2,667 research outputs found
Efficiency of gas cooling and accretion at the disc-corona interface
In star-forming galaxies, stellar feedback can have a dual effect on the
circumgalactic medium both suppressing and stimulating gas accretion. The
trigger of gas accretion can be caused by disc material ejected into the halo
in the form of fountain clouds and by its interaction with the surrounding hot
corona. Indeed, at the disc-corona interface, the mixing between the
cold/metal-rich disc gas (T ~ 10^6 K)
can dramatically reduce the cooling time of a portion of the corona and produce
its condensation and accretion. We studied the interaction between fountain
clouds and corona in different galactic environments through parsec-scale
hydrodynamical simulations, including the presence of thermal conduction, a key
mechanism that influences gas condensation. Our simulations showed that the
coronal gas condensation strongly depends on the galactic environment, in
particular it is less efficient for increasing virial temperature/mass of the
haloes where galaxies reside and it is fully ineffective for objects with
virial masses larger than 10^13 Msun. This result implies that the coronal gas
cools down quickly in haloes with low-intermediate virial mass (Mvir <~ 3 x
10^12 Msun) but the ability to cool the corona decreases going from late-type
to early-type disc galaxies, potentially leading to the switching off of
accretion and the quenching of star formation in massive systems.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Highly nonlinear pulse splitting and recombination in a two-dimensional granular network
The propagation of highly nonlinear signals in a branched two-dimensional granular system was investigated experimentally and numerically for a system composed of chains of spherical beads of different materials. The system studied consists of a double Y-shaped guide in which high- and low-modulus/mass chains of spheres are arranged in various geometries. We observed the transformation of a single or a train of solitary pulses crossing the interface between branches. We report fast splitting of the initial pulse, rapid chaotization of the signal and impulse redirection and bending. Pulse and energy trapping was also observed in the branches. Numerical analysis based on Hertzian interaction between the particles and the side walls of the guide was found in agreement with the experimental data, except for nonsymmetric arrangements of particles excited by a large mass striker
Multiscale mass-spring models of carbon nanotube foams
This article is concerned with the mechanical properties of dense, vertically aligned CNT foams subject to one-dimensional compressive loading. We develop a discrete model directly inspired by the micromechanical response reported experimentally for CNT foams, where infinitesimal portions of the tubes are represented by collections of uniform bi-stable springs. Under cyclic loading, the given model predicts an initial elastic deformation, a non-homogeneous buckling regime, and a densification response, accompanied by a hysteretic unloading path. We compute the dynamic dissipation of such a model through an analytic approach. The continuum limit of the microscopic spring chain defines a mesoscopic dissipative element (micro-meso transition) which represents a finite portion of the foam thickness. An upper-scale model formed by a chain of non-uniform mesoscopic springs is employed to describe the entire CNT foam. A numerical approximation illustrates the main features of the proposed multiscale approach. Available experimental results on the compressive response of CNT foams are fitted with excellent agreement
GSATools: Analysis of allosteric communication and functional local motions using a structural alphabet
Motivation: GSATools is a free software package to analyze conformational ensembles and to detect functional motions in proteins by means of a structural alphabet. The software integrates with the widely used GROMACS simulation package and can generate a range of graphical outputs. Three applications can be supported: (i) investigation of the conformational variability of local structures; (ii) detection of allosteric communication; and (iii) identification of local regions that are critical for global functional motions. These analyses provide insights into the dynamics of proteins and allow for targeted design of functional mutants in theoretical and experimental studies. Availability: The C source code of the GSATools, along with a set of pre-compiled binaries, is freely available under GNU General Public License from http://mathbio.nimr.mrc.ac.uk/wiki/GSATools. Contact: alessandro.pandini@kcl. ac.uk or [email protected] Supplementary information: Supplementary data are available at Bioinformatics online. © 2013 The Author 2013. Published by Oxford University Press
Stationary models for the extra-planar gas in disc galaxies
The kinematics of the extra-planar neutral and ionised gas in disc galaxies
shows a systematic decline of the rotational velocity with height from the
plane (vertical gradient). This feature is not expected for a barotropic gas,
whilst it is well reproduced by baroclinic fluid homogeneous models. The
problem with the latter is that they require gas temperatures (above K)
much higher than the temperatures of the cold and warm components of the
extra-planar gas layer. In this paper, we attempt to overcome this problem by
describing the extra-planar gas as a system of gas clouds obeying the Jeans
equations. In particular, we consider models having the observed extra-planar
gas distribution and gravitational potential of the disc galaxy NGC 891: for
each model we construct pseudo-data cubes and we compare them with the HI data
cube of NGC 891. In all cases the rotational velocity gradients are in
qualitative agreement with the observations, but the synthetic and the observed
data cubes of NGC 891 show systematic differences that cannot be accommodated
by any of the explored models. We conclude that the extra-planar gas in disc
galaxies cannot be satisfactorily described by a stationary Jeans-like system
of gas clouds.Comment: 14 pages, 7 figures, accepted for pubblication in MNRA
The stellar mass-halo mass relation of isolated field dwarfs: a critical test of CDM at the edge of galaxy formation
We fit the rotation curves of isolated dwarf galaxies to directly measure the
stellar mass-halo mass relation () over the mass range . By accounting for cusp-core
transformations due to stellar feedback, we find a monotonic relation with
little scatter. Such monotonicity implies that abundance matching should yield
a similar if the cosmological model is correct. Using the 'field
galaxy' stellar mass function from the Sloan Digital Sky Survey (SDSS) and the
halo mass function from the Cold Dark Matter Bolshoi simulation, we
find remarkable agreement between the two. This holds down to M, and to M if we
assume a power law extrapolation of the SDSS stellar mass function below M.
However, if instead of SDSS we use the stellar mass function of nearby galaxy
groups, then the agreement is poor. This occurs because the group stellar mass
function is shallower than that of the field below M,
recovering the familiar 'missing satellites' and 'too big to fail' problems.
Our result demonstrates that both problems are confined to group environments
and must, therefore, owe to 'galaxy formation physics' rather than exotic
cosmology.
Finally, we repeat our analysis for a Warm Dark Matter cosmology,
finding that it fails at 68% confidence for a thermal relic mass of keV, and keV if we use the power law extrapolation
of SDSS. We conclude by making a number of predictions for future surveys based
on these results.Comment: 22 pages; 2 Tables; 10 Figures. This is the version accepted for
publication in MNRAS. Key changes: (i) added substantially more information
on the surveys used to measure the stellar mass functions; (ii) added tests
of the robustness of our results. Results and conclusions unchanged from
previously. Minor typos corrected from previous versio
Galactic fountains and gas accretion
Star-forming disc galaxies such as the Milky Way need to accrete \gsim 1
of gas each year to sustain their star formation. This gas
accretion is likely to come from the cooling of the hot corona, however it is
still not clear how this process can take place. We present simulations
supporting the idea that this cooling and the subsequent accretion are caused
by the passage of cold galactic-fountain clouds through the hot corona. The
Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas
causes coronal gas to condense in the cloud's wake. For likely parameters of
the Galactic corona and of typical fountain clouds we obtain a global accretion
rate of the order of that required to feed the star formation.Comment: 2 pages, 1 figure, to appear in "Hunting for the Dark: The Hidden
Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista &
C.C. Popescu, AIP Conf. Se
The survival of gas clouds in the Circumgalactic Medium of Milky Way-like galaxies
Observational evidence shows that low-redshift galaxies are surrounded by
extended haloes of multiphase gas, the so-called 'circumgalactic medium' (CGM).
To study the survival of relatively cool gas (T < 10^5 K) in the CGM, we
performed a set of hydrodynamical simulations of cold (T = 10^4 K) neutral gas
clouds travelling through a hot (T = 2x10^6 K) and low-density (n = 10^-4
cm^-3) coronal medium, typical of Milky Way-like galaxies at large
galactocentric distances (~ 50-150 kpc). We explored the effects of different
initial values of relative velocity and radius of the clouds. Our simulations
were performed on a two-dimensional grid with constant mesh size (2 pc) and
they include radiative cooling, photoionization heating and thermal conduction.
We found that for large clouds (radii larger than 250 pc) the cool gas survives
for very long time (larger than 250 Myr): despite that they are partially
destroyed and fragmented into smaller cloudlets during their trajectory, the
total mass of cool gas decreases at very low rates. We found that thermal
conduction plays a significant role: its effect is to hinder formation of
hydrodynamical instabilities at the cloud-corona interface, keeping the cloud
compact and therefore more difficult to destroy. The distribution of column
densities extracted from our simulations are compatible with those observed for
low-temperature ions (e.g. SiII and SiIII) and for high-temperature ions (OVI)
once we take into account that OVI covers much more extended regions than the
cool gas and, therefore, it is more likely to be detected along a generic line
of sight.Comment: 12 pages, 10 figures. Accepted for publication in MNRA
Fountain-driven gas accretion by the Milky Way
Accretion of fresh gas at a rate of ~ 1 M_{sun} yr^{-1} is necessary in
star-forming disc galaxies, such as the Milky Way, in order to sustain their
star-formation rates. In this work we present the results of a new hydrodynamic
simulation supporting the scenario in which the gas required for star formation
is drawn from the hot corona that surrounds the star-forming disc. In
particular, the cooling of this hot gas and its accretion on to the disc are
caused by the passage of cold galactic fountain clouds through the corona.Comment: 2 pages, 1 figure. To appear in the proceedings of the conference
"Assembling the Puzzle of the Milky Way", Le Grand-Bornand 17-22 April 2011,
European Physical Journal, editors C. Reyl\'e, A. Robin and M. Schulthei
Modelling the HI halo of the Milky Way
Aims: we studied the global distribution and kinematics of the extra-planar
neutral gas in the Milky Way. Methods: we built 3D models for a series of
Galactic HI layers, projected them for an inside view, and compared them with
the Leiden-Argentina-Bonn 21-cm observations. Results: we show that the Milky
Way disk is surrounded by an extended halo of neutral gas with a vertical
scale-height of 1.6[+0.6/-0.4] kpc and an HI mass of 3.2[+1.0/-0.9]x10^8 solar
masses, which is 5-10% of the total Galactic HI. This HI halo rotates more
slowly than the disk with a vertical velocity gradient of -15[+/-4] km/s/kpc.
We found evidence for a global infall motion, both vertical (20[+5/-7] km/s)
and radial (30[+7/-5]km/s). Conclusions: the Milky Way HI halo shows properties
similar to the halos of external galaxies and is compatible with being
predominantly produced by supernova explosions in the disk. It is most likely
composed of distinct gas complexes with masses of 10^4-10^5 solar masses of
which the Intermediate Velocity Clouds are the local manifestations. The
classical High Velocity Clouds appear to be a separate population.Comment: 13 pages, 9 figures, accepted for publication in A&
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