5,954 research outputs found
A lower bound in Nehari's theorem on the polydisc
By theorems of Ferguson and Lacey (d=2) and Lacey and Terwilleger (d>2),
Nehari's theorem is known to hold on the polydisc D^d for d>1, i.e., if H_\psi
is a bounded Hankel form on H^2(D^d) with analytic symbol \psi, then there is a
function \phi in L^\infty(\T^d) such that \psi is the Riesz projection of \phi.
A method proposed in Helson's last paper is used to show that the constant C_d
in the estimate \|\phi\|_\infty\le C_d \|H_\psi\| grows at least exponentially
with d; it follows that there is no analogue of Nehari's theorem on the
infinite-dimensional polydisc
Constraints on the Baryonic Compression and Implications for the Fraction of Dark Halo Lenses
We predict the fraction of dark halo lenses, that is, the fraction of lens
systems produced by the gravitational potential of dark halos, on the basis of
a simple parametric model of baryonic compression. The fraction of dark halo
lenses primarily contains information on the effect of baryonic compression and
the density profile of dark halos, and is expected to be insensitive to
cosmological parameters and source population. The model we adopt comprises the
galaxy formation probability p_g(M) which describes the global efficiency of
baryonic compression and the ratio of circular velocities of galaxies to virial
velocities of dark halos gamma_v=v_c/v_{vir} which means how the inner
structure of dark halos is modified due to baryonic compression. The model
parameters are constrained from the velocity function of galaxies and the
distribution of image separations in gravitational lensing, although the
degeneracy between model parameters still remains. We show that the fraction of
dark halo lenses depends strongly on gamma_v and the density profile of dark
halos such as inner slope alpha. This means that the observation of the
fraction of dark halos can break the degeneracy between model parameters if the
density profile of dark halo lenses is fully settled. On the other hand, by
restricting gamma_v to physically plausible range we can predict the lower
limit of the fraction of dark halo lenses on the basis of our model. Our result
indicates that steeper inner cusps of dark halos (alpha >~ 1.5) or too
centrally concentrated dark halos are inconsistent with the lack of dark halo
lenses in observations.Comment: 10 pages, 9 figures, emulateapj5, accepted for publication in Ap
Dark-Halo Cusp: Asymptotic Convergence
We propose a model for how the buildup of dark halos by merging satellites
produces a characteristic inner cusp, of a density profile \rho \prop r^-a with
a -> a_as > 1, as seen in cosmological N-body simulations of hierarchical
clustering scenarios. Dekel, Devor & Hetzroni (2003) argue that a flat core of
a<1 exerts tidal compression which prevents local deposit of satellite
material; the satellite sinks intact into the halo center thus causing a rapid
steepening to a>1. Using merger N-body simulations, we learn that this cusp is
stable under a sequence of mergers, and derive a practical tidal mass-transfer
recipe in regions where the local slope of the halo profile is a>1. According
to this recipe, the ratio of mean densities of halo and initial satellite
within the tidal radius equals a given function psi(a), which is significantly
smaller than unity (compared to being 1 according to crude resonance criteria)
and is a decreasing function of a. This decrease makes the tidal mass transfer
relatively more efficient at larger a, which means steepening when a is small
and flattening when a is large, thus causing converges to a stable solution.
Given this mass-transfer recipe, linear perturbation analysis, supported by toy
simulations, shows that a sequence of cosmological mergers with homologous
satellites slowly leads to a fixed-point cusp with an asymptotic slope a_as>1.
The slope depends only weakly on the fluctuation power spectrum, in agreement
with cosmological simulations. During a long interim period the profile has an
NFW-like shape, with a cusp of 1<a<a_as. Thus, a cusp is enforced if enough
compact satellite remnants make it intact into the inner halo. In order to
maintain a flat core, satellites must be disrupted outside the core, possibly
as a result of a modest puffing up due to baryonic feedback.Comment: 37 pages, Latex, aastex.cls, revised, ApJ, 588, in pres
Há alternativas ao uso dos transgênicos?
A existĂŞncia ou nĂŁo de alternativa ao uso de transgĂŞnicos capaz de satisfazer a demanda mundial por alimento e nutrientes Ă© uma questĂŁo que permanece aberta Ă investigação cientĂfica.A importância dos transgĂŞnicos ainda nĂŁo está bem fundamentada no conhecimento cientĂfico disponĂvel,em parte porque as conquistas e o potencial da agroecologia nĂŁo foram objeto de atenção cientĂfica suficiente
The effect of gravitational-wave recoil on the demography of massive black holes
The coalescence of massive black hole (MBH) binaries following galaxy mergers
is one of the main sources of low-frequency gravitational radiation. A
higher-order relativistic phenomenon, the recoil as a result of the non-zero
net linear momentum carried away by gravitational waves, may have interesting
consequences for the demography of MBHs at the centers of galaxies. We study
the dynamics of recoiling MBHs and its observational consequences. The
``gravitational rocket'' may: i) deplete MBHs from late-type spirals, dwarf
galaxies, and stellar clusters; ii) produce off-nuclear quasars, including
unusual radio morphologies during the recoil of a radio-loud source; and iii)
give rise to a population of interstellar and intergalactic MBHs.Comment: emulateapj, 5 pages, 2 figures, to appear in the ApJ Letter
The mass function
We present the mass functions for different mass estimators for a range of
cosmological models. We pay particular attention to how universal the mass
function is, and how it depends on the cosmology, halo identification and mass
estimator chosen. We investigate quantitatively how well we can relate observed
masses to theoretical mass functions.Comment: 14 pages, 12 figures, to appear in ApJ
Complexity of diatom response to Lateglacial and Holocene climate and environmental change in ancient, deep and oligotrophic Lake Ohrid (Macedonia and Albania)
© Author(s) 2016. Lake Ohrid (Macedonia and Albania) is a rare example of a deep, ancient Mediterranean lake and is a key site for palaeoclimate research in the northeastern Mediterranean region. This study conducts the analysis of diatoms as a proxy for Lateglacial and Holocene climate and environmental change in Lake Ohrid at a higher resolution than in previous studies. While Lake Ohrid has the potential to be sensitive to water temperature change, the data demonstrate a highly complex diatom response, probably comprising a direct response to temperature-induced lake productivity in some phases and an indirect response to temperaturerelated lake stratification or mixing and epilimnetic nutrient availability in others. The data also demonstrate the possible influence of physical limnological (e.g. the influence of wind stress on stratification or mixing) and chemical processes (e.g. the influence of catchment dynamics on nutrient input) in mediating the complex response of diatoms. During the Lateglacial (ca. 12 300-11 800 cal yr BP), the low-diversity dominance of hypolimnetic Cyclotella fottii indicates low lake productivity, linked to low water temperature. Although the subsequent slight increase in small, epilimnetic C. minuscula during the earliest Holocene (ca. 11 800-10 600 cal yr BP) suggests climate warming and enhanced stratification, diatom concentration remains as low as during the Lateglacial, suggesting that water temperature increase was muted across this major transition. The early Holocene (ca. 10 600-8200 cal yr BP) is characterised by a sustained increase in epilimnetic taxa, with mesotrophic C. ocellata indicating high water-temperature-induced productivity between ca. 10 600-10 200 cal yr BP and between ca. 9500-8200 cal yr BP and with C. minuscula in response to low nutrient availability in the epilimnion between ca. 10 200-9500 cal yr BP. During the middle Holocene (ca. 8200-2600 cal yr BP), when sedimentological and geochemical proxies provide evidence for maximum Holocene water temperature, anomalously low C. ocellata abundance is probably a response to epilimnetic nutrient limitation, almost mimicking the Lateglacial flora apart from the occurrence of mesotrophic Stephanodiscus transylvanicus in the hypolimnion. During the late Holocene (ca. 2600 cal yr BP-present), high abundance and fluctuating composition of epilimnetic taxa are probably a response more to enhanced anthropogenic nutrient input, particularly nitrogen enrichment, than to climate. Overall, the data indicate that previous assumptions concerning the linearity of diatom response in this deep, ancient lake are invalid, and multi-proxy analysis is essential to improve understanding of palaeolimnological dynamics in future research on the long, Quaternary sequence
Flavor ordering of elliptic flows at high transverse momentum
Based on the quark coalescence model for the parton-to-hadron phase
transition in ultra-relativistic heavy ion collisions, we relate the elliptic
flow () of high \pt hadrons to that of high \pt quarks. For high \pt
hadrons produced from an isospin symmetric and quark-antiquark symmetric
partonic matter, magnitudes of their elliptic flows follow a flavor ordering as
if strange quarks have a
smaller elliptic flow than light quarks. The elliptic flows of high \pt
hadrons further follow a simple quark counting rule if strange quarks and light
quarks have same high \pt spectrum and coalescence probability.Comment: 4 pages, 1 figure, revte
Where can we really find the First Stars' Remnants today?
A number of recent numerical investigations concluded that the remnants of
rare structures formed at very high redshift, such as the very first stars and
bright redshift z~6 QSOs, are preferentially located at the center of the most
massive galaxy clusters at redshift z=0. In this paper we readdress this
question using a combination of cosmological simulations of structure formation
and extended Press-Schechter formalism and we show that the typical remnants of
Population III stars are instead more likely to be found in a group
environment, that is in dark matter halos of mass ~2x10^{13} h^{-1}M_sun.
Similarly, the descendants of the brightest z~6 QSOs are expected to be in
medium-sized clusters (mass of a few 10^{14} h^{-1}M_sun), rather than in the
most massive superclusters (M>10^{15} h^{-1}M_sun) found within the typical 1
Gpc^3 cosmic volume where a bright z~6 QSO lives. The origin of past claims
that the most massive clusters preferentially host these remnants is rooted in
the numerical method used to initialize their numerical simulations: Only a
small region of the cosmological volume of interest was simulated with
sufficient resolution to identify low-mass halos at early times, and this
region was chosen to host the most massive halo in the cosmological volume at
late times. The conclusion that the earliest structures formed in the entire
cosmological volume evolve into the most massive halo at late times was thus
arrived at by construction. We demonstrate that, to the contrary, the first
structures to form in a cosmological region evolve into relatively typical
objects at later times. We propose alternative numerical methods for simulating
the earliest structures in cosmological volumes.Comment: 18 pages, 5 figures, ApJ accepted, high resolution version of the
paper available at http://www.stsci.edu/~trenti/papers/halo_evolution.pd
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