4,483 research outputs found
Metallicity in damped Lyman-alpha systems: evolution or bias?
Assuming that damped Lyman-alpha(DLA) systems are galactic discs, we
calculate the corresponding evolution of metal abundances. We use detailed
multi-zone models of galactic chemical evolution (reproducing successfully the
observed properties of disc galaxies) and appropriate statistics (including
geometrical propability factors) to calculate the average metallicity as a
function of redshift. The results are compatible with available observations,
provided that observational biases are taken into account, as suggested by
Boisse et al. (1998). In particular, high column density and high metallicity
systems are not detected because the light of backround quasars is severely
extinguished, while low column density and low metallicity systems are not
detectable through their absorption lines by current surveys. We show that
these observational constraints lead to a ``no-evolution'' picture for the DLA
metallicity, which does not allow to draw strong conclusions about the nature
of those systems or about their role in ``cosmic chemical evolution''.Comment: 7 pages, 5 figures, MNRAS in pres
On the relative frequencies of core-collapse supernovae sub-types: the role of progenitor metallicity
We show that the observed ratio of the subtypes Ib,c and II core-collapse
supernovae depends on the metallicity of the host galaxy, as expected on
theoretical grounds. However, the observed relation differs considerably from
expectations based on non-rotating models of single stars with mass loss. We
argue that the predictions of recent models with rotation offer a much better
agreement with observations, at least for progenitor stars with solar
metallicity; calculations of models with higher and lower metallicities are
required in order to substantiate these conclusions. We also suggest that
systematic surveys of core collapse supernovae up to redshift of 1 with the
SNAP satellite would allow to probe the effect of metallicity on supernovae
properties during the past history of the universe.Comment: 7 pages, 5 figs. ; accepted for publication in Astronomy and
Astrophysic
Chemo-spectrophotometric evolution of spiral galaxies: III. Abundance and colour gradients in discs
We study the relations between luminosity and chemical abundance profiles of
spiral galaxies, using detailed models for the chemical and spectro-photometric
evolution of galactic discs. The models are ``calibrated'' on the Milky Way
disc and are successfully extended to other discs with the help of simple
``scaling'' relations, obtained in the framework of semi-analytic models of
galaxy formation. We find that our models exhibit oxygen abundance gradients
that increase in absolute value with decreasing disc luminosity (when expressed
in dex/kpc) and are independent of disc luminosity (when expressed in
dex/scalelength), both in agreement with observations. We notice an important
strong correlation between abundance gradient and disc scalelength. These
results support the idea of ``homologuous evolution'' of galactic discs.Comment: 9 pages, 6 postscript figures, MNRAS in pres
Chemical evolution and depletion pattern in Damped Lyman-alpha (DLA) systems
In this paper we point out a previously unnoticed anticorrelation between the
observed abundance ratio [X/Zn] (where Zn is assumed to be undepleted and X
stands for the refractories Fe, Cr and Ni) and metal column density
([Zn/H]+log(N{HI})) in DLAs. We suggest that this trend is an unambiguous sign
of dust depletion, since metal column density is a measure of the amount of
dust along the line of sight. Assuming that DLAs are (proto-)galactic disks and
using detailed chemical evolution models with metallicity dependent yields we
study chemical evolution and dust depletion patterns for alpha and iron-peak
elements in DLAs. When observational constraints on the metal column density of
DLAs are taken into account (as suggested in Boisse et al. 1998) we find that
our models reproduce fairly well the observed mild redshift evolution of the
abundances of 8 elements (Al, Si, S, Cr, Mn, Fe, Zn and Ni) as well as the
observed scatter at a given redshift. By considering the aforementioned
dependence of abundance ratios on metal column density, we further explore the
general dust depletion pattern in DLAs, comparing to our model results and to a
solar reference pattern. We suggest that further measurements of the key
elements, i.e. Zn, S and Mn, will help to gain more insight into the nature of
DLAs. In any case, the presently uncertain nucleosynthesis of Zn in massive
stars (on which a large part of these conclusions is based) should be carefully
scrutinised.Comment: 12 pages, 4 figures, Astronomy and Astrophysics, in pres
Stellar Mass and Velocity Functions of Galaxies: Backward evolution and the fate of Milky Way siblings
We attempt in this paper to check the consistency of the observed Stellar
Mass Function (SMF), SFR functions and the cosmic star formation rate density
with simple backward evolutionary models. Starting from observed SMF for
star-forming galaxies, we use backwards models to predict the evolution of a
number of quantities, such as the SFR function, the cosmic SFR density and the
Velocity Function. The velocity being a parameter attached to a galaxy during
its history (contrary to the stellar mass), this approach allows us to quantify
the number density evolution of galaxies of a given velocity, e.g. of the Milky
Way siblings. Observations suggest that the SMF of star forming galaxies is
constant between redshift 0 and 1. In order to reproduce this result, we must
quench star formation in a number of star forming galaxies. The SMF of these
quenched galaxies is consistent with available data concerning the increase in
the population of quiescent galaxies in the same redshift interval. The SMF of
quiescent galaxies is then mainly determined by the distribution of active
galaxies that must stop star formation, with a modest mass redistribution
during mergers. The cosmic SFR density, and the evolution of the SFR functions
are relatively well recovered, although they provide some clue for a small
evolution of the SMF of star forming galaxies at the lowest redshifts. We thus
consider that we have obtained in a simple way a relatively consistent picture
of the evolution of galaxies at intermediate redshifts. We note that if this
picture is correct, 50 percent of the Milky-Way sisters (galaxies with the same
velocity as our Galaxy, i.e. 220 km/s) have quenched their star formation since
redshift 1 (and an even larger fraction for larger velocities). We discuss the
processes that might be responsible for this transformation.Comment: 12 pages, 14 figures, accepted in Astronomy and Astrophysic
{Chemo-spectrophotometric evolution of spiral galaxies: IV. Star formation efficiency and effective ages of spirals
We study the star formation history of normal spirals by using a large and
homogeneous data sample of local galaxies. For our analysis we utilise detailed
models of chemical and spectrophotometric galactic evolution, calibrated on the
Milky Way disc. We find that star formation efficiency is independent of
galactic mass, while massive discs have, on average, lower gas fractions and
are redder than their low mass counterparts; put together, these findings
convincingly suggest that massive spirals are older than low mass ones. We
evaluate the effective ages of the galaxies of our sample and we find that
massive spirals must be several Gyr older than low mass ones. We also show that
these galaxies (having rotational velocities in the 80-400 km/s range) cannot
have suffered extensive mass losses, i.e. they cannot have lost during their
lifetime an amount of mass much larger than their current content of gas+stars.Comment: 11 pages, 8 figures, MNRAS in pres
No compelling evidence of distributed production of CO in comet C/1995 O1 (Hale-Bopp) from millimeter interferometric data and a reanalysis of near-IR lines
Based on long-slit infrared spectroscopic observations, it has been suggested
that half of the carbon monoxide present in the atmosphere of comet C/1995 O1
(Hale-Bopp) close to perihelion was released by a distributed source in the
coma, whose nature (dust or gas) remains unidentified. We re-assess the origin
of CO in Hale-Bopp's coma from millimeter interferometric data and a
re-analysis of the IR lines.
Simultaneous observations of the CO J(1-0) (115 GHz) and J(2-1) (230 GHz)
lines were undertaken with the IRAM interferometer in single-dish and
interferometric modes. The diversity of angular resolutions (from 1700 to 42000
km diameter at the comet) is suitable to study the radial distribution of CO
and detect the extended source observed in the infrared. We used excitation and
radiative transfer models to simulate the observations. Various CO density
distributions were considered, including 3D time-dependent hydrodynamical
simulations which reproduce a CO rotating jet. The CO J(1-0) and J(2-1)
observations can be consistently explained by a nuclear production of CO.
Composite 50:50 nuclear/extended productions with characteristic scale lengths
of CO parent L_p > 1500 km are rejected.
Based on similar radiation transfer calculations, we show that the CO v = 1-0
ro-vibrational lines observed in comet Hale-Bopp at heliocentric distances less
than 1.5 AU are severely optically thick. The broad extent of the CO brightness
distribution in the infrared is mainly due to optical depth effects. Additional
factors can be found in the complex structure of the CO coma, and non-ideal
slit positioning caused by the anisotropy of dust IR emission.
We conclude that both CO millimeter and infrared lines do not provide
compelling evidence for a distributed source of CO in Hale-Bopp's atmosphere.Comment: Accepted for publication in Icarus (55 pages, 13 figures
A Framework for Normative MultiAgent Organisations
The social and organisational aspects of agency have led to a good amount of theoretical work in terms of formal models and theories. From these different works normative multiagent systems and multiagent organisations are particularily considered in this paper. Embodying such models and theories in the conception and engineering of proper infrastructures that achieve requirements of openness and adaptation, is still an open issue. In this direction, this paper presents and discusses a framework for normative multiagent organisations. Based on the Agents and Artifacts meta-model (A&A), it introduces organisational artifacts as first class entities to instrument the normative organisation for supporting agents activities within it
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