105 research outputs found
Gas accretion in galactic disks
Evidence for the accretion of material in spiral galaxies has grown over the
past years and clear signatures can be found in HI observations of galaxies.
We describe here new detailed and sensitive HI synthesis observations of a
few nearby galaxies (NGC 3359, NGC 4565 and NGC 6946) which show that indeed
accretion of small amounts of gas is taking place.
These should be regarded as examples illustrating a general phenomenon of gas
infall in galaxies. Such accretion may also be at the origin of the gaseous
halos which are being found around spirals. Probably it is the same kind of
phenomenon of material infall as observed in the stellar streams in the halo
and outer parts of our galaxy and M 31Comment: To appear in the proceedings of the symposium "Extraplanar Gas", ASP
Conference series, editor R. Braun, 8 pages + 5 figure
Evidence for gas accretion in galactic disks
Studies of the HI in galaxies have clearly shown that subtle details of the
HI distribution and kinematics often harbour key information for understanding
the structure and evolution of galaxies. Evidence for the accretion of material
has grown over the past many years and clear signatures can be found in HI
observations of galaxies. We have obtained new detailed and sensitive HI
synthesis observations of three nearby galaxies which are suspected of
capturing small amounts of HI and show that indeed accretion of small amounts
of gas is taking place in these galaxies. This could be the same kind of
phenomenon of material infall as observed in the stellar streams in the halo
and outer parts of our galaxy and M31Comment: To appear in the proceedings of the IAU Symposium #217, Recycling
Intergalactic and Interstellar Matter, eds. P.-A. Duc, J. Braine, and E.
Brinks, 8 page
Redundancy Calibration of Phased Array Stations
Our aim is to assess the benefits and limitations of using the redundant
visibility information in regular phased array systems for improving the
calibration.
Regular arrays offer the possibility to use redundant visibility information
to constrain the calibration of the array independent of a sky model and a beam
models of the station elements. It requires a regular arrangement in the
configuration of array elements and identical beam patterns.
We revised a calibration method for phased array stations using the redundant
visibility information in the system and applied it successfully to a LOFAR
station. The performance and limitations of the method were demonstrated by
comparing its use on real and simulated data. The main limitation is the mutual
coupling between the station elements, which leads to non-identical beams and
stronger baseline dependent noise. Comparing the variance of the estimated
complex gains with the Cramer-Rao Bound (CRB) indicates that redundancy is a
stable and optimum method for calibrating the complex gains of the system.
Our study shows that the use of the redundant visibility does improve the
quality of the calibration in phased array systems. In addition it provides a
powerful tool for system diagnostics. Our results demonstrate that designing
redundancy in both the station layout and the array configuration of future
aperture arrays is strongly recommended. In particular in the case of the
Square Kilometre Array with its dynamic range requirement which surpasses any
existing array by an order of magnitude.Comment: 16 pages, 15 figures, accepted for publication in the A&A in Section
13, acceptance date: 1st May 2012. NOTE: Please contact the first author for
high resolution figure
The effect of stellar feedback on a Milky Way-like galaxy and its gaseous halo
We present the study of a set of N-body+smoothed particle hydrodynamics simulations of a Milky Way-like system produced by the radiative cooling of hot gas embedded in a dark matter halo. The galaxy and its gaseous halo evolve for 10 Gyr in isolation, which allows us to study how internal processes affect the evolution of the system. We show how the morphology, the kinematics and the evolution of the galaxy are affected by the input supernova feedback energy ESN, and we compare its properties with those of the Milky Way. Different values of ESN do not significantly affect the star formation history of the system, but the disc of cold gas gets thicker and more turbulent as feedback increases. Our main result is that, for the highest value of ESN considered, the galaxy shows a prominent layer of extraplanar cold (log (T/K) < 4.3) gas extended up to a few kiloparsec above the disc at column densities of 1019 cm-2. The kinematics of this material is in agreement with that inferred for the H I haloes of our Galaxy and NGC 891, although its mass is lower. Also, the location, the kinematics and the typical column densities of the hot (5.3 < log (T/K) < 5.7) gas are in good agreement with those determined from the O VI absorption systems in the halo of the Milky Way and external galaxies. In contrast with the observations, however, gas at log (T/K) < 5.3 is lacking in the circumgalactic region of our systems
The Cosmically Depressed: Life, Sociology and Identity of Voids
We review and discuss aspects of Cosmic Voids that form the background for
our Void Galaxy Survey (see accompanying paper by Stanonik et al.). Following a
sketch of the general characteristics of void formation and evolution, we
describe the influence of the environment on their development and structure
and the characteristic hierarchical buildup of the cosmic void population. In
order to be able to study the resulting tenuous void substructure and the
galaxies populating the interior of voids, we subsequently set out to describe
our parameter free tessellation-based watershed void finding technique. It
allows us to trace the outline, shape and size of voids in galaxy redshift
surveys. The application of this technique enables us to find galaxies in the
deepest troughs of the cosmic galaxy distribution, and has formed the basis of
our void galaxy program.Comment: 10 pages, 4 figures, proceedings "Galaxies in Isolation" (May 2009,
Granada, Spain), eds. L. Verdes-Montenegro, ASP (this is a colour, extended
and combined version; accompanying paper to Stanonik et al., arXiv:0909.2869,
in same volume
Non-parametric estimation of morphological lopsidedness
Asymmetries in the neutral hydrogen gas distribution and kinematics of galaxies are thought to be indicators for both gas accretion and gas removal processes. These are of fundamental importance for galaxy formation and evolution. Upcoming large blind H I surveys will provide tens of thousands of galaxies for a study of these asymmetries in a proper statistical way. Due to the large number of expected sources and the limited resolution of the majority of objects, detailed modelling is not feasible for most detections. We need fast, automatic and sensitive methods to classify these objects in an objective way. Existing non-parametric methods suffer from effects like the dependence on signal to noise, resolution and inclination. Here we show how to correctly take these effects into account and show ways to estimate the precision of the methods. We will use existing and modelled data to give an outlook on the performance expected for galaxies observed in the various sky surveys planned for e.g. WSRT/APERTIF and ASKAP
An observational and theoretical view of the radial distribution of HI gas in galaxies
We analyze the radial distribution of HI gas for 23 disk galaxies with
unusually high HI content from the Bluedisk sample, along with a similar-sized
sample of "normal" galaxies. We propose an empirical model to fit the radial
profile of the HI surface density, an exponential function with a depression
near the center. The radial HI surface density profiles are very homogeneous in
the outer regions of the galaxy; the exponentially declining part of the
profile has a scale-length of R1, where R1 is the radius where the
column density of the HI is 1 M pc. This holds for all
galaxies, independent of their stellar or HI mass. The homogenous outer
profiles, combined with the limited range in HI surface density in the
non-exponential inner disk, results in the well-known tight relation between HI
size and HI mass. By comparing the radial profiles of the HI-rich galaxies with
those of the control systems, we deduce that in about half the galaxies, most
of the excess gas lies outside the stellar disk, in the exponentially declining
outer regions of the HI disk. In the other half, the excess is more centrally
peaked. We compare our results with existing smoothed-particle hydrodynamical
simulations and semi-analytic models of disk galaxy formation in a
Cold Dark Matter universe. Both the hydro simulations and the semi-analytic
models reproduce the HI surface density profiles and the HI size-mass relation
without further tuning of the simulation and model inputs. In the semi-analytic
models, the universal shape of the outer HI radial profiles is a consequence of
the {\em assumption} that infalling gas is always distributed exponentially.
The conversion of atomic gas to molecular form explains the limited range of HI
surface densities in the inner disk. These two factors produce the tight HI
mass-size relation.Comment: 15 pages, 14 figures, submitted to MNRA
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