Showing papers by "David Bacon published in 2008"
TL;DR: In this paper, the properties of Optically Passive Spirals and dusty red galaxies in the A901/2 cluster complex at redshift 0.17 were investigated using restframe near-UV-optical SEDs, 24 micron IR data and HST morphologies from the STAGES dataset.
Abstract: We investigate the properties of optically passive spirals and dusty red galaxies in the A901/2 cluster complex at redshift ~0.17 using restframe near-UV-optical SEDs, 24 micron IR data and HST morphologies from the STAGES dataset. The cluster sample is based on COMBO-17 redshifts with an rms precision of sigma_cz~2000 km/sec. We find that 'dusty red galaxies' and 'optically passive spirals' in A901/2 are largely the same phenomenon, and that they form stars at a substantial rate, which is only 4x lower than that in blue spirals at fixed mass. This star formation is more obscured than in blue galaxies and its optical signatures are weak. They appear predominantly in the stellar mass range of log M*/Msol=[10,11] where they constitute over half of the star-forming galaxies in the cluster; they are thus a vital ingredient for understanding the overall picture of star formation quenching in clusters. We find that the mean specific SFR of star-forming galaxies in the cluster is clearly lower than in the field, in contrast to the specific SFR properties of blue galaxies alone, which appear similar in cluster and field. Such a rich red spiral population is best explained if quenching is a slow process and morphological transformation is delayed even more. At log M*/Msol<10, such galaxies are rare, suggesting that their quenching is fast and accompanied by morphological change. We note, that edge-on spirals play a minor role; despite being dust-reddened they form only a small fraction of spirals independent of environment.
206 citations
TL;DR: In this paper, the authors explore the amount of obscured star-formation as a function of environment in the A901/902 supercluster at z=0.165 in conjunction with a field sample drawn from the HST as part of the STAGES and GEMS surveys.
Abstract: We explore the amount of obscured star-formation as a function of environment in the A901/902 supercluster at z=0.165 in conjunction with a field sample drawn from the A901 and CDFS fields, imaged with HST as part of the STAGES and GEMS surveys. We combine the COMBO-17 near-UV/optical SED with Spitzer 24um photometry to estimate both the unobscured and obscured star formation in galaxies with Mstar>10^{10}Msun. We find that the star formation activity in massive galaxies is suppressed in dense environments, in agreement with previous studies. Yet, nearly 40% of the star-forming galaxies have red optical colors at intermediate and high densities. These red systems are not starbursting; they have star formation rates per unit stellar mass similar to or lower than blue star-forming galaxies. More than half of the red star-forming galaxies have low IR-to-UV luminosity ratios, relatively high Sersic indices and they are equally abundant at all densities. They might be gradually quenching their star-formation, possibly but not necessarily under the influence of gas-removing environmental processes. The other >40% of the red star-forming galaxies have high IR-to-UV luminosity ratios, indicative of high dust obscuration. They have relatively high specific star formation rates and are more abundant at intermediate densities. Our results indicate that while there is an overall suppression in the star-forming galaxy fraction with density, the small amount of star formation surviving the cluster environment is to a large extent obscured, suggesting that environmental interactions trigger a phase of obscured star formation, before complete quenching.
128 citations
TL;DR: In this article, the authors investigate the interaction of edge dislocations with interstitial dislocation loops in α-iron at 300 K and show that dislocation reactions are wide-ranging and complex, but can be described in terms of conventional dislocations reactions in which Burgers vector is conserved.
109 citations
01 Jan 2008
TL;DR: In this article, a new cosmic shear survey carried out with the Echelle Spectrograph and Imager on the Keck II telescope is presented, which covers a total of 0.6 square degrees in 173 fields probing independent lines of sight, hence minimizing the impact of sample variance.
Abstract: The recent measurements of weak lensing by large-scale structure present significant new opportunities for studies of the matter distribution in the universe. Here, we present a new cosmic shear survey carried out with the Echelle Spectrograph and Imager on the Keck II telescope. This covers a total of 0.6 square degrees in 173 fields probing independent lines of sight, hence minimising the impact of sample variance. We also extend our measurements of cosmic shear with the William Herschel Telescope (Bacon, Refregier & Ellis 2000) to a survey area of 1 square degree. The joint measurements with two independent telescopes allow us to assess the impact of instrument-specific systematics, one of the major difficulties in cosmic shear measurements. For both surveys, we carefully account for effects such as smearing by the point spread function and shearing due to telescope optics. We find negligible residuals in both cases and recover mutually consistent cosmic shear signals, significant at the 5.1� level. We present a simple method to compute the statistical error in the shear correlation function, including non-gaussian sample variance and the covariance between different angular bins. We measure shear correlation functions for all fields and use these to ascertain the amplitude of the matter power spectrum, finding �8 m 0.3
103 citations
TL;DR: Ackland et al. as mentioned in this paper investigated the effect of the dislocation segment in the 70°-mixed orientation with the interatomic potential on the critical line shape at which the dislocations breaks from the void and found that the effect on the line shape does not arise from anisotropy of the elastic line tension.
Abstract: Atomic processes and strengthening effects due to interaction between edge dislocations and voids in α-iron have been investigated by means of molecular dynamics with a recently developed interatomic potential (Ackland et al 2004 J. Phys.: Condens. Matter 16 S2629) and compared with those obtained earlier with an older potential (Ackland et al 1997 Phil. Mag. A 75 713). Differences between the interactions for the two models are insignificant at temperature T≥100 K, thereby confirming the validity of the previous results. In particular, voids are relatively strong obstacles because for large voids and/or low temperature, the initially straight edge dislocation is pulled into screw orientation before it breaks away at the critical shear stress, τc. Differences between the core structures and glide planes of the screw dislocation for the two potentials do not affect τc in this temperature range. The only significant difference between the dislocation–void interactions in the two models occurs at low temperature in static or pseudo-static conditions (T≤1 K). It arises from the influence of the dislocation segment in the 70°-mixed orientation with the (Ackland et al 2004 J. Phys.: Condens. Matter 16 S2629) potential and is seen in the critical line shape at which the dislocation breaks from the void. It affects τc for some combinations of void size and spacing. The effect on the line shape does not arise from anisotropy of the elastic line tension: it is due to the high Peierls stress of the 70° dislocation. When this effect does not control breakaway, the dependence of τc on void size and spacing follows an equation first found by modelling the Orowan process in the approximation of linear elasticity.
100 citations
01 Oct 2008
TL;DR: In this paper, the interaction of an edge dislocation with interstitial dislocation loops in α-iron at 300 K was studied systematically for different loop positions and Burgers vector orientations, and results were compared for two different interatomic potentials.
Abstract: Abstract Atomic-level simulations are used to investigate the interaction of an edge dislocation with 〈1 0 0〉 interstitial dislocation loops in α-iron at 300 K. Dislocation reactions are studied systematically for different loop positions and Burgers vector orientations, and results are compared for two different interatomic potentials. Reactions are wide-ranging and complex, but can be described in terms of conventional dislocation reactions in which Burgers vector is conserved. The fraction of interstitials left behind after dislocation breakaway varies from 25 to 100%. The nature of the reactions requiring high applied stress for breakaway is identified. The obstacle strengths of 〈1 0 0〉 loops, 1/2〈1 1 1〉 loops and voids containing the same number (169) of point defects are compared. 〈1 0 0〉 loops with Burgers vector parallel to the dislocation glide plane are slightly stronger than 〈1 0 0〉 and 1/2〈1 1 1〉 loops with inclined Burgers vector: voids are about 30% weaker than the stronger loops. However, small voids are stronger than small 1/2〈1 1 1〉 loops. The complexity of some reactions and the variety of obstacle strengths poses a challenge for the development of continuum models of dislocation behaviour in irradiated iron.
96 citations
Institut d'Astrophysique de Paris1, University of British Columbia2, University of Nottingham3, Herzberg Institute of Astrophysics4, Space Telescope Science Institute5, Max Planck Society6, University of Oxford7, Institute of Cosmology and Gravitation, University of Portsmouth8, University of Waterloo9, École Polytechnique Fédérale de Lausanne10, University of Innsbruck11, University of Texas at Austin12, University of Massachusetts Amherst13, University of Edinburgh14, Chinese Academy of Sciences15
TL;DR: In this paper, a high-resolution dark matter reconstruction of the z = 0.165 Abell 901/902 supercluster from a weak lensing analysis of the Hubble Space Telescope STAGES survey is presented.
Abstract: We present a high-resolution dark matter reconstruction of the z = 0.165 Abell 901/902 supercluster from a weak lensing analysis of the Hubble Space Telescope STAGES survey. We detect the four main structures of the supercluster at high significance, resolving substructure within and between the clusters. We find that the distribution of dark matter is well traced by the cluster galaxies, with the brightest cluster galaxies marking out the strongest peaks in the dark matter distribution. We also find a significant extension of the dark matter distribution of Abell 901a in the direction of an infalling X-ray group Abell 901 alpha. We present mass, mass-to-light and mass-to-stellar mass ratio measurements of the structures and substructures that we detect. We find no evidence for variation of the mass-to-light and mass-to-stellar mass ratio between the different clusters. We compare our space-based lensing analysis with an earlier ground-based lensing analysis of the supercluster to demonstrate the importance of space-based imaging for future weak lensing dark matter 'observations'.
90 citations
TL;DR: In this article, a high resolution dark matter reconstruction of the z=0.165 Abell 901/902 supercluster from a weak lensing analysis of the HST STAGES survey is presented.
Abstract: We present a high resolution dark matter reconstruction of the z=0.165 Abell 901/902 supercluster from a weak lensing analysis of the HST STAGES survey. We detect the four main structures of the supercluster at high significance, resolving substructure within and between the clusters. We find that the distribution of dark matter is well traced by the cluster galaxies, with the brightest cluster galaxies marking out the strongest peaks in the dark matter distribution. We also find a significant extension of the dark matter distribution of Abell 901a in the direction of an infalling X-ray group Abell 901alpha. We present mass, mass-to-light and mass-to-stellar mass ratio measurements of the structures and substructures that we detect. We find no evidence for variation of the mass-to-light and mass-to-stellar mass ratio between the different clusters. We compare our space-based lensing analysis with an earlier ground-based lensing analysis of the supercluster to demonstrate the importance of space-based imaging for future weak lensing dark matter 'observations'.
86 citations
TL;DR: In this paper, the authors used atomic-scale computer simulation to investigate the primary damage created by displacement cascades in copper over a wide range of temperature (100 K T 900 K) and primary knock-on atom energy (5 keV EPKA 25 keV).
79 citations
TL;DR: The Space Telescope A901/2 Galaxy Evolution Survey (STAGES) as mentioned in this paper is a multi-wavelength project designed to probe physical drivers of galaxy evolution across a wide range of environments and luminosity.
Abstract: We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). STAGES is a multiwavelength project designed to probe physical drivers of galaxy evolution across a wide range of environments and luminosity. A complex multi-cluster system at z~0.165 has been the subject of an 80-orbit F606W HST/ACS mosaic covering the full 0.5x0.5 (~5x5 Mpc^2) span of the supercluster. Extensive multiwavelength observations with XMM-Newton, GALEX, Spitzer, 2dF, GMRT, and the 17-band COMBO-17 photometric redshift survey complement the HST imaging. Our survey goals include simultaneously linking galaxy morphology with other observables such as age, star-formation rate, nuclear activity, and stellar mass. In addition, with the multiwavelength dataset and new high resolution mass maps from gravitational lensing, we are able to disentangle the large-scale structure of the system. By examining all aspects of environment we will be able to evaluate the relative importance of the dark matter halos, the local galaxy density, and the hot X-ray gas in driving galaxy transformation. This paper describes the HST imaging, data reduction, and creation of a master catalogue. We perform Sersic fitting on the HST images and conduct associated simulations to quantify completeness. In addition, we present the COMBO-17 photometric redshift catalogue and estimates of stellar masses and star-formation rates for this field. We define galaxy and cluster sample selection criteria which will be the basis for forthcoming science analyses, and present a compilation of notable objects in the field. Finally, we describe the further multiwavelength observations and announce public access to the data and catalogues.
TL;DR: In this paper, molecular dynamics simulations of displacement cascades created in α-iron (Fe) by primary knock-on atoms (PKAs) with energy from 5 to 20 keV and mass chosen to represent C, Fe and Bi are presented.
Abstract: Results are presented from molecular dynamics (MD) simulations of displacement cascades created in α-iron (Fe) by primary knock-on atoms (PKAs) with energy from 5 to 20 keV and mass chosen to represent C, Fe and Bi The PKA-Fe interaction potential at short range has been varied, and damage by molecular Bi2 has been simulated using two Bi PKAs Four effects are reported First, the PKA mass has a major effect on the damage produced in individual cascades while the PKA-Fe potential has little influence Second, the total number of point defects produced in a cascade decreases with increasing PKA mass This fact is not accounted for in models used conventionally for estimating damage Third, interstitial loops of type and both vacancy and interstitial loops of ⟨100⟩ type are formed, the latter being observed in MD simulation for the first time The probability of ⟨100⟩ loop appearance increases with increasing PKA mass as well as energy Finally, there is a correlation between production of large vacancy an
TL;DR: In this paper, the structure and properties of planar interstitial and vacancy clusters in α-zirconium containing up to ≈300 defects were studied by atomic-scale computer modelling.
TL;DR: In this article, the authors used molecular dynamics simulations to investigate defect production by displacement cascades in iron with carbon (C) in solution and found that significant fractions of single SIAs and vacancies are trapped by C in the cascade process, irrespective of cascade energy.
TL;DR: In this article, a new image distortion with two distinct descriptions, the twist and the turn, was introduced, which is not activated gravitationally to first order, but will be activated by systematic effects.
Abstract: We account for all the image distortions relevant to weak gravitational lensing to second order. Besides the familiar shear, convergence, rotation and flexions, we find a new image distortion with two distinct descriptions, the twist and the turn. Like rotation, this distortion is not activated gravitationally to first order, but will be activated by systematic effects. We examine the rotational properties of twist and turn, and their effect on images in real and shapelet space. We construct estimators for the new distortion, taking into account the centroid shift which it generates. We then use these estimators to make first constraints on twist using the STAGES HST survey; we find that the mean twist estimator is consistent with zero. We measure correlation functions for our twist estimator on the survey, again finding no evidence of systematic effects.
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TL;DR: In this paper, the authors used the STAGES ACS HST survey of the Abell 901/902 supercluster (z~0.165), along with earlier field studies based on the SDSS and the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS).
Abstract: In dense clusters, higher densities at early epochs as well as physical processes, such as ram pressure stripping and tidal interactions become important, and can have direct consequences for the evolution of bars and their host disks. To study bars and disks as a function of environment, we are using the STAGES ACS HST survey of the Abell 901/902 supercluster (z~0.165), along with earlier field studies based the SDSS and the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS). We explore the limitations of traditional methods for characterizing the bar fraction, and in particular highlight uncertainties in disk galaxy selection in cluster environments. We present an alternative approach for exploring the proportion of bars, and investigate the properties of bars as a function of host galaxy color, Sersic index, stellar mass, star formation rate (SFR), specific SFR, and morphology.
01 Aug 2008
TL;DR: In this article, the authors used the STAGES ACS HST survey of the Abell 901/902 supercluster (z~0.165), along with earlier field studies based on the SDSS and the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS).
Abstract: In dense clusters, higher densities at early epochs as well as physical processes, such as ram pressure stripping and tidal interactions become important, and can have direct consequences for the evolution of bars and their host disks. To study bars and disks as a function of environment, we are using the STAGES ACS HST survey of the Abell 901/902 supercluster (z~0.165), along with earlier field studies based the SDSS and the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS). We explore the limitations of traditional methods for characterizing the bar fraction, and in particular highlight uncertainties in disk galaxy selection in cluster environments. We present an alternative approach for exploring the proportion of bars, and investigate the properties of bars as a function of host galaxy color, Sersic index, stellar mass, star formation rate (SFR), specific SFR, and morphology.
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TL;DR: In this article, the authors present a study of galaxies in the STAGES survey of the Abell 901/902 supercluster at z~0.165, based on HST ACS F606W, COMBO-17, Spitzer 24um, XMM-Newton X-ray, and gravitational lensing maps.
Abstract: We present a study of galaxies in the STAGES survey of the Abell 901/902 supercluster at z~0.165, based on HST ACS F606W, COMBO-17, Spitzer 24um, XMM-Newton X-ray, and gravitational lensing maps. We characterize galaxies with strong, externally-triggered morphological distortions and normal, relatively undisturbed galaxies, using visual classification and quantitative CAS parameters. We compare normal and distorted galaxies in terms of their frequency, distribution within the cluster, star formation properties, and relationship to dark matter (DM) or surface mass density, and intra-cluster medium (ICM) density. We report here our preliminary results.