280 research outputs found
High resolution simulations of the reionization of an isolated Milky Way - M31 galaxy pair
We present the results of a set of numerical simulations aimed at studying
reionization at galactic scale. We use a high resolution simulation of the
formation of the Milky Way-M31 system to simulate the reionization of the local
group. The reionization calculation was performed with the post-processing
radiative transfer code ATON and the underlying cosmological simulation was
performed as part of the CLUES project. We vary the source models to bracket
the range of source properties used in the literature. We investigate the
structure and propagation of the galatic ionization fronts by a visual
examination of our reionization maps. Within the progenitors we find that
reionization is patchy, and proceeds locally inside out. The process becomes
patchier with decreasing source photon output. It is generally dominated by one
major HII region and 1-4 additional isolated smaller bubbles, which eventually
overlap. Higher emissivity results in faster and earlier local reionization. In
all models, the reionization of the Milky Way and M31 are similar in duration,
i.e. between 203 Myr and 22 Myr depending on the source model, placing their
zreion between 8.4 and 13.7. In all models except the most extreme, the MW and
M31 progenitors reionize internally, ignoring each other, despite being
relatively close to each other even during the epoch of reionization. Only in
the case of strong supernova feedback suppressing star formation in haloes less
massive than 10^9 M_sun, and using our highest emissivity, we find that the MW
is reionized by M31.Comment: Accepted for publication in ApJ. 14 pages, 4 figures, 1 tabl
Spectroscopic ages and metallicities of stellar populations: validation of full spectrum fitting
Fitting whole spectra at intermediate spectral resolution (R = 1000 -- 3000),
to derive physical properties of stellar populations, appears as an optimized
alternative to methods based on spectrophotometric indices: it uses all the
redundant information contained in the signal. This paper addresses the
validation of the method and it investigates the quality of the population
models together with the reliability of the fitting procedures. We are using
two algorithms: STECKMAP, a non-parametric regularized program and NBURSTS a
parametric non-linear minimization. We compare three spectral synthesis models
for single stellar populations: Pegase-HR, Galaxev (BC03) and Vazdekis/Miles,
and we analyse spectra of Galactic clusters whose populations are known from
studies of color-magnitude diagrams (CMD) and spectroscopy of individual stars.
We find that: (1) The quality of the models critically depends on the stellar
library they use. Pegase-HR and Vazdekis/Miles are consistent, while the
comparison between Pegase-HR and BC03 shows some systematics reflecting the
limitations of the stellar library (STELIB) used to generate the latter models;
(2) The two fitting programs are consistent; (3) For globular clusters and M67
spectra, the method restitutes metallicities in agreement with spectroscopy of
stars within 0.14 dex; (4) The spectroscopic ages are very sensitive to the
presence of a blue horizontal branch (BHB) or of blue stragglers. A BHB
morphology results in a young SSP-equivalent age. Fitting a free amount of blue
stars in addition to the SSP model to mimic the BHB improves and stabilizes the
fit and restores ages in agreement with CMDs studies. This method is
potentially able to disentangle age or BHB effects in extragalactic clusters.Comment: accepted in MNRAS; Full version available at
http://www-obs.univ-lyon1.fr/labo/perso/prugniel/mina/koleva.pd
The SPHINX cosmological simulations of the first billion years: The impact of binary stars on reionization
We present the SPHINX suite of cosmological adaptive mesh refinement
simulations, the first radiation-hydrodynamical simulations to simultaneously
capture large-scale reionization and the escape of ionizing radiation from
thousands of resolved galaxies. Our and co-moving Mpc volumes resolve
haloes down to the atomic cooling limit and model the inter-stellar medium with
better than pc resolution. The project has numerous goals in
improving our understanding of reionization and making predictions for future
observations. In this first paper we study how the inclusion of binary stars in
computing stellar luminosities impacts reionization, compared to a model that
includes only single stars. Owing to the suppression of galaxy growth via
strong feedback, our galaxies are in good agreement with observational
estimates of the galaxy luminosity function. We find that binaries have a
significant impact on the timing of reionization: with binaries, our boxes are
percent ionized by volume at , while without them our
volumes fail to reionize by . These results are robust to changes in
volume size, resolution, and feedback efficiency. The escape of ionizing
radiation from individual galaxies varies strongly and frequently. On average,
binaries lead to escape fractions of percent, about times
higher than with single stars only. The higher escape fraction is a result of a
shallower decline in ionizing luminosity with age, and is the primary reason
for earlier reionization, although the higher integrated luminosity with
binaries also plays a sub-dominant role
II.5 Where to find the CoRoT data?
This book is dedicated to all the people interested in the CoRoT mission and the beautiful data that were delivered during its six year duration. Either amateurs, professional, young or senior researchers, they will find treasures not only at the time of this publication but also in the future twenty or thirty years. It presents the data in their final version, explains how they have been obtained, how to handle them, describes the tools necessary to understand them, and where to find them. It also highlights the most striking first results obtained up to now. CoRoT has opened several unexpected directions of research and certainly new ones still to be discovered
Mergers and Mass Accretion Rates in Galaxy Assembly: The Millennium Simulation Compared to Observations of z~2 Galaxies
Recent observations of UV-/optically selected, massive star forming galaxies
at z~2 indicate that the baryonic mass assembly and star formation history is
dominated by continuous rapid accretion of gas and internal secular evolution,
rather than by major mergers. We use the Millennium Simulation to build new
halo merger trees, and extract halo merger fractions and mass accretion rates.
We find that even for halos not undergoing major mergers the mass accretion
rates are plausibly sufficient to account for the high star formation rates
observed in z~2 disks. On the other hand, the fraction of major mergers in the
Millennium Simulation is sufficient to account for the number counts of
submillimeter galaxies (SMGs), in support of observational evidence that these
are major mergers. When following the fate of these two populations in the
Millennium Simulation to z=0, we find that subsequent mergers are not frequent
enough to convert all z~2 turbulent disks into elliptical galaxies at z=0.
Similarly, mergers cannot transform the compact SMGs/red sequence galaxies at
z~2 into observed massive cluster ellipticals at z=0. We argue therefore, that
secular and internal evolution must play an important role in the evolution of
a significant fraction of z~2 UV-/optically and submillimeter selected galaxy
populations.Comment: 5 pages, 4 figures, Accepted for publication in Ap
Impact of the Specific Mutation in KRAS Codon 12 Mutated Tumors on Treatment Efficacy in Patients with Metastatic Colorectal Cancer Receiving Cetuximab-Based First-Line Therapy: A Pooled Analysis of Three Trials
Purpose: This study investigated the impact of specific mutations in codon 12 of the Kirsten-ras (KRAS) gene on treatment efficacy in patients with metastatic colorectal cancer (mCRC). Patients: Overall, 119 patients bearing a KRAS mutation in codon 12 were evaluated. All patients received cetuximab-based first-line chemotherapy within the Central European Cooperative Oncology Group (CECOG), AIO KRK-0104 or AIO KRK-0306 trials. Results: Patients with KRAS codon 12 mutant mCRC showed a broad range of outcome when treated with cetuximab-based first-line regimens. Patients with tumors bearing a KRAS p.G12D mutation showed a strong trend to a more favorable outcome compared to other mutations (overall survival 23.3 vs. 14-18 months; hazard ratio 0.66, range 0.43-1.03). An interaction model illustrated that KRAS p.G12C was associated with unfavorable outcome when treated with oxaliplatin plus cetuximab. Conclusion: The present analysis suggests that KRAS codon 12 mutation may not represent a homogeneous entity in mCRC when treated with cetuximab-based first-line therapy. Copyright (C) 2012 S. Karger AG, Base
Differential stellar population models: how to reliably measure [Fe/H] and [alpha/Fe] in galaxies
We present differential stellar population models, which allow improved
determinations of the ages, iron and alpha-element abundances of old stellar
populations from spectral fitting. These new models are calibrated at solar
abundances using the predictions from classical, semi-empirical stellar
population models. We then use the predictive power of fully synthetic models
to compute predictions for different [Fe/H] and [alpha/Fe]. We show that these
new differential models provide remarkably accurate fits to the integrated
optical spectra of the bulge globular clusters NGC6528 and NGC6553, and that
the inferred [Fe/H] and [alpha/Fe] agree with values derived elsewhere from
stellar photometry and spectroscopy. The analysis of a small sample of SDSS
early-type galaxies further confirms that our alpha-enhanced models provide a
better fit to the spectra of massive ellipticals than the solar-scaled ones.
Our approach opens new opportunities for precision measurements of abundance
ratios in galaxies.Comment: 5 pages, 5 figures, MNRAS in pres
Analytic and numerical realisations of a disk galaxy
Recent focus on the importance of cold, unshocked gas accretion in galaxy
formation -- not explicitly included in semi-analytic studies -- motivates the
following detailed comparison between two inherently different modelling
techniques: direct hydrodynamical simulation and semi-analytic modelling. By
analysing the physical assumptions built into the Gasoline simulation, formulae
for the emergent behaviour are derived which allow immediate and accurate
translation of these assumptions to the Galform semi-analytic model. The
simulated halo merger history is then extracted and evolved using these
equivalent equations, predicting a strikingly similar galactic system. This
exercise demonstrates that it is the initial conditions and physical
assumptions which are responsible for the predicted evolution, not the choice
of modelling technique. On this level playing field, a previously published
Galform model is applied (including additional physics such as chemical
enrichment and feedback from active galactic nuclei) which leads to starkly
different predictions.Comment: 15 pages, 15 figure
B2 0902+34: A Collapsing Protogiant Elliptical Galaxy at z=3.4
We have used the visible integral-field replicable unit spectrograph
prototype (VIRUS-P), a new integral field spectrograph, to study the spatially
and spectrally resolved Lyman-alpha emission line structure in the radio galaxy
B2 0902+34 at z=3.4. We observe a halo of Lyman-alpha emission with a velocity
dispersion of 250 km/s extending to a radius of 50 kpc. A second feature is
revealed in a spatially resolved region where the line profile shows
blueshifted structure. This may be viewed as either HI absorption at -450 km/s
or secondary emission at -900 km/s from the primary peak. Our new data, in
combination with the 21 cm absorption, suggest two important and unexplained
discrepancies. First, nowhere in the line profiles of the Lyman-alpha halo is
the 21 cm absorber population evident. Second, the 21 cm absorption redshift is
higher than the Lyman-alpha emission redshift. In an effort to explain these
two traits, we have undertaken the first three dimensional Monte Carlo
simulations of resonant scattering in radio galaxies. Though simple, the model
produces the features in the Lyman-alpha data and predicts the 21 cm
properties. To reach agreement between this model and the data, global infall
of the HI is strictly necessary. The amount of gas necessary to match the model
and data is surprisingly high, >= 10E12 solar masses, an order of magnitude
larger than the stellar mass. The collapsing structure and large gas mass lead
us to interpret B2 0902+34 as a protogiant elliptical galaxy.Comment: 30 pages, 8 figures, 4 tables, accepted in Ap
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