6,864 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
ISM properties in hydrodynamic galaxy simulations: Turbulence cascades, cloud formation, role of gravity and feedback
We study the properties of ISM substructure and turbulence in hydrodynamic
(AMR) galaxy simulations with resolutions up to 0.8 pc and 5x10^3 Msun. We
analyse the power spectrum of the density distribution, and various components
of the velocity field. We show that the disk thickness is about the average
Jeans scale length, and is mainly regulated by gravitational instabilities.
From this scale of energy injection, a turbulence cascade towards small-scale
is observed, with almost isotropic small-scale motions. On scales larger than
the disk thickness, density waves are observed, but there is also a full range
of substructures with chaotic and strongly non-isotropic gas velocity
dispersions. The power spectrum of vorticity in an LMC-sized model suggests
that an inverse cascade of turbulence might be present, although energy input
over a wide range of scales in the coupled gaseous+stellar fluid could also
explain this quasi-2D regime on scales larger than the disk scale height.
Similar regimes of gas turbulence are also found in massive high-redshift disks
with high gas fractions. Disk properties and ISM turbulence appear to be mainly
regulated by gravitational processes, both on large scales and inside dense
clouds. Star formation feedback is however essential to maintain the ISM in a
steady state by balancing a systematic gas dissipation into dense and small
clumps. Our galaxy simulations employ a thermal model based on a barotropic
Equation of State (EoS) aimed at modelling the equilibrium of gas between
various heating and cooling processes. Denser gas is typically colder in this
approach, which is shown to correctly reproduce the density structures of a
star-forming, turbulent, unstable and cloudy ISM down to scales of a few
parsecs.Comment: MNRAS in pres
High-velocity hot CO emission close to Sgr A*: Herschel/HIFI submillimeter spectral survey toward Sgr A*
The properties of molecular gas, the fuel that forms stars, inside the cavity
of the circumnuclear disk (CND) are not well constrained. We present results of
a velocity-resolved submillimeter scan (~480 to 1250 GHz}) and [CII]158um line
observations carried out with Herschel/HIFI toward Sgr A*; these results are
complemented by a ~2'x2' CO (J=3-2) map taken with the IRAM 30 m telescope at
~7'' resolution. We report the presence of high positive-velocity emission (up
to about +300 km/s) detected in the wings of CO J=5-4 to 10-9 lines. This wing
component is also seen in H2O (1_{1,0}-1_{0,1}) a tracer of hot molecular gas;
in [CII]158um, an unambiguous tracer of UV radiation; but not in [CI]492,806
GHz. This first measurement of the high-velocity CO rotational ladder toward
Sgr A* adds more evidence that hot molecular gas exists inside the cavity of
the CND, relatively close to the supermassive black hole (< 1 pc). Observed by
ALMA, this velocity range appears as a collection of CO (J=3-2) cloudlets lying
in a very harsh environment that is pervaded by intense UV radiation fields,
shocks, and affected by strong gravitational shears. We constrain the physical
conditions of the high positive-velocity CO gas component by comparing with
non-LTE excitation and radiative transfer models. We infer T_k~400 K to 2000 K
for n_H~(0.2-1.0)x10^5 cm^-3. These results point toward the important role of
stellar UV radiation, but we show that radiative heating alone cannot explain
the excitation of this ~10-60 M_Sun component of hot molecular gas inside the
central cavity. Instead, strongly irradiated shocks are promising candidates.Comment: Accepted for publication in A&A Letters ( this v2 includes
corrections by language editor
BiTeCl and BiTeBr: a comparative high-pressure optical study
We here report a detailed high-pressure infrared transmission study of BiTeCl
and BiTeBr. We follow the evolution of two band transitions: the optical
excitation between two Rashba-split conduction bands, and the
absorption across the band gap. In the low pressure range, ~GPa,
for both compounds is approximately constant with pressure and
decreases, in agreement with band structure calculations. In BiTeCl, a clear
pressure-induced phase transition at 6~GPa leads to a different ground state.
For BiTeBr, the pressure evolution is more subtle, and we discuss the
possibility of closing and reopening of the band gap. Our data is consistent
with a Weyl phase in BiTeBr at 56~GPa, followed by the onset of a structural
phase transition at 7~GPa.Comment: are welcom
3D simulations of supernova remnants evolution including non-linear particle acceleration
If a sizeable fraction of the energy of supernova remnant shocks is channeled into energetic particles (commonly identified with Galactic cosmic rays), then the morphological evolution of the remnants must be distinctly modified. Evidence of such modifications has been recently obtained with the Chandra and XMM-Newton X-ray satellites. To investigate these effects, we coupled a semi-analytical kinetic model of shock acceleration with a 3D hydrodynamic code (by means of an effective adiabatic index). This enables us to study the time-dependent compression of the region between the forward and reverse shocks due to the back reaction of accelerated particles, concomitantly with the development of the Rayleigh-Taylor hydrodynamic instability at the contact discontinuity. Density profiles depend critically on the injection level η of particles: for η âČ 10-4 modifications are weak and progressive, for η Ë 10-3 modifications are strong and immediate. Nevertheless, the extension of the Rayleigh-Taylor unstable region does not depend on the injection rate. A first comparison of our simulations with observations of Tycho's remnant strengthens the case for efficient acceleration of protons at the forward shock
Optical evidence for heavy charge carriers in FeGe
The optical spectrum of the cubic helimagnetic metal FeGe has been
investigated in the frequency range from 0.01 - 3.1 eV for different
temperatures from 30 K to 296 K. The optical conductivity shows the evolution
of a low energy (0.22 eV) interband transition and the development of a narrow
free carrier response with a strong energy and temperature dependence. The
frequency dependent effective mass and scattering rate derived from the optical
data indicate the formation of dressed quasi-particles with a mass
renormalization factor of 12. Similar to FeSi the spectral weight in FeGe is
not recovered over a broad frequency range, an effect usually attributed to the
influence of the on-site Coulomb interaction.Comment: 5 pages, 5 figure
Incorporating Ambipolar and Ohmic Diffusion in the AMR MHD code RAMSES
We have implemented non-ideal Magneto-Hydrodynamics (MHD) effects in the
Adaptive Mesh Refinement (AMR) code RAMSES, namely ambipolar diffusion and
Ohmic dissipation, as additional source terms in the ideal MHD equations. We
describe in details how we have discretized these terms using the adaptive
Cartesian mesh, and how the time step is diminished with respect to the ideal
case, in order to perform a stable time integration. We have performed a large
suite of test runs, featuring the Barenblatt diffusion test, the Ohmic
diffusion test, the C-shock test and the Alfven wave test. For the latter, we
have performed a careful truncation error analysis to estimate the magnitude of
the numerical diffusion induced by our Godunov scheme, allowing us to estimate
the spatial resolution that is required to address non-ideal MHD effects
reliably. We show that our scheme is second-order accurate, and is therefore
ideally suited to study non-ideal MHD effects in the context of star formation
and molecular cloud dynamics
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