608 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
Consequences of spectrograph illumination for the accuracy of radial-velocimetry
For fiber-fed spectrographs with a stable external wavelength source,
scrambling properties of optical fibers and, homogeneity and stability of the
instrument illumination are important for the accuracy of radial-velocimetry.
Optical cylindric fibers are known to have good azimuthal scrambling. In
contrast, the radial one is not perfect. In order to improve the scrambling
ability of the fiber and to stabilize the illumination, optical double
scrambler are usually coupled to the fibers. Despite that, our experience on
SOPHIE and HARPS has lead to identified remaining radial-velocity limitations
due to the non-uniform illumination of the spectrograph. We conducted tests on
SOPHIE with telescope vignetting, seeing variation and centering errors on the
fiber entrance. We simulated the light path through the instrument in order to
explain the radial velocity variation obtained with our tests. We then
identified the illumination stability and uniformity has a critical point for
the extremely high-precision radial velocity instruments (ESPRESSO@VLT,
CODEX@E-ELT). Tests on square and octagonal section fibers are now under
development and SOPHIE will be used as a bench test to validate these new feed
optics.Comment: to appear in the Proceedings conference "New Technologies for Probing
the Diversity of Brown Dwarfs and Exoplanets", Shanghai, 200
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
SOPHIE+: First results of an octagonal-section fiber for high-precision radial velocity measurements
High-precision spectrographs play a key role in exoplanet searches and
Doppler asteroseismology using the radial velocity technique. The 1 m/s level
of precision requires very high stability and uniformity of the illumination of
the spectrograph. In fiber-fed spectrographs such as SOPHIE, the fiber-link
scrambling properties are one of the main conditions for high precision. To
significantly improve the radial velocity precision of the SOPHIE spectrograph,
which was limited to 5-6 m/s, we implemented a piece of octagonal-section fiber
in the fiber link. We present here the scientific validation of the upgrade of
this instrument, demonstrating a real improvement. The upgraded instrument,
renamed SOPHIE+, reaches radial velocity precision in the range of 1-2 m/s. It
is now fully efficient for the detection of low-mass exoplanets down to 5-10
Earth mass and for the identification of acoustic modes down to a few tens of
cm/s.Comment: 12 pages, 11 figures, accepted in Astronomy and Astrophysic
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
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
The Gravitational Lens Candidate FBQ 1633+3134
We present our ground-based optical imaging, spectral analysis, and high
resolution radio mapping of the gravitational lens candidate FBQ 1633+3134.
This z=1.52, B=17.7 quasar appears double on CCD images with an image
separation of 0.66 arcseconds and a flux ratio of ~3:1 across BVRI filters. A
single 0.27 mJy radio source is detected at 8.46 GHz, coincident to within an
arcsecond of both optical components, but no companion at radio wavelengths is
detected down to a flux level of 0.1 mJy (3 sigma). Spectral observations
reveal a rich metal-line absorption system consisting of a strong Mg II doublet
and associated Fe I and Fe II absorption features, all at an intervening
redshift of z=0.684, suggestive of a lensing galaxy. Point spread function
subtraction however shows no obvious signs of a third object between the two
quasar images, and places a detection limit of I > 23.0 if such an object
exists. Although the possibility that FBQ 1633+3134 is a binary quasar cannot
be ruled out, the evidence is consistent with it being a single quasar lensed
by a faint, metal-rich galaxy.Comment: 24 pages, 5 figures. Accepted by AJ. A calibration error affecting B
and V band apparent magnitudes has been corrected. The conclusions of the
paper are not change
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