652 research outputs found
Mg II Absorber Number Density at z~0.05: Implications for Omega_DLA Evolution
An unbiased sample of 147 quasar/AGN spectra, obtained with the FOS/HST, has
been searched for intervening MgII absorbers over the redshift range 0<z<0.15.
The total redshift path searched is 18.8, with the survey being 80% complete to
a 5-sigma rest-frame equivalent width, W_r(2796), of 0.6 Ang. Main results of
this work are: [1] Four systems were found, with a mean redshift of =0.06,
yielding a redshift number density dN/dz=0.22(+0.12)(-0.09) for absorbers with
W_r(2796)>0.6 Ang. This is consistent with the value expected if these systems
do not evolve from higher redshifts (z=2.2). [2] No systems with W_r(2796)<0.6
Ang were found. It is a 2-sigma result to have a null detection of smaller
W_r(2796) systems. If this implies a turnover in the low W_r(2796) region of
the equivalent width distribution at z~0, then there is at least a 25%
reduction in the average galaxy gas cross section from z<0.2 galaxies. [3]
These systems have strong FeII absorption and are good candidates for damped
Ly-alpha absorbers DLAs (see Rao & Turnshek 2000, ApJS, 130, 1). This
translates to a redshift number density of dN/dz=0.08(+0.09)(-0.05) for DLAs at
z~0. In tandem with the data analyzed by Rao & Turnshek, these results indicate
that the redshift number density of DLAs does not evolve from z~4 to z~0. If
the HI mass function does not evolve from z~0.5 to z~0, then the cosmological
HI mass density is also deduced to not evolve from z~4 to z~0. These z~0
results for MgII absorption-selected DLAs are at odds with those based upon
21-cm emission from HI galaxies by a factor of five to six.Comment: 23 pages, 7 Figures, accepted to ApJ. Replaced version includes
additional figures and tables and substantial modifications to the tex
Analytical Approximations for Calculating the Escape and Absorption of Radiation in Clumpy Dusty Environments
We present analytical approximations for calculating the scattering,
absorption and escape of nonionizing photons from a spherically symmetric
two-phase clumpy medium, with either a central point source of isotropic
radiation, a uniform distribution of isotropic emitters, or uniformly
illuminated by external sources. The analytical approximations are based on the
mega-grains model of two-phase clumpy media, as proposed by Hobson & Padman,
combined with escape and absorption probability formulae for homogeneous media.
The accuracy of the approximations is examined by comparison with 3D Monte
Carlo simulations of radiative transfer, including multiple scattering. Our
studies show that the combined mega-grains and escape/absorption probability
formulae provide a good approximation of the escaping and absorbed radiation
fractions for a wide range of parameters characterizing the medium. A realistic
test is performed by modeling the absorption of a starlike source of radiation
by interstellar dust in a clumpy medium, and by calculating the resulting
equilibrium dust temperatures and infrared emission spectrum of both the clumps
and the interclump medium. In particular, we find that the temperature of dust
in clumps is lower than in the interclump medium if clumps are optically thick.
Comparison with Monte Carlo simulations of radiative transfer in the same
environment shows that the analytic model yields a good approximation of dust
temperatures and the emerging UV to FIR spectrum of radiation for all three
types of source distributions mentioned above. Our analytical model provides a
numerically expedient way to estimate radiative transfer in a variety of
interstellar conditions and can be applied to a wide range of astrophysical
environments, from star forming regions to starburst galaxies.Comment: 55 pages, 27 figures. ApJ 523 (1999), in press. Corrected equations
and text so as to be same as ApJ versio
The bias-extension test for the analysis of in-plane shear properties of textile composite reinforcements and prepregs: a review
The bias-extension test is a rather simple experiment aiming to determine in-plane shear properties of textile composite reinforcements. However the mechanics during the test involves fibrous material at large shear strains and large rotations of the fibres. Several aspects are still being studied and are not yet modeled in a consensual manner. The standard analysis of the test is based on two assumptions: inextensibility of the fibers and rotations at the yarn crossovers without slippage. They lead to the development of zones with constant fibre orientations proper to the bias-extension test. Beyond the analysis of the test within these basic assumptions, the paper presents studies that have been carried out on the lack of verification of these hypothesis (slippage, tension in the yarns, effects of fibre bending). The effects of temperature, mesoscopic modeling and tension locking are also considered in the case of the bias-extension test
Discovery of Damped Lyman-Alpha Systems at Redshifts Less Than 1.65 and Results on their Incidence and Cosmological Mass Density
We report results on the incidence and cosmological mass density of damped
Lyman-alpha (DLA) systems at redshifts less that 1.65. We used HST and an
efficient non-traditional (but unbiased) survey technique to discover DLA
systems at redshifts z<1.65, where we observe the Lyman-alpha line in known
MgII absorption-line systems. We uncovered 14 DLA lines including 2
serendipitously. We find that (1) The DLA absorbers are drawn almost
exclusively from the population of MgII absorbers which have rest equivalent
widths W(2796)>0.6A. (2) The incidence of DLA systems per unit redshift,
n(DLA), is observed to decrease with decreasing redshift. (3) On the other
hand, the cosmological mass density of neutral gas in low-redshift DLA
absorbers, Omega(DLA), is observed to be comparable to that observed at high
redshift. (4) The low-redshift DLA absorbers exhibit a significantly larger
fraction of very high column density systems in comparison to determinations at
both high redshift and locally.Comment: 47 pages in LaTeX - emulateapj style with included tables and
encapsulated postscript figures. Accepted for Publication in Astrophysical
Journal Supplements. Results unchanged, text revise
The first radial velocity measurements of a microlensing event: no evidence for the predicted binary
The gravitational microlensing technique allows the discovery of exoplanets
around stars distributed in the disk of the galaxy towards the bulge. However,
the alignment of two stars that led to the discovery is unique over the
timescale of a human life and cannot be re-observed. Moreover, the target host
is often very faint and located in a crowded region. These difficulties hamper
and often make impossible the follow-up of the target and study of its possible
companions. Gould et al. (2013) predicted the radial-velocity curve of a binary
system, OGLE-2011-BLG-0417, discovered and characterised from a microlensing
event by Shin et al. (2012). We used the UVES spectrograph mounted at the VLT,
ESO to derive precise radial-velocity measurements of OGLE-2011-BLG-0417. To
gather high-precision on faint targets of microlensing events, we proposed to
use the source star as a reference to measure the lens radial velocities. We
obtained ten radial velocities on the putative V=18 lens with a dispersion of
~100 m/s, spread over one year. Our measurements do not confirm the
microlensing prediction for this binary system. The most likely scenario is
that the assumed V=18 mag lens is actually a blend and not the primary lens
that is 2 magnitude fainter. Further observations and analyses are needed to
understand the microlensing observation and infer on the nature and
characteristics of the lens itself.Comment: submitted on 3rd June 2015 to A&ALette
Damped Lyman Alpha Systems at z<1.65: The Expanded SDSS HST Sample
We present results of our HST Cycle 11 Survey for low-redshift (z<1.65) DLAs
in the UV spectra of quasars selected from the SDSS Early Data Release. These
quasars have strong intervening MgII-FeII systems which are known signatures of
high column density neutral gas. In total, UV observations of Ly-alpha
absorption in 197 MgII systems with z<1.65 and rest equivalent width (REW)
W2796 \ge 0.3A have now been obtained. The main results are: (1) 36(+/- 6)% of
systems with W2796 \ge 0.5 A and FeII W2600 \ge 0.5 A are DLAs. This increases
to 42(+/- 7)% for systems with W2796/W2600 0.1 A. (2) The
mean N(HI) of MgII systems with 0.3 A \le W2796 < 0.6 A is a factor of ~36
lower than that of systems with W2796 \ge 0.6 A. (3) The DLA incidence per unit
redshift is consistent with no evolution for z <~ 2 (Omega_L=0.7, Omega_M =
0.3), but exhibits significant evolution for z >~ 2. (4) Omega_{DLA} is
constant for 0.5<z<5.0 to within the uncertainties. This is larger than
Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution
does not change significantly with redshift. However, the low redshift
distribution is marginally flatter due to the higher fraction of high N(HI)
systems in our sample. (6) Finally, using the precision of MgII survey
statistics, we find that there may be evidence of a decreasing Omega_{DLA} from
z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very
high columns of neutral gas might be missed by DLA surveys because of their
very small cross sections, and therefore, that Omega_{DLA} might not include
the bulk of the neutral gas mass in the Universe. (Abridged)Comment: Accepted for publication in ApJ. 22 pages, 22 figure
Metallicity Evolution of Damped Lyman-Alpha Galaxies
We have reanalyzed the existing data on Zinc abundances in damped Ly-alpha
(DLA) absorbers to investigate whether their mean metallicity evolves with
time. Most models of cosmic chemical evolution predict that the mass- weighted
mean interstellar metallicity of galaxies should rise with time from a low
value ~ 1/30 solar at z ~ 3 to a nearly solar value at z ~ 0. However, several
previous analyses have suggested that there is little or no evolution in the
global metallicity of DLAs. We have used a variety of statistical techniques to
quantify the global metallicity-redshift relation and its uncertainties, taking
into account both measurement and sampling errors. Three new features of our
analysis are: (a) an unbinned N(H I)-weighted nonlinear chi-square fit to an
exponential relation; (b) survival analysis to treat the large number of limits
in the existing data; and (c) a comparison of the data with several models of
cosmic chemical evolution based on an unbinned N(H I)-weighted chi-square. We
find that a wider range of evolutionary rates is allowed by the present data
than claimed in previous studies. The slope of the exponential fit to the N(H
I)-weighted mean Zn metallicity vs. redshift relation is -0.20 plus minus 0.11
counting limits as detections and -0.27 plus minus 0.12 counting limits as
zeros. Similar results are also obtained if the data are binned in redshift,
and if survival analysis is used. These slopes are marginally consistent with
no evolution, but are also consistent with the rates predicted by several
models of cosmic chemical evolution. Finally, we outline some future
measurements necessary to improve the statistics of the global
metallicity-redshift relation.Comment: 25 pages, 1 figure, accepted for publication in the Astrophysical
Journa
The CIV-MgII Kinematics Connection in <z>~0.7 Galaxies
We have examined Faint Object Spectrograph data from the Hubble Space
Telescope Archive for CIV 1548,1550 absorption associated with 40 MgII
2796,2803 absorption-selected galaxies at 0.4 < z < 1.4. We report a strong
correlation between MgII kinematics, measured in 6 km/s resolution HIRES/Keck
spectra, and W_r(1548); this implies a physical connection between the
processes that produce "outlying velocity" MgII clouds and high ionization
galactic/halo gas. We found no trend in ionization condition,
W_r(1548)/W_r(2796), with galaxy-QSO line-of-sight separation for 13 systems
with confirmed associated galaxies, suggesting no obvious ionization gradient
with galactocentric distance in these higher redshift galaxies. We find
tentative evidence (2-sigma) that W_r(1548)/W_r(2796) is anti-correlated with
galaxy color; if further data corroborate this trend, in view of the
strong CIV-MgII kinematics correlation, it could imply a connection between
stellar populations, star formation episodes, and the kinematics and ionization
conditions of halo gas at z~1.Comment: Accepted to Astrophysical Journal Letters; 4 pages; 3 figures;
emulateapj.st
A new analysis of the WASP-3 system: no evidence for an additional companion
In this work we investigate the problem concerning the presence of additional
bodies gravitationally bounded with the WASP-3 system. We present eight new
transits of this planet and analyse all the photometric and radial velocity
data published so far. We did not observe significant periodicities in the
Fourier spectrum of the observed minus calculated (O-C) transit timing and
radial velocity diagrams (the highest peak having false-alarm probabilities of
56 per cent and 31 per cent, respectively) or long-term trends. Combining all
the available information, we conclude that the radial velocity and transit
timing techniques exclude, at 99 per cent confidence limit, any perturber more
massive than M \gtrsim 100 M_Earth with periods up to 10 times the period of
the inner planet. We also investigate the possible presence of an exomoon on
this system and determined that considering the scatter of the O-C transit
timing residuals a coplanar exomoon would likely produce detectable transits.
This hypothesis is however apparently ruled out by observations conducted by
other researchers. In case the orbit of the moon is not coplanar the accuracy
of our transit timing and transit duration measurements prevents any
significant statement. Interestingly, on the basis of our reanalysis of SOPHIE
data we noted that WASP-3 passed from a less active (log R'_hk=-4.95) to a more
active (log R'_hk=-4.8) state during the 3 yr monitoring period spanned by the
observations. Despite no clear spot crossing has been reported for this system,
this analysis claims for a more intensive monitoring of the activity level of
this star in order to understand its impact on photometric and radial velocity
measurements.Comment: MNRAS accepted (14/08/2012
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