1,287 research outputs found
Resonantly enhanced filamentation in gases
In this Letter, a low-loss Kerr-driven optical filament in Krypton gas is
experimentally reported in the ultraviolet. The experimental findings are
supported by ab initio quantum calculations describing the atomic optical
response. Higher-order Kerr effect induced by three-photon resonant transitions
is identified as the underlying physical mechanism responsible for the
intensity stabilization during the filamentation process, while ionization
plays only a minor role. This result goes beyond the commonly-admitted paradigm
of filamentation, in which ionization is a necessary condition of the filament
intensity clamping. At resonance, it is also experimentally demonstrated that
the filament length is greatly extended because of a strong decrease of the
optical losses
La question de Sein et de Haben dans le parfait actif allemand : esquisse d'une méthodologie
Résumé
        Pour rendre compte des parfaits pĂ©riphrastiques de lâallemand contemporain â et en particulier des cas « inexpliquĂ©s » que les grammaires qualifient dâ« exceptions » â, il faut reconsidĂ©rer la question dans son ensemble, câest-Ă -dire intĂ©grer dâune part les donnĂ©es diachroniques, (car lâ« état des lieux » sur lequel se basent les analyses syntaxiques ou sĂ©mantiques des grammaires a Ă©tĂ© faussĂ© par la force niveleuse de lâanalogie qui uniformise) et dâautre part reconsidĂ©rer la structure mĂȘme de ces formes verbales dans lâassiette propositionnelle dans laquelle elles sont insĂ©rĂ©es.
        Ces formes pĂ©riphrastiques en sein et haben se rencontrent dans des propositions oĂč opĂšrent des structures attributives (voir les travaux de J. M. Zemb) dont Sein et haben assurent la base verbale. Ils y fonctionnent comme des verbes attributifs, opĂ©rant une attribution dâun « contenu » rĂ©sultatif ou non-rĂ©sultatif sur un support (le sujet dans la cas de lâattribution directe, lâobjet dans le cas de lâattribution indirecte).
        La question de ces formes périphrastiques du parfait se rapporte ainsi à deux questions de nature attributive :
 1. Quelle est la nature aspectuelle de lâattribut (la forme participiale) qui doit ĂȘtre attribuĂ© ? La considĂ©ration dâ« entier » vs. « fragmentaire » (du sujet comme de lâattribut) de mĂȘme que celle lâ« unique » (singulier vs. pluriel),  qui permettent de fines analyses sĂ©mantiques se rĂ©vĂšlent ĂȘtre essentielles.      2. Quels sont les critĂšres de lâattribution sur tel ou tel support (sujet ou objet) ? Et de quelle façon le support en est-il affectĂ© ? Le critĂšre de son degrĂ© dâaffectation apparaĂźt et est corrĂ©lĂ© avec celui de sa dĂ©termination.
Stellar and Quasar Feedback in Concert: Effects on AGN Accretion, Obscuration, and Outflows
We study the interaction of feedback from active galactic nuclei (AGN) and a
multi-phase interstellar medium (ISM), in simulations including explicit
stellar feedback, multi-phase cooling, accretion-disk winds, and Compton
heating. We examine radii ~0.1-100 pc around a black hole (BH), where the
accretion rate onto the BH is determined and where AGN-powered winds and
radiation couple to the ISM. We conclude: (1) The BH accretion rate is
determined by exchange of angular momentum between gas and stars in
gravitational instabilities. This produces accretion rates ~0.03-1 Msun/yr,
sufficient to power luminous AGN. (2) The gas disk in the galactic nucleus
undergoes an initial burst of star formation followed by several Myrs where
stellar feedback suppresses the star formation rate (SFR). (3) AGN winds
injected at small radii with momentum fluxes ~L/c couple efficiently to the ISM
and have dramatic effects on ISM properties within ~100 pc. AGN winds suppress
the nuclear SFR by factors ~10-30 and BH accretion rate by factors ~3-30. They
increase the outflow rate from the nucleus by factors ~10, consistent with
observational evidence for galaxy-scale AGN-driven outflows. (4) With AGN
feedback, the predicted column density distribution to the BH is consistent
with observations. Absent AGN feedback, the BH is isotropically obscured and
there are not enough optically-thin sightlines to explain Type-I AGN. A
'torus-like' geometry arises self-consistently as AGN feedback evacuates gas in
polar regions.Comment: 17 pages, 12 figures, MNRAS accepted (revised to match published
version
The formation of massive, quiescent galaxies at cosmic noon
The cosmic noon (z~1.5-3) marked a period of vigorous star formation for most
galaxies. However, about a third of the more massive galaxies at those times
were quiescent in the sense that their observed stellar populations are
inconsistent with rapid star formation. The reduced star formation activity is
often attributed to gaseous outflows driven by feedback from supermassive black
holes, but the impact of black hole feedback on galaxies in the young Universe
is not yet definitively established. We analyze the origin of quiescent
galaxies with the help of ultra-high resolution, cosmological simulations that
include feedback from stars but do not model the uncertain consequences of
black hole feedback. We show that dark matter halos with specific accretion
rates below ~0.25-0.4 per Gyr preferentially host galaxies with reduced star
formation rates and red broad-band colors. The fraction of such halos in large
dark matter only simulations matches the observed fraction of massive quiescent
galaxies (~10^10-10^11 Msun). This strongly suggests that halo accretion rate
is the key parameter determining which massive galaxies at z~1.5-3 become
quiescent. Empirical models that connect galaxy and halo evolution, such as
halo occupation distribution or abundance matching models, assume a tight link
between galaxy properties and the masses of their parent halos. These models
will benefit from adding the specific accretion rate of halos as a second model
parameter.Comment: 5 pages, 5 figures, to appear in MNRAS Letter
Stellar feedback sets the universal acceleration scale in galaxies
It has been established for decades that rotation curves deviate from the Newtonian gravity expectation given baryons alone below a characteristic acceleration scale gâ âŒ10â»âž cm sâ»ÂČâ , a scale promoted to a new fundamental constant in MOND. In recent years, theoretical and observational studies have shown that the star formation efficiency (SFE) of dense gas scales with surface density, SFE âŒ ÎŁ/ÎŁ_(crit) with ÎŁ_(crit)âŒâšpË/mââ©/(ÏG)âŒ1000 M_â pcâ»ÂČ (where âšpË/mââ© is the momentum flux output by stellar feedback per unit stellar mass in a young stellar population). We argue that the SFE, more generally, should scale with the local gravitational acceleration, i.e. that SFE âŒg_(tot)/g_(crit) ⥠(GM_(tot)/RÂČ)/âšpË/mââ©â , where M_(tot) is the total gravitating mass and g_(crit) = âšpË/mââ© = ÏGÎŁ_(crit) â 10â»âž cm sâ»ÂČ â gâ . Hence, the observed gâ may correspond to the characteristic acceleration scale above which stellar feedback cannot prevent efficient star formation, and baryons will eventually come to dominate. We further show how this may give rise to the observed acceleration scaling g_(obs) ⌠(g_(baryon)gâ )^(1/2) (where g_(baryon) is the acceleration due to baryons alone) and flat rotation curves. The derived characteristic acceleration gâ can be expressed in terms of fundamental constants (gravitational constant, proton mass, and Thomson cross-section): gâ âŒ0.1Gmp_/Ï_Tâ
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