From kinetic theory to dissipative fluid dynamics

We present the results of deriving the Israel-Stewart equations of relativistic dissipative fluid dynamics from kinetic theory via Grad's 14-moment expansion. Working consistently to second order in the Knudsen number, these equations contain several new terms which are absent in previous treatments.Comment: 7 pages, proceedings of the Erice School on Nuclear Physics "Heavy Ion collisions from the Coulomb Barrier up to the Quark Gluon Plasma", Erice, Sicily, Sep. 16 - 24, 200

A rotating helical filament in the L1251 dark cloud

(Abridged) Aims. We derive the physical properties of a filament discovered in the dark cometary-shaped cloud L1251. Methods. Mapping observations in the NH3(1,1) and (2,2) inversion lines, encompassing 300 positions toward L1251, were performed with the Effelsberg 100-m telescope at a spatial resolution of 40 arcsec and a spectral resolution of 0.045 km/s. Results. The filament L1251A consists of three condensations (alpha, beta, and gamma) of elongated morphology, which are combined in a long and narrow structure covering a 38 arcmin by 3 arcmin angular range. The opposite chirality (dextral and sinistral) of the alpha+beta and gamma condensations indicates magnetic field helicities of two types, negative and positive, which were most probably caused by dynamo mechanisms. We estimated the magnetic Reynolds number Rm > 600 and the Rossby number R < 1, which means that dynamo action is important.Comment: 21 pages, 10 figures, 1 table. Accepted for publication in A&

Star-forming regions of the Aquila rift cloud complex. II. Turbulence in molecular cores probed by NH3 emission

(Abridged) Aims. We intend to derive statistical properties of stochastic gas motion inside the dense low mass star forming molecular cores traced by NH3(1,1) and (2,2) emission lines. Methods. We use the spatial two-point autocorrelation (ACF) and structure functions calculated from maps of the radial velocity fields. Results. We find oscillating ACFs which eventually decay to zero with increasing lags on scales of 0.04 <= l <= 0.5 pc. The current paradigm supposes that the star formation process is controlled by the interplay between gravitation and turbulence, the latter preventing molecular cores from a rapid collapse due to their own gravity. Thus, oscillating ACFs may indicate a damping of the developed turbulent flows surrounding the dense but less turbulent core - a transition to dominating gravitational forces and, hence, to gravitational collapse.Comment: 11 pages, 16 figures, 3 tables, to be published in Astronomy and Astrophysic

Lattice two-point functions and conformal invariance

A new realization of the conformal algebra is studied which mimics the behaviour of a statistical system on a discrete albeit infinite lattice. The two-point function is found from the requirement that it transforms covariantly under this realization. The result is in agreement with explicit lattice calculations of the $(1+1)D$ Ising model and the $d-$dimensional spherical model. A hard core is found which is not present in the continuum. For a semi-infinite lattice, profiles are also obtained.Comment: 5 pages, plain Tex with IOP macros, no figure

DGSAT: Dwarf Galaxy Survey with Amateur Telescopes II. A catalogue of isolated nearby edge-on disk galaxies and the discovery of new low surface brightness systems

The connection between the bulge mass or bulge luminosity in disk galaxies and the number, spatial and phase space distribution of associated dwarf galaxies is a discriminator between cosmological simulations related to galaxy formation in cold dark matter and generalized gravity models. Here, a nearby sample of isolated Milky Way class edge-on galaxies is introduced, to facilitate observational campaigns to detect the associated families of dwarf galaxies at low surface brightness. Three galaxy pairs with at least one of the targets being edge-on are also introduced. About 60% of the catalogued isolated galaxies contain bulges of different size, while the remaining objects appear to be bulge-less. Deep images of NGC 3669 (small bulge, with NGC 3625 at the edge of the image) and NGC 7814 (prominent bulge), obtained with a 0.4-m aperture, are also presented, resulting in the discovery of two new dwarf galaxy candidates, NGC3669-DGSAT-3 and NGC7814-DGSAT-7. Eleven additional low surface brightness galaxies are identified, previously notified with low quality measurement flags in the Sloan Digital Sky Survey (SDSS). Integrated magnitudes, surface brightnesses, effective radii, Sersic indices, axis ratios, and projected distances to their putative major hosts are displayed. At least one of the galaxies, NGC3625-DGSAT-4, belongs with a surface brightness of approximately 26 mag per arcsec^2 and effective radius >1.5 kpc to the class of ultra-diffuse galaxies (UDGs). NGC3669-DGSAT-3, the galaxy with lowest surface brightness in our sample, may also be an UDG.Comment: 12 pages including 6 figures, 4 tables, a brief appendix, accepted for publication in Astronomy & Astrophysics (A&A). Paper slightly modified after A&A language editing, updating very few references and correcting a small typo at the start of the Appendi

Noise-induced dynamical transition in systems with symmetric absorbing states

We investigate the effect of noise strength on the macroscopic ordering dynamics of systems with symmetric absorbing states. Using an explicit stochastic microscopic model, we present evidence for a phase transition in the coarsening dynamics, from an Ising-like to a voter-like behavior, as the noise strength is increased past a nontrivial critical value. By mapping to a thermal diffusion process, we argue that the transition arises due to locally-absorbing states being entered more readily in the high-noise regime, which in turn prevents surface tension from driving the ordering process.Comment: v2 with improved introduction and figures, to appear in PRL. 4 pages, 4 figure

A Survey for H2O Megamasers III. Monitoring Water Vapor Masers in Active Galaxies

We present single-dish monitoring of the spectra of 13 extragalactic water megamasers taken over a period of 9 years and a single epoch of sensitive spectra for 7 others. Our data include the first K-band science observations taken with the new 100 m Green Bank Telescope (GBT). In the context of a circumnuclear, molecular disk model, our results suggest that either (a) the maser lines seen are systemic features subject to a much smaller acceleration than present in NGC 4258, presumably because the gas is farther from the nuclear black hole, or (b) we are detecting ``satellite'' lines for which the acceleration is in the plane of the sky. We also report a search for water vapor masers towards the nuclei of 58 highly inclined, nearby galaxies.Comment: accepted by ApJ

Three-dimensional Roton-Excitations and Supersolid formation in Rydberg-excited Bose-Einstein Condensates

We study the behavior of a Bose-Einstein condensate in which atoms are weakly coupled to a highly excited Rydberg state. Since the latter have very strong van der Waals interactions, this coupling induces effective, nonlocal interactions between the dressed ground state atoms, which, opposed to dipolar interactions, are isotropically repulsive. Yet, one finds partial attraction in momentum space, giving rise to a roton-maxon excitation spectrum and a transition to a supersolid state in three-dimensional condensates. A detailed analysis of decoherence and loss mechanisms suggests that these phenomena are observable with current experimental capabilities.Comment: 4 pages, 5 figure