1,074 research outputs found
Hierarchical octree and k-d tree grids for 3D radiative transfer simulations
A crucial ingredient for numerically solving the 3D radiative transfer
problem is the choice of the grid that discretizes the transfer medium. Many
modern radiative transfer codes, whether using Monte Carlo or ray tracing
techniques, are equipped with hierarchical octree-based grids to accommodate a
wide dynamic range in densities. We critically investigate two different
aspects of octree grids in the framework of Monte Carlo dust radiative
transfer. Inspired by their common use in computer graphics applications, we
test hierarchical k-d tree grids as an alternative for octree grids. On the
other hand, we investigate which node subdivision-stopping criteria are optimal
for constructing of hierarchical grids. We implemented a k-d tree grid in the
3D radiative transfer code SKIRT and compared it with the previously
implemented octree grid. We also considered three different node
subdivision-stopping criteria (based on mass, optical depth, and density
gradient thresholds). Based on a small suite of test models, we compared the
efficiency and accuracy of the different grids, according to various quality
metrics. For a given set of requirements, the k-d tree grids only require half
the number of cells of the corresponding octree. Moreover, for the same number
of grid cells, the k-d tree is characterized by higher discretization accuracy.
Concerning the subdivision stopping criteria, we find that an optical depth
criterion is not a useful alternative to the more standard mass threshold,
since the resulting grids show a poor accuracy. Both criteria can be combined;
however, in the optimal combination, for which we provide a simple approximate
recipe, this can lead to a 20% reduction in the number of cells needed to reach
a certain grid quality. An additional density gradient threshold criterion can
be added that solves the problem of poorly resolving sharp edges and...
(abridged).Comment: 10 pages, 6 figures. Accepted for publication in A&
The dynamical structure of isotropic spherical galaxies with a central black hole
We discuss the kinematical structure of a two-parameter family of isotropic
models with a central black hole. The family contains the slope of the central
density cusp and the relative black hole mass as parameters. Most of the basic
kinematical quantities of these models can be expressed analytically. This
family contains three distinct models where also the distribution function,
differential energy distribution and spatial LOSVDs can be expressed completely
analytically. Each of these models show a drastically different behaviour of
the distribution function. Although the effect of a black hole on the
distribution function is very strong, in particular for models with a shallow
density cusp, the differential energy distribution is only marginally affected.
We discuss in detail the effects of a central black hole on the LOSVDs. The
projected velocity dispersion increases with black hole mass at small projected
radii, but the effect of a black hole on the shape of the LOSVDs (characterized
by the h4 parameter) is less straightforward to interpret. Too much reliance on
the wings of the LOSVDs and the value of the h4 parameter to determine black
hole masses might hence be dangerous.Comment: 12 pages, 3 figures, accepted for publication in A&
Radial stability of a family of anisotropic Hernquist models with and without a supermassive black hole
We present a method to investigate the radial stability of a spherical
anisotropic system that hosts a central supermassive black hole (SBH). Such
systems have never been tested before for stability, although high anisotropies
have been considered in the dynamical models that were used to estimate the
masses of the central putative supermassive black holes. A family of analytical
anisotropic spherical Hernquist models with and without a black hole were
investigated by means of N-body simulations. A clear trend emerges that the
supermassive black hole has a significant effect on the overall stability of
the system, i.e. an SBH with a mass of a few percent of the total mass of the
galaxy can prevent or reduce the bar instabilities in anisotropic systems. Its
mass not only determines the strength of the instability reduction, but also
the time in which this occurs. These effects are most significant for models
with strong radial anisotropies. Furthermore, our analysis shows that unstable
systems with similar SBH but with different anisotropy radii evolve
differently: highly radial systems become oblate, while more isotropic models
tend to form into prolate structures. In addition to this study, we also
present a Monte-Carlo algorithm to generate particles in spherical anisotropic
systems.Comment: 16 pages, 12 figures, accepted for publication in MNRAS (some figures
have a lowered resolution
G2C2-III: Structural parameters for Galactic globular clusters in SDSS passbands
We use our Galactic Globular Cluster Catalog (G2C2) photometry for 111
Galactic globular clusters (GC) in g and z, as well as r and i photometry for a
subset of 60 GCs and u photometry for 22 GCs, to determine the structural
parameters assuming King (1962) models.
In general, the resulting core radii are in good comparison with the current
literature values. However, our half-light radii are slightly lower than the
literature. The concentrations (and therefore also the tidal radii) are poorly
constrained mostly because of the limited radial extent of our imaging.
Therefore, we extensively discuss the effects of a limited field-of-view on the
derived parameters using mosaicked SDSS data, which do not suffer from this
restriction. We also illustrate how red giant branch (RGB) stars in cluster
cores can stochastically induce artificial peaks in the surface brightness
profiles. The issues related to these bright stars are scrutinised based on
both our photometry and simulated clusters. We also examine colour gradients
and find that the strongest central colour gradients are caused by central RGB
stars and thus not representative for the cluster light or colour distribution.
We recover the known relation between the half-light radius and the
Galactocentric distance in the g-band, but find a lower slope for redder
filters. We did not find a correlation between the scatter on this relation and
other cluster properties. We find tentative evidence for a correlation between
the half-light radii and the [Fe/H], with metal-poor GCs being larger than
metal-rich GCs. However, we conclude that this trend is caused by the position
of the clusters in the Galaxy, with metal-rich clusters being more centrally
located.Comment: 17 pages, 23 figures, 3 tables. Accepted to MNRAS. The online
appendix includes the structural parameters and the SB profile fits for all
the sample cluster
Large and small-scale structures and the dust energy balance problem in spiral galaxies
The interstellar dust content in galaxies can be traced in extinction at
optical wavelengths, or in emission in the far-infrared. Several studies have
found that radiative transfer models that successfully explain the optical
extinction in edge-on spiral galaxies generally underestimate the observed
FIR/submm fluxes by a factor of about three. In order to investigate this
so-called dust energy balance problem, we use two Milky Way-like galaxies
produced by high-resolution hydrodynamical simulations. We create mock optical
edge-on views of these simulated galaxies (using the radiative transfer code
SKIRT), and we then fit the parameters of a basic spiral galaxy model to these
images (using the fitting code FitSKIRT). The basic model includes smooth
axisymmetric distributions along a S\'ersic bulge and exponential disc for the
stars, and a second exponential disc for the dust. We find that the dust mass
recovered by the fitted models is about three times smaller than the known dust
mass of the hydrodynamical input models. This factor is in agreement with
previous energy balance studies of real edge-on spiral galaxies. On the other
hand, fitting the same basic model to less complex input models (e.g. a smooth
exponential disc with a spiral perturbation or with random clumps), does
recover the dust mass of the input model almost perfectly. Thus it seems that
the complex asymmetries and the inhomogeneous structure of real and
hydrodynamically simulated galaxies are a lot more efficient at hiding dust
than the rather contrived geometries in typical quasi-analytical models. This
effect may help explain the discrepancy between the dust emission predicted by
radiative transfer models and the observed emission in energy balance studies
for edge-on spiral galaxies.Comment: 9 pages, 5 figures, accepted for publication in A&
A low-frequency study of recently identified double-double radio galaxies
In order to understand the possible mechanisms of recurrent jet activity in
radio galaxies and quasars, which are still unclear, we have identified such
sources with a large range of linear sizes (220 917 kpc), and hence time
scales of episodic activity. Here we present high-sensitivity 607-MHz Giant
Metrewave Radio Telescope (GMRT) images of 21 possible double-double radio
galaxies (DDRGs) identified from the FIRST survey to confirm their episodic
nature. These GMRT observations show that none of the inner compact components
suspected to be hot-spots of the inner doubles are cores having a flat radio
spectrum, confirming the episodic nature of these radio sources. We have
indentified a new DDRG with a candidate quasar, and have estimated the upper
spectral age limits for eight sources which showed marginal evidence of
steepening at higher frequencies. The estimated age limits (11 52 Myr) are
smaller than those of the large-sized ( 1 Mpc) DDRGs.Comment: Accepted for publication in MNRAS. 14 pages, 7 figure
Radiative equilibrium in Monte Carlo radiative transfer using frequency distribution adjustment
The Monte Carlo method is a powerful tool for performing radiative
equilibrium calculations, even in complex geometries. The main drawback of the
standard Monte Carlo radiative equilibrium methods is that they require
iteration, which makes them numerically very demanding. Bjorkman & Wood
recently proposed a frequency distribution adjustment scheme, which allows
radiative equilibrium Monte Carlo calculations to be performed without
iteration, by choosing the frequency of each re-emitted photon such that it
corrects for the incorrect spectrum of the previously re-emitted photons.
Although the method appears to yield correct results, we argue that its
theoretical basis is not completely transparent, and that it is not completely
clear whether this technique is an exact rigorous method, or whether it is just
a good and convenient approximation. We critically study the general problem of
how an already sampled distribution can be adjusted to a new distribution by
adding data points sampled from an adjustment distribution. We show that this
adjustment is not always possible, and that it depends on the shape of the
original and desired distributions, as well as on the relative number of data
points that can be added. Applying this theorem to radiative equilibrium Monte
Carlo calculations, we provide a firm theoretical basis for the frequency
distribution adjustment method of Bjorkman & Wood, and we demonstrate that this
method provides the correct frequency distribution through the additional
requirement of radiative equilibrium. We discuss the advantages and limitations
of this approach, and show that it can easily be combined with the presence of
additional heating sources and the concept of photon weighting. However, the
method may fail if small dust grains are included... (abridged)Comment: 17 pages, 2 figures, accepted for publication in New Astronom
Photo-centric variability of quasars caused by variations in their inner structure: Consequences on Gaia measurements
We study the photocenter position variability due to variations in the quasar
inner structure. We consider variability in the accretion disk emissivity and
torus structure variability due to different illumination by the central
source. We discuss possible detection of these effects by Gaia. Observations of
the photocenter variability in two AGNs, SDSS J121855+020002 and SDSS
J162011+1724327 have been reported and discussed. With investigation of the
variations in the quasar inner structure we explore how much this effect can
affect the position determination and whether it can be (or not) detected with
Gaia mission. We used (a) a model of a relativistic disk, including the
perturbation that can increase brightness of a part of the disk, and
consequently offset the photocenter position, and (b) a model of a dusty torus
which absorbs and re-emits the incoming radiation from accretion disk. We
estimated the value of the photocenter offset due to these two effects. We
found that perturbations in the inner structure can significantly offset the
photocenter. It depends on the characteristics of perturbation and accretion
disk and structure of the torus. In the case of two considered QSOs the
observed photocenter offsets cannot be explained by variations in the accretion
disk and other effects should be considered. We discussed possibility of
exploding stars very close to the AGN source, and also possibility that there
are two variable sources in the center of these two AGNs that may indicate a
binary super-massive black hole system on a kpc (pc) scale. The Gaia mission
seems to be very perspective, not only for astrometry, but also for exploring
the inner structure of AGNs. We conclude that variations in the quasar inner
structure can affect the observed photocenter (up to several mas). There is a
chance to observe such effect in the case of bright and low-redshifted QSOs.Comment: 12 pages, 8 figures, 3 tables. Accepted for publication in Astronomy
and Astrophysics. Language improved, typos correcte
G2C2 II: Integrated colour-metallicity relations for Galactic Globular Clusters in SDSS passbands
We use our integrated SDSS photometry for 96 globular clusters in and
, as well as and photometry for a subset of 56 clusters, to derive
the integrated colour-metallicity relation (CMR) for Galactic globular
clusters. We compare this relation to previous work, including extragalactic
clusters, and examine the influence of age, present-day mass function
variations, structural parameters and the morphology of the horizontal branch
on the relation. Moreover, we scrutinise the scatter introduced by foreground
extinction (including differential reddening) and show that the scatter in the
colour-metallicity relation can be significantly reduced combining two
reddening laws from the literature. In all CMRs we find some low-reddening
young GCs that are offset to the CMR. Most of these outliers are associated
with the Sagittarius system. Simulations show that this is due less to age than
to a different enrichment history. Finally, we introduce colour-metallicity
relations based on the infrared Calcium triplet, which are clearly non-linear
when compared to and colours.Comment: Accepted for publication in MNRAS. 17 pages, 18 figure
Far-infrared and dust properties of present-day galaxies in the EAGLE simulations
The Evolution and Assembly of GaLaxies and their Environments (EAGLE) cosmological simulations reproduce the observed galaxy stellar mass function and many galaxy properties. In this work, we study the dust-related properties of present-day EAGLE galaxies through mock observations in the far-infrared and submm wavelength ranges obtained with the 3D dust radiative transfer code SKIRT. To prepare an EAGLE galaxy for radiative transfer processing, we derive a diffuse dust distribution from the gas particles and we re-sample the star-forming gas particles and the youngest star particles into star-forming regions that are assigned dedicated emission templates. We select a set of redshift-zero EAGLE galaxies that matches the K-band luminosity distribution of the galaxies in the Herschel Reference Survey (HRS), a volume-limited sample of about 300 normal galaxies in the Local Universe. We find overall agreement of the EAGLE dust scaling relations with those observed in the HRS, such as the dust-to-stellar mass ratio versus stellar mass and versus NUV–r colour relations. A discrepancy in the f250/f350 versus f350/f500 submm colour–colour relation implies that part of the simulated dust is insufficiently heated, likely because of limitations in our sub-grid model for star-forming regions. We also investigate the effect of adjusting the metal-to-dust ratio and the covering factor of the photodissociation regions surrounding the star-forming cores. We are able to constrain the important dust-related parameters in our method, informing the calculation of dust attenuation for EAGLE galaxies in the UV and optical domain
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