QSO Lyman-Alpha Absorbers and Galaxy Halos

We review the current status of our survey of galaxies in fields of HST target QSOs, which has allowed us to identify the galaxies responsible for a number of Lyman-α absorption systems. We emphasize the use of QSO absorption lines to study the structure and kinematics of the large gaseous halos that virtually all galaxies appear to possess. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamical friction in dwarf galaxies

We present a simplified analytic approach to the problem of the spiralling of a massive body orbiting within the dark halo of a dwarf galaxy. This dark halo is treated as the core region of a King distribution of dark matter particles, in consistency with the observational result of dwarf galaxies having solid-body rotation curves. Thus we derive a simple formula which provides a reliable and general first-order solution to the problem, totally analogous to the one corresponding to the dynamical friction problem in an isothermal halo. This analytic approach allows a clear handling and a transparent understanding of the physics and the scaling of the problem. A comparison with the isothermal case shows that in the core regions of a King sphere, dynamical friction proceeds at a different rate, and is sensitive to the total core radius. Thus, in principle, observable consequences may result. In order to illustrate the possible effects, we apply this formula to the spiralling of globular cluster orbits in dwarf galaxies, and show how present-day globular cluster systems could, in principle, be used to derive better limits on the structure of dark haloes around dwarf galaxies, when the observational situation improves. As a second application, we study the way a massive black hole population forming a fraction of these dark haloes would gradually concentrate towards the centre, with a consequent deformation of an originally solid-body rotation curve. This effect allows us to set limits on the fraction/mass of any massive black hole minority component of the dark haloes of dwarf galaxies. In essence, we take advantage of the way the global matter distribution fixes the local distribution function for the dark matter particles, which in turn determines the dynamical friction problem.     

Circular polarization in pulsar integrated profiles

We present a systematic study of the circular polarization in pulsar integrated profiles, based on published polarization data. For core components, we find no significant correlation between the sense change of circular polarization and the sense of linear position-angle variation. Circular polarization is not restricted to core components and, in some cases, reversals of circular polarization sense are observed across the conal emission. In conal double profiles, the sense of circular polarization is found to be correlated with the sense of position-angle variation. Pulsars with a high degree of linear polarization often have one hand of circular polarization across the whole profile. For most pulsars, the sign of circular polarization is the same at 50-cm and 20-cm wavelengths, and the degree of polarization is similar, albeit with a wide scatter. However, at least two cases of frequency-dependent sign reversals are known. This diverse behaviour may require more than one mechanism to generate circular polarization.     

Discovery of an X-ray-selected radio-loud obscured AGN at z = 1.246

We have discovered an obscured active galaxy at redshift z  = 1.246 identified with the ROSAT X-ray source RX J1011.2+5545. We report on multiwavelength observations of this source and discuss its X-ray, optical and radio properties. This is the first X-ray-selected, obscured active galaxy at high redshift to be shown to be radio-loud, with a radio counterpart exhibiting a classical double-lobe morphology.     

Dynamic Characteristics of Granite Subjected to Intermediate Loading Rate

Summary A large diameter split Hopkinson pressure bar (SHPB) has been developed. This equipment is briefly described, together with a shaped striker that initiates a half-sine incident waveform to obtain the complete stress-strain relationship of the Bukit Timah granite at medium strain rate. Good constant strain rate was derived, and the dynamic complete stress-strain curves and energy absorption of the granite were measured at a strain rate between 20 and 60 per second. Repeated impact between 60–90% of the static strength of the granite was also conducted. Results from the tests show that the cumulative damage of the granite depends on the peak stress of the dynamic loads with a fixed duration. The dynamic fracture strength of the granite loaded at medium strain rate is directly proportional to the cube root of the strain rate. For the granite loaded at this strain rate, Youngs modulus is unchanged. Energy absorption of the samples loading to fragmentation determined its fragmented size distribution. At high strain rate, the rock possesses large energy absorption and the particle size of the fragments is much smaller.     

The Schwabe and Gleissberg Periods in the Wolf Sunspot Numbers and the Group Sunspot Numbers

Three wavelet functions: the Morlet wavelet, the Paul wavelet, and the DOG wavelet have been respectively performed on both the monthly Wolf sunspot numbers (R_z) from January 1749 to May 2004 and the monthly group sunspot numbers (R_g) from June 1795 to December 1995 to study the evolution of the Gleissberg and Schwabe periods of solar activity. The main results obtained are (1) the two most obvious periods in both the R_z and R_g are the Schwabe and Gleissberg periods. The Schwabe period oscillated during the second half of the eighteenth century and was steady from the 1850s onward. No obvious drifting trend of the Schwabe period exists. (2) The Gleissberg period obviously drifts to longer periods the whole consideration time, and the drifting speed of the Gleissberg period is larger for R_z than for R_g. (3) Although the Schwabe-period values for R_z and R_g are about 10.7 years, the value for R_z seems slightly larger than that for R_g. The Schwabe period of R_z is highly significant after the 1820s, and the Schwabe period of R_g is highly significant over almost the whole consideration time except for about 20 years around the 1800s. The evolution of the Schwabe period for both R_z and R_g in time is similar to each other. (4) The Gleissberg period in R_z and R_g is highly significant during the whole consideration time, but this result is unreliable at the two ends of each of the time series of the data. The evolution of the Gleissberg period in R_z is similar to that in R_g.     

ISO2002: an analytical stochastic model of multi-difference GPS carrier-phase data

The differencing technique is useful in global positioning system (GPS) positioning when two or more GPS receivers collect simultaneous observables from common satellites at each epoch, and all carrier-phase observables have the same normal distribution. An analytical probability distribution of the single-, double-, triple- and multi-difference GPS observables is obtained. This analytical model, called ISO2002, has a good matrix structure, in which I indicates the number of receivers, S indicates the number of observed satellites, and O indicates the number of epochs. The variance–covariance matrix can be expressed as the Kronecker product of several small matrices, so its inverse is equal to the Kronecker product of the inverses of these sub-matrices. Moreover, these small matrices are circulant or symmetric diagonal Toeplitz matrices, so their inverses have analytical solutions. The analytical model ISO2002 proposed to compute the inverse variance–covariance matrix is shown to be very effective.