Large scale simulations of the jet-IGM interaction

In a parameter study extending to jet densities of $10^{-5}$ times the ambient one, I have recently shown that light large scale jets start their lives in a spherical bow shock phase. This allows an easy description of the sideways bow shock propagation in that phase. Here, I present new, bipolar, simulations of very light jets in 2.5D and 3D, reaching the observationally relevant scale of $>200$ jet radii. Deviations from the early bow shock propagation law are expected because of various effects. The net effect is, however, shown to remain small. I calculate the X-ray appearance of the shocked cluster gas and compare it to Cygnus A and 3C 317. Rings, bright spots and enhancements inside the radio cocoon may be explained.Comment: 8 pages, 5 figures, ApSS accepted, proceedings of the virtual jets 2003 conference in Dogliani/Italy, v3: funny and unimportant bug corrected, one reference adde

Theory of a refined earth model

Refined equations are derived relating the variations of the earths gravity and radius as functions of longitude and latitude. They particularly relate the oblateness coefficients of the old harmonics and the difference of the polar radii /respectively, ellipticities and polar gravity accelerations/ in the Northern and Southern Hemispheres

Transport coefficients for the shear dynamo problem at small Reynolds numbers

We build on the formulation developed in Sridhar & Singh (JFM, 664, 265, 2010), and present a theory of the \emph{shear dynamo problem} for small magnetic and fluid Reynolds numbers, but for arbitrary values of the shear parameter. Specializing to the case of a mean magnetic field that is slowly varying in time, explicit expressions for the transport coefficients, $\alpha_{il}$ and $\eta_{iml}$, are derived. We prove that, when the velocity field is non helical, the transport coefficient $\alpha_{il}$ vanishes. We then consider forced, stochastic dynamics for the incompressible velocity field at low Reynolds number. An exact, explicit solution for the velocity field is derived, and the velocity spectrum tensor is calculated in terms of the Galilean--invariant forcing statistics. We consider forcing statistics that is non helical, isotropic and delta-correlated-in-time, and specialize to the case when the mean-field is a function only of the spatial coordinate $X_3$ and time $\tau\,$; this reduction is necessary for comparison with the numerical experiments of Brandenburg, R{\"a}dler, Rheinhardt & K\"apyl\"a (ApJ, 676, 740, 2008). Explicit expressions are derived for all four components of the magnetic diffusivity tensor, $\eta_{ij}(\tau)\,$. These are used to prove that the shear-current effect cannot be responsible for dynamo action at small \re and \rem, but for all values of the shear parameter.Comment: 27 pages, 5 figures, Published in Physical Review

Late growth stages and post-growth diffusion in organic epitaxy: PTCDA on Ag(111)

The late growth stages and the post-growth diffusion of crystalline organic thin films have been investigated for 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Ag(111), a model system in organic epitaxy. In situ x-ray measurements at the anti-Bragg point during the growth show intensity oscillations followed by a time-independent intensity which is independent of the growth temperature. At T > 350 K, the intensity increases after growth up to a temperature-dependent saturation value due to a post-growth diffusion process. The time-independent intensity and the subsequent intensity recovery have been reproduced by models based on the morphology change as a function of the growth temperature. The morphology found after the post-growth diffusion processes has been studied by specular rod measurements.Comment: 9 pages, 8 figures, accepted for publication in Surface Scienc

Numerical modelling of the lobes of radio galaxies in cluster environments -- IV. Remnant radio galaxies

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.We examine the remnant phase of radio galaxies using three-dimensional hydrodynamical simulations of relativistic jets propagating through cluster environments. By switching the jets off once the lobes have reached a certain length we can study how the energy distribution between the lobes and shocked intra-cluster medium compares to that of an active source, as well as calculate synchrotron emission properties of the remnant sources. We see that as a result of disturbed cluster gas beginning to settle back into the initial cluster potential, streams of dense gas are pushed along the jet axis behind the remnant lobes, causing them to rise out of the cluster faster than they would due to buoyancy. This leads to increased adiabatic losses and a rapid dimming. The rapid decay of total flux density and surface brightness may explain the small number of remnant sources found in samples with a high flux density limit and may cause analytic models to overestimate the remnant fraction expected in sensitive surveys such as those now being carried out with LOFAR.Peer reviewedFinal Accepted Versio

Turbulent transport and dynamo in sheared MHD turbulence with a non-uniform magnetic field

We investigate three-dimensional magnetohydrodynamics turbulence in the presence of velocity and magnetic shear (i.e., with both a large-scale shear flow and a nonuniform magnetic field). By assuming a turbulence driven by an external forcing with both helical and nonhelical spectra, we investigate the combined effect of these two shears on turbulence intensity and turbulent transport represented by turbulent diffusivities (turbulent viscosity, α and β effect) in Reynolds-averaged equations. We show that turbulent transport (turbulent viscosity and diffusivity) is quenched by a strong flow shear and a strong magnetic field. For a weak flow shear, we further show that the magnetic shear increases the turbulence intensity while decreasing the turbulent transport. In the presence of a strong flow shear, the effect of the magnetic shear is found to oppose the effect of flow shear (which reduces turbulence due to shear stabilization) by enhancing turbulence and transport, thereby weakening the strong quenching by flow shear stabilization. In the case of a strong magnetic field (compared to flow shear), magnetic shear increases turbulence intensity and quenches turbulent transport

Probing gaseous halos of galaxies with radio jets

Reproduced with permission from Astronomy & Astrophysics. © 2019 ESOContext. Gaseous halos play a key role in understanding inflow, feedback, and the overall baryon budget in galaxies. Literature models predict transitions of the state of the gaseous halo between cold and hot accretion, winds, fountains, and hydrostatic halos at certain galaxy masses. Since luminosities of radio AGN are sensitive to halo densities, any significant transition would be expected to show up in the radio luminosities of large samples of galaxies. The LOw Frequency ARray (LOFAR) Two-Metre Sky Survey (LoTSS) has identified a galaxy stellar mass scale, 10 11 M ⊙, above which the radio luminosities increase disproportionately. Aims. We investigate if radio luminosities of galaxies, especially the marked rise at galaxy masses around 10 11 M ⊙, can be explained with standard assumptions regarding jet powers, scaling between black hole mass and galaxy mass, and gaseous halos. Methods. Based on observational data and theoretical constraints, we developed models for the radio luminosity of radio AGN in halos under infall, galactic wind, and hydrostatic conditions. We compared these models to LoTSS data for a large sample of galaxies in the mass range between 10 8.5 M ⊙ and 10 12 M ⊙. Results. Under the assumption that the same characteristic upper limit to jet powers known from high galaxy masses holds at all masses, we find the maximum radio luminosities for the hydrostatic gas halos to lie close to the upper envelope of the distribution of the LOFAR data. The marked rise in radio luminosity at 10 11 M ⊙ is matched in our model and is related to a significant change in halo gas density around this galaxy mass, which is a consequence of lower cooling rates at a higher virial temperature. Wind and infall models overpredict the radio luminosities for small galaxy masses and have no particular steepening of the run of the radio luminosities predicted at any galaxy mass. Conclusions. Radio AGN could have the same characteristic Eddington-scaled upper limit to jet powers in galaxies of all masses in the sample if the galaxies have hydrostatic gas halos in phases when radio AGN are active. We find no evidence of a change of the type of galaxy halo with the galaxy mass. Galactic winds and quasi-spherical cosmological inflow phases cannot frequently occur at the same time as powerful jet episodes unless the jet properties in these phases are significantly different from what we assumed in our model.Peer reviewedFinal Accepted Versio

The atom-molecule reaction D plus H2 yields HD plus H studied by molecular beams

Collisions between deuterium atoms and hydrogen molecules were studied in a modulated crossed beam experiment. The relative signal intensity and the signal phase for the product HD from reactive collisions permitted determination of both the angular distribution and HD mean velocity as a function of angle. From these a relative differential reactive scattering cross section in center-of-mass coordinates was deduced. The experiment indicates that reactively formed HD which has little or no internal excitation departs from the collision anisotropically, with maximum amplitude 180 deg from the direction of the incident D beam in center-of-mass coordinates, which shows that the D-H-H reacting configuration is short-lived compared to its rotation time. Non reactive scattering of D by H2 was used to assign absolute values to the differential reactive scattering cross sections

Excitation of Na D-line radiation in collisions of sodium atoms with internally excited H2, D2, and N2

Excitation of D-line radiation in collisions of Na atoms with vibrationally excited N2, H2 and D2 was studied in two modulated crossed beam experiments. In both experiments, the vibrational excitation of the molecules was provided by heating the molecular beam source to temperatures in the range of 2000 to 3000 K, which was assumed to give populations according to the Boltzmann expression. In the first experiment, a total rate coefficient was measured as a function of molecular beam temperature, with absolute calibration of the photon detector being made using the black body radiation from the heated molecular beam source. Since heating affects both the internal energy and the collisional kinetic energy, the first experiment could not determine the relative contributions of internal energy transfer versus collisional excitation. The second experiment achieved partial separation of internal versus kinetic energy transfer effects by using a velocity-selected molecular beam. Using two simple models for the kinetic energy dependence of the transfer cross section for a given change in vibrational quantum number, the data from both experiments were used to determine parameters in the models