Galaxy evolution, cosmology and dark energy with the Square Kilometer Array

The present-day Universe is seemingly dominated by dark energy and dark matter, but mapping the normal (baryonic) content remains vital for both astrophysics – understanding how galaxies form – and astro-particle physics – inferring properties of the dark components.The Square Kilometer Array (SKA) will provide the only means of studying the cosmic evolution of neutral hydrogen (HI) which, alongside information on star formation from the radio continuum, is needed to understand how stars formed from gas within dark-matter over-densities and the rôles of gas accretion and galaxy merging.‘All hemisphere’ HI redshift surveys to z 1.5 are feasible with wide-field-of-view realizations of the SKA and, by measuring the galaxy power spectrum in exquisite detail, will allow the first precise studies of the equation-of-state of dark energy. The SKA will be capable of other uniquely powerful cosmological studies including the measurement of the dark-matter power spectrum using weak gravitational lensing, and the precise measurement of H₀ using extragalactic water masers.The SKA is likely to become the premier dark-energy-measuring machine, bringing breakthroughs in cosmology beyond those likely to be made possible by combining CMB (e.g. Planck), optical (e.g. LSST, SNAP) and other early-21st-century datasets.     

On the Saturation of Astrophysical Dynamos: Numerical Experiments with the No-Cosines Flow

In the context of astrophysical dynamos we illustrate that the no-cosines flow, with zero mean helicity, can drive fast dynamo action and we study the dynamo’s mode of operation during both the linear and non-linear saturation regimes. It turns out that in addition to a high growth rate in the linear regime, the dynamo saturates at a level significantly higher than normal turbulent dynamos, namely at exact equipartition when the magnetic Prandtl number Pr_m∼ 1. Visualization of the magnetic and velocity fields at saturation will help us to understand some of the aspects of the non-linear dynamo problem.     

On The Concept Of Periapsis In Hill’s Problem

The concept of closest approach is analyzed in Hill’s problem, resulting in a partitioning of the position space. The different behavior between the direct and retrograde motion is explained analytically, resulting in a simple estimate of the variation of Hill’s periodic and quasi-circular orbits as a function of the Jacobi constant. The local behavior of the orbits on the zero velocity surfaces and an analytical definition of local escape and capture in Hill’s problem are also given.     

Conditions for the Formation and Maintenance of Filaments – (Invited Review)

Observational conditions for the formation and maintenance of filaments are reviewed since 1989 in the light of recent findings on their structure, chirality, inferred magnetic topology, and mass flows. Recent observations confirm the necessary conditions previously cited: (1) their location at a boundary between opposite-polarity magnetic fields (2) a system of overlying coronal loops, (3) a magnetically-defined channel beneath, (4) the convergence of the opposite-polarity network magnetic fields towards their common boundary within the channel and (5) cancellation of magnetic flux at the common polarity boundary. Evidence is put forth for three additional conditions associated with fully developed filaments: (A) field-aligned mass flows parallel with their fine structure (B) a multi-polar background source of small-scale magnetic fields necessary for the formation of the filament barbs and (C) a handedness property known as chirality which requires them to be either of two types, dextral or sinistral. One-to-one relationships have been established between the chirality of filaments and the chirality of their filament channels and overlying coronal arcades. These findings reinforce earlier evidence that every filament magnetic field is separate from the magnetic field of the overlying arcade but both are parts of a larger magnetic field system. The larger system has at least quadrupolar footprints in the photosphere and includes the filament channel and subphotospheric magnetic fields, This ‘systems’ view of filaments and their environment enables new perspectives on why arcades and channels are invariable conditions for their existence. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005026814076     

On the existence of prevalent westward shifts in the solar atmosphere

A statistical analysis of the deviations of the sideral angular rotation velocities () of single sunspot groups from the averaged rotation curve , shows that positive deviations tend to be greater than negative ones: we can therefore deduce the existence of prevalent westward shifts in the solar atmosphere. This effect is only partially due to the sunspot group proper motions at the beginning of their evolution, and it could indicate the prevalence of westward currents in the solar photosphere.     

Diffuse interstellar bands in low E (B – V) objects

We analyse spectra of slightly reddened, early-type stars in which the major diffuse interstellar bands at 5870 and 5797 Å are either strong or weak in relation to E(B – V) . It is demonstrated that among the low E(B – V) objects one can find stars obscured by 'sigma' as well as 'zeta' type clouds. The profiles of the diffuse bands seem not to be dependent on the total opacities of the interstellar clouds. We also discuss the physical conditions leading to the formation of the diffuse band carriers in low-opacity interstellar clouds; the carriers are apparently formed in media in which one can observe complex velocity fields.     

Frequency map analysis of the orbital structure in elliptical galaxies

We present an application of the frequency map analysis to an elliptical galaxy which is represented by a generalization of a double-power-law spherical mass model. The density distribution of this model varies as r ^−γ close to the centre and as r ⁻⁴ at large radii. We study the case with γ = 1, which is known as the 'weak-cusp' model and which represents well the density profile of the 'core' galaxies observed by the Hubble Space Telescope . The final objective of our work is to improve our understanding of the dynamics of elliptical galaxies in a similar way to Merritt &38; Fridman, finding the regions of stochasticity, looking for resonances that might play an important role in sustaining the triaxial morphology, and analysing the diffusion of orbits. To this end, we use the frequency map analysis of Laskar, which has been applied widely in the field of celestial mechanics but which is a relatively new technique in the area of galactic dynamics. Finally, we show some useful features of this method in understanding the global dynamical structure of the system.     

Fast variability of BL Lacertae at 1 μm

We present the results of photometric observations of BL Lacertae at 1 μm performed during the strong outburst that occurred in the summer of 1997. Measurements were carried out using the F99 filter of the Arizona system. Simultaneous observations in the B V bands were made with other telescopes. Large and fast variations were observed on the night of August 2 when the brightness of BL Lacertae changed by ∼ 0.5 mag in about 2 h: the variation amplitude is greater at higher frequency. Some implications of these results for the origin of the fast variations are discussed. We also report the first F99 calibration for three reference stars in the BL Lac field.     

Magnetospheric accretion and pre-main-sequence stellar masses

We present a method of determining lower limits on the masses of pre-main-sequence (PMS) stars and so constraining the PMS evolutionary tracks. This method uses the redshifted absorption feature observed in some emission-line profiles of T Tauri stars, indicative of infall. The maximum velocity of the accreting material measures the potential energy at the stellar surface, which, combined with an observational determination of the stellar radius, yields the stellar mass. This estimate is a lower limit owing to uncertainties in the geometry and projection effects. Using available data, we show that the computed lower limits can be larger than the masses derived from PMS evolutionary tracks for M   0.5 M. Our analysis also supports the notion that accretion streams do not impact near the stellar poles but probably hit the stellar surface at moderate latitudes.