12]. For most of reionization, star
formation was dominated by haloes between 10^10 - 10^11 Msun , so low-mass halo
suppression was not reflected by a distinct feature in the global star
formation history. (Abridged)Comment: 26 pages, 16 figures, accepted in MNRA
Cosmic Variance and the Inhomogeneous UV Luminosity Function of Galaxies During Reionization
When the first galaxies formed and starlight escaped into the intergalactic
medium to reionize it, galaxy formation and reionization were both highly
inhomogeneous in time and space, and fully-coupled by mutual feedback. To show
how this imprinted the UV luminosity function (UVLF) of reionization-era
galaxies, we use our large-scale, radiation-hydrodynamics simulation CoDa II to
derive the time- and space-varying halo mass function and UVLF, from
-15. That UVLF correlates strongly with local reionization redshift:
earlier-reionizing regions have UVLFs that are higher, more extended to
brighter magnitudes, and flatter at the faint end than later-reionizing regions
observed at the same . In general, as a region reionizes, the faint-end
slope of its local UVLF flattens, and, by (when reionization ended), the
global UVLF, too, exhibits a flattened faint-end slope, `rolling-over' at
. CoDa II's UVLF is broadly consistent with
cluster-lensed galaxy observations of the Hubble Frontier Fields at -8,
including the faint end, except for the faintest data point at , based on
one galaxy at . According to CoDa II, the probability of
observing the latter is . However, the effective volume searched at
this magnitude is very small, and is thus subject to significant cosmic
variance. We find that previous methods adopted to calculate the uncertainty
due to cosmic variance underestimated it on such small scales by a factor of
2-4, primarily by underestimating the variance in halo abundance when the
sample volume is small.Comment: 16 pages, 16 figures, accepted by MNRAS 07/20/23, comments welcom
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