Modern Physics Letters A

We review the latest attempts to determine the accretion geometry in radio-loud active galactic nuclei (AGN). These objects, which comprise ~ 10–20% of the AGN population, produce powerful collimated radio jets that can extend thousands of parsecs from the center of the host galaxy. Recent multiwavelength surveys have shown that radio-loudness is more common in low-luminosity AGN than in higher luminosity Seyfert galaxies or quasars. These low-luminosity AGN have small enough accretion rates that they are most likely accreting via a geometrically thick and radiatively inefficient accretion flow. In contrast, X-ray spectroscopic observations of three higher luminosity broad-line radio galaxies (3C 120, 4C+74.26 and PG 1425+267) have found evidence for an untruncated thin disk extending very close to the black hole. These tentative detections indicate that, for this class of radio-loud AGN, the accretion geometry is very similar to their radio-quiet counterparts. These observations suggest that there are three conditions to jet formation that must be satisfied: the presence of a rapidly spinning black hole, an accretion flow with a large H/r ratio, and a favorable magnetic field geometry.

Damped Lyman-α systems (DLAs) are useful probes of star formation and galaxy formation at high-redshift (hereafter high-z). We study the physical properties of DLAs and their relationship to Lyman break galaxies (LBGs) using cosmological hydrodynamic simulations based on the concordance Λ cold dark matter model. Fundamental statistics such as global neutral hydrogen (H I) mass density, H I column density distribution function, DLA rate-of-incidence and mean halo mass of DLAs are reproduced reasonably well by the simulations, but with some deviations that need to be understood better in the future. We discuss the feedback effects by supernovae and galactic winds on the DLA distribution. We also compute the [C II] emission from neutral gas in high-z galaxies, and make predictions for the future observations by the Atacama Large Millimeter Array (ALMA) and Space Infrared Telescope for Cosmology and Astrophysics (SPICA). Agreement and disagreement between simulations and observations are discussed, as well as the future directions of our DLA research.

The infrared behavior of the gluon propagator is directly related to confinement in QCD. Indeed, the Gribov–Zwanziger scenario of confinement predicts an infrared vanishing (transverse) gluon propagator in Landau-like gauges, implying violation of reflection positivity and gluon confinement. Finite-volume effects make it very difficult to observe (in the minimal Landau gauge) an infrared suppressed gluon propagator in lattice simulations of the four-dimensional case. Here we report results for the SU(2) gluon propagator in a gauge that interpolates between the minimal Landau gauge (for gauge parameter λ equal to 1) and the minimal Coulomb gauge (corresponding to λ = 0). For small values of λ we find that the spatially-transverse gluon propagator D^tr(0, |p|), considered as a function of the spatial momenta |p|, is clearly infrared suppressed. This result is in agreement with the Gribov–Zwanziger scenario and with previous numerical results in the minimal Coulomb gauge. We also discuss the nature of the limit λ→0 (complete Coulomb gauge) and its relation to the standard Coulomb gauge (λ = 0). Our findings are corroborated by similar results in the three-dimensional case, where the infrared suppression is observed for all considered values of λ.

We find a (4+1)-dimensional stationary vacuum hyper-cylindrical solution which is spherically symmetric in three dimensions and invariant under the translation along the fifth coordinate. The solution is characterized by three parameters, mass, tension, and conserved momentum along the fifth coordinate. The metric is locally equivalent to the known static solution. We briefly discuss its physical properties.

We discuss different aspects of the (2+2)-signature from the point of view of the quatl theory. In particular, we compare two alternative approaches to such a spacetime signature, namely the (1+1)-matrix-brane and the (2+2)-target spacetime of a string. This analysis also reveals hidden discrete symmetries of the (2+2)-brane action associated with the (2+2)-dimensional sector of a (2+10)-dimensional target background.

It is shown how the two experimentally measurable properties of the mixing matrix V, the asymmetry Δ(V) = |V₁₂|² - |V₂₁|² of V with respect to the main diagonal and the Jarlskog invariant , can be exploited to obtain constraints on possible structures of mass matrices in the quark sector. Specific mass matrices are examined in detail as an illustration.

In this paper, based on the gauge potential decomposition and the ϕ-mapping theories, we study the topological structures and properties of the cosmic strings that arise in the Abelian–Higgs gauge theory in the zero-thickness limit. After a detailed discussion, we conclude that the topological tensor current introduced in our model is a better and more basic starting point than the generally used Nambu–Goto effective action for studying cosmic strings.