657 research outputs found
Continuous Fields and Discrete Samples: Reconstruction through Delaunay Tessellations
Here we introduce the Delaunay Density Estimator Method. Its purpose is
rendering a fully volume-covering reconstruction of a density field from a set
of discrete data points sampling this field. Reconstructing density or
intensity fields from a set of irregularly sampled data is a recurring key
issue in operations on astronomical data sets, both in an observational context
as well as in the context of numerical simulations. Our technique is based upon
the stochastic geometric concept of the Delaunay tessellation generated by the
point set. We shortly describe the method, and illustrate its virtues by means
of an application to an N-body simulation of cosmic structure formation. The
presented technique is a fully adaptive method: automatically it probes high
density regions at maximum possible resolution, while low density regions are
recovered as moderately varying regions devoid of the often irritating
shot-noise effects. Of equal importance is its capability to sharply and
undilutedly recover anisotropic density features like filaments and walls. The
prominence of such features at a range of resolution levels within a
hierarchical clustering scenario as the example of the standard CDM scenario is
shown to be impressively recovered by our scheme.Comment: 4 pages, 2 figures, accepted for publication in Astronomy &
Astrophysics Letter
The vertical extent and kinematics of the HI in NGC 2403
The neutral hydrogen line profiles along the major axis of the nearby spiral
galaxy NGC 2403 show a wing towards the sytemic velocity. This asymmetry can be
explained with the presence of an abnormally thick HI disk (FWHM ~ 5 kpc) or
with a two-component structure: a thin disk and a slowly rotating, thicker (1-3
kpc) HI layer. The latter model gives a better representation of the
observations. These results throw a new light on the disk-halo connection. In
particular, the decrease of rotational velocity with height above the plane may
be the result of a galactic fountain flow. A vertically extended, slowly
rotating HI layer may be common among spiral galaxies with high levels of star
formation.Comment: 4 pages, 2 figures, accepted for publication in Astronomy and
Astrophysics Letter
High Latitude HI in NGC 2613: Buoyant Disk-Halo Outflow
We combine new VLA D array HI data of NGC 2613 with previous high resolution
data to show new disk-halo features in this galaxy. The global HI distribution
is modeled in detail using a technique which can disentangle the effects of
inclination from scale height and can also solve for the average volume density
distribution in and perpendicular to the disk. The model shows that the
galaxy's inclination is on the low end of the range given by Chaves & Irwin
(2001) and that the HI disk is thin (z_e = 188 pc), showing no evidence for
halo. Numerous discrete disk-halo features are observed, however, achieving z
heights up to 28 kpc from mid-plane. One prominent feature in particular, of
mass, 8X10^7 Msun and height, 22 kpc, is seen on the advancing side of the
galaxy at a projected galactocentric radius of 15.5 kpc. If this feature
achieves such high latitudes because of events in the disk alone, then input
energies of order ~ 10^{56} ergs are required. We have instead investigated the
feasibility of such a large feature being produced via buoyancy (with drag)
within a hot, pre-existing X-ray corona. Reasonable plume densities,
temperatures, stall height (~ 11 kpc), outflow velocities and ages can indeed
be achieved in this way. The advantage of this scenario is that the input
energy need only be sufficient to produce blow-out, a condition which requires
a reduction of three orders of magnitude in energy. If this is correct, there
should be an observable X-ray halo around NGC 2613.Comment: 32 pages 7 gif figures, accepted by Ap
High-Latitude HI in the Low Surface Brightness Galaxy UGC7321
From the analysis of sensitive HI 21-cm line observations, we find evidence
for vertically extended HI emission (|z|<~2.4 kpc) in the edge-on, low surface
brightness spiral galaxy UGC7321. Three-dimensional modelling suggests that the
HI disk of UGC7321 is both warped and flared, but that neither effect can fully
reproduce the spatial distribution and kinematics of the highest z-height gas.
We are able to model the high-latitude emission as an additional HI component
in the form of a ``thick disk'' or ``halo'' with a FWHM~3.3 kpc. We find
tentative evidence that the vertically extended gas declines in rotational
velocity as a function of z, although we are unable to completely rule out
models with constant V(z). In spite of the low star formation rate of UGC7321,
energy from supernovae may be sufficient to sustain this high-latitude gas.
However, alternative origins for this material, such as slow, sustained infall,
cannot yet be excluded.Comment: to appear in the August 20 Astrophysical Journal; 17 pages; version
with full resolution figures available at
http://cfa-www.harvard.edu/~lmatthew
Deprojection of luminosity functions of galaxies in the Coma cluster
We use a simple analytic model to deproject 2-d luminosity functions (LF) of
galaxies in the Coma cluster measured by Beijersbergen et al. 2002. We
demonstrate that the shapes of the LFs change after deprojection. It is
therefore essential to correct LFs for projection effects. The deprojected LFs
of the central area have best-fitting Schechter parameters of
M^{*}_U=-18.31^{+0.08}_{-0.08} and \alpha_U=-1.27^{+0.018}_{-0.018},
M^{*}_B=-19.79^{+0.14}_{-0.15} and \alpha_B=-1.44^{+0.016}_{-0.016} and
M^{*}_r=-21.77^{+0.20}_{-0.28} and \alpha_r=-1.27^{+0.012}_{-0.012}. The
corrections are not significant enough to change the previously observed trend
of increasing faint end slopes with increasing distance to the cluster center.
The weighted U, B, and r band slopes of the deprojected LFs show a slightly
weaker steepening with increasing projected cluster radius.Comment: Accepted for publication in A&A as a Research Not
Generation of Vorticity and Velocity Dispersion by Orbit Crossing
We study the generation of vorticity and velocity dispersion by orbit
crossing using cosmological numerical simulations, and calculate the
backreaction of these effects on the evolution of large-scale density and
velocity divergence power spectra. We use Delaunay tessellations to define the
velocity field, showing that the power spectra of velocity divergence and
vorticity measured in this way are unbiased and have better noise properties
than for standard interpolation methods that deal with mass weighted
velocities. We show that high resolution simulations are required to recover
the correct large-scale vorticity power spectrum, while poor resolution can
spuriously amplify its amplitude by more than one order of magnitude. We
measure the scalar and vector modes of the stress tensor induced by orbit
crossing using an adaptive technique, showing that its vector modes lead, when
input into the vorticity evolution equation, to the same vorticity power
spectrum obtained from the Delaunay method. We incorporate orbit crossing
corrections to the evolution of large scale density and velocity fields in
perturbation theory by using the measured stress tensor modes. We find that at
large scales (k~0.1 h/Mpc) vector modes have very little effect in the density
power spectrum, while scalar modes (velocity dispersion) can induce percent
level corrections at z=0, particularly in the velocity divergence power
spectrum. In addition, we show that the velocity power spectrum is smaller than
predicted by linear theory until well into the nonlinear regime, with little
contribution from virial velocities.Comment: 27 pages, 14 figures. v2: reorganization of the material, new
appendix. Accepted by PR
Imaging Fabry-Perot Spectroscopy of NGC 5775: Kinematics of the Diffuse Ionized Gas Halo
We present imaging Fabry-Perot observations of Halpha emission in the nearly
edge-on spiral galaxy NGC 5775. We have derived a rotation curve and a radial
density profile along the major axis by examining position-velocity (PV)
diagrams from the Fabry-Perot data cube as well as a CO 2-1 data cube from the
literature. PV diagrams constructed parallel to the major axis are used to
examine changes in azimuthal velocity as a function of height above the
midplane. The results of this analysis reveal the presence of a vertical
gradient in azimuthal velocity. The magnitude of this gradient is approximately
1 km/s/arcsec, or about 8 km/s/kpc, though a higher value of the gradient may
be appropriate in localized regions of the halo. The evidence for an azimuthal
velocity gradient is much stronger for the approaching half of the galaxy,
although earlier slit spectra are consistent with a gradient on both sides.
There is evidence for an outward radial redistribution of gas in the halo. The
form of the rotation curve may also change with height, but this is not
certain. We compare these results with those of an entirely ballistic model of
a disk-halo flow. The model predicts a vertical gradient in azimuthal velocity
which is shallower than the observed gradient, indicating that an additional
mechanism is required to further slow the rotation speeds in the halo.Comment: 18 pages, 18 figures. Uses emulateapj.cls. Accepted for publication
in Ap
Spin alignment of dark matter haloes in filaments and walls
The MMF technique is used to segment the cosmic web as seen in a cosmological
N-body simulation into wall-like and filament-like structures. We find that the
spins and shapes of dark matter haloes are significantly correlated with each
other and with the orientation of their host structures. The shape orientation
is such that the halo minor axes tend to lie perpendicular to the host
structure, be it a wall or filament. The orientation of the halo spin vector is
mass dependent. Low mass haloes in walls and filaments have a tendency to have
their spins oriented within the parent structure, while higher mass haloes in
filaments have spins that tend to lie perpendicular to the parent structure.Comment: 4 pages, 2 figure
Cosmic Voids: structure, dynamics and galaxies
In this review we discuss several aspects of Cosmic Voids. Voids are a major
component of the large scale distribution of matter and galaxies in the
Universe. They are of instrumental importance for understanding the emergence
of the Cosmic Web. Their relatively simple shape and structure makes them into
useful tools for extracting the value of a variety cosmic parameters, possibly
including even that of the influence of dark energy. Perhaps most promising and
challenging is the issue of the galaxies found within their realm. Not only
does the pristine environment of voids provide a promising testing ground for
assessing the role of environment on the formation and evolution of galaxies,
the dearth of dwarf galaxies may even represent a serious challenge to the
standard view of cosmic structure formation.Comment: 29 pages, 12 figures, invited review COSPA2008, Pohang, Korea. Modern
Physics Letters A, accepted. For high-res version see
http://www.astro.rug.nl/~weygaert/voids.cospa2008.weygaert.pd
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