251 research outputs found
The impact of binary-star yields on the spectra of galaxies
One of the complexities in modelling integrated spectra of stellar populations is the effect of interacting binary stars besides Type Ia supernovae (SNeIa). These include common envelope systems, cataclysmic variables, novae, and are usually ignored in models predicting the chemistry and spectral absorption line strengths in galaxies. In this paper, predictions of chemical yields from populations of single and binary stars are incorporated into a galactic chemical evolution model to explore the significance of the effects of these other binary yields. Effects on spectral line strengths from different progenitor channels of SNeIa are also explored. Small systematic effects are found when the yields from binaries, other than SNeIa, are included, for a given star formation history. These effects are, at present, within the observational uncertainties on the line strengths. More serious differences can arise in considering different types of SNIa models, their rates and contributions
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
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
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
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
An analysis of the composite stellar population in M32
We obtained long-slit spectra of high signal-to-noise ratio of the galaxy M32
with the GMOS spectrograph at the GEMINI North telescope. We analysed the
integrated spectra by means of full spectral fitting in order to extract the
mixture of stellar populations that best represents its composite nature. Three
different galactic radii were analysed, from the nuclear region out to 2 arcmin
from the centre. This allows us to compare, for the first time, the results of
integrated light spectroscopy with those of resolved colour-magnitude diagrams
from the literature. As our main result, we propose that an ancient and an
intermediate-age population coexist in M32, and that the balance between these
two populations change between the nucleus and outside 1 effective radius in
the sense that the contribution from the intermediate population is larger at
the nuclear region. We retrieve a smaller signal of a young population at all
radii whose origin is unclear and may be a contamination from horizontal-branch
stars, such as the ones identified by Brown et al. in the nuclear region. We
compare our metallicity distribution function for a region 1 to 2 arcmin from
the centre to the one obtained with photometric data by Grillmair et al. Both
distributions are broad, but our spectroscopically derived distribution has a
significant component with [Z/Z_{\sun}] \leq -1, which is not found by
Grillmair et al.Comment: accepted for publication in MNRA
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
The nature of high [O III]88 μ m/[C II]158 μm galaxies in the epoch of reionization: Low carbon abundance and a top-heavy IMF?
ALMA observations of z > 6 galaxies have revealed abnormally high [O III]88 μm/[C II]158 μm ratios and [C II]158 μm deficits compared to local galaxies. The origin of this behaviour is unknown. Numerous solutions have been proposed including differences in C and O abundance ratios, observational bias, and differences in ISM properties, including ionization parameter, gas density, or photodissociation region (PDR) covering fraction. In order to elucidate the underlying physics that drives this high-redshift phenomenon, we employ SPHINX20, a state-of-the-art, cosmological radiation–hydrodynamics simulation, that resolves detailed ISM properties of thousands of galaxies in the epoch of reionization which has been post-processed with CLOUDY to predict emission lines. We find that the observed z > 6 [O III]88 μm–SFR and [C II]158 μm–SFR relations can only be reproduced when the C/O abundance ratio is ∼8 × lower than Solar and the total metal production is ∼4 × higher than that of a Kroupa IMF. This implies that high-redshift galaxies are potentially primarily enriched by low-metallicity core–collapse supernovae with a more top-heavy IMF. As AGB stars and type-Ia supernova begin to contribute to the galaxy metallicity, both the [C II]158 μm–SFR and [C II]158 μm luminosity functions are predicted to converge to observed values at z ∼ 4.5. While we demonstrate that ionization parameter, LyC escape fraction, ISM gas density, and CMB attenuation all drive galaxies towards higher [O III]88 μm/[C II]158 μm, observed values at z > 6 can only be reproduced with substantially lower C/O abundances compared to Solar. The combination of [C II]158 μm and [O III]88 μm can be used to predict the values of ionization parameter, ISM gas density, and LyC escape fraction and we provide estimates of these quantities for nine observed z > 6 galaxies. Finally, we demonstrate that [O I]63 μm can be used as a replacement for [C II]158 μ m in high-redshift galaxies where [C II]158 μ m is unobserved and argue that more observation time should be used to target [O I]63 μm at z > 6. Future simulations will be needed to self-consistently address the numerous uncertainties surrounding a varying IMF at high redshift and the associated metal returns
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