4,738 research outputs found
Initial temperature and EoS of quark matter from direct photons
The time evolution of the quark gluon plasma created in gold-gold collisions
of the Relativistic Heavy Ion Collider (RHIC) can be described by
hydrodynamical models. Distribution of hadrons reflects the freeze-out state of
the matter. To investigate the time evolution one needs to analyze penetrating
probes, such as direct photon spectra. Distributions of low energy photons was
published in 2010 by PHENIX. In this paper we analyze a 3+1 dimensional
solution of relativistic hydrodynamics and calculate momentum distribution of
direct photons. Using earlier fits of this model to hadronic spectra, we
compare photon calculations to measurements and find that the initial
temperature of the center of the fireball is at least 519+-12 MeV, while for
the equation of state we get c_s= 0.36+-0.02.Comment: Talk at the VI Workshop on Particle Correlations and Femtoscopy,
Kiev, September 14-18, 2010. 6 pages, 1 figure. This work was supported by
the OTKA grant NK73143 and M. Csanad's Bolyai scholarshi
Designing a Green Roof for Ireland
A model is presented for the gravity-driven flow of rainwater descending through the soil layer of a green roof, treated as a porous medium on a at permeable surface representing an efficient drainage layer. A fully saturated zone is shown to occur. It is typically a thin layer, relative to the total soil thickness, and lies at the bottom of the soil layer. This provides a bottom boundary condition for the partially saturated upper zone. It is shown that after the onset of rainfall, well-defined fronts of water can descend through the soil layer. Also the rainwater flow is relatively quick compared with the moisture uptake by the roots of the plants in the roof. In a separate model the exchanges of water are described between the (smaller-scale) porous granules of soil, the roots and the rainwater in the inter-granule pores
Blending bias impacts the host halo masses derived from a cross-correlation analysis of bright sub-millimetre galaxies
Placing bright sub-millimetre galaxies (SMGs) within the broader context of
galaxy formation and evolution requires accurate measurements of their
clustering, which can constrain the masses of their host dark matter halos.
Recent work has shown that the clustering measurements of these galaxies may be
affected by a `blending bias,' which results in the angular correlation
function of the sources extracted from single-dish imaging surveys being
boosted relative to that of the underlying galaxies. This is due to confusion
introduced by the coarse angular resolution of the single-dish telescope and
could lead to the inferred halo masses being significantly overestimated. We
investigate the extent to which this bias affects the measurement of the
correlation function of SMGs when it is derived via a cross-correlation with a
more abundant galaxy population. We find that the blending bias is essentially
the same as in the auto-correlation case and conclude that the best way to
reduce its effects is to calculate the angular correlation function using SMGs
in narrow redshift bins. Blending bias causes the inferred host halo masses of
the SMGs to be overestimated by a factor of when a redshift interval of
is used. However, this reduces to a factor of for . The broadening of photometric redshift probability distributions with
increasing redshift can therefore impart a mild halo `downsizing' effect onto
the inferred host halo masses, though this trend is not as strong as seen in
recent observational studies.Comment: 10 pages, 9 figures, 1 table. Accepted to MNRA
Transfer of autocollimator calibration for use with scanning gantry profilometers for accurate determination of surface slope and curvature of state of the art x ray mirrors
X ray optics, desired for beamlines at free electron laser and diffraction limited storage ring x ray light sources, must have almost perfect surfaces, capable of delivering light to experiments without significant degradation of brightness and coherence. To accurately characterize such optics at an optical metrology lab, two basic types of surface slope profilometers are used the long trace profilers LTPs and nanometer optical measuring NOM like angular deflectometers, based on electronic autocollimator AC ELCOMAT 3000. The inherent systematic errors of the instrument s optical sensors set the principle limit to their measuring performance. Where autocollimator of a NOM like profiler may be calibrated at a unique dedicated facility, this is for a particular configuration of distance, aperture size, and angular range that does not always match the exact use in a scanning measurement with the profiler. Here we discuss the developed methodology, experimental set up, and numerical methods of transferring the calibration of one reference AC to the scanning AC of the Optical Surface Measuring System OSMS , recently brought to operation at the ALS Xray Optics Laboratory. We show that precision calibration of the OSMS performed in three steps, allows us to provide high confidence and accuracy low spatial frequency metrology and not print into measurements the inherent systematic error of tool in use. With the examples of the OSMS measurements with a state of the art x ray aspherical mirror, available from one of the most advanced vendors of X ray optics, we demonstrate the high efficacy of the developed calibration procedure. The results of our work are important for obtaining high reliability data, needed for sophisticated numerical simulations of beamline performance and optimization of beamline usage of the optics. This work was supported by the U. S. Department of Energy under contract number DE AC02 05CH1123
The far infra-red SEDs of main sequence and starburst galaxies
We compare observed far infra-red/sub-millimetre (FIR/sub-mm) galaxy spectral
energy distributions (SEDs) of massive galaxies (
M) derived through a stacking analysis with predictions from
a new model of galaxy formation. The FIR SEDs of the model galaxies are
calculated using a self-consistent model for the absorption and re-emission of
radiation by interstellar dust based on radiative transfer calculations and
global energy balance arguments. Galaxies are selected based on their position
on the specific star formation rate (sSFR) - stellar mass () plane.
We identify a main sequence of star-forming galaxies in the model, i.e. a well
defined relationship between sSFR and , up to redshift . The
scatter of this relationship evolves such that it is generally larger at higher
stellar masses and higher redshifts. There is remarkable agreement between the
predicted and observed average SEDs across a broad range of redshifts
() for galaxies on the main sequence. However, the
agreement is less good for starburst galaxies at , selected here to
have elevated sSFRs the main sequence value. We find that the
predicted average SEDs are robust to changing the parameters of our dust model
within physically plausible values. We also show that the dust temperature
evolution of main sequence galaxies in the model is driven by star formation on
the main sequence being more burst-dominated at higher redshifts.Comment: 20 pages, 13 figures. Accepted to MNRA
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
A Mathematical Model for Flash Sintering
A mathematical model is presented for the Joule heating that occurs in a
ceramic powder compact during the process of flash sintering. The ceramic is
assumed to have an electrical conductivity that increases with temperature, and
this leads to the possibility of runaway heating that could facilitate and
explain the rapid sintering seen in experiments. We consider reduced models
that are sufficiently simple to enable concrete conclusions to be drawn about
the mathematical nature of their solutions. In particular we discuss how
different local and non-local reaction terms, which arise from specified
experimental conditions of fixed voltage and current, lead to thermal runaway
or to stable conditions. We identify incipient thermal runaway as a necessary
condition for the flash event, and hence identify the conditions under which
this is likely to occur.Comment: 14 pages, 9 figure
A Collision of Subclusters in Abell 754
We present direct evidence of a collision of subclusters in the galaxy
cluster Abell 754. Our comparison of new optical data and archival ROSAT PSPC
X-ray data reveal three collision signatures predicted by n-body/hydrodynamical
simulations of hierarchical cluster evolution. First, there is strong evidence
of a non-hydrostatic process; neither of the two major clumps in the galaxy
distribution lies on the off-center peak of the X-ray emission from the
intracluster gas. Second, the peak of the X-ray emission is elongated
perpendicular to the collision axis defined by the centroids of the two galaxy
clumps. Third, there is evidence of compression-heated gas; one of A754's two
X-ray temperature components (Henry & Briel 1995) is among the hottest observed
in any cluster and hotter than that inferred from the velocity dispersion of
the associated galaxy clump. These signatures are consistent with the
qualitative features of simulations (Evrard 1990a,b) in which two subclusters
have collided in the plane of the sky during roughly the last Gyr. The
detection of such collisions is crucial for understanding both the dynamics of
individual clusters and the underlying cosmology. First, for systems like A754,
estimating the cluster X-ray mass from assumptions of hydrostatic equilibrium
and isothermality is incorrect and may produce the discrepancies sometimes
found between X-ray masses and those derived from gravitational lens models
(Babul & Miralda-Escude 1994). Second, the fraction of nearby clusters in which
subclusters have collided in the last Gyr is especially sensitive to the mean
mass density parameter Omega_0 (cf. Richstone et al. 1992; Evrard et al. 1993;
Lacey & Cole 1993). With a large, well-defined cluster sample, it will be
possible to place a new and powerful constraint on cosmological models.Comment: 4 pages + 1 color figure (Postscript). Accepted for Publication in
ApJ Letter
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