256 research outputs found
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
Comparison of different spectral population models
We have compared simple stellar populations (SSPs) generated with different population synthesis tools: BC03, Vazdekis and Pegase.HR and different stellar libraries: ELODIE3.1, SteLib and MILES. We find that BC03/SteLib SSPs are biased toward solar metallicity, however Pegase.HR/ELODIE3.1 and Vazdekis/MILES are extremely consistent. The extensive coverage of the space of atmospheric parameters in the large stellar libraries allows precise synthesis for a large range of ages (0.1 .. 10 Gyr) and metallicities (-2 .. +0.4 dex) limited by the quality of the determination of stellar parameters (like temperature scale of the giants)
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
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
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
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
The inhomogeneous reionization times of present-day galaxies
Today's galaxies experienced cosmic reionization at different times in different locations. For the first time, reionization (50% ionized) redshifts, z R , at the location of their progenitors are derived from new, fully coupled radiation-hydrodynamics simulation of galaxy formation and reionization at z > 6, matched to N-body simulation to z = 0. Constrained initial conditions were chosen to form the well-known structures of the local universe, including the Local Group and Virgo, in a (91 Mpc)3 volume large enough to model both global and local reionization. Reionization simulation CoDa I-AMR, by CPU-GPU code EMMA, used (2048)3 particles and (2048)3 initial cells, adaptively refined, while N-body simulation CoDa I-DM2048, by Gadget2, used (2048)3 particles, to find reionization times for all galaxies at z = 0 with masses M(z = 0) ≥ 108 M ⊙. Galaxies with reionized earlier than the universe as a whole, by up to ~500 Myr, with significant scatter. For Milky Way–like galaxies, z R ranged from 8 to 15. Galaxies with typically reionized as late or later than globally averaged 50% reionization at , in neighborhoods where reionization was completed by external radiation. The spread of reionization times within galaxies was sometimes as large as the galaxy-to-galaxy scatter. The Milky Way and M31 reionized earlier than global reionization but later than typical for their mass, neither dominated by external radiation. Their most-massive progenitors at z > 6 had z R =9.8 (MW) and 11 (M31), while their total masses had z R = 8.2 (both)
Lopsided galaxies: the case of NGC 891
It has been known for a long time that a large fraction of disc galaxies are
lopsided. We simulate three different mechanisms that can induce lopsidedness:
flyby interactions, gas accretion from cosmological filaments and ram pressure
from the intergalactic medium. Comparing the morphologies, HI spectrum,
kinematics and m=1 Fourier components, we find that all of these mechanisms can
induce lopsidedness in galaxies, although in different degrees and with
observable consequences. The timescale over which lopsidedness persists
suggests that flybys can contribute to ~20 per cent of lopsided galaxies. We
focus our detailed comparison on the case of NGC 891, a lopsided, edge-on
galaxy with a nearby companion (UGC 1807). We find that the main properties of
NGC 891 (morphology, HI spectrum, rotation curve, existence of a gaseous
filament pointing towards UGC 1807) favour a flyby event for the origin of
lopsidedness in this galaxy.Comment: 12 pages, 12 figures, MNRAS, accepte
Galaxy stellar mass functions of different morphological types in clusters, and their evolution between z=0.8 and z=0
We present the galaxy stellar mass function (MF) and its evolution in
clusters from z~0.8 to the current epoch, based on the WIde-field Nearby
Galaxy-cluster Survey (WINGS) (0.04<z<0.07), and the ESO Distant Cluster Survey
(EDisCS) (0.4<z <0.8). We investigate the total MF and find it evolves
noticeably with redshift. The shape at M*>10^11 M' does not evolve, but below
M*~10^10.8 M' the MF at high redshift is flat, while in the Local Universe it
flattens out at lower masses. The population of M* = 10^10.2 - 10^10.8 M'
galaxies must have grown significantly between z=0.8 and z=0. We analyze the MF
of different morphological types (ellipticals, S0s and late-types), and find
that also each of them evolves with redshift. All types have proportionally
more massive galaxies at high- than at low-z, and the strongest evolution
occurs among S0 galaxies. Examining the morphology-mass relation (the way the
proportion of galaxies of different morphological types changes with galaxy
mass), we find it strongly depends on redshift. At both redshifts, ~40% of the
stellar mass is in elliptical galaxies. Another ~43% of the mass is in S0
galaxies in local clusters, while it is in spirals in distant clusters. To
explain the observed trends, we discuss the importance of those mechanisms that
could shape the MF. We conclude that mass growth due to star formation plays a
crucial role in driving the evolution. It has to be accompanied by infall of
galaxies onto clusters, and the mass distribution of infalling galaxies might
be different from that of cluster galaxies. However, comparing with high-z
field samples, we do not find conclusive evidence for such an environmental
mass segregation. Our results suggest that star formation and infall change
directly the MF of late-type galaxies in clusters and, indirectly, that of
early-type galaxies through subsequent morphological transformations.Comment: MNRAS in press, 24 pages, 19 figures and 8 table
Stellar populations of bulges at low redshift
This chapter summarizes our current understanding of the stellar population
properties of bulges and outlines important future research directions.Comment: Review article to appear in "Galactic Bulges", Editors: Laurikainen
E., Peletier R., Gadotti D., Springer Publishing. 34 pages, 12 figure
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