1,446 research outputs found
The Westerbork HI Survey of Spiral and Irregular Galaxies I. HI Imaging of Late-type Dwarf Galaxies
Neutral hydrogen observations with the Westerbork Synthesis Radio Telescope
are presented for a sample of 73 late-type dwarf galaxies. These observations
are part of the WHISP project (Westerbork HI Survey of Spiral and Irregular
Galaxies). Here we present HI maps, velocity fields, global profiles and radial
surface density profiles of HI, as well as HI masses, HI radii and line widths.
For the late-type galaxies in our sample, we find that the ratio of HI extent
to optical diameter, defined as 6.4 disk scale lengths, is on average 1.8+-0.8,
similar to that seen in spiral galaxies. Most of the dwarf galaxies in this
sample are rich in HI, with a typical M_HI/L_B of 1.5. The relative HI content
M_HI/L_R increases towards fainter absolute magnitudes and towards fainter
surface brightnesses. Dwarf galaxies with lower average HI column densities
also have lower average optical surface brightnesses. We find that lopsidedness
is as common among dwarf galaxies as it is in spiral galaxies. About half of
the dwarf galaxies in our sample have asymmetric global profiles, a third has a
lopsided HI distribution, and about half shows signs of kinematic lopsidedness.Comment: Accepted for publication in A&A. 18 pages. 39 MB version with all
figures is available http://www.robswork.net/publications/WHISPI.ps.g
IC 4200: a gas-rich early-type galaxy formed via a major merger
We present the result of radio and optical observations of the S0 galaxy IC
4200. We find that the galaxy hosts 8.5 billion solar masses of HI rotating on
a ~90 deg warped disk extended out to 60 kpc from the centre of the galaxy.
Optical spectroscopy reveals a simple-stellar-population-equivalent age of 1.5
Gyr in the centre of the galaxy and V- and R-band images show stellar shells.
Ionised gas is observed within the stellar body and is kinematically decoupled
from the stars and characterised by LINER-like line ratios.We interpret these
observational results as evidence for a major merger origin of IC 4200, and
date the merger back to 1-3 Gyr ago.Comment: Accepted for publication in Astronomy & Astrophysics; 18 pages, 13
figures; the tables of Appendix C can be downloaded at
http://www.astro.rug.nl/~pserra/IC420
Cooperative scattering and radiation pressure force in dense atomic clouds
We consider the collective scattering by a cloud of two-level atoms
driven by an uniform radiation field. Dense atomic clouds can be described by a
continuous density and the problem reduces to deriving the spectrum of the
atom-atom coupling operator. For clouds much larger than the optical
wavelength, the spectrum is treated as a continuum, and analytical expressions
for several macroscopic quantities, such as scattered radiation intensity and
radiation pressure force, are derived. The analytical results are then compared
to the exact -body solution and with those obtained assuming a symmetric
timed Dicke state. In contrast with the symmetric timed Dicke state, our
calculations takes account of the back action of the atoms on the driving field
leading to phase shifts due to the finite refraction of the cloud
Examining alternatives to wavelet de-noising for astronomical source finding
The Square Kilometre Array and its pathfinders ASKAP and MeerKAT will produce
prodigious amounts of data that necessitate automated source finding. The
performance of automated source finders can be improved by pre-processing a
dataset. In preparation for the WALLABY and DINGO surveys, we have used a test
HI datacube constructed from actual Westerbork Telescope noise and WHISP HI
galaxies to test the real world improvement of linear smoothing, the {\sc
Duchamp} source finder's wavelet de-noising, iterative median smoothing and
mathematical morphology subtraction, on intensity threshold source finding of
spectral line datasets. To compare these pre-processing methods we have
generated completeness-reliability performance curves for each method and a
range of input parameters. We find that iterative median smoothing produces the
best source finding results for ASKAP HI spectral line observations, but
wavelet de-noising is a safer pre-processing technique.
In this paper we also present our implementations of iterative median
smoothing and mathematical morphology subtraction.Comment: 7 pages, 4 figures, 3 colour figures. Accepted as part of the
Publications of the Astronomical Society of Australia's special issue on
source finding and visualisatio
Observation of a Cooperative Radiation Force in the Presence of Disorder
Cooperative scattering of light by an extended object such as an atomic
ensemble or a dielectric sphere is fundamentally different from scattering from
many point-like scatterers such as single atoms. Homogeneous distributions tend
to scatter cooperatively, whereas fluctuations of the density distribution
increase the disorder and suppress cooperativity. In an atomic cloud, the
amount of disorder can be tuned via the optical thickness, and its role can be
studied via the radiation force exerted by the light on the atomic cloud.
Monitoring cold atoms released from a magneto-optical trap, we
present the first experimental signatures of radiation force reduction due to
cooperative scattering. The results are in agreement with an analytical
expression interpolating between the disorder and the cooperativity-dominated
regimes
Modelling of laboratory data of bi-directional reflectance of regolith surface containing Alumina
Bidirectional reflectance of a surface is defined as the ratio of the
scattered radiation at the detector to the incident irradiance as a function of
geometry. The accurate knowledge of the bidirectional reflection function (BRF)
of layers composed of discrete, randomly positioned scattering particles is
very essential for many remote sensing, engineering, biophysical applications
and in different areas of Astrophysics. The computations of BRF's for plane
parallel particulate layers are usually reduced to solve the radiative transfer
equation (RTE) by the existing techniques. In this work we present our
laboratory data on bidirectional reflectance versus phase angle for two sample
sizes of 0.3 and 1 of Alumina for the He-Ne laser at 632.8 nm (red) and
543.5nm(green) wavelength. The nature of the phase curves of the asteroids
depends on the parameters like- particle size, composition, porosity, roughness
etc. In our present work we analyse the data which are being generated using
single scattering phase function i.e. Mie theory considering particles to be
compact sphere. The well known Hapke formula will be considered along with
different particle phase function such as Mie and Henyey Greenstein etc to
model the laboratory data obtained at the asteroid laboratory of Assam
University.Comment: 5 pages, 5 figures [accepted for publication in Publications of the
Astronomical Society of Australia (PASA) on 8 June, 2011
Optical extinction due to intrinsic structural variations of photonic crystals
Unavoidable variations in size and position of the building blocks of
photonic crystals cause light scattering and extinction of coherent beams. We
present a new model for both 2 and 3-dimensional photonic crystals that relates
the extinction length to the magnitude of the variations. The predicted lengths
agree well with our new experiments on high-quality opals and inverse opals,
and with literature data analyzed by us. As a result, control over photons is
limited to distances up to 50 lattice parameters (m) in
state-of-the-art structures, thereby impeding large-scale applications such as
integrated circuits. Conversely, scattering in photonic crystals may lead to
novel physics such as Anderson localization and non-classical diffusion.Comment: 10 pages, 3 figures. Changes include: added Lagendijk as author;
simplified and generalized the tex
Resonances in Mie scattering by an inhomogeneous atomic cloud
Despite the quantum nature of the process, collective scattering by dense
cold samples of two-level atoms can be interpreted classically describing the
sample as a macroscopic object with a complex refractive index. We demonstrate
that resonances in Mie theory can be easily observable in the cooperative
scattering by tuning the frequency of the incident laser field or the atomic
number. The solution of the scattering problem is obtained for spherical atomic
clouds who have the parabolic density characteristic of BECs, and the
cooperative radiation pressure force calculated exhibits resonances in the
cloud displacement for dense clouds. At odds from uniform clouds which show a
complex structure including narrow peaks, these densities show resonances, yet
only under the form of quite regular and contrasted oscillations
Self-Segregation vs. Clustering in the Evolutionary Minority Game
Complex adaptive systems have been the subject of much recent attention. It
is by now well-established that members (`agents') tend to self-segregate into
opposing groups characterized by extreme behavior. However, while different
social and biological systems manifest different payoffs, the study of such
adaptive systems has mostly been restricted to simple situations in which the
prize-to-fine ratio, , equals unity. In this Letter we explore the dynamics
of evolving populations with various different values of the ratio , and
demonstrate that extreme behavior is in fact {\it not} a generic feature of
adaptive systems. In particular, we show that ``confusion'' and
``indecisiveness'' take over in times of depression, in which case cautious
agents perform better than extreme ones.Comment: 4 pages, 4 figure
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