229 research outputs found
On the nature of cosmological time
Time is a parameter playing a central role in our most fundamental modeling
of natural laws. Relativity theory shows that the comparison of times measured
by different clocks depends on their relative motions and on the strength of
the gravitational field in which they are embedded. In standard cosmology, the
time parameter is the one measured by fundamental clocks, i.e. clocks at rest
with respect to the expanding space. This proper time is assumed to flow at a
constant rate throughout the whole history of the Universe. We make the
alternative hypothesis that the rate at which cosmological time flows depends
on the dynamical state of the Universe. In thermodynamics, the arrow of time is
strongly related to the second law, which states that the entropy of an
isolated system will always increase with time or, at best, stay constant.
Hence, we assume that time measured by fundamental clocks is proportional to
the entropy of the region of the Universe that is causally connected to them.
Under that simple assumption, we build a cosmological model that explains the
Type Ia Supernovae data (the best cosmological standard candles) without the
need for exotic dark matter nor dark energy.Comment: 5 pages, 4 figures, submitted to MNRA
Analysis of luminosity distributions of strong lensing galaxies: subtraction of diffuse lensed signal
Strong gravitational lensing gives access to the total mass distribution of
galaxies. It can unveil a great deal of information about the lenses dark
matter content when combined with the study of the lenses light profile.
However, gravitational lensing galaxies, by definition, appear surrounded by
point-like and diffuse lensed signal that is irrelevant to the lens flux.
Therefore, the observer is most often restricted to studying the innermost
portions of the galaxy, where classical fitting methods show some
instabilities. We aim at subtracting that lensed signal and at characterising
some lenses light profile by computing their shape parameters. Our objective is
to evaluate the total integrated flux in an aperture the size of the Einstein
ring in order to obtain a robust estimate of the quantity of ordinary matter in
each system. We are expanding the work we started in a previous paper that
consisted in subtracting point-like lensed images and in independently
measuring each shape parameter. We improve it by designing a subtraction of the
diffuse lensed signal, based only on one simple hypothesis of symmetry. This
extra step improves our study of the shape parameters and we refine it even
more by upgrading our half-light radius measurement. We also calculate the
impact of our specific image processing on the error bars. The diffuse lensed
signal subtraction makes it possible to study a larger portion of relevant
galactic flux, as the radius of the fitting region increases by on average
17\%. We retrieve new half-light radii values that are on average 11\% smaller
than in our previous work, although the uncertainties overlap in most cases.
This shows that not taking the diffuse lensed signal into account may lead to a
significant overestimate of the half-light radius. We are also able to measure
the flux within the Einstein radius and to compute secure error bars to all of
our results
Detection of the lensing galaxy in HE 1104-1805
peer reviewedWe report on deep IR imaging of the double quasar HE 1104-1805. A new image deconvolution technique has been applied to the data in order to optimally combine the numerous frames obtained. The resulting J and K' images allow us to detect and study the lensing galaxy between the two lensed QSO images. The near infrared images not only confirm the lensed nature of this double quasar, but also support the previous redshift estimate of z=1.66 for the lensing galaxy. No obvious overdensity of galaxies is detected in the immediate region surrounding the lens, down to limiting magnitudes of J=22 and K=20. The geometry of the system, together with the time delays expected for this lensed quasar, make HE 1104-1805 a remarkable target for future photometric monitoring programs, for the study of microlensing and for the determination of the cosmological parameters in the IR and optical domains. Based on observations obtained at ESO, La Silla, Chil
The chemical composition of the extreme halo stars. III - Equivalent widths of 20 dwarfs
peer reviewedIn the first two papers of this series the chemical composition of 20 extreme halo stars was discussed. The abundances of a number of elements were determined using model atmosphere analysis of equivalent widths. In this paper a tabulation of the equivalent width data is presented. A comparison of the measurements with the equivalent widths from different sources is used to assess the quality of the data
Barium isotopes in the very metal-poor star HD 140283
peer reviewedThe fractions of odd and even isotopes of barium in the metal-poor star HD140283 are determined from an analysis of the profile of the Ba II resonance line, taking advantage of the differences in hyperfine structure splitting between the different isotopes. Although one would expect an increased contribution of the r-process in very metal-poor stars, it is found that the isotopic ratio is very close to solar
Very Low Upper Limits on the Strength of Interstellar Lithium Lines Toward Supernova 1987A
Evidence of a type 1/type 2 dichotomy in the correlation between quasar optical polarization and host-galaxy/extended emission position angles
peer reviewedAims.For Seyfert galaxies, the AGN unification model provides a simple and well-established explanation of the type 1/type 2 dichotomy through orientation-based effects. The generalization of this unification model to the higher luminosity AGNs that quasars are remains a key question. The recent detection of type 2 radio-quiet quasars seems to support such an extension. We propose a further test of this scenario. Methods: On the basis of a compilation of quasar host-galaxy position angles consisting of previously published data and of new measurements performed using HST Archive images, we investigate the possible existence of a correlation between the linear polarization position angle and the host-galaxy/extended emission position angle of quasars. Results: We find that the orientation of the rest-frame UV/blue extended emission is correlated to the direction of the quasar polarization. For type 1 quasars, the polarization is aligned with the extended UV/blue emission, while these two quantities are perpendicular in type 2 objects. This result is independent of the quasar radio loudness. We interpret this (anti-)alignment effect in terms of scattering in a two-component polar+equatorial model that applies to both type 1 and type 2 objects. Moreover, the orientation of the polarization -and then of the UV/blue scattered light- does not appear correlated to the major axis of the stellar component of the host-galaxy measured from near-IR images
TRAPPIST monitoring of comet C/2012 F6 (Lemmon)
Comet C/2012 F6 is a long-period comet that reached perihelion on March 23, 2012. The unexpected brightness of this comet since December 2012 allowed us to obtain narrowband photometry and to study its chemical composition as well as its rotation
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