Laboratoire de Radioastronomie ENS - LERMA UMR 8112

Journal Club//2011 1e trimestre

Journal Club on 2011/03/15

(Ce texte n'est pas disponible en français, nous nous en excusons.)

A Study of Magnetic Helicity in Forced and Decaying 3D-MHD Turbulence

By Kumar Shiva Malapaka (LJLL, Paris)

Room D18 (2nd floor) at ENS, 24 rue Lhomond, 13:30 to 14:30


Large-scale magnetic structure formation in the universe is one of the open problems in modern astrophysics. A possible explanation for the formation of such structures could be offered from a property of 3D- magnetohydrodynamic (MHD) turbulence, namely inverse cascade of magnetic helicity. Magnetic helicity is defined as the volume integral of the dot product of the magnetic field and the magnetic vector potential. Inverse cascade means the transfer of that quantity spectrally from small scales to large scales, with a constant flux.
We report some of the important results form the spectral and structural study of this property using high resolution Direct Numerical Simulations (DNS) in two setups namely, forced 3D-MHD turbulence and decaying 3D-MHD turbulence. These results include self-similar behavior in various quantities of the turbulent flow in their spectra; which were hitherto unknown. Dimensional analysis of magnetic heilicity equation from the eddy damped quasi normal Markovian approximation (EDQNM) equation set, yields a relation which relates magnetic helicity and kinetic helicity, along with the kinetic and magnetic energies, in the spectral space. This relation is verified using the simulation data of both forced and decaying MHD turbulences. This relation brings out the importance of kinetic helicity in the transfer of magnetic helicity to large scales.
Statistical analysis of structures in the the turbulent fields in both the cases will also be presented.



Dernière mise à jour 14-03-2011 15:33 / Marc Joos

Journal Club on 2011/03/23

(Ce texte n'est pas disponible en français, nous nous en excusons.)

From large scale gas compression to cluster formation in the Antennae overlap region

By Cinthya Herrera (IAS, Orsay)

Room D18 (2nd floor) at ENS, 24 rue Lhomond, 13:30 to 14:30

Galaxy mergers underlie the hierarchical growth of galaxies. Interaction between gas-rich galaxies are known to trigger IR luminous bursts of star formation, but our understanding of how star formation is physically sparked is unclear. Observations of the nearby Antennae merger show that star formation occurs in the overlap region, where the two galaxies collide and where numerous Super-Star Clusters (SSCs) with masses up to a few 10^6 M? are found. What is not yet understood is how these clusters form?

The early phases of their evolution and formation - very young clusters still deeply embedded in their parent cloud, and, above all, the preceding stages of fragmentation and collapse of molecular gas - have not yet been observed. Stars must form where the mechanical energy of the collision is being dissipated. Recent observations of interacting galaxies show that this dissipation is traced by H2 line emission. Guided by Spitzer observations of the H2 rotational line emission, we selected to observe a 600 pc wide field around one of the brightest and youngest SSCs in the Antennae overlap region, with the VLT/SINFONI spectro-imager. I will present the observations and our data interpretation which connects the large scale gas dynamics from the galaxies interaction to the formation and early evolution of massive clusters. I highlight here the most spectacular result.
The data reveal a compact source (60 pc diameter) with a K-band spectrum showing bright, spectrally resolved, H2 lines, but no Brgamma nor continuum emission. The source exceptional H2 line luminosity (6x10^6 L?) and line widths (FWHM = 200 km/s) may be accounted for by the rapid (1 Myr) collapse of a massive (virial mass 3x10^7 M?) cloud. If this interpretation is right we may have discovered a pre-cluster cloud, which is in its final evolution towards the formation of a SSC. We will propose ALMA observations of mass tracers, molecular and dust emission, to test this interpretation.



Dernière mise à jour 08-03-2011 09:29 / Marc Joos

Journal Club on 2011/03/30

(Ce texte n'est pas disponible en français, nous nous en excusons.)

Spectra Line Energy Distributions (SLEDs) from ground to space and back...

By Padelis Papadopoulos (Bonn University)

Room L269 (former D18, 2nd floor)  at ENS, 24 rue Lhomond, 13:30 to 14:30


I will be presenting results from a CO and HCN multi-J line survey of local IR-luminous galaxies and QSOs conducted over the last 5 years with the JCMT and the IRAM 30-m telescope, and recently extended to Space with the Herschel Space Observatory (key project HerCULES). Local benchmarks on SLEDs allow a revisit of the sparsely sampled CO SLEDs of high-z systems such as submm galaxies (SMGs), BzK disk galaxies and powerful radio galaxies and the state of their molecular ISM. I will then: a) stick out some predictions, and b) offer some general directions for ALMA science.


Dernière mise à jour 07-03-2011 10:29 / Marc Joos

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