174 research outputs found
The extended ROSAT-ESO Flux Limited X-ray Galaxy Cluster Survey (REFLEX II)\\ II. Construction and Properties of the Survey
Galaxy clusters provide unique laboratories to study astrophysical processes
on large scales and are important probes for cosmology. X-ray observations are
currently the best means of detecting and characterizing galaxy clusters. In
this paper we describe the construction of the REFLEX II galaxy cluster survey
based on the southern part of the ROSAT All-Sky Survey. REFLEX II extends the
REFLEX I survey by a factor of about two down to a flux limit of erg s cm (0.1 - 2.4 keV). We describe the determination
of the X-ray parameters, the process of X-ray source identification, and the
construction of the survey selection function. The REFLEX II cluster sample
comprises currently 915 objects. A standard selection function is derived for a
lower source count limit of 20 photons in addition to the flux limit. The
median redshift of the sample is . Internal consistency checks and
the comparison to several other galaxy cluster surveys imply that REFLEX II is
better than 90\% complete with a contamination less than 10\%.Comment: Astronomy and Astrophysics Vol. 555, A30 - 15 pages, 20 figure
Exploring the relations between bulges and central black holes in unusual galaxies
This thesis reports on the measurement of the masses of supermassive black holes in the centres of four galaxies using stellar dynamics. It is based on observations in the near-infrared (1.9-2.5\mum) with the integral-field spectrograph SINFONI at the Very Large Telescope (VLT). These data were used to determine the motions of stars in the very centre of the galaxies, which then were modelled to derive the mass of the central black hole using the Schwarzschild method. Such measurements were until now restricted to massive and dust-free galaxies, observed with the Hubble Space Telescope in the optical wavelength range. With SINFONI and the technique of adaptive optics it is now possible to measure the mass of supermassive black holes also in dusty, low-mass and faint galaxies. This is essential in order to understand the correlations between the evolution of galaxies and the growth of central black holes.
The faint elliptical galaxy NGC4486a contains a nuclear disc of dust and stars. With a velocity dispersion of 110km/s it belongs to the class of low-mass galaxies. The mass of the central black hole is M_BH=1.26x10^7 M_sun. Fornax A, a radio galaxy located in the outskirts of the Fornax galaxy cluster, experienced a recent major merger. A mass of M_BH=1.5x10^8 M_sun was derived for the central supermassive black hole. NGC3368 and NGC3489 are both galaxies which host a composite pseudobulge, i.e. a central disclike component grown via secular evolution and an additional small classical bulge. The derived masses of the central black holes are M_BH=7.5x10^6 M_sun for NGC3368 and
M_BH=6.00x10^6 M_sun for NGC3489. The tight correlation between M_BH and the velocity dispersion of the galaxy bulge, which is mainly based on elliptical galaxies, seems to be also a good indicator of the black hole mass for other galaxy types. All black hole masses derived here are in agreement with this relation. The correlation between M_BH and bulge mass, however, only seems to be valid for old classical bulges. It predicts several times larger black hole masses for the merger remnant Fornax A and for the two pseudobulge galaxies. The masses of the small classical bulge components in NGC3368 and NGC3489 seem to be a much better indicator of M_BH
Detection of Very-High Energy Gamma-Rays from the BL Lac Object PG 1553+113 with the MAGIC Telescope
The MAGIC telescope has observed very-high energy gamma-ray emission from the
BL Lac object PG 1553+113 in 2005 and 2006 at an overall significance is 8.8
sigma. The light curve shows no significant flux variations on a daily
timescale. The flux level during 2005 was, however, significantly higher as
compared to 2006. The differential energy spectrum between approx. 90 GeV and
500 GeV is well described by a power law with a spectral index of -4.2+-0.3.
The photon energy spectrum and spectral modeling allow to pose upper limits of
z=0.74 and z=0.56, respectively, on the yet undetermined redshift of PG
1553+113. Recent VLT observations of this blazar show featureless spectra in
the near-IR, thus no direct redshift could be determined from these
measurements.Comment: 4 pages, 4 figures, to appear in the proceedings of the 30th
International Cosmic Ray Conference, Merida, July 200
Using 3D Spectroscopy to Probe the Orbital Structure of Composite Bulges
Detailed imaging and spectroscopic analysis of the centers of nearby S0 and
spiral galaxies shows the existence of "composite bulges", where both classical
bulges and disky pseudobulges coexist in the same galaxy. As part of a search
for supermassive black holes in nearby galaxy nuclei, we obtained VLT-SINFONI
observations in adaptive-optics mode of several of these galaxies.
Schwarzschild dynamical modeling enables us to disentangle the stellar orbital
structure of the different central components, and to distinguish the differing
contributions of kinematically hot (classical bulge) and kinematically cool
(pseudobulge) components in the same galaxy.Comment: LaTeX, 2 pages, 1 PDF figure. To appear in "Proceedings of IAU
Symposium 309: Galaxies in 3D across the Universe", eds. B. L. Ziegler, F.
Combes, H. Dannerbauer, and M. Verdug
Kinetics of liquid bridges and formation of satellite droplets: Difference between micellar and bi-layer forming solutions
AbstractThe process of drop detachment from a capillary tip and formation of satellite droplets is studied for solutions of trisiloxane surfactants above the critical aggregation concentration. Two of the studied surfactants self-assemble in bilayer based phases, whereas the third forms micelles. The difference in the aggregates formed results in an essential difference in the rate of equilibration between the surface and the bulk solution and in a different behaviour near the pinch-off point. The difference in behaviour becomes pronounced when the viscosity of solutions increases 2–6 times (and therefore diffusion coefficients decrease correspondingly). In particular, when surfactant solutions are prepared in a water/glycerol mixture with a viscosity six times larger than water, the size of satellite droplets formed by the micellar solutions increases more than twice, whereas the size of droplets formed by the bilayer-forming solutions remains almost constant over a range of concentration covering two orders of magnitude. The bilayers forming solutions demonstrate a decrease in the effective surface tension near to pinch-off which can be related to the Marangoni stresses generated by surface flow during the thinning of the capillary bridge connecting the main drop with the liquid in the capillary
Effect of surfactant concentration and viscosity of outer phase during the coalescence of a surfactant-laden drop with a surfactant-free drop
AbstractThis work focuses on the coalescence of two water drops, one of which contains surfactant. The coalescence was carried out in surrounding silicone oils of various viscosities. It has been found that, in all the studied cases, the outward motion of the liquid neck follows the power scaling law with exponent ∼0.5 with the pre-factor dependent upon the surfactant concentration and viscosity of outer phase. Interfacial Marangoni flows arising at coalescence were visualised and quantified. Considerable convective bulk motion was observed by coalescence of surfactant-laden and surfactant-free drops with patterns depending on viscosity of outer phase. No noticeable convection was observed during the coalescence of two surfactant-free drops or similar surfactant-laden drops. It was confirmed that the rate of growth of the liquid bridge was reduced when the surfactant was present in the drops due to the lower interfacial tension in the bridge. Dependence of the reduction on surfactant concentration and viscosity of surrounding liquid is discussed. For the coalescence of a surfactant-free drop and a surfactant-laden drop, the curvature of the meniscus was different on either side of the growing bridge
The Chandra Source Catalog
The Chandra Source Catalog (CSC) is a general purpose virtual X-ray
astrophysics facility that provides access to a carefully selected set of
generally useful quantities for individual X-ray sources, and is designed to
satisfy the needs of a broad-based group of scientists, including those who may
be less familiar with astronomical data analysis in the X-ray regime. The first
release of the CSC includes information about 94,676 distinct X-ray sources
detected in a subset of public ACIS imaging observations from roughly the first
eight years of the Chandra mission. This release of the catalog includes point
and compact sources with observed spatial extents <~ 30''. The catalog (1)
provides access to the best estimates of the X-ray source properties for
detected sources, with good scientific fidelity, and directly supports
scientific analysis using the individual source data; (2) facilitates analysis
of a wide range of statistical properties for classes of X-ray sources; and (3)
provides efficient access to calibrated observational data and ancillary data
products for individual X-ray sources, so that users can perform detailed
further analysis using existing tools. The catalog includes real X-ray sources
detected with flux estimates that are at least 3 times their estimated 1 sigma
uncertainties in at least one energy band, while maintaining the number of
spurious sources at a level of <~ 1 false source per field for a 100 ks
observation. For each detected source, the CSC provides commonly tabulated
quantities, including source position, extent, multi-band fluxes, hardness
ratios, and variability statistics, derived from the observations in which the
source is detected. In addition to these traditional catalog elements, for each
X-ray source the CSC includes an extensive set of file-based data products that
can be manipulated interactively.Comment: To appear in The Astrophysical Journal Supplement Series, 53 pages,
27 figure
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