188 research outputs found
Outskirts of Distant Galaxies In Absorption
QSO absorption spectroscopy provides a sensitive probe of both the neutral
medium and diffuse ionized gas in the distant Universe. It extends 21cm maps of
gaseous structures around low-redshift galaxies both to lower gas column
densities and to higher redshifts. Combining galaxy surveys with
absorption-line observations of gas around galaxies enables comprehensive
studies of baryon cycles in galaxy outskirts over cosmic time. This Chapter
presents a review of the empirical understanding of the cosmic neutral gas
reservoir from studies of damped Lya absorbers (DLAs). It describes the
constraints on the star formation relation and chemical enrichment history in
the outskirts of distant galaxies from DLA studies. A brief discussion of
available constraints on the ionized circumgalactic gas from studies of lower
column density Lya absorbers and associated ionic absorption transitions is
presented at the end.Comment: 45 pages, 7 figures, invited review, Book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
BBN and the Primordial Abundances
The relic abundances of the light elements synthesized during the first few
minutes of the evolution of the Universe provide unique probes of cosmology and
the building blocks for stellar and galactic chemical evolution, while also
enabling constraints on the baryon (nucleon) density and on models of particle
physics beyond the standard model. Recent WMAP analyses of the CBR temperature
fluctuation spectrum, combined with other, relevant, observational data, has
yielded very tight constraints on the baryon density, permitting a detailed,
quantitative confrontation of the predictions of Big Bang Nucleosynthesis with
the post-BBN abundances inferred from observational data. The current status of
this comparison is presented, with an emphasis on the challenges to astronomy,
astrophysics, particle physics, and cosmology it identifies.Comment: To appear in the Proceedings of the ESO/Arcetri Workshop on "Chemical
Abundances and Mixing in Stars in the Milky Way and its Satellites", eds., L.
Pasquini and S. Randich (Springer-Verlag Series, "ESO Astrophysics Symposia"
A New Approach to Systematic Uncertainties and Self-Consistency in Helium Abundance Determinations
Tests of big bang nucleosynthesis and early universe cosmology require
precision measurements for helium abundance determinations. However, efforts to
determine the primordial helium abundance via observations of metal poor H II
regions have been limited by significant uncertainties. This work builds upon
previous work by providing an updated and extended program in evaluating these
uncertainties. Procedural consistency is achieved by integrating the hydrogen
based reddening correction with the helium based abundance calculation, i.e.,
all physical parameters are solved for simultaneously. We include new atomic
data for helium recombination and collisional emission based upon recent work
by Porter et al. and wavelength dependent corrections to underlying absorption
are investigated. The set of physical parameters has been expanded here to
include the effects of neutral hydrogen collisional emission. Because of a
degeneracy between the solutions for density and temperature, the precision of
the helium abundance determinations is limited. Also, at lower temperatures (T
\lesssim 13,000 K) the neutral hydrogen fraction is poorly constrained
resulting in a larger uncertainty in the helium abundances. Thus the derived
errors on the helium abundances for individual objects are larger than those
typical of previous studies. The updated emissivities and neutral hydrogen
correction generally raise the abundance. From a regression to zero
metallicity, we find Y_p as 0.2561 \pm 0.0108, in broad agreement with the WMAP
result. Tests with synthetic data show a potential for distinct improvement,
via removal of underlying absorption, using higher resolution spectra. A small
bias in the abundance determination can be reduced significantly and the
calculated helium abundance error can be reduced by \sim 25%.Comment: 51 pages, 13 figure
Physical conditions in QSO absorbers from fine-structure absorption lines
We calculate theoretical population ratios of the ground fine-structure
levels of some atoms/ions which typically exhibit UV lines in the spectra of
QSO absorbers redward the Ly-alpha forest: C0, C+, O0, Si+ and Fe+. The most
reliable atomic data available is employed and a variety of excitation
mechanisms considered: collisions with several particles in the medium, direct
excitation by photons from the cosmic microwave background radiation (CMBR) and
fluorescence induced by a UV field present.
The theoretical population ratios are confronted with the corresponding
column density ratios of C I and C II lines observed in damped Ly-alpha (DLA)
and Lyman Limit (LL) systems collected in the recent literature to infer their
physical conditions.
The volumetric density of neutral hydrogen in DLA systems is constrained to
be lower than tens of cm^-3 (or a few cm^-3 in the best cases) and the UV
radiation field intensity must be lower than two orders of magnitude the
radiation field of the Galaxy (one order of magnitude in the best cases). Their
characteristic sizes are higher than a few pc (tens of pc in the best cases)
and lower limits for their total masses vary from 10^0 to 10^5 solar masses.
For the only LL system in our sample, the electronic density is constrained
to be n_e<0.15 cm^-3. We suggest that the fine-structure lines may be used to
discriminate between the current accepted picture of the UV extragalatic
background as the source of ionization in these systems against a local origin
for the ionizing radiation as supported by some authors.
We also investigate the validity of the temperature-redshift relation of the
CMBR predicted by the standard model and study the case for alternative models.Comment: 16 pages, 12 figure
A Homogeneous Sample of Sub-DLAs I: Construction of the Sample and Chemical Abundance Measurements
In this first paper of a series, we report on the use of quasar spectra
obtained with the UVES high resolution spectrograph and available through the
ESO VLT archive to build the first sample of sub-DLA systems, absorbers with HI
column densities > 10^{19} cm^{-2} but lower than the classical definition of
damped Ly-alpha systems (DLAs) 2x10^{20} cm^{-2}. A systematic investigation of
the properties of these systems and a comparison with those of the DLAs (Paper
II of this series; Peroux et al. 2003b) is expected to provide new clues on the
association of high column density absorbers with galaxies and on the overall
evolution of the neutral hydrogen gas mass and metal content in the Universe.
In the spectra of 22 quasars which were found suitable for a sub-DLA search, we
identified 12 sub-DLAs and 1 borderline case between the DLA and sub-DLA
systems in the redshift interval z = 1.8-4.3. We measured the column densities
of HI and up to 16 ions of low-, intermediate- and high-ionization. We further
investigated the significance of the ionization corrections in the
determination of the chemical abundances from the low-ionization ions in the
sub-DLA HI column density range. Using the predictions of different ion ratios
as a function of the ionization parameter computed with the CLOUDY software
package, we have estimated that with the exception of one case, the ionization
corrections to the abundances of 9 systems for which we were able to constrain
the ionization parameter, are lower than 0.2 dex for all of the elements except
AlII and ZnII down to HI column densities of log N(HI) = 19.3 cm^{-2}. We
finally present the first sub-DLA chemical abundance database which contains
the abundance measurements of 11 different elements (O, C, Si, N, S, Mg, Al,
Fe, Ni, Zn, and Cr).Comment: 35 pages, 43 figures, Accepted for publication in MNRAS
(high-resolution figures available on request from the authors or in the
journal
Primordial black holes in braneworld cosmologies: astrophysical constraints
In two recent papers we explored the modifications to primordial black hole
physics when one moves to the simplest braneworld model, Randall--Sundrum type
II. Both the evaporation law and the cosmological evolution of the population
can be modified, and additionally accretion of energy from the background can
be dominant over evaporation at high energies. In this paper we present a
detailed study of how this impacts upon various astrophysical constraints,
analyzing constraints from the present density, from the present high-energy
photon background radiation, from distortion of the microwave background
spectrum, and from processes affecting light element abundances both during and
after nucleosynthesis. Typically, the constraints on the formation rate of
primordial black holes weaken as compared to the standard cosmology if black
hole accretion is unimportant at high energies, but can be strengthened in the
case of efficient accretion.Comment: 17 pages RevTeX4 file with three figures incorporated; final paper in
series astro-ph/0205149 and astro-ph/0208299. Minor changes to match version
accepted by Physical Review
Projeto RiskBenefit4EU - uma estratégia para a avaliação de risco-benefício de alimentos em Portugal
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment
This paper describes an analysis of the angular distribution of W->enu and
W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with
the ATLAS detector at the LHC in 2010, corresponding to an integrated
luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and
the missing transverse energy, the W decay angular distribution projected onto
the transverse plane is obtained and analysed in terms of helicity fractions
f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV
and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw
> 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour,
are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017
+/- 0.030, where the first uncertainties are statistical, and the second
include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables,
revised author list, matches European Journal of Physics C versio
Observation of a new chi_b state in radiative transitions to Upsilon(1S) and Upsilon(2S) at ATLAS
The chi_b(nP) quarkonium states are produced in proton-proton collisions at
the Large Hadron Collider (LHC) at sqrt(s) = 7 TeV and recorded by the ATLAS
detector. Using a data sample corresponding to an integrated luminosity of 4.4
fb^-1, these states are reconstructed through their radiative decays to
Upsilon(1S,2S) with Upsilon->mu+mu-. In addition to the mass peaks
corresponding to the decay modes chi_b(1P,2P)->Upsilon(1S)gamma, a new
structure centered at a mass of 10.530+/-0.005 (stat.)+/-0.009 (syst.) GeV is
also observed, in both the Upsilon(1S)gamma and Upsilon(2S)gamma decay modes.
This is interpreted as the chi_b(3P) system.Comment: 5 pages plus author list (18 pages total), 2 figures, 1 table,
corrected author list, matches final version in Physical Review Letter
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