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Hiemer A.,Institute for Astro and Particle Physics | Barden M.,Institute for Astro and Particle Physics | Kelvin L.S.,Institute for Astro and Particle Physics | Haussler B.,University of Oxford | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present GALAPAGOS-C, a code designed to process a complete set of survey images through automation of source detection (via SEXTRACTOR), postage stamp cutting, object mask preparation, sky background estimation and complex two-dimensional light profile Sérsic modelling (via GALFIT). GALAPAGOS-C is designed around the concept of MPI-parallelization, allowing the processing of large data sets in a quick and efficient manner. Further, GALAPAGOS-C is capable of fitting multiple-Sérsic profiles to each galaxy, each representing distinct galaxy components (e.g. bulge, disc, bar), in addition to the option to fit asymmetric Fourier mode distortions. The modelling reliability of our core single-Sérsic fitting capability is tested thoroughly using image simulations.We apply GALAPAGOS-C to the Space Telescope A901/902 Galaxy Evolution Survey to investigate the evolution of galaxy structure with cosmic time and the dependence on environment. We measure the distribution of Sérsic indices as a function of local object density in the A901/902 cluster sample to provide one of the first measures of the Sérsic index- density relation. We find that the fraction of galaxies with a high Sérsic index (2.5 < n < 7.0) is higher in denser environments (35 per cent), halving towards sparsely populated regions (15 per cent). The population of low Sérsic index galaxies (0.4 < n< 1.6) is lower in denser environments (35 per cent), increasing towards sparsely populated regions (60 per cent). The population of intermediate Sérsic index galaxies (1.6 < n < 2.5) approximately follows the trend of the high Sérsic index types. © 2014.

Khurana S.,Center for Computation and Technology | Brener N.,Center for Computation and Technology | Karki B.,Center for Computation and Technology | Benger W.,Institute for Astro and Particle Physics | Ritter M.,Institute of Basic science in Civil Engineering
20th International Conference in Central Europe on Computer Graphics, Visualization and Computer Vision, WSCG 2012 - Conference Proceedings | Year: 2012

This paper presents a multi scale color coding technique which enhances the visualization of scalar fields along integration lines in vector fields. In particular, this multi scale technique enables one to see detailed variations in selected small ranges of the scalar field while at the same time allowing one to observe the entire range of values of the scalar field. This type of visualization, observing small variations as well as the entire range of values, is usually not possible with uniform color coding. This multi scale approach, which is linear within each division of the scale (piecewise linear), is a general visualization technique that can be applied to many different scalar fields of interest. As an example, in this paper we apply it to the visualization of fluid flow mixing indicators along pathlines in computational fluid dynamics (CFD) simulations. Pathlines are the trajectories of fluid particles over time in the CFD simulations, and applying multi scale color coding on the pathlines brings out quantities of interest in the flow such as curvature, torsion and specific measure of length generation, which are indicators of the degree of mixing in the fluid system. In contrast to uniform color coding, this multi scale scheme can display small variations in the mixing indicators and still show the entire range of values of these indicators. In this paper, the mixing indicators are computed and displayed only along line structures (pathlines) in the flow field rather than at all points in the flow field.

Mcdonald I.,Jodrell Bank Center for Astrophysics | White J.R.,Jodrell Bank Center for Astrophysics | Zijlstra A.A.,Jodrell Bank Center for Astrophysics | Guzman Ramirez L.,Jodrell Bank Center for Astrophysics | And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present spectra of 1142 colour-selected stars in the direction of the Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy, of which 1058 were taken with VLT/FLAMES multi-object spectrograph and 84 were taken with the SAAO Radcliffe 1.9-m telescope grating spectrograph. Spectroscopic membership is confirmed (at >99 per cent confidence) for 592 stars on the basis of their radial velocity, and spectral types are given. Very slow rotation is marginally detected around the galaxy's major axis. We identify five S stars and 23 carbon stars, of which all but four carbon stars are newly determined and all but one (PQ Sgr) are likely Sgr dSph members. We examine the onset of carbon richness in this metal-poor galaxy in the context of stellar models. We compare the stellar death rate (one star per 1000-1700 yr) with the known planetary nebula dynamical ages and find that the bulk population produce the observed (carbon-rich) planetary nebulae. We compute average lifetimes of S and carbon stars as 60-250 and 130-500 kyr, compared to a total thermal-pulsing asymptotic giant branch lifetime of 530-1330 kyr. We conclude by discussing the return of carbon-rich material to the interstellar medium. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Bohm A.,Institute for Astro and Particle Physics | Ziegler B.L.,Institute of Astronomy
Astronomy and Astrophysics | Year: 2016

Aims. Galaxy scaling relations such as the Tully-Fisher relation (between the maximum rotation velocity Vmax and luminosity) and the velocity-size relation (between Vmax and the disk scale length) are powerful tools to quantify the evolution of disk galaxies with cosmic time. Methods. We took spatially resolved slit spectra of 261 field disk galaxies at redshifts up to z ≈ 1 using the FORS instruments of the ESO Very Large Telescope. The targets were selected from the FORS Deep Field and William Herschel Deep Field. Our spectroscopy was complemented with HST/ACS imaging in the F814W filter. We analyzed the ionized gas kinematics by extracting rotation curves from the two-dimensional spectra. Taking into account all geometrical, observational, and instrumental effects, these rotation curves were used to derive the intrinsic Vmax. Results. Neglecting galaxies with disturbed kinematics or insufficient spatial rotation curve extent, Vmax was reliably determined for 124 galaxies covering redshifts 0.05 < z < 0.97. This is one of the largest kinematic samples of distant disk galaxies to date. We compared this data set to the local B-band Tully-Fisher relation and the local velocity-size relation. The scatter in both scaling relations is a factor of ∼2 larger at z ≈ 0.5 than at z ≈ 0. The deviations of individual distant galaxies from the local Tully-Fisher relation are systematic in the sense that the galaxies are increasingly overluminous toward higher redshifts, corresponding to an overluminosity ΔMB = -(1.2 ± 0.5) mag at z = 1. This luminosity evolution at given Vmax is probably driven by younger stellar populations of distant galaxies with respect to their local counterparts, potentially combined with modest changes in dark matter mass fractions. The analysis of the velocity-size relation reveals that disk galaxies of a given Vmax have grown in size by a factor of ∼1.5 over the past ∼8 Gyr, most likely through accretion of cold gas and/or small satellites. From scrutinizing the combined evolution in luminosity and size, we find that the galaxies that show the strongest evolution toward smaller sizes at z ≈ 1 are not those that feature the strongest evolution in luminosity, and vice versa. © CSIRO 2016.

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