33,008 research outputs found
The Way We Measure: Comparison of Methods to Derive Radial Surface Brightness Profiles
The breaks and truncations in the luminosity profile of face-on spiral
galaxies offer valuable insights in their formation history. The traditional
method of deriving the surface photometry profile for face-on galaxies is to
use elliptical averaging. In this paper, we explore the question whether
elliptical averaging is the best way to do this. We apply two additional
surface photometry methods, one new: principle axis summation, and one old that
has become seldom used: equivalent profiles. These are compared to elliptically
averaged profiles using a set of 29 face-on galaxies. We find that the
equivalent profiles match extremely well with elliptically averaged profiles,
confirming the validity of using elliptical averaging. The principle axis
summation offers a better comparison to edge-on galaxies.Comment: Accepted for publication by Monthly Notices of the R.A.S. A hi-res
version is available at http://www.astro.rug.nl/~vdkruit/Petersetal-VI.pd
The Shape of Dark Matter Haloes IV. The Structure of Stellar Discs in Edge-on Galaxies
We present optical and near-infrared archival observations of eight edge-on
galaxies. These observations are used to model the stellar content of each
galaxy using the FitSKIRT software package. Using FitSKIRT, we can
self-consistently model a galaxy in each band simultaneously while treating for
dust. This allows us to accurately measure both the scale length and scale
height of the stellar disc, plus the shape parameters of the bulge. By
combining this data with the previously reported integrated magnitudes of each
galaxy, we can infer their true luminosities. We have successfully modelled
seven out of the eight galaxies in our sample. We find that stellar discs can
be modelled correctly, but have not been able to model the stellar bulge
reliably. Our sample consists for the most part of slow rotating galaxies, and
we find that the average dust layer is much thicker than what is reported for
faster rotating galaxies.Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version
available at www.astro.rug.nl/~vdkruit/Petersetal-IV.pd
The Shape of Dark Matter Haloes II. The Galactus HI Modelling & Fitting Tool
We present a new HI modelling tool called \textsc{Galactus}. The program has
been designed to perform automated fits of disc-galaxy models to observations.
It includes a treatment for the self-absorption of the gas. The software has
been released into the public domain. We describe the design philosophy and
inner workings of the program. After this, we model the face-on galaxy NGC2403,
using both self-absorption and optically thin models, showing that
self-absorption occurs even in face-on galaxies. It is shown that the maximum
surface brightness plateaus seen in Paper I of this series are indeed signs of
self-absorption. The apparent HI mass of an edge-on galaxy can be drastically
lower compared to that same galaxy seen face-on. The Tully-Fisher relation is
found to be relatively free from self-absorption issues.Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version
available at www.astro.rug.nl/~vdkruit/Petersetal-II.pd
Building research capacity in a practitioner network: the National Action Research Network on researching and evaluating Personal Development Plannin
The SILCC (SImulating the LifeCycle of molecular Clouds) project: I. Chemical evolution of the supernova-driven ISM
The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a
more self-consistent understanding of the interstellar medium (ISM) on small
scales and its link to galaxy evolution. We simulate the evolution of the
multi-phase ISM in a 500 pc x 500 pc x 10 kpc region of a galactic disc, with a
gas surface density of .
The Flash 4.1 simulations include an external potential, self-gravity, magnetic
fields, heating and radiative cooling, time-dependent chemistry of H and CO
considering (self-) shielding, and supernova (SN) feedback. We explore SN
explosions at different (fixed) rates in high-density regions (peak), in random
locations (random), in a combination of both (mixed), or clustered in space and
time (clustered). Only random or clustered models with self-gravity (which
evolve similarly) are in agreement with observations. Molecular hydrogen forms
in dense filaments and clumps and contributes 20% - 40% to the total mass,
whereas most of the mass (55% - 75%) is in atomic hydrogen. The ionised gas
contributes <10%. For high SN rates (0.5 dex above Kennicutt-Schmidt) as well
as for peak and mixed driving the formation of H is strongly suppressed.
Also without self-gravity the H fraction is significantly lower (
5%). Most of the volume is filled with hot gas (90% within 2 kpc).
Only for random or clustered driving, a vertically expanding warm component of
atomic hydrogen indicates a fountain flow. Magnetic fields have little impact
on the final disc structure. However, they affect dense gas () and delay H formation. We highlight that individual chemical
species, in particular atomic hydrogen, populate different ISM phases and
cannot be accurately accounted for by simple temperature-/density-based phase
cut-offs.Comment: 30 pages, 23 figures, submitted to MNRAS. Comments welcome! For
movies of the simulations and download of selected Flash data see the SILCC
website: http://www.astro.uni-koeln.de/silc
Interaction of massive black hole binaries with their stellar environment: II. Loss-cone depletion and binary orbital decay
We study the long-term evolution of massive black hole binaries (MBHBs) at
the centers of galaxies using detailed scattering experiments to solve the full
three-body problem. Ambient stars drawn from a isotropic Maxwellian
distribution unbound to the binary are ejected by the gravitational slingshot.
We construct a minimal, hybrid model for the depletion of the loss cone and the
orbital decay of the binary, and show that secondary slingshots - stars
returning on small impact parameter orbits to have a second super-elastic
scattering with the MBHB - may considerably help the shrinking of the pair in
the case of large binary mass ratios. In the absence of loss-cone refilling by
two-body relaxation or other processes, the mass ejected before the stalling of
a MBHB is half the binary reduced mass. About 50% of the ejected stars are
expelled ejected in a "burst" lasting ~1E4 yrs M_6^1/4, where M_6 is the binary
mass in units of 1E6 Msun. The loss cone is completely emptied in a few bulge
crossing timescales, 1E7 yrs M_6^1/4. Even in the absence of two-body
relaxation or gas dynamical processes, unequal mass and/or eccentric binaries
with M_6 >0.1 can shrink to the gravitational wave emission regime in less than
a Hubble time, and are therefore "safe" targets for the planned Laser
Interferometer Space Antenna (LISA).Comment: Minor revision. 10 pages, 7 figures, ApJ in pres
The SILCC project: III. Regulation of star formation and outflows by stellar winds and supernovae
We study the impact of stellar winds and supernovae on the multi-phase
interstellar medium using three-dimensional hydrodynamical simulations carried
out with FLASH. The selected galactic disc region has a size of (500 pc) x
5 kpc and a gas surface density of 10 M/pc. The simulations
include an external stellar potential and gas self-gravity, radiative cooling
and diffuse heating, sink particles representing star clusters, stellar winds
from these clusters which combine the winds from indi- vidual massive stars by
following their evolution tracks, and subsequent supernova explosions. Dust and
gas (self-)shielding is followed to compute the chemical state of the gas with
a chemical network. We find that stellar winds can regulate star (cluster)
formation. Since the winds suppress the accretion of fresh gas soon after the
cluster has formed, they lead to clusters which have lower average masses
(10 - 10 M) and form on shorter timescales (10 -
10 Myr). In particular we find an anti-correlation of cluster mass and
accretion time scale. Without winds the star clusters easily grow to larger
masses for ~5 Myr until the first supernova explodes. Overall the most massive
stars provide the most wind energy input, while objects beginning their
evolution as B-type stars contribute most of the supernova energy input. A
significant outflow from the disk (mass loading 1 at 1 kpc) can be
launched by thermal gas pressure if more than 50% of the volume near the disc
mid-plane can be heated to T > 3x10 K. Stellar winds alone cannot create a
hot volume-filling phase. The models which are in best agreement with observed
star formation rates drive either no outflows or weak outflows.Comment: 23 pages; submitted to MNRA
Serious Games in Cultural Heritage
Although the widespread use of gaming for leisure purposes has been well documented, the use of games to support cultural heritage purposes, such as historical teaching and learning, or for enhancing museum visits, has been less well considered. The state-of-the-art in serious game technology is identical to that of the state-of-the-art in entertainment games technology. As a result the field of serious heritage games concerns itself with recent advances in computer games, real-time computer graphics, virtual and augmented reality and artificial intelligence. On the other hand, the main strengths of serious gaming applications may be generalised as being in the areas of communication, visual expression of information, collaboration mechanisms, interactivity and entertainment. In this report, we will focus on the state-of-the-art with respect to the theories, methods and technologies used in serious heritage games. We provide an overview of existing literature of relevance to the domain, discuss the strengths and weaknesses of the described methods and point out unsolved problems and challenges. In addition, several case studies illustrating the application of methods and technologies used in cultural heritage are presented
Four-Body Effects in Globular Cluster Black Hole Coalescence
In the high density cores of globular clusters, multibody interactions are
expected to be common, with the result that black holes in binaries are
hardened by interactions. It was shown by Sigurdsson & Hernquist (1993) and
others that 10 solar mass black holes interacting exclusively by three-body
encounters do not merge in the clusters themselves, because recoil kicks the
binaries out of the clusters before the binaries are tight enough to merge.
Here we consider a new mechanism, involving four-body encounters. Numerical
simulations by a number of authors suggest that roughly 20-50% of binary-binary
encounters will eject one star but leave behind a stable hierarchical triple.
If the orbital plane of the inner binary is strongly tilted with respect to the
orbital plane of the outer object, a secular Kozai resonance, first
investigated in the context of asteroids in the Solar System, can increase the
eccentricity of the inner body significantly. We show that in a substantial
fraction of cases the eccentricity is driven to a high enough value that the
inner binary will merge by gravitational radiation, without a strong
accompanying kick. Thus the merged object remains in the cluster; depending on
the binary fraction of black holes and the inclination distribution of
newly-formed hierarchical triples, this mechanism may allow massive black holes
to accumulate through successive mergers in the cores of globular clusters. It
may also increase the likelihood that stellar-mass black holes in globular
clusters will be detectable by their gravitational radiation.Comment: Submitted to ApJ Letters (includes emulateapj.sty
Nearly horizon skimming orbits of Kerr black holes
An unusual set of orbits about extreme Kerr black holes resides at the
Boyer-Lindquist radius , the coordinate of the hole's event horizon.
These ``horizon skimming'' orbits have the property that their angular momentum
{\it increases} with inclination angle, opposite to the familiar behavior
one encounters at larger radius. In this paper, I show that this behavior is
characteristic of a larger family of orbits, the ``nearly horizon skimming''
(NHS) orbits. NHS orbits exist in the very strong field of any black hole with
spin a\agt 0.952412M. Their unusual behavior is due to the locking of
particle motion near the event horizon to the hole's spin, and is therefore a
signature of the Kerr metric's extreme strong field. An observational hallmark
of NHS orbits is that a small body spiraling into a Kerr black hole due to
gravitational-wave emission will be driven into orbits of progressively smaller
inclination angle, toward the equator. This is in contrast to the ``normal''
behavior. For circular orbits, the change in inclination is very small, and
unlikely to be of observational importance. I argue that the change in
inclination may be considerably larger when one considers the evolution of
inclined eccentric orbits. If this proves correct, then the gravitational waves
produced by evolution through the NHS regime may constitute a very interesting
and important probe of the strong-field nature of rotating black holes.Comment: 9 pages, 5 figures, accepted for publication in PR
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