5,916 research outputs found
Globular Cluster Formation in the Virgo Cluster
Metal poor globular clusters (MPGCs) are a unique probe of the early
universe, in particular the reionization era. Systems of globular clusters in
galaxy clusters are particularly interesting as it is in the progenitors of
galaxy clusters that the earliest reionizing sources first formed. Although the
exact physical origin of globular clusters is still debated, it is generally
admitted that globular clusters form in early, rare dark matter peaks (Moore et
al. 2006; Boley et al. 2009). We provide a fully numerical analysis of the
Virgo cluster globular cluster system by identifying the present day globular
cluster system with exactly such early, rare dark matter peaks. A popular
hypothesis is that that the observed truncation of blue metal poor globular
cluster formation is due to reionization (Spitler et al. 2012; Boley et al.
2009; Brodie & Strader 2006); adopting this view, constraining the formation
epoch of MPGCs provides a complementary constraint on the epoch of
reionization. By analyzing both the line of sight velocity dispersion and the
surface density distribution of the present day distribution we are able to
constrain the redshift and mass of the dark matter peaks. We find and quantify
a dependence on the chosen line of sight of these quantities, whose strength
varies with redshift, and coupled with star formation efficiency arguments find
a best fitting formation mass and redshift of and . We predict intracluster MPGCs in
the Virgo cluster. Our results confirm the techniques pioneered by Moore et al.
(2006) when applied to the the Virgo cluster and extend and refine the analytic
results of Spitler et al. (2012) numerically.Comment: 13 Pages, 13 Figures, submitted to MNRA
Galactic star formation in parsec-scale resolution simulations
The interstellar medium (ISM) in galaxies is multiphase and cloudy, with
stars forming in the very dense, cold gas found in Giant Molecular Clouds
(GMCs). Simulating the evolution of an entire galaxy, however, is a
computational problem which covers many orders of magnitude, so many
simulations cannot reach densities high enough or temperatures low enough to
resolve this multiphase nature. Therefore, the formation of GMCs is not
captured and the resulting gas distribution is smooth, contrary to
observations. We investigate how star formation (SF) proceeds in simulated
galaxies when we obtain parsec-scale resolution and more successfully capture
the multiphase ISM. Both major mergers and the accretion of cold gas via
filaments are dominant contributors to a galaxy's total stellar budget and we
examine SF at high resolution in both of these contexts.Comment: 4 pages, 4 figures. To appear in the proceedings for IAU Symposium
270: Computational Star Formation (eds. Alves, Elmegreen, Girart, Trimble
Modelling CO emission from hydrodynamic simulations of nearby spirals, starbursting mergers, and high-redshift galaxies
We model the intensity of emission lines from the CO molecule, based on
hydrodynamic simulations of spirals, mergers, and high-redshift galaxies with
very high resolutions (3pc and 10^3 Msun) and detailed models for the
phase-space structure of the interstellar gas including shock heating, stellar
feedback processes and galactic winds. The simulations are analyzed with a
Large Velocity Gradient (LVG) model to compute the local emission in various
molecular lines in each resolution element, radiation transfer and opacity
effects, and the intensity emerging from galaxies, to generate synthetic
spectra for various transitions of the CO molecule. This model reproduces the
known properties of CO spectra and CO-to-H2 conversion factors in nearby
spirals and starbursting major mergers. The high excitation of CO lines in
mergers is dominated by an excess of high-density gas, and the high turbulent
velocities and compression that create this dense gas excess result in broad
linewidths and low CO intensity-to-H2 mass ratios. When applied to
high-redshift gas-rich disks galaxies, the same model predicts that their
CO-to-H2 conversion factor is almost as high as in nearby spirals, and much
higher than in starbursting mergers. High-redshift disk galaxies contain giant
star-forming clumps that host a high-excitation component associated to gas
warmed by the spatially-concentrated stellar feedback sources, although CO(1-0)
to CO(3-2) emission is overall dominated by low-excitation gas around the
densest clumps. These results overall highlight a strong dependence of CO
excitation and the CO-to-H2 conversion factor on galaxy type, even at similar
star formation rates or densities. The underlying processes are driven by the
interstellar medium structure and turbulence and its response to stellar
feedback, which depend on global galaxy structure and in turn impact the CO
emission properties.Comment: A&A in pres
Initial Conditions for Large Cosmological Simulations
This technical paper describes a software package that was designed to
produce initial conditions for large cosmological simulations in the context of
the Horizon collaboration. These tools generalize E. Bertschinger's Grafic1
software to distributed parallel architectures and offer a flexible alternative
to the Grafic2 software for ``zoom'' initial conditions, at the price of large
cumulated cpu and memory usage. The codes have been validated up to resolutions
of 4096^3 and were used to generate the initial conditions of large
hydrodynamical and dark matter simulations. They also provide means to generate
constrained realisations for the purpose of generating initial conditions
compatible with, e.g. the local group, or the SDSS catalog.Comment: 12 pages, 11 figures, submitted to ApJ
Collapse, outflows and fragmentation of massive, turbulent and magnetized prestellar barotropic cores
Stars and more particularly massive stars, have a drastic impact on galaxy
evolution. Yet the conditions in which they form and collapse are still not
fully understood. In particular, the influence of the magnetic field on the
collapse of massive clumps is relatively unexplored, it is thus of great
relevance in the context of the formation of massive stars to investigate its
impact. We perform high resolution, MHD simulations of the collapse of hundred
solar masses, turbulent and magnetized clouds, using the adaptive mesh
refinement code RAMSES. We compute various quantities such as mass
distribution, magnetic field and angular momentum within the collapsing core
and study the episodic outflows and the fragmentation that occurs during the
collapse. The magnetic field has a drastic impact on the cloud evolution. We
find that magnetic braking is able to substantially reduce the angular momentum
in the inner part of the collapsing cloud. Fast and episodic outflows are being
launched with typical velocities of the order of 3-5 km s although the
highest velocities can be as high as 30-40 km s. The fragmentation in
several objects, is reduced in substantially magnetized clouds with respect to
hydrodynamical ones by a factor of the order of 1.5-2. We conclude that
magnetic fields have a significant impact on the evolution of massive clumps.
In combination with radiation, magnetic fields largely determine the outcome of
massive core collapse. We stress that numerical convergence of MHD collapse is
a challenging issue. In particular, numerical diffusion appears to be important
at high density therefore possibly leading to an over-estimation of the number
of fragments.Comment: accepted for publication in A&
Evolution of the mass, size, and star formation rate in high-redshift merging galaxies MIRAGE - A new sample of simulations with detailed stellar feedback
We aim at addressing the questions related to galaxy mass assembly through
major and minor wet merging processes in the redshift range 1<z<2. A consequent
fraction of Milky Way like galaxies are thought to have undergone an unstable
clumpy phase at this early stage. Using the adaptive mesh refinement code
RAMSES, with a recent physically-motivated implementation of stellar feedback,
we build the Merging and Isolated high-Redshift Adaptive mesh refinement
Galaxies (MIRAGE) sample. It is composed of 20 mergers and 3 isolated idealized
disks simulations with global physical properties in accordance with the 1<z<2
mass complete sample MASSIV. The numerical hydrodynamical resolution reaches 7
parsecs in the smallest Eulerian cells. Our simulations include: star
formation, metal line cooling, metallicity advection, and a recent
implementation of stellar feedback which encompasses OB-type stars radiative
pressure, photo-ionization heating, and supernovae. The initial conditions are
set to match the z~2 observations, thanks to a new public code DICE. The
numerical resolution allows us to follow the formation and evolution of giant
clumps formed in-situ from Jeans instabilities triggered by high initial gas
fraction. The star formation history of isolated disks shows stochastic star
formation rate, which proceeds from the complex behavior of the giant clumps.
Our minor and major gas-rich merger simulations do not trigger starbursts,
suggesting a saturation of the star formation in a turbulent and clumpy
interstellar medium fed by substantial accretion from the circum-galactic
medium. Our simulations are close to the normal regime of the disk-like star
formation on a Schmidt-Kennicutt diagram. The mass-size relation and its rate
of evolution matches observations, suggesting that the inside-out growth
mechanisms of the stellar disk do not necessarily require to be achieved
through a cold accretion.Comment: 18 pages, 12 figures. Accepted in A&
Chameleon f(R) gravity on the Virgo cluster scale
Models of modified gravity offer promising alternatives to the concordance Λ cold dark matter (ΛCDM) cosmology to explain the late-time acceleration of the universe. A popular such model is f(R) gravity, in which the Ricci scalar in the Einstein-Hilbert action is replaced by a general function of it. We study the f(R) model of Hu & Sawicki, which recovers standard general relativity in high-density regimes, while reproducing the desired late time acceleration at cosmological scales. We run a suite of high-resolution zoom simulations using the ecosmog code to examine the effect of f(R) gravity on the properties of a halo that is analogous to the Virgo cluster. We show that the velocity dispersion profiles can potentially discriminate between f(R) models and ΛCDM, and provide complementary analysis of lensing signal profiles to explore the possibility to further distinguish the different f(R) models. Our results confirm the techniques explored by Cabré etal. to quantify the effect of environment in the behaviour of f(R) gravity, and we extend them to study halo satellites at various redshifts. We find that the modified gravity effects in our models are most observable at low redshifts, and that effects are generally stronger for satellites far from the centre of the main halo. We show that the screening properties of halo satellites trace very well that of dark matter particles, which means that low-resolution simulations in which subhaloes are not very well resolved can in principle be used to study satellite properties. We discuss observables, particularly for halo satellites, that can potentially be used to constrain the observational viability of f(R) gravit
Optical properties of Bi2Te2Se at ambient and high pressure
The temperature dependence of the complex optical properties of the
three-dimensional topological insulator Bi2Te2Se is reported for light
polarized in the a-b planes at ambient pressure, as well as the effects of
pressure at room temperature. This material displays a semiconducting character
with a bulk optical gap of 300 meV at 295 K. In addition to the two expected
infrared-active vibrations observed in the planes, there is additional fine
structure that is attributed to either the removal of degeneracy or the
activation of Raman modes due to disorder. A strong impurity band located at
200 cm^{-1} is also observed. At and just above the optical gap, several
interband absorptions are found to show a strong temperature and pressure
dependence. As the temperature is lowered these features increase in strength
and harden. The application of pressure leads to a very abrupt closing of the
gap above 8 GPa, and strongly modifies the interband absorptions in the
mid-infrared spectral range. While ab initio calculations fail to predict the
collapse of the gap, they do successfully describe the size of the band gap at
ambient pressure, and the magnitude and shape of the optical conductivity.Comment: 8 pages, 7 figure
The impact of ISM turbulence, clustered star formation and feedback on galaxy mass assembly through cold flows and mergers
Two of the dominant channels for galaxy mass assembly are cold flows (cold
gas supplied via the filaments of the cosmic web) and mergers. How these
processes combine in a cosmological setting, at both low and high redshift, to
produce the whole zoo of galaxies we observe is largely unknown. Indeed there
is still much to understand about the detailed physics of each process in
isolation. While these formation channels have been studied using
hydrodynamical simulations, here we study their impact on gas properties and
star formation (SF) with some of the first simulations that capture the
multiphase, cloudy nature of the interstellar medium (ISM), by virtue of their
high spatial resolution (and corresponding low temperature threshold). In this
regime, we examine the competition between cold flows and a
supernovae(SNe)-driven outflow in a very high-redshift galaxy (z {\approx} 9)
and study the evolution of equal-mass galaxy mergers at low and high redshift,
focusing on the induced SF. We find that SNe-driven outflows cannot reduce the
cold accretion at z {\approx} 9 and that SF is actually enhanced due to the
ensuing metal enrichment. We demonstrate how several recent observational
results on galaxy populations (e.g. enhanced HCN/CO ratios in ULIRGs, a
separate Kennicutt Schmidt (KS) sequence for starbursts and the population of
compact early type galaxies (ETGs) at high redshift) can be explained with
mechanisms captured in galaxy merger simulations, provided that the multiphase
nature of the ISM is resolved.Comment: To appear in the proceedings of IAUS 277, 'Tracing the ancestry of
galaxies', eds Carignan, Freeman & Combes. 4 pages, 2 figure
Coupled basin-detachment systems as paleoaltimetry archives of the western North American Cordillera
Stable isotope paleoaltimetry data from the Snake Range metamorphic core complex (MCC) and Sacramento Pass Basin (NV, USA) document that extensional mylonite zones and kinematically linked syntectonic basins reliably record paleotopography in the continental interior of western North America when compared to a sea-level reference. Here we show that this basin-MCC pair tracks meteoric fluid flow at different levels of actively extending crust in a high-topography region during Oligo-Miocene extension of the Basin and Range Province. For paleoaltimetry purposes we compare multi-proxy oxygen (δ 18O) and hydrogen (δD) isotope data as well as geochronological information from the Snake Range MCC to a time-equivalent (ca. 20Ma) stable isotopic proxy record from the Buckskin Mountains MCC (AZ, USA), which developed next to the Pacific Coast near Miocene sea level. We complement this paleoaltimetry study by comparing the Buckskin Mountains MCC data with older (~35Ma) lacustrine stable isotope and paleofloral records from the nearby House Range (UT, USA), whose paleoelevation has been determined independently through paleobotanical analysis. Each of the investigated compartments of the paleohydrologic system within the Snake Range MCC depicts a coherent scenario of low Oligo-Miocene δ 18O and δD values of meteoric water that reflect precipitation sourced at high elevation. A 77‰ difference in δD water between the Snake Range (δD water~-113‰) and the Buckskin Mountains (δD water~-36‰) is consistent with minimum mean paleoelevation of the Snake Range of about 3850±650m above Miocene sea level. Additional support for such elevations comes from a comparison between the Buckskin Mountains MCC and the Eocene House Range basin (UT, USA) where differences in δ 18O water values are consistent with 2300±500m minimum paleoelevation of the House Range. Based on the presence of brecciated rock-avalanche deposits within the Sacramento Pass Basin, we conclude that the Snake Range was a topographic high and locus of significant relief during regional scale extension within the Cordilleran hinterland. © 2012 Elsevier B.V
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