1,484 research outputs found
Muons tomography applied to geosciences and volcanology
Imaging the inner part of large geological targets is an important issue in
geosciences with various applications. Dif- ferent approaches already exist
(e.g. gravimetry, electrical tomography) that give access to a wide range of
informations but with identified limitations or drawbacks (e.g. intrinsic
ambiguity of the inverse problem, time consuming deployment of sensors over
large distances). Here we present an alternative and complementary tomography
method based on the measurement of the cosmic muons flux attenuation through
the geological structures. We detail the basics of this muon tomography with a
special emphasis on the photo-active detectors.Comment: Invited talk at the 6th conference on New Developments In
Photodetection (NDIP'11), Lyon-France, July 4-8, 2011; Nuclear Instruments
and Methods in Physics Research Section A, 201
Astrophysics: Most distant cosmic blast seen
The most distant -ray burst yet sighted is the earliest astronomical object
ever observed in cosmic history. This ancient beacon offers a glimpse of the
little-known cosmic dark ages.Comment: Published in Nature News & View
The effect of beam inclination on the performance of a passive vibration isolator using buckled beams
Passive vibration isolators are desired to have both high static stiffness to support large static load and low local stiffness to reduce the displacement transmissibility at frequencies greater than resonance. Utilization of a vertical buckled beam as a spring component is one way to realize such a stiffness characteristic since it exhibits a smaller ratio of local stiffness to static stiffness than that of a linear spring. This paper investigates the behaviour of a vibration isolator using inclined beams as well as vertical ones and examines the effect of beam inclination for the purpose of improving the isolation performance. The experimental system investigated has anisolated mass which is supported by a combination of two types of beams: buckled beams and constraining beams. The buckled beams can be inclined from the vertical by attachment devices, and the constraining beams are employed to prevent off-axis motion of the isolated mass. The results demonstrate that the inclination of the buckled beams reduces the resonance frequency and improves the displacement transmissibility at frequencies greater than resonance
Hyperfine Populations Prior to Muon Capture
It is shown that the 1S level hyperfine populations prior to muon capture
will be statistical when either target or beam are unpolarised independent of
the atomic level at which the hyperfine interaction becomes appreciable. This
assertion holds in the absence of magnetic transitions during the cascade and
is true because of minimal polarisation after atomic capture and selective
feeding during the cascade.Comment: (revtex, 6 preprint pages, no figures
The History of Cosmological Star Formation: Three Independent Approaches and a Critical Test Using the Extragalactic Background Light
Taking three independent approaches, we investigate the simultaneous
constraints set on the cosmic star formation history from various observations,
including stellar mass density and extragalactic background light (EBL). We
compare results based on: 1) direct observations of past light-cone, 2) a model
using local fossil evidence constrained by SDSS observations at z~0 (the
`Fossil' model), and 3) theoretical ab initio models from three calculations of
cosmic star formation history: (a) new (1024)^3 Total Variation Diminishing
(TVD) cosmological hydrodynamic simulation, (b) analytic expression of
Hernquist & Springel based on cosmological Smoothed Particle Hydrodynamics
(SPH) simulations, and (c) semi-analytic model of Cole et al. We find good
agreement among the three independent approaches up to the order of
observational errors, except that all the models predict bolometric EBL of
I_tot ~= 37-52 nW m^-2 sr^-1, which is at the lower edge of the the
observational estimate by Hauser & Dwek. We emphasize that the Fossil model
that consists of two components -- spheroids and disks --, when normalized to
the local observations, provides a surprisingly simple but accurate description
of the cosmic star formation history and other observable quantities. Our
analysis suggests that the consensus global parameters at z=0 are: Omega_* =
0.0023+-0.0004, I_EBL = 43+-7 nW m^-2 sr^-1 rho_SFR=(1.06+-0.22)e-2 Msun yr^-1
Mpc^-3, j_bol = (3.1+-0.2)e8 Lsun Mpc^-3.Comment: 40 page, 10 figures. ApJ in press. Matched to the accepted versio
The K20 survey. VI. The Distribution of the Stellar Masses in Galaxies up to z~2
We present a detailed analysis of the stellar mass content of galaxies up to
z=2.5 in the K20 galaxy sample, that has a 92% spectroscopic completeness and a
complete multicolor coverage. We find that the M/L ratio decreases
with redshift: in particular, the average M/L ratio of early type galaxies
decreases with , with a scatter that is indicative of a range of
star--formation time-scales and redshift of formation. More important, the
typical M/L of massive early type galaxies is larger than that of less massive
ones, suggesting that their stellar population formed at higher z. The final
K20 galaxy sample spans a range of stellar masses from M*=10^9Msun to
M*=10^12Msun, with massive galaxies ($M*>10^11Msun) detected up to z~2. We
compute the Galaxy Stellar Mass Function at various z, of which we observe only
a mild evolution (i.e. by 20-30%) up to z~1. At z>1, the evolution of the GSMF
appears to be much faster: at z~2, about 35% of the present day stellar mass in
objects with M*~10^11Msun appear to have assembled. We also detect a change in
the physical nature of the most massive galaxies, since at z>1 a population of
massive star--forming galaxies progressively appears. We finally analyze our
results in the framework of Lambda-CDM hierarchical models. First, we show that
the large number of massive galaxies detected at high z does not violate any
fundamental Lambda-CDM constraint based on the number of massive DM halos.
Then, we compare our results with the predictions of renditions of both
semianalytic and hydro-dynamical models, that range from severe underestimates
to slight overestimates of the observed mass density at z<~2. We discuss how
the differences among these models are due to the different implementation of
the main physical processes. (Abridged)Comment: Accepted for publication on Astronomy & Astrophysic
Properties of Disks and Bulges of Spiral and Lenticular Galaxies in the Sloan Digital Sky Survey
A bulge-disk decomposition is made for 737 spiral and lenticular galaxies
drawn from a SDSS galaxy sample for which morphological types are estimated. We
carry out the bulge-disk decomposition using the growth curve fitting method.
It is found that bulge properties, effective radius, effective surface
brightness, and also absolute magnitude, change systematically with the
morphological sequence; from early to late types, the size becomes somewhat
larger, and surface brightness and luminosity fainter. In contrast disks are
nearly universal, their properties remaining similar among disk galaxies
irrespective of detailed morphologies from S0 to Sc. While these tendencies
were often discussed in previous studies, the present study confirms them based
on a large homogeneous magnitude-limited field galaxy sample with morphological
types estimated. The systematic change of bulge-to-total luminosity ratio,
, along the morphological sequence is therefore not caused by disks but
mostly by bulges. It is also shown that elliptical galaxies and bulges of
spiral galaxies are unlikely to be in a single sequence. We infer the stellar
mass density (in units of the critical mass density) to be 0.0021 for
spheroids, i.e., elliptical galaxies plus bulges of spiral galaxies, and
0.00081 for disks.Comment: 30 pages, 9 figure
The Galaxy Mass Function up to z=4 in the GOODS-MUSIC sample: into the epoch of formation of massive galaxies
The goal of this work is to measure the evolution of the Galaxy Stellar Mass
Function and of the resulting Stellar Mass Density up to redshift ~4, in order
to study the assembly of massive galaxies in the high redshift Universe. We
have used the GOODS-MUSIC catalog, containing ~3000 Ks-selected galaxies with
multi-wavelength coverage extending from the U band to the Spitzer 8 micron
band, of which 27% have spectroscopic redshifts and the remaining fraction have
accurate photometric redshifts. On this sample we have applied a standard
fitting procedure to measure stellar masses. We compute the Galaxy Stellar Mass
Function and the resulting Stellar Mass Density up to redshift ~4, taking into
proper account the biases and incompleteness effects. Within the well known
trend of global decline of the Stellar Mass Density with redshift, we show that
the decline of the more massive galaxies may be described by an exponential
timescale of ~6 Gyrs up to z~1.5, and proceeds much faster thereafter, with an
exponential timescale of ~0.6 Gyrs. We also show that there is some evidence
for a differential evolution of the Galaxy Stellar Mass Function, with low mass
galaxies evolving faster than more massive ones up to z~1-1.5 and that the
Galaxy Stellar Mass Function remains remarkably flat (i.e. with a slope close
to the local one) up to z~1-1.3. The observed behaviour of the Galaxy Stellar
Mass Function is consistent with a scenario where about 50% of present-day
massive galaxies formed at a vigorous rate in the epoch between redshift 4 and
1.5, followed by a milder evolution until the present-day epoch.Comment: accepted for publication in A&A. Uses aa.cls, 15 pages, 11 figures.
The observed mass functions are available in electronic form at
http://lbc.oa-roma.inaf.it/goods/massfunctio
Topology of structure in the Sloan Digital Sky Survey: model testing
We measure the three-dimensional topology of large-scale structure in the
Sloan Digital Sky Survey (SDSS). This allows the genus statistic to be measured
with unprecedented statistical accuracy. The sample size is now sufficiently
large to allow the topology to be an important tool for testing galaxy
formation models. For comparison, we make mock SDSS samples using several
state-of-the-art N-body simulations: the Millennium run of Springel et al.
(2005)(10 billion particles), Kim & Park (2006) CDM models (1.1 billion
particles), and Cen & Ostriker (2006) hydrodynamic code models (8.6 billion
cell hydro mesh). Each of these simulations uses a different method for
modeling galaxy formation. The SDSS data show a genus curve that is broadly
characteristic of that produced by Gaussian random phase initial conditions.
Thus the data strongly support the standard model of inflation where Gaussian
random phase initial conditions are produced by random quantum fluctuations in
the early universe. But on top of this general shape there are measurable
differences produced by non-linear gravitational effects (cf. Matsubara 1994),
and biasing connected with galaxy formation. The N-body simulations have been
tuned to reproduce the power spectrum and multiplicity function but not
topology, so topology is an acid test for these models. The data show a
``meatball'' shift (only partly due to the Sloan Great Wall of Galaxies; this
shift also appears in a sub-sample not containing the Wall) which differs at
the 2.5\sigma level from the results of the Millennium run and the Kim & Park
dark halo models, even including the effects of cosmic variance.Comment: 13 Apj pages, 7 figures High-resolution stereo graphic available at
http://www.astro.princeton.edu/~dclayh/stereo50.ep
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