6,211 research outputs found
Clustering and collision of inertial particles in random velocity fields
The influence of clustering on the collision rate of inertial particles in a
smooth random velocity field, mimicking the smaller scales of a turbulent flow,
is analyzed. For small values of the the ratio between the relaxation time of
the particle velocity and the characteristic time of the field, the effect of
clusters is to make more energetic collisions less likely. The result is
independent of the flow dimensionality and is due only to the origin of
collisions in the process of caustic formation.Comment: 4 pages, 3 figures, revtex
Concentration fluctuations of large Stokes number particles in a one-dimensional random velocity field
We analyze the behavior of an ensemble of inertial particles in a
one-dimensional smooth Gaussian velocity field, in the limit of large inertia,
but considering a finite correlation time for the random field. We derive in
this limit a perturbative scheme for the calculation of the concentration
correlation and of the particle relative velocity distribution, providing
analytical expressions for the concentration fluctuation amplitude, its
correlation length, and the modification in the particle pair relative velocity
variance. The amplitude of the concentration fluctuations is characterized by
slow decay at large inertia and a much larger correlation length than that of
the random field. The fluctuation structure in velocity space is very different
from predictions from short-time correlated random velocity fields, with only
few particle pairs crossing at sufficiently small relative velocity to produce
correlations. Concentration fluctuations are associated with depletion of the
relative velocity variance of colliding particles.Comment: 8 pages, 1 figure, revtex
The ROSAT Deep Cluster Survey: the X-ray Luminosity Function out to z=0.8
We present the X-ray Luminosity Function (XLF) of the ROSAT Deep Cluster
Survey (RDCS) sample over the redshift range 0.05-0.8. Our results are derived
from a complete flux-limited subsample of 70 galaxy clusters, representing the
brightest half of the total sample, which have been spectroscopically
identified down to the flux limit of 4*10^{-14} erg/cm^2/s (0.5-2.0 keV) and
have been selected via a serendipitous search in ROSAT-PSPC pointed
observations. The redshift baseline is large enough that evolutionary effects
can be studied within the sample. The local XLF (z < 0.25) is found to be in
excellent agreement with previous determinations using the ROSAT All-Sky Survey
data. The XLF at higher redshifts, when combined with the deepest number counts
constructed to date (f>2*10^{-14} arg/cm^2/s), reveal no significant evolution
at least out to z=0.8, over a luminosity range 2*10^{42}-3*10^{44} erg/s in the
[0.5-2 keV] band. These findings extend the study of cluster evolution to the
highest redshifts and the faintest fluxes probed so far in X-ray surveys. They
complement and do not necessarily conflict with those of the Einstein Extended
Medium Sensitivity Survey, leaving the possibility of negative evolution of the
brightest end of the XLF at high redshifts.Comment: 12 pages, 4 figures, LaTeX (aasms4.sty). To appear in ApJ Letter
Globular Cluster Formation from Colliding Substructure
We investigate a scenario where the formation of Globular Clusters (GCs) is
triggered by high-speed collisions between infalling atomic-cooling subhalos
during the assembly of the main galaxy host, a special dynamical mode of star
formation that operates at high gas pressures and is intimately tied to LCDM
hierarchical galaxy assembly. The proposed mechanism would give origin to
"naked" globulars, as colliding dark matter subhalos and their stars will
simply pass through one another while the warm gas within them clashes at
highly supersonic speed and decouples from the collisionless component, in a
process reminiscent of the Bullet galaxy cluster. We find that the resulting
shock-compressed layer cools on a timescale that is typically shorter than the
crossing time, first by atomic line emission and then via fine-structure
metal-line emission, and is subject to gravitational instability and
fragmentation. Through a combination of kinetic theory approximation and
high-resolution -body simulations, we show that this model may produce: (a)
a GC number-halo mass relation that is linear down to dwarf galaxy scales and
agrees with the trend observed over five orders of magnitude in galaxy mass;
(b) a population of old globulars with a median age of 12 Gyr and an age spread
similar to that observed; (c) a spatial distribution that is biased relative to
the overall mass profile of the host; and (d) a bimodal metallicity
distribution with a spread similar to that observed in massive galaxies.Comment: 15 pages, 5 figures, accepted for publication by the Astrophysical
Journa
Early Enrichment of the Intergalactic Medium and its Feedback on Galaxy Formation
Supernova-driven outflows from early galaxies may have had a large impact on
the kinetic and chemical structure of the intergalactic medium (IGM). We use
three-dimensional Monte Carlo cosmological realizations of a simple linear
peaks model to track the time evolution of such metal-enriched outflows and
their feedback on galaxy formation. We find that at most 30% of the IGM by
volume is enriched to values above 10^-3 solar in models that only include
objects that cool by atomic transitions. The majority of enrichment occurs
relatively early (5 < z < 12) and resulting in a mass-averaged cosmological
metallicity between 10^-3 and 10^-1.5 solar. The inclusion of Population III
objects that cool through H2 line emission has only a minor impact on these
results: increasing the mean metallicity and filling factor by at most a factor
of 1.4, and moving the dawn of the enrichment epoch to a redshift of
approximately 14 at the earliest. Thus enrichment by outflowing galaxies is
likely to have been incomplete and inhomogeneous, biased to the areas near the
starbursting galaxies themselves. Models with a 10% star formation efficiency
can satisfactorily reproduce the nearly constant (2 < z < 5, Z approximately
3.5 x 10^-4 solar) metallicity of the low column density Ly-alpha forest
derived by Songaila (2001), an effect of the decreasing efficiency of metal
loss from larger galaxies. Finally, we show that IGM enrichment is intimately
tied to the ram-pressure stripping of baryons from neighboring perturbations.
This results in the suppression of at least 20% of the dwarf galaxies in the
mass range 10^8.5 to 10^9.5 solar, in all models with filling factors greater
than 2%, and an overall suppression of approximately 50% of dwarf galaxies in
the most observationally-favored model.Comment: 8 pages, 5 figures, accepted to Ap
Cosmological Reionization
In popular cosmological scenarios, some time beyond a redshift of 10, stars
within protogalaxies created the first heavy elements; these systems, together
perhaps with an early population of quasars, generated the ultraviolet
radiation and mechanical energy that reheated and reionized the cosmos. The
history of the Universe during and soon after these crucial formative stages is
recorded in the all-pervading intergalactic medium (IGM), which contains most
of the ordinary baryonic material left over from the big bang. Throughout the
epoch of structure formation, the IGM becomes clumpy and acquires peculiar
motions under the influence of gravity, and acts as a source for the gas that
gets accreted, cools, and forms stars within galaxies, and as a sink for the
metal enriched material, energy, and radiation which they eject.Comment: LateX, 13 pages, 4 figures, slightly revised version (corrected
several typos), to appear in Phil. Trans. R. Soc. London A (2000) 35
Interaction of massive black hole binaries with their stellar environment: II. Loss-cone depletion and binary orbital decay
We study the long-term evolution of massive black hole binaries (MBHBs) at
the centers of galaxies using detailed scattering experiments to solve the full
three-body problem. Ambient stars drawn from a isotropic Maxwellian
distribution unbound to the binary are ejected by the gravitational slingshot.
We construct a minimal, hybrid model for the depletion of the loss cone and the
orbital decay of the binary, and show that secondary slingshots - stars
returning on small impact parameter orbits to have a second super-elastic
scattering with the MBHB - may considerably help the shrinking of the pair in
the case of large binary mass ratios. In the absence of loss-cone refilling by
two-body relaxation or other processes, the mass ejected before the stalling of
a MBHB is half the binary reduced mass. About 50% of the ejected stars are
expelled ejected in a "burst" lasting ~1E4 yrs M_6^1/4, where M_6 is the binary
mass in units of 1E6 Msun. The loss cone is completely emptied in a few bulge
crossing timescales, 1E7 yrs M_6^1/4. Even in the absence of two-body
relaxation or gas dynamical processes, unequal mass and/or eccentric binaries
with M_6 >0.1 can shrink to the gravitational wave emission regime in less than
a Hubble time, and are therefore "safe" targets for the planned Laser
Interferometer Space Antenna (LISA).Comment: Minor revision. 10 pages, 7 figures, ApJ in pres
Orthognathic surgery and rhinoplasty in Binder syndrome
El síndrome de Binder es una patología caracterizada por hipoplasia nariz-maxilar, ángulo naso-frontal plano, senos frontales hipoplasicos, ausencia de la espina nasal anterior, columela corta y ángulo nasolabial agudo. El tratamiento del los pacientes con síndrome de Binder puede ser ortodóntico o quirúrgico según la gravedad de la malformación. En este trabajo hemos realizado una revision bibliográfica sobre la etiología, el diagnóstico diferential y el tratamiento de la sindrome de Binder y presentamos un caso clínico de un paciente binderiano sometido a intervención de cirugía ortognática y rinoplastia con injerto de cartílago costal para recostruir el dorso y la punta nasal. Binder syndrome is a disorder characterized by nasomaxillary hypoplasia that results in a short nose, a frontonasal angle of almost 180 degrees, hypoplasia of the frontal sinuses, an absent anterior nasal spine, a short columella and an acute nasolabial angle. The patient can be treated orthodontically or surgically depending on the seriousness of the malformation. We review the literature on the etiology, differential diagnosis and treatment of Binder syndrome. We present the case of a boy with this syndrome surgically treated with orthognatic surgery and rhinoplasty with an L-shaped rib cartilage graft
Viscoelasticity and Stokes-Einstein relation in repulsive and attractive colloidal glasses
We report a numerical investigation of the visco-elastic behavior in models
for steric repulsive and short-range attractive colloidal suspensions, along
different paths in the attraction-strength vs packing fraction plane. More
specifically, we study the behavior of the viscosity (and its frequency
dependence) on approaching the repulsive glass, the attractive glass and in the
re-entrant region where viscosity shows a non monotonic behavior on increasing
attraction strength. On approaching the glass lines, the increase of the
viscosity is consistent with a power-law divergence with the same exponent and
critical packing fraction previously obtained for the divergence of the density
fluctuations. Based on mode-coupling calculations, we associate the increase of
the viscosity with specific contributions from different length scales. We also
show that the results are independent on the microscopic dynamics by comparing
newtonian and brownian simulations for the same model. Finally we evaluate the
Stokes-Einstein relation approaching both glass transitions, finding a clear
breakdown which is particularly strong for the case of the attractive glass.Comment: 12 pages; sent to J. Chem. Phy
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