2,366 research outputs found
Long-term variability of CO2 and O in the Mars upper atmosphere from MRO radio science data
We estimate the annual variability of CO2 and O partial density using approximately 6years of Mars Reconnaissance Orbiter (MRO) radio science data from August 2006 to January 2012, which cover three full Martian years (from the northern hemisphere summer of 28 to the northern hemisphere summer of 31). These two elements are the dominant species at the MRO periapsis altitude, constituting about 70-80% of the total density. We report the recovered annual cycle of CO2 and the annual and seasonal cycle of O in the upper atmosphere. Although no other observations are available at those altitudes, our results are in good agreement with the density measurements of the Mars Express Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars, which uses stellar occultations between 60 and 130km to determine the CO2 variability, and with the Mars Global Reference Atmospheric Model 2010 for the O annual and seasonal variabilities. Furthermore, the updated model provides more reasonable MRO drag coefficients (CD), which are estimated to absorb mismodeling in the atmospheric density prediction. The higher content of dust in the atmosphere due to dust storms increases the density, so the CDs should compensate for this effect. The correlation between the drag coefficient and the dust optical depth, measured by the Mars Odyssey Thermal Emission Imaging System (THEMIS) instrument, increases from 0.4 to 0.8 with the a priori and adjusted models, respectively. The trend of CDs not only confirms a substantial improvement in the prediction of the atmospheric density with the updated model but also provides useful information for local dust storms, near MRO periapsis, that cannot be measured by the opacity level since THEMIS does not always sample the southern hemisphere evenly
Properties of Faint Distant Galaxies as seen through Gravitational Telescopes
This paper reviews the most recent developments related to the use of lensing
clusters of galaxies as Gravitational Telescopes in deep Universe studies. We
summarize the state of the art and the most recent results aiming at studying
the physical properties of distant galaxies beyond the limits of conventional
spectroscopy. The application of photometric redshift techniques in the context
of gravitational lensing is emphasized for the study of both lensing structures
and the background population of lensed galaxies. A presently ongoing search
for the first building blocks of galaxies behind lensing clusters is presented
and discussed.Comment: Review lecture given at "Gravitational Lensing: a unique tool for
cosmology",Aussois, France, January 2003. To appear in ASP Conf. S., eds. D.
Valls-Gabaud & J.-P. Kneib, 26 pages, 8 figure
Trans-Planckian Dark Energy?
It has recently been proposed by Mersini et al. 01, Bastero-Gil and Mersini
02 that the dark energy could be attributed to the cosmological properties of a
scalar field with a non-standard dispersion relation that decreases
exponentially at wave-numbers larger than Planck scale (k_phys > M_Planck). In
this scenario, the energy density stored in the modes of trans-Planckian
wave-numbers but sub-Hubble frequencies produced by amplification of the vacuum
quantum fluctuations would account naturally for the dark energy. The present
article examines this model in detail and shows step by step that it does not
work. In particular, we show that this model cannot make definite predictions
since there is no well-defined vacuum state in the region of wave-numbers
considered, hence the initial data cannot be specified unambiguously. We also
show that for most choices of initial data this scenario implies the production
of a large amount of energy density (of order M_Planck^4) for modes with
momenta of order M_Planck, far in excess of the background energy density. We
evaluate the amount of fine-tuning in the initial data necessary to avoid this
back-reaction problem and find it is of order H/M_Planck. We also argue that
the equation of state of the trans-Planckian modes is not vacuum-like.
Therefore this model does not provide a suitable explanation for the dark
energy.Comment: RevTeX - 15 pages, 7 figures: final version to appear in PRD, minor
changes, 1 figure adde
The optical depth of the Universe to ultrahigh energy cosmic ray scattering in the magnetized large scale structure
This paper provides an analytical description of the transport of ultrahigh
energy cosmic rays in an inhomogeneously magnetized intergalactic medium. This
latter is modeled as a collection of magnetized scattering centers such as
radio cocoons, magnetized galactic winds, clusters or magnetized filaments of
large scale structure, with negligible magnetic fields in between. Magnetic
deflection is no longer a continuous process, it is rather dominated by
scattering events. We study the interaction between high energy cosmic rays and
the scattering agents. We then compute the optical depth of the Universe to
cosmic ray scattering and discuss the phenomological consequences for various
source scenarios. For typical parameters of the scattering centers, the optical
depth is greater than unity at 5x10^{19}eV, but the total angular deflection is
smaller than unity. One important consequence of this scenario is the
possibility that the last scattering center encountered by a cosmic ray be
mistaken with the source of this cosmic ray. In particular, we suggest that
part of the correlation recently reported by the Pierre Auger Observatory may
be affected by such delusion: this experiment may be observing in part the last
scattering surface of ultrahigh energy cosmic rays rather than their source
population. Since the optical depth falls rapidly with increasing energy, one
should probe the arrival directions of the highest energy events beyond
10^{20}eV on an event by event basis to circumvent this effect.Comment: version to appear in PRD; substantial improvements: extended
introduction, sections added on angular images and on direction dependent
effects with sky maps of optical depth, enlarged discussion of Auger results
(conclusions unchanged); 27 pages, 9 figure
Integral field spectroscopy with SINFONI of VVDS galaxies. II. The mass-metallicity relation at 1.2 < z < 1.6
This work aims to provide a first insight into the mass-metallicity (MZ)
relation of star-forming galaxies at redshift z~1.4. To reach this goal, we
present a first set of nine VVDS galaxies observed with the NIR integral-field
spectrograph SINFONI on the VLT. Oxygen abundances are derived from empirical
indicators based on the ratio between strong nebular emission-lines (Halpha,
[NII]6584 and [SII]6717,6731). Stellar masses are deduced from SED fitting with
Charlot & Bruzual (2007) population synthesis models, and star formation rates
are derived from [OII]3727 and Halpha emission-line luminosities. We find a
typical shift of 0.2-0.4 dex towards lower metallicities for the z~1.4
galaxies, compared to the MZ-relation in the local universe as derived from
SDSS data. However, this small sample of eight galaxies does not show any clear
correlation between stellar mass and metallicity, unlike other larger samples
at different redshift (z~0, z~0.7, and z~2). Indeed, our galaxies lie just
under the relation at z~2 and show a small trend for more massive galaxies to
be more metallic (~0.1 logarithmic slope). There are two possible explanations
to account for these observations. First, the most massive galaxies present
higher specific star formation rates when compared to the global VVDS sample
which could explain the particularly low metallicity of these galaxies as
already shown in the SDSS sample. Second, inflow of metal-poor gas due to tidal
interactions could also explain the low metallicity of these galaxies as two of
these three galaxies show clear signatures of merging in their velocity fields.
Finally, we find that the metallicity of 4 galaxies is lower by ~0.2 to 0.4 dex
if we take into account the N/O abundance ratio in their metallicity estimate.Comment: 7 pages, 4 figures, accepted in A&A Comments: Comments: more accurate
results with better stellar mass estimate
Magnetic Fields from Phase Transitions
The generation of primordial magnetic fields from cosmological phase
transitions is discussed, paying particular attention to the electroweak
transition and to the various definitions of the `average' field that have been
put forward. It is emphasised that only the volume average has dynamical
significance as a seed for galactic dynamos. On rather general grounds of
causality and energy conservation, it is shown that, in the absence of MHD
effects that transfer power in the magnetic field from small to large scales,
processes occurring at the electroweak transition cannot generate fields
stronger than Gauss on a scale of 0.5 Mpc. However, it is
implausible that this upper bound could ever be reached, as it would require
all the energy in the Universe to be turned into a magnetic field coherent at
the horizon scale. Non-linear MHD effects seem therefore to be necessary if the
electroweak transition is to create a primordial seed field.Comment: 6pp RevTeX. Correct finished version supplie
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