6C radio galaxies at z~1: The influence of radio power on the alignment effect

Powerful radio galaxies often display enhanced optical/UV continuum emission and extended emission line regions, elongated and aligned with the radio jet axis. The expansion of the radio source strongly affects the gas clouds in the surrounding IGM, and the kinematic and ionization properties of the extended emission line regions display considerable variation over the lifetime of individual sources, as well as with cosmic epoch. We present the results of deep rest-frame UV and optical imaging and UV spectroscopy of high redshift 6C radio galaxies. The interdependence of the host galaxy and radio source properties are discussed, considering: (i) the relative contribution of shocks associated with the expanding radio source to the observed emission line gas kinematics, and their effect on the ionization state of the gas; (ii) the similarities and differences between the morphologies of the host galaxies and aligned emission for a range of radio source powers; and (iii) the influence of radio power on the strength of the observed alignment effect.Comment: LaTeX, 6 pages, 5 figures, Elsevier Science format. To appear in "Radio galaxies: past, present & future". eds. M. Jarvis et al., Leiden, Nov 200

Lexical acquisition in elementary science classes

The purpose of this study was to further researchers' understanding of lexical acquisition in the beginning primary schoolchild by investigating word learning in small-group elementary science classes. Two experiments were conducted to examine the role of semantic scaffolding (e.g., use of synonymous terms) and physical scaffolding (e.g., pointing to referents) in children's acquisition of novel property terms. Children's lexical knowledge was assessed using multiple tasks (naming, comprehension, and definitional). Children struggled to acquire meanings of adjectives without semantic or physical scaffolding (Experiment 1), but they were successful in acquiring extensive lexical knowledge when offered semantic scaffolding (Experiment 2). Experiment 2 also shows that semantic scaffolding used in combination with physical scaffolding helped children acquire novel adjectives and that children who correctly named pictures of adjectives had acquired definitions

Children's acquisition of science terms: simple exposure is insufficient

The ability of school children (N = 233) to acquire new scientific vocabulary was examined. Children from two age groups (M = 4;8 and M = 6;5) were introduced to previously unknown words in an educational video. Word knowledge was assessed through accuracy and latency for production and comprehension over a nine month period. A draw and write task assessed acquisition of domain knowledge. Word learning was poorer than has previously been reported in the literature, and subject to influences of word type (domain-specificity) and word class. The results indicate that the acquisition of scientific terms is a complex process moderated by lexical, semantic and pragmatic factors

An excess of sub-millimetre sources towards z~1 clusters

Deep sub-millimetre observations using SCUBA are presented of the central regions of four high redshift clusters which have been extensively studied optically: CL0023+0423 (z=0.84), J0848+4453 (z=1.27), CL1604+4304 (z=0.90) and CL1604+4321 (z=0.92). 10 sub-millimetre sources are securely detected towards these four clusters at 850 microns, with two further tenuous detections; the raw 850 micron source counts exceed those determined from blank-field surveys by a factor of 3-4. In particular, towards CL1604+4304, 6 sources are detected with S(850) > 4 mJy making this the richest sub-mm field discovered to date. Corrections for gravitational lensing by these high redshift clusters reduce these excess sources counts, but are unlikely to account for more than about half of the excess, with the remainder presumably directly associated with cluster galaxies. The 450 to 850 micron flux density ratios of the detected sources are systematically higher (at a significance level > 98%) than those determined for blank-field selected sources, consistent with them being at the cluster redshifts. If subsequent identifications confirm cluster membership, these results will demonstrate that the optical Butcher-Oemler effect is also observed at sub-mm wavelengths.Comment: MNRAS, in consideration. 11 pages, including 6 figures. Minor typos correcte

The Search for AGN in Distant Galaxy Clusters

We are undertaking the first systematic study of the prevalence of AGN activity in a large sample of high redshift galaxy clusters. Local clusters contain mainly red elliptical galaxies, and have little or no luminous AGN activity. However, recent studies of some moderate to high redshift clusters have revealed significant numbers of luminous AGN within the cluster. This effect may parallel the Butcher-Oemler effect - the increase in the fraction of blue galaxies in distant clusters compared to local clusters. Our aim is to verify and quantify recent evidence that AGN activity in dense environments increases with redshift, and to evaluate the significance of this effect. As cluster AGN are far less prevalent than field sources, a large sample of over 120 cluster fields at z > 0.1 has been selected from the Chandra archives and is being analysed for excess point sources. The size of the excess, the radial distribution and flux of the sources and the dependence of these on cluster redshift and luminosity will reveal important information about the triggering and fueling of AGN.Comment: 2 pages, to appear in proceedings of 'Multi-wavelength AGN surveys', Cozumel, 200

QCD on \alpha-Clusters

It is shown that the 21264 Alpha processor can reach about 20% sustained efficiency for the inversion of the Wilson-Dirac operator. Since fast ethernet is not sufficient to get balancing between computation and communication on reasonable lattice- and system-sizes, an interconnection using Myrinet is discussed. We find a price/performance ratio comparable with state-of-the-art SIMD-systems for lattice QCD.Comment: LATTICE99(machines), 3 page

The triggering probability of radio-loud AGN: A comparison of high and low excitation radio galaxies in hosts of different colors

Low luminosity radio-loud active galactic nuclei (AGN) are generally found in massive red elliptical galaxies, where they are thought to be powered through gas accretion from their surrounding hot halos in a radiatively inefficient manner. These AGN are often referred to as "low-excitation" radio galaxies (LERGs). When radio-loud AGN are found in galaxies with a young stellar population and active star formation, they are usually high-power radiatively-efficient radio AGN ("high-excitation", HERG). Using a sample of low-redshift radio galaxies identified within the Sloan Digital Sky Survey (SDSS), we determine the fraction of galaxies that host a radio-loud AGN, $f_{RL}$, as a function of host galaxy stellar mass, $M_*$, star formation rate, color (defined by the 4000 \angstrom break strength), radio luminosity and excitation state (HERG/LERG). We find the following: 1. LERGs are predominantly found in red galaxies. 2. The radio-loud AGN fraction of LERGs hosted by galaxies of any color follows a $f^{LE}_{RL} \propto M^{2.5}_*$ power law. 3. The fraction of red galaxies hosting a LERG decreases strongly for increasing radio luminosity. For massive blue galaxies this is not the case. 4. The fraction of green galaxies hosting a LERG is lower than that of either red or blue galaxies, at all radio luminosities. 5. The radio-loud AGN fraction of HERGs hosted by galaxies of any color follows a $f^{HE}_{RL} \propto M^{1.5}_*$ power law. 6. HERGs have a strong preference to be hosted by green or blue galaxies. 7. The fraction of galaxies hosting a HERG shows only a weak dependence on radio luminosity cut. 8. For both HERGs and LERGs, the hosting probability of blue galaxies shows a strong dependence on star formation rate. This is not observed in galaxies of a different color.[abridged]Comment: 7 pages, 6 figure

Luminosity function, sizes and FR dichotomy of radio-loud AGN

The radio luminosity function (RLF) of radio galaxies and radio-loud quasars is often modelled as a broken power-law. The break luminosity is close to the dividing line between the two Fanaroff-Riley (FR) morphological classes for the large-scale radio structure of these objects. We use an analytical model for the luminosity and size evolution of FRII-type objects together with a simple prescription for FRI-type sources to construct the RLF. We postulate that all sources start out with an FRII-type morphology. Weaker jets subsequently disrupt within the quasi-constant density cores of their host galaxies and develop turbulent lobes of type FRI. With this model we recover the slopes of the power laws and the break luminosity of the RLF determined from observations. The rate at which AGN with jets of jet power $Q$ appear in the universe is found to be proportional to $Q^{-1.6}$. The model also roughly predicts the distribution of the radio lobe sizes for FRII-type objects, if the radio luminosity of the turbulent jets drops significantly at the point of disruption. We show that our model is consistent with recent ideas of two distinct accretion modes in jet-producing AGN, if radiative efficiency of the accretion process is correlated with jet power.Comment: 13 pages, 1 figure, accepted by MNRA