513 research outputs found
Dynamical models and numerical simulations of incomplete violent relaxation
N-body simulations of collisionless collapse have offered important clues to
the construction of realistic stellar dynamical models of elliptical galaxies.
Such simulations confirm and quantify the qualitative expectation that rapid
collapse of a self-gravitating collisionless system, initially cool and
significantly far from equilibrium, leads to incomplete relaxation. In this
paper we revisit the problem, by comparing the detailed properties of a family
of distribution functions derived from statistical mechanics arguments to those
of the products of collisionless collapse found in N-body simulations.Comment: 4 pages, 2 figures, to appear in the proceedings of the 39th
Rencontres de Moriond, "Exploring the Universe," La Thuile, Italy, March 28 -
April 4, 200
Dynamical properties of a family of collisionless models of elliptical galaxies
N-body simulations of collisionless collapse have offered important clues to
the construction of realistic stellar dynamical models of elliptical galaxies.
Such simulations confirm and quantify the qualitative expectation that rapid
collapse of a self-gravitating collisionless system, initially cool and
significantly far from equilibrium, leads to incomplete relaxation, that is to
a quasi-equilibrium configuration characterized by isotropic, quasi-Maxwellian
distribution of stellar orbits in the inner regions and by radially biased
anisotropic pressure in the outer parts. In earlier studies, as illustrated in
a number of papers several years ago (see Bertin et al. 1993 and references
therein), the attention was largely focused on the successful comparison
between the models (constructed under the qualitative clues offered by the
N-body simulations mentioned above) and the observations. In this paper we
revisit the problem of incomplete violent relaxation, by making a direct
comparison between the detailed properties of a family of distribution
functions and those of the products of collisionless collapse found in N-body
simulations.Comment: to appear in "Plasmas in the Laboratory and in the Universe: new
insights and new challenges", G. Bertin, D. Farina, R. Pozzoli eds., AIP
Conference Proceedings, Vol. XXX, pp. YY
Thermodynamical description of a family of partially relaxed stellar systems
We examine the thermodynamical properties of a family of partially relaxed,
anisotropic stellar systems, derived earlier from the Boltzmann entropy under
the assumption that a third quantity Q is conserved in addition to the total
energy and the total number of stars. We now show that the family of models
conforms to the paradigm of the gravothermal catastrophe, which is expected to
occur (in the presence of adequate energy transport mechanisms) when the
one-parameter equilibrium sequence attains sufficiently high values of the
concentration parameter; these are the values for which the models are well
fitted by the R^(1/4) law. In the intermediate concentration regime the models
belonging to the sequence exhibit significant deviations from the R^(1/4) law.
Curiously, in the low-concentration regime, the global thermodynamical
temperature associated with the models becomes negative when the models become
too anisotropic so that they are unstable against the radial orbit instability;
this latter behavior, while offering a new clue to the physical interpretation
of the radial orbit instability, is at variance with respect to the
low-concentration limit of the classical case of the isotropic, isothermal
sphere investigated by Bonnor (1956) and Lynden-Bell & Wood (1968).Comment: 16 pages, 4 figures, accepted for publication in The Astrophysical
Journal (Part 1
Expanded Search for z~10 Galaxies from HUDF09, ERS, and CANDELS Data: Evidence for Accelerated Evolution at z>8?
We search for z~10 galaxies over ~160 arcmin^2 of WFC3/IR data in the Chandra
Deep Field South, using the public HUDF09, ERS, and CANDELS surveys, that reach
to 5sigma depths ranging from 26.9 to 29.4 in H_160 AB mag. z>~9.5 galaxy
candidates are identified via J_125-H_160>1.2 colors and non-detections in any
band blueward of J_125. Spitzer IRAC photometry is key for separating the
genuine high-z candidates from intermediate redshift (z~2-4) galaxies with
evolved or heavily dust obscured stellar populations. After removing 16 sources
of intermediate brightness (H_160~24-26 mag) with strong IRAC detections, we
only find one plausible z~10 galaxy candidate in the whole data set, previously
reported in Bouwens et al. (2011). The newer data cover a 3x larger area and
provide much stronger constraints on the evolution of the UV luminosity
function (LF). If the evolution of the z~4-8 LFs is extrapolated to z~10, six
z~10 galaxies are expected in our data. The detection of only one source
suggests that the UV LF evolves at an accelerated rate before z~8. The
luminosity density is found to increase by more than an order of magnitude in
only 170 Myr from z~10 to z~8. This increase is >=4x larger than expected from
the lower redshift extrapolation of the UV LF. We are thus likely witnessing
the first rapid build-up of galaxies in the heart of cosmic reionization.
Future deep HST WFC3/IR data, reaching to well beyond 29 mag, can enable a more
robust quantification of the accelerated evolution around z~10.Comment: 13 pages, 11 figures, ApJ resubmitted after referee repor
First Frontier Field Constraints on the Cosmic Star-Formation Rate Density at z~10 - The Impact of Lensing Shear on Completeness of High-Redshift Galaxy Samples
We search the complete Hubble Frontier Field dataset of Abell 2744 and its
parallel field for z~10 sources to further refine the evolution of the cosmic
star-formation rate density (SFRD) at z>8. We independently confirm two images
of the recently discovered triply-imaged z~9.8 source by Zitrin et al. (2014)
and set an upper limit for similar z~10 galaxies with red colors of
J_125-H_160>1.2 in the parallel field of Abell 2744. We utilize extensive
simulations to derive the effective selection volume of Lyman-break galaxies at
z~10, both in the lensed cluster field and in the adjacent parallel field.
Particular care is taken to include position-dependent lensing shear to
accurately account for the expected sizes and morphologies of highly-magnified
sources. We show that both source blending and shear reduce the completeness at
a given observed magnitude in the cluster, particularly near the critical
curves. These effects have a significant, but largely overlooked, impact on the
detectability of high-redshift sources behind clusters, and substantially
reduce the expected number of highly-magnified sources. The detections and
limits from both pointings result in a SFRD which is higher by 0.4+-0.4 dex
than previous estimates at z~10 from blank fields. Nevertheless, the
combination of these new results with all other estimates remain consistent
with a rapidly declining SFRD in the 170 Myr from z~8 to z~10 as predicted by
cosmological simulations and dark-matter halo evolution in LambdaCDM. Once
biases introduced by magnification-dependent completeness are accounted for,
the full six cluster and parallel Frontier Field program will be an extremely
powerful new dataset to probe the evolution of the galaxy population at z>8
before the advent of the JWST.Comment: 10 pages, 7 figures, changed to match accepted version to appear in
Ap
The dynamical state of the Globular Cluster M10 (NGC 6254)
Studying the radial variation of the stellar mass function in globular
clusters (GCs) has proved a valuable tool to explore the collisional dynamics
leading to mass segregation and core collapse. In order to study the radial
dependence of the luminosity and mass function of M 10, we used ACS/HST deep
high resolution archival images, reaching out to approximately the cluster's
half-mass radius (rhm), combined with deep WFPC2 images that extend our radial
coverage to more than 2 rhm. From our photometry, we derived a radial mass
segregation profile and a global mass function that we compared with those of
simulated clusters containing different energy sources (namely hard binaries
and/or an IMBH) able to halt core collapse and to quench mass segregation. A
set of direct N-body simulations of GCs, with and without an IMBH of mass 1% of
the total cluster mass, comprising different initial mass functions (IMFs) and
primordial binary fractions, was used to predict the observed mass segregation
profile and mass function. The mass segregation profile of M 10 is not
compatible with cluster models without either an IMBH or primordial binaries,
as a source of energy appears to be moderately quenching mass segregation in
the cluster. Unfortunately, the present observational uncertainty on the binary
fraction in M10 does not allow us to confirm the presence of an IMBH in the
cluster, since an IMBH, a dynamically non-negligible binary fraction (~ 5%), or
both can equally well explain the radial dependence of the cluster mass
function.Comment: 15 pages, 8 figures, accepted for publication on Ap
On the Fundamental Line of Galactic and Extragalactic Globular Clusters
In a previous paper we found that the Globular Clusters of our Galaxy lie
around a line in the log(Re), SBe, log(sigma) parameter space, with a moderate
degree of scatter and remarkable axi-symmetry. This implies the existence of a
purely photometric scaling law obtained by projecting such a line onto the
log(Re), SBe plane. Such photometric quantities are readily available for large
samples of clusters, as opposed to stellar velocity dispersion data. We study a
sample of 129 Galactic and extragalactic clusters on such photometric plane in
the V-band. We look for a linear relation between SBe and log(Re) and study how
the scatter around it is influenced by age and dynamical environment. We
interpret our results as a test on the evolutionary versus primordial origin of
the Fundamental Line. We perform a detailed analysis of surface brightness
profiles, which allows us to present a catalogue of structural properties,
without relying on a given dynamical model. We find a linear relation between
SBe and log(Re), in the form SBe = (5.25 +- 0.44) log(Re) + (15.58 +- 0.28),
where SBe is measured in mag/arcsec^2 and Re in parsec. Both young and old
clusters lie on the scaling law, with a scatter of approximately 1 mag in SBe.
However, young clusters display more scatter and a clear trend of such scatter
with age, which old clusters do not. Such trend becomes tighter if cluster age
is measured in units of the cluster half-light relaxation time. Two-body
relaxation therefore plays a major role, together with passive stellar
population evolution, in shaping the relation between SBe, log(Re), and cluster
age. We argue that the log(Re)-SBe relation and hence the Fundamental Line
scaling law is not primordially set at cluster formation, but rather is the
result of combined stellar evolution and collisional dynamical evolution.Comment: Accepted for publication on Astronomy and Astrophysics, official
acceptance date November 2, 200
Probing the Dawn of Galaxies at z~9-12: New Constraints from HUDF12/XDF and CANDELS Data
We present a comprehensive analysis of z>8 galaxies based on ultra-deep
WFC3/IR data. We constrain the evolution of the UV luminosity function (LF) and
luminosity densities from z~11 to z~8 by exploiting all the WFC3/IR data over
the Hubble Ultra-Deep Field from the HUDF09 and the new HUDF12 program, in
addition to the HUDF09 parallel field data, as well as wider area WFC3/IR
imaging over GOODS-South. Galaxies are selected based on the Lyman Break
Technique in three samples centered around z~9, z~10 and z~11, with seven z~9
galaxy candidates, and one each at z~10 and z~11. We confirm a new z~10
candidate (with z=9.8+-0.6) that was not convincingly identified in our first
z~10 sample. The deeper data over the HUDF confirms all our previous z>~7.5
candidates as genuine high-redshift candidates, and extends our samples to
higher redshift and fainter limits (H_160~29.8 mag). We perform one of the
first estimates of the z~9 UV LF and improve our previous constraints at z~10.
Extrapolating the lower redshift UV LF evolution should have revealed 17 z~9
and 9 z~10 sources, i.e., a factor ~3x and 9x larger than observed. The
inferred star-formation rate density (SFRD) in galaxies above 0.7 M_sun/yr
decreases by 0.6+-0.2 dex from z~8 to z~9, in good agreement with previous
estimates. The low number of sources found at z>8 is consistent with a very
rapid build-up of galaxies across z~10 to z~8. From a combination of all
current measurements, we find a best estimate of a factor 10x decrease in the
SFRD from z~8 to z~10, following (1+z)^(-11.4+-3.1). Our measurements thus
confirm our previous finding of an accelerated evolution beyond z~8, and
signify a rapid build-up of galaxies with M_UV<-17.7 within only ~200 Myr from
z~10 to z~8, in the heart of cosmic reionization.Comment: 21 pages, 13 figures, 6 tables; submitted to Ap
Chemical defence by sterols in the freshwater ciliate Stentor polymorphus
Heterotrich ciliates typically retain toxic substances in specialized ejectable organelles, called extrusomes, which are used in predator-prey interactions. In this study, we analysed the chemical defence strategy of the freshwater heterotrich ciliate Stentor polymorphus against the predatory ciliate Coleps hirtus, and the microturbellarian flatworm Stenostomum sphagnetorum. The results showed that S. polymorphus is able to defend itself against these two predators by deploying a mix of bioactive sterols contained in its extrusomes. Sterols were isolated in vivo and characterized by liquid chromatography-mass spectrometry (LC-MS), and nuclear magnetic resonance (NMR), as ergosterol, 7-dehydroporiferasterol, and their two peroxidized analogues. The assessment of the toxicity of ergosterol and ergosterol peroxide against various organisms, indicated that these sterols are essential for the effectiveness of the chemical defence in S. polymorphus
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