449 research outputs found
Molecular lines studies at redshift greater than 1
Observations of CO molecules in the millimetrer domain at high redshift
(larger than 1), have provided interesting informations about star formation
efficiency, and its evolution with redshift. Due to the difficulty of the
detections, selection effects are important. The detection if often due to
gravitational amplification. Objects selected by their (far)infrared flux, are
in general associated to ULIRGS, mergers with starburst in the nuclear regions.
Quasars have been selected as powerful optical sources, and have been found to
be associated to starbursts, rich in gas. The gas fraction appears to be much
higher at redshift greater than 1.
Quasars allow to probe the end of the reionisation period, and the relation
between bulge and black hole mass. However these selection bias could have led
us to miss some gaseous galaxies, with low-efficiency of star formation, such
as the more quiescent objects selected by their BzK colors at z=1.5 or 2.Comment: 3 pages, to appear in IAU highlights, JD14 "FIR2009, The ISM of
galaxies in the Far-infrared and Sub-millimeter
ALMA Observations of Molecular Absorption in the Gravitational Lens PMN 0134-0931
We report the detection of molecular absorption lines at z=0.7645 towards the
radio-loud QSO PMN 0134-0931. The CO J=2-1 and HCO+ J=2-1 lines are seen in
absorption along two different lines of sight to lensed images of the
background QSO. The lines of sight are separated by ~0.7", corresponding to 5
kpc in the lens plane. PMN 0134-0931 represents one out of only five known
molecular absorption line systems at cosmologically significant distances.
Moreover, it is also one of three such systems where the absorption occurs in a
galaxy acting as a gravitational lens. The absorption lines through the two
lines of sight are shifted by 215+/-8 km/s, possibly representing rotational
motion in one of the lensing galaxies. The absorption profiles are wide, ~200
km/s, suggesting that the absorption occurs in a highly inclined disk galaxy
with a flat rotation curve and a cloud-cloud velocity dispersion ~30 km/s.
Gravitational lens models require two equal mass galaxies to account for the
observed configuration of lensed images. The presence of two galaxies in close
proximity means that they might be interacting and potentially merging and the
kinematics of the molecular gas may not reflect ordered rotational motion. The
column densities of both CO and HCO+ are normal for diffuse molecular gas
towards one of the lensed images, but significantly higher towards the other.
Also, the abundance ratio N(CO)/N(HCO+) is 2-3 times higher than in typical
diffuse molecular gas. It is plausible that the second line of sight probes
denser molecular gas than what is normally the case for absorption.Comment: 17 pages, 4 figures, 4 table
Local stability of a gravitating filament: a dispersion relation
Filamentary structures are ubiquitous in astrophysics and are observed at
various scales. On a cosmological scale, matter is usually distributed along
filaments, and filaments are also typical features of the interstellar medium.
Within a cosmic filament, matter can contract and form galaxies, whereas an
interstellar gas filament can clump into a series of bead-like structures which
can then turn into stars. To investigate the growth of such instabilities, we
derive a local dispersion relation for an idealized self-gravitating filament,
and study some of its properties. Our idealized picture consists of an infinite
self-gravitating and rotating cylinder with pressure and density related by a
polytropic equation of state. We assume no specific density distribution, treat
matter as a fluid, and use hydrodynamics to derive the linearized equations
that govern the local perturbations. We obtain a dispersion relation for
axisymmetric perturbations and study its properties in the (k_R, k_z) phase
space, where k_R and k_z are respectively the radial and longitudinal
wavenumbers. While the boundary between the stable and unstable regimes is
symmetrical in k_R and k_z and analogous to the Jeans criterion, the most
unstable mode displays an asymmetry that could constrain the shape of the
structures that form within the filament. Here the results are applied to a
fiducial interstellar filament, but could be extended for more astrophysical
systems such as cosmological filaments and tidal tails.Comment: 8 pages, 1 figure, published in A&
Increased Exposure of China to Asymmetric External Shocks: Is Fiscal Federalism an Efficient Answer?
The aim of this paper is to examine whether there is a co-insurance mechanism against provincial aggregate income fluctuations between the Chinese provinces. Our theoretical argument relies on the existence of an efficient allocation of risk between the Chinese provinces. According to this analysis, an institutional arrangement between the provinces allows the perfect smoothing of provincial private consumption. In this case, changes in provincial private consumption depend rather on changes in aggregate Chinese income than on asymmetric changes in provincial income. We test this hypothesis on the 1989-2000 period for 30 Chinese provinces using the GMM estimator. Econometric evidence highlights the weakness of co-insurance mechanisms between the Chinese provinces. First, we reject the hypothesis of perfect insurance. Second, there does not seem to exist a significant, though imperfect, insurance mechanism. Indeed, the provinces’ private consumption reactions are the same either after a shock affecting all the provinces or after an asymmetric shock.China., Fiscal Federalism, Regional Economics Risk Coping, Co-insurance
Increased Exposure of China to Asymmetric External Shocks: Is Fiscal Federalism an Efficient Answer?
The aim of this paper is to examine whether there is a co-insurance mechanism against provincial aggregate income fluctuations between the Chinese provinces. Our theoretical argument relies on the existence of an efficient allocation of risk between the Chinese provinces. According to this analysis, an institutional arrangement between the provinces allows the perfect smoothing of provincial private consumption. In this case, changes in provincial private consumption depend rather on changes in aggregate Chinese income than on asymmetric changes in provincial income. We test this hypothesis on the 1989-2000 period for 30 Chinese provinces using the GMM estimator. Econometric evidence highlights the weakness of co-insurance mechanisms between the Chinese provinces. First, we reject the hypothesis of perfect insurance. Second, there does not seem to exist a significant, though imperfect, insurance mechanism. Indeed, the provinces’ private consumption reactions are the same either after a shock affecting all the provinces or after an asymmetric shock.China., Fiscal Federalism, Regional Economics Risk Coping, Co-insurance
Transculturing and/or Pluricultural Competence?
This contribution aims to lead the reader to an answer to the question in the title. It must be seen as a meta-reflection on the results of a series of courses set up as action (intervention) research. These courses primarily revealed that it was important to understand the internal processes that take place before discourse is produced and trigger its production in situations where the interactants come from different cultural backgrounds. The courses will be briefly described, followed by a transdisciplinary theoretical development. A final course taking on board the results of our theoretical and field work will then be analyzed. The need for some form of introspection to understand what is described as transcultural behavior will justify the answer to the question in the title.Cette contribution a pour objectif d’amener le lecteur à comprendre la réponse apportée à la question du titre. Elle doit être perçue comme une métaréflexion sur les résultats de plusieurs formations organisées sous forme de recherches-interventions. Ces formations ont révélé qu’il était important de comprendre comment fonctionnent les processus internes qui précèdent la production de discours et qui déclenchent celle-ci. Une brève analyse de ces formations suivra, complétée par un développe¬ment théorique transdisciplinaire. Une dernière formation prenant en compte l’ensemble des résultats de notre travail théorique et pratique sera ensuite abordée. Le besoin d’une forme d’introspection pour comprendre ce que l’on pourrait appeler un comportement transculturel justifiera la réponse que nous donnons à la question du titre.Peer Reviewe
Are There Spillover Effects Between Coastal and Non-Coastal Regions in China ?
The evolution of regional policy between the Mao era and the Deng era generated much debate concerning inter-provincial disparities and the trade-off between efficiency and equity. The aim of this paper is to explore the existence of regional growth spillover effects looked for Deng’s policy. Indeed, the main objective was the spread of coastal provinces’ growth onto inland provinces’ growth. After reviewing the theoretical underpinnings of such effects, their existence is tested with panel data, for the period 1981-1998. Moreover, the hypothesis of an equal distribution of these effects over all the inland provinces is also tested. A relative failure to boost development of the western provinces from the coastal provinces’ growth is observed. Hence, it would seen to be an error to wait for spillover effects to be sufficient to reduce disparities between Chinese provinces in the short run.panel data., spillover effects, regions, growth, China
On the Lengths, Colours and Ages of Bars
In an effort to obtain further observational evidences for secular evolution
processes in galaxies, as well as observational constraints to current
theoretical models of secular evolution, we have used BVRI and Ks images of a
sample of 18 barred galaxies to measure the lengths and colours of bars, create
colour maps and estimate global colour gradients. In addition, applying a
method we developed in a previous article, we could distinguish for 7 galaxies
in our sample those whose bars have been recently formed from the ones with
already evolved bars. We estimated an average difference in the optical colours
between young and evolved bars that may be translated to an age difference of
the order of 10 Gyr, meaning that bars may be long standing structures.
Moreover, our results show that, on average, evolved bars are longer than young
bars. This seems to indicate that, during its evolution, a bar grows longer by
capturing stars from the disk, in agreement with recent numerical and
analytical results.Comment: To appear in Galaxy Evolution Across the Hubble Time, proceedings of
the IAU Symp. 235, F. Combes and J. Palous (eds.); 1 page; the poster can be
found at http://www.mpa-garching.mpg.de/~dimitri/iauga.pd
The interplay between a galactic bar and a supermassive black hole: nuclear fueling in a sub-parsec resolution galaxy simulation
We study the connection between the large-scale dynamics and the gas fueling
toward a central black hole via the analysis of a Milky Way-like simulation at
sub-parsec resolution. This allows us to follow a set of processes at various
scales (e.g., the triggering of inward gas motion towards inner resonances via
the large-scale bar, the connection to the central black hole via mini spirals)
in a self-consistent manner. This simulation provides further insights on the
role of shear for the inhibition of star formation within the bar in regions
with significant amount of gas. We also witness the decoupling of the central
gas and nuclear cluster from the large-scale disc, via interactions with the
black hole. This break of symmetry in the mass distribution triggers the
formation of gas clumps organised in a time-varying 250 pc ring-like structure,
the black hole being offset by about 70 pc from its centre. Some clumps form
stars, while most get disrupted or merge. Supernovae feedback further creates
bubbles and filaments, some of the gas being expelled to 100 pc or higher above
the galaxy plane. This helps remove angular momentum from the gas, which gets
closer to the central dark mass. Part of the gas raining down is being
accreted, forming a 10~pc polar disc-like structure around the black hole,
leading to an episode of star formation. This gives rise to multiple stellar
populations with significantly different angular momentum vectors, and may lead
to a natural intermittence in the fueling of the black hole.Comment: Accepted for publication in MNRAS, 15 pages, 13 figure
Galaxies et cosmologie
Enseignement Cours et séminaires – Dynamique des galaxies, spirales et barres, interactions et fusions Introduction La formation et l’évolution des galaxies dépendent essentiellement de leur dynamique, de leur gravité et de l’hydrodynamique du gaz. Ces phénomènes contrôlent la formation d’étoiles, et tous les processus associés, y compris les rétroactions dues à l’énergie des supernovae. Chaque galaxie croît en symbiose avec son trou noir supermassif, tapi en son centre, qui devient actif par..
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