4,177 research outputs found
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
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
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
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 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
Leading Order Calculation of Shear Viscosity in Hot Quantum Electrodynamics from Diagrammatic Methods
We compute the shear viscosity at leading order in hot Quantum
Electrodynamics. Starting from the Kubo relation for shear viscosity, we use
diagrammatic methods to write down the appropriate integral equations for
bosonic and fermionic effective vertices. We also show how Ward identities can
be used to put constraints on these integral equations. One of our main results
is an equation relating the kernels of the integral equations with functional
derivatives of the full self-energy; it is similar to what is obtained with
two-particle-irreducible effective action methods. However, since we use Ward
identities as our starting point, gauge invariance is preserved. Using these
constraints obtained from Ward identities and also power counting arguments, we
select the necessary diagrams that must be resummed at leading order. This
includes all non-collinear (corresponding to 2 to 2 scatterings) and collinear
(corresponding to 1+N to 2+N collinear scatterings) rungs responsible for the
Landau-Pomeranchuk-Migdal effect. We also show the equivalence between our
integral equations obtained from quantum field theory and the linearized
Boltzmann equations of Arnold, Moore and Yaffe obtained using effective kinetic
theory.Comment: 45 pages, 22 figures (note that figures 7 and 14 are downgraded in
resolution to keep this submission under 1000kb, zoom to see them correctly
Simulated observations of sub-millimetre galaxies: the impact of single-dish resolution and field variance
Recent observational evidence suggests that the coarse angular resolution [âŒ20 arcsec full width at half maximum (FWHM)] of single-dish telescopes at sub-mm wavelengths has biased the observed galaxy number counts by blending together the sub-mm emission from multiple sub-mm galaxies (SMGs). We use lightcones computed from an updated implementation of the galform semi-analytic model to generate 50 mock sub-mm surveys of 0.5 deg2 at 850 ÎŒm, taking into account the effects of the finite single-dish beam in a more accurate way than has been done previously. We find that blending of SMGs does lead to an enhancement of source extracted number counts at bright fluxes (S850âÎŒm âł 1 mJy). Typically, three to six galaxies contribute 90âperâcent of the flux of an S850âÎŒm = 5 mJy source and these blended galaxies are physically unassociated. We find that field-to-field variations are comparable to Poisson fluctuations for our S850âÎŒm > 5 mJy SMG population, which has a median redshift z50 = 2.0, but are greater than Poisson for the S850âÎŒm > 1 mJy population (z50 = 2.8). In a detailed comparison to a recent interferometric survey targeted at single-dish detected sources, we reproduce the difference between single-dish and interferometer number counts and find a median redshift (z50 = 2.5) in excellent agreement with the observed value (z50 = 2.5 ± 0.2). We also present predictions for single-dish survey number counts at 450 and 1100 ÎŒm, which show good agreement with observational data
Reexamination of the galaxy formation-regulated gas evolution model in groups and clusters
As an alternative explanation of the entropy excess and the steepening of the
X-ray luminosity-temperature relation in groups and clusters, the galaxy
formation-regulated gas evolution (GG) model proposed recently by Bryan makes
an attempt to incorporate the formation of galaxies into the evolution of gas
without additional heating by nongravitational processes. This seems to provide
a unified scheme for our understanding of the structures and evolution of both
galaxies and gas in groups and clusters. In this paper, we present an extensive
comparison of the X-ray properties of groups and clusters predicted by the GG
model and those revealed by current X-ray observations, using various large
data sources in the literature and also taking the observational selection
effects into account. These include an independent check of the fundamental
working hypothesis of the GG model, i.e., galaxy formation was less efficient
in rich clusters than in groups, a new test of the radial gas distributions
revealed by both the gas mass fraction and the X-ray surface brightness
profiles, and an reexamination of the X-ray luminosity-temperature and
entropy-temperature relations. In particular, it shows that the overall X-ray
surface brightness profiles predicted by the GG model are very similar in
shape, insensitive to the X-ray temperature, and the shallower X-ray surface
brightness profiles seen at low-temperature systems may arise from the current
observational selection effect. This can be used as the simplest approach to
distinguishing between the GG model and the preheating scenario. The latter
yields an intrinsically shallower gas distribution in groups than in rich
clusters.Comment: 30 pages, 10 figures, accepted for publication in Ap
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