215 research outputs found
The Millennium Galaxy Catalogue: Star counts and the Structure of the Galactic Stellar Halo
We derive a star catalogue generated from the images taken as part of the
37.5 sq. deg Millennium Galaxy Catalogue. These data, alone and together with
colours gained from the Sloan Digital Sky Survey Early Data Release, allow the
analysis of faint star counts (B(MGC) < 20) at high Galactic latitude (41 < b <
63), as a function of Galactic longitude (239 < l < 353). We focus here on the
inner stellar halo, providing robust limits on the amplitude of substructure
and on the large-scale flattening. In line with previous results, the thick
disk, an old, intermediate-metallicity population, is clearly seen in the
colour-magnitude diagram. We find that the Galactic stellar halo within ~10 kpc
(the bulk of the stellar mass) is significantly flattened, with an axial ratio
of (c/a) =0.56 +/- 0.01, again consistent with previous results. Our analysis
using counts-in-cells, angular correlation functions and the Lee 2D statistic,
confirms tidal debris from the Sagittarius dwarf but finds little evidence for
other substructure in the inner halo, at heliocentric distances of < 5 kpc.
This new quantification of the smoothness in coordinate space limits the
contribution of recent accretion/disruption to the build-up of the bulk of the
stellar halo.Comment: Accepted for publication in MNRAS (figs 16 and 17 degraded here
Design of an Intelligent, Modular IGBT/SiC Inverter Platform up to 400 kW for Fast Realization of New Test-Bench Concepts
This paper presents an intelligent, modular two level, three phase inverter platform for up to 1200 V DC-link voltage and 400 kW continuous power at 10 kHz switching frequency. It features an integrated signal processing system and various sensors, which allow standalone as well as cross-linked operation. Customizable software of the signal processing system allows easy adaption to different applications such as Active-Front-End (AFE), DC/DC-converters, Dual-Active-Bridges or Drive Inverters. Focus of this paper is a design guideline for an inverter platform which fulfills requirements of various applications with regard to sensor setup, control, failure management and monitoring. Design goal of the platform is a fast setup of new testbench concepts for academic research and novel applications
From Espresso to Codex
CODEX and ESPRESSO are concepts for ultra-stable, high-resolution
spectrographs at the E-ELT and VLT, respectively. Both instruments are well
motivated by distinct sets of science drivers. However, ESPRESSO will also be a
stepping stone towards CODEX both in a scientific as well as in a technical
sense. Here we discuss this role of ESPRESSO with respect to one of the most
exciting CODEX science cases, i.e. the dynamical determination of the cosmic
expansion history.Comment: To appear in the Proceedings of the Workshop "Science with the VLT in
the ELT era", 8-12 October 2007, Garching, A. Moorwood, e
High Precision Astrometry with MICADO at the European Extremely Large Telescope
In this article we identify and discuss various statistical and systematic
effects influencing the astrometric accuracy achievable with MICADO, the
near-infrared imaging camera proposed for the 42-metre European Extremely Large
Telescope (E-ELT). These effects are instrumental (e.g. geometric distortion),
atmospheric (e.g. chromatic differential refraction), and astronomical
(reference source selection). We find that there are several phenomena having
impact on ~100 micro-arcsec scales, meaning they can be substantially larger
than the theoretical statistical astrometric accuracy of an optical/NIR
42m-telescope. Depending on type, these effects need to be controlled via
dedicated instrumental design properties or via dedicated calibration
procedures. We conclude that if this is done properly, astrometric accuracies
of 40 micro-arcsec or better - with 40 micro-arcsec/year in proper motions
corresponding to ~20 km/s at 100 kpc distance - can be achieved in one epoch of
actual observationsComment: 15 pages, 9 figures, 3 tables. Accepted by MNRA
Large-scale structure in the Lyman-alpha forest II: analysis of a group of ten QSOs
The spatial distribution of Ly-alpha forest absorption systems towards ten
QSOs has been analysed to search for large-scale structure over the redshift
range 2.2 < z < 3.4. The QSOs form a closely spaced group on the sky and are
concentrated within a 1 deg^2 field. We have employed a technique based on the
first and second moments of the transmission probability density function which
is capable of identifying and assessing the significance of regions of over- or
underdense Ly-alpha absorption. We find evidence for large-scale structure in
the distribution of Ly-alpha forest absorption at the > 99 per cent confidence
level. In individual spectra we find overdense Ly-alpha absorption on scales of
up to 1200 km s^-1. There is also strong evidence for correlated absorption
across line of sight pairs separated by < 3 h^-1 proper Mpc (q_0 = 0.5). For
larger separations the cross-correlation signal becomes progressively less
significant.Comment: 15 pages, LaTeX, 6 Postscript figures, accepted for publication in
MNRA
GRB 050223: A dark GRB in a dusty starburst galaxy
Aims: We aim at detecting and determining the properties of the host galaxy
of the dark GRB 050223.
Methods: We use VLT optical/NIR images coupled to Swift X-ray positioning,
and optical spectra of the host galaxy to measure its properties.
Results: We find a single galaxy within the Swift error box of GRB 050223. It
is located at z = 0.584 and its luminosity is L ~ 0.4 L*. Emission lines in the
galaxy spectrum imply an intrinsic SFR > 7 Msun/yr, and a large extinction A_V
> 2 mag within it. We also detect absorption lines, which reveal an underlying
stellar population with an age between 40 Myr and 1.5 Gyr.
Conclusions: The identification of a host galaxy with atypical properties
using only the X-ray transient suggests that a bias may be present in the
former sample of host galaxies. Dust obscuration together with intrinsic
faintness are the most probable causes for the darkness of this burst.Comment: 4 pages, 2 figures, accepted for publication in Astronomy &
Astrophysic
Elliptic and hyperelliptic magnetohydrodynamic equilibria
The present study is a continuation of a previous one on "hyperelliptic"
axisymmetric equilibria started in [Tasso and Throumoulopoulos, Phys. Plasmas
5, 2378 (1998)].
Specifically, some equilibria with incompressible flow nonaligned with the
magnetic field and restricted by appropriate side conditions like "isothermal"
magnetic surfaces, "isodynamicity" or P + B^2/2 constant on magnetic surfaces
are found to be reducible to elliptic integrals. The third class recovers
recent equilibria found in [Schief, Phys. Plasmas 10, 2677 (2003)]. In contrast
to field aligned flows, all solutions found here have nonzero toroidal magnetic
field on and elliptic surfaces near the magnetic axis.Comment: 9 page
The detection of the imprint of filaments on cosmic microwave background lensing
Galaxy redshift surveys, such as 2dF, SDSS, 6df, GAMA and VIPERS, have shown
that the spatial distribution of matter forms a rich web, known as the cosmic
web. The majority of galaxy survey analyses measure the amplitude of galaxy
clustering as a function of scale, ignoring information beyond a small number
of summary statistics. Since the matter density field becomes highly
non-Gaussian as structure evolves under gravity, we expect other statistical
descriptions of the field to provide us with additional information. One way to
study the non-Gaussianity is to study filaments, which evolve non-linearly from
the initial density fluctuations produced in the primordial Universe. In our
study, we report the first detection of CMB (Cosmic Microwave Background)
lensing by filaments and we apply a null test to confirm our detection.
Furthermore, we propose a phenomenological model to interpret the detected
signal and we measure how filaments trace the matter distribution on large
scales through filament bias, which we measure to be around 1.5. Our study
provides a new scope to understand the environmental dependence of galaxy
formation. In the future, the joint analysis of lensing and Sunyaev-Zel'dovich
observations might reveal the properties of `missing baryons', the vast
majority of the gas which resides in the intergalactic medium and has so far
evaded most observations
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