The Numerical Simulation of Radiative Shocks I: The elimination of numerical shock instabilities using a localized oscillation filter

We address a numerical instability that arises in the directionally split computation of hydrodynamic flows when shock fronts are parallel to a grid plane. Transverse oscillations in pressure, density and temperature are produced that are exacerbated by thermal instability when cooling is present, forming post--shock `stripes'. These are orthogonal to the classic post--shock 'ringing' fluctuations. The resulting post--shock `striping' substantially modifies the flow. We discuss three different methods to resolve this problem. These include (1) a method based on artificial viscosity; (2) grid--jittering and (3) a new localized oscillation filter that acts on specific grid cells in the shock front. These methods are tested using a radiative wall shock problem with an embedded shear layer. The artificial viscosity method is unsatisfactory since, while it does reduce post--shock ringing, it does not eliminate the stripes and the excessive shock broadening renders the calculation of cooling inaccurate, resulting in an incorrect shock location. Grid--jittering effectively counteracts striping. However, elsewhere on the grid, the shear layer is unphysically diffused and this is highlighted in an extreme case. The oscillation filter method removes stripes and permits other high velocity gradient regions of the flow to evolve in a physically acceptable manner. It also has the advantage of only acting on a small fraction of the cells in a two or three dimensional simulation and does not significantly impair performance.Comment: 20 pages, 6 figures, revised version submitted to ApJ Supplement Serie

Radio-optically selected clusters of galaxies. I. The radiogalaxy sample

In order to study the status and the possible evolution of clusters of galaxies at intermediate redshifts (z ~ 0.1 - 0.3), as well as their spatial correlation and relationship with the local environment, we built a sample of candidate groups and clusters of galaxies using radiogalaxies as tracers of dense environments. This technique - complementary to purely optical or X-ray cluster selection methods - represents an interesting tool for the selection of clusters in a wide range of richness, so to make it possible to study the global properties of groups and clusters of galaxies, such as their morphological content, dynamical status and number density, as well as the effect of the environment on the radio emission phenomena. In this paper we describe the compilation of a catalogue of 16000 radio sources in the region of the South Galactic Pole extracted from the publicly available NRAO VLA Sky Survey maps, and the optical identification procedure with galaxies brighter than b_J=20.0 in the EDSGC Catalogue. The radiogalaxy sample, valuable for the study of radio source populations down to low flux levels, consists of 1288 identifications and has been used to detect candidate groups and clusters associated to NVSS radio sources. In a companion paper we will discuss the cluster detection method, the cluster sample as well as first spectroscopic results.Comment: 15 pages, 6 Postscript and 1 GIF figures. Accepted for publication in A&

PKS B1545-321: Bow shocks of a relativistic jet?

Sensitive, high resolution images of the double-double radio galaxy PKS B1545-321 reveal detailed structure, which we interpret in the light of previous work on the interaction of restarted jets with pre-existing relict cocoons. We have also examined the spectral and polarization properties of the source, the color distribution in the optical host and the environment of this galaxy in order to understand its physical evolution. We propose that the restarted jets generate narrow bow shocks and that the inner lobes are a mixture of cocoon plasma reaccelerated at the bow shock and new jet material reaccelerated at the termination shock. The dynamics of the restarted jets implies that their hot spots advance at mildly relativistic speeds with external Mach numbers of at least 5. The existence of supersonic hot spot Mach numbers and bright inner lobes is the result of entrainment causing a reduction in the sound speed of the pre-existing cocoon. The interruption to jet activity in PKS B1545-321 has been brief - lasting less than a few percent of the lifetime $\sim (0.3-2)\times 10^{8} yr$ of the giant radio source. The host galaxy is located at the boundary of a large scale filamentary structure, and shows blue patches in color distribution indicative of a recent merger, which may have triggered the Mpc-scale radio galaxy.Comment: 26 pages including 1 table and 16 figures. To appear in MNRA

Correlation of the Cd-to-Te ratio on CdTe surfaces with the surface structure

We report here that reconstruction on (100), (1lIlA, and (1l1lB CdTe surfaces is either C(2X2), (2X2), and (l X I) or (2X I), (l X I), and (l X I) when they are Cd or Te stabilized, respectively. There is a mixed region between Cd and Te stabilization in which the reflected high-energy electron-diffraction (RHEED) patterns contain characteristics of both Cd- and Te-stabilized surfaces. We have also found that the Cd-to-Te ratio of the x-ray photoelectron intensities of their 3d$_{3/ 2}$ core levels is about 20% larger for a Cd-stabilized (1lIlA, (1lIlB, or (100) CdTe surface than for a Te-stabilized one. According to a simple model calculation, which was normalized by means of the photoelectron intensity ratio of a Cd-stabilized (lll)A and aTe-stabilized (1l1lB CdTe surface, the experimental data for CdTe surfaces can be explained by a linear dependence of the photoelectron-intensity ratio on the fraction of Cd in the uppermost monatomic layer. This surface composition can be correlated with the surface structure, i.e., the corresponding RHEED patterns. This correlation can in turn be employed to determine Te and Cd evaporation rates. The Te reevaporation rate is increasingly slower for the Te-stabilized (Ill) A, (l1l)B, and (100) surfaces, while the opposite is true for Cd from Cd-stabilized (Ill) A and (Ill)B surfaces. In addition, Te is much more easily evaporated from all the investigated surfaces than is Cd, if the substrate is kept at normal molecular-beam-epitaxy growth temperatures ranging from 2oo·C to 300 ·C

Three episodes of jet activity in the FRII radio galaxy B0925+420

We present Very Large Array images of a "Double-Double Radio Galaxy", a class of objects in which two pairs of lobes are aligned either side of the nucleus. In this object, B0925+420, we discover a third pair of lobes, close to the core and again in alignment with the other lobes. This first-known "Triple-Double" object strongly increases the likelihood that these lobes represent mutiple episodes of jet activity, as opposed to knots in an underlying jet. We model the lobes in terms of their dynamical evolution. We find that the inner pair of lobes is consistent with the outer pair having been displaced buoyantly by the ambient medium. The middle pair of lobes is more problematic - to the extent where an alternative model interpreting the middle and inner "lobes" as additional bow shocks within the outer lobes may be more appropriate - and we discuss the implications of this on our understanding of the density of the ambient medium.Comment: Accepted for publication in MNRAS. Figure 2 is best viewed in colou