428 research outputs found
Three-dimensional extinction mapping using Gaussian random fields
We present a scheme for using stellar catalogues to map the three-dimensional
distributions of extinction and dust within our Galaxy. Extinction is modelled
as a Gaussian random field, whose covariance function is set by a simple
physical model of the ISM that assumes a Kolmogorov-like power spectrum of
turbulent fluctuations. As extinction is modelled as a random field, the
spatial resolution of the resulting maps is set naturally by the data
available; there is no need to impose any spatial binning. We verify the
validity of our scheme by testing it on simulated extinction fields and show
that its precision is significantly improved over previous dust-mapping
efforts. The approach we describe here can make use of any photometric,
spectroscopic or astrometric data; it is not limited to any particular survey.
Consequently, it can be applied to a wide range of data from both existing and
future surveys.Comment: 16 pages, 12 figures. Submitted for publication in MNRAS. Text
revise
Marginal likelihoods of distances and extinctions to stars: computation and compact representation
We present a method for obtaining the likelihood function of distance and
extinction to a star given its photometry. The other properties of the star
(its mass, age, metallicity and so on) are marginalised assuming a simple
Galaxy model. We demonstrate that the resulting marginalised likelihood
function can be described faithfully and compactly using a Gaussian mixture
model. For dust mapping applications we strongly advocate using monochromatic
over bandpass extinctions, and provide tables for converting from the former to
the latter for different stellar types.Comment: 14 pages, 12 figures. Accepted for publication in MNRAS. Source code
is available at https://github.com/stuartsal
Dynamical modelling of luminous and dark matter in 17 Coma early-type galaxies
Dynamical models for 17 Coma early-type galaxies are presented. The galaxy
sample consists of flattened, rotating as well as non-rotating early-types
including cD and S0 galaxies with luminosities between M=-18.79 and M=-22.56.
Kinematical long-slit observations cover at least the major and minor axis and
extend to 1-4 effective radii. Axisymmetric Schwarzschild models are used to
derive stellar mass-to-light ratios and dark halo parameters. In every galaxy
models with a dark matter halo match the data better than models without. The
statistical significance is over 95 percent for 8 galaxies, around 90 percent
for 5 galaxies and for four galaxies it is not significant. For the highly
significant cases systematic deviations between observed and modelled
kinematics are clearly seen; for the remaining galaxies differences are more
statistical in nature. Best-fit models contain 10-50 percent dark matter inside
the half-light radius. The central dark matter density is at least one order of
magnitude lower than the luminous mass density. The central phase-space density
of dark matter is often orders of magnitude lower than in the luminous
component, especially when the halo core radius is large. The orbital system of
the stars along the major-axis is slightly dominated by radial motions. Some
galaxies show tangential anisotropy along the minor-axis, which is correlated
with the minor-axis Gauss-Hermite coefficient H4. Changing the balance between
data-fit and regularisation constraints does not change the reconstructed mass
structure significantly. Model anisotropies tend to strengthen if the weight on
regularisation is reduced, but the general property of a galaxy to be radially
or tangentially anisotropic, respectively, does not change. (abridged)Comment: 31 pages, 34 figures; accepted for publication in MNRA
Quasar Jets and their Fields
Observations of jets from quasars and other types of accreting black hole are
briefly summarized. The importance of beaming and -ray observations for
understanding the origin of these jets is emphasised. It is argued that both
the power source and the collimation are likely to be magnetic in origin,
although the details remain controversial. Ultrarelativistic jets may be formed
by the spinning hole and collimated by a hydromagnetic disc wind. Progress in
understanding jets has been handicapped by our inadequate knowledge of how
magnetic field really behaves under cosmic conditions. Fortunately, significant
insights are coming from solar observations, numerical simulation and
laboratory plasma experiments. Some possible, evolutionary ramifications are
briefly discussed and it is suggested that it is the mass of the black hole
relative to that of the galaxy which determines the eventual galaxy morphology.Comment: Latex. 17pages Proc Discusison Meeting on Magnetic Activity in Stars,
Discs and Quasars. Ed. D. Lynden-Bell, E. R. Priest and N. O. Weiss. To
appear in Phil. Trans. Roy. Soc.
Axisymmetric Three-Integral Models for Galaxies
We describe an improved, practical method for constructing galaxy models that
match an arbitrary set of observational constraints, without prior assumptions
about the phase-space distribution function (DF). Our method is an extension of
Schwarzschild's orbit superposition technique. As in Schwarzschild's original
implementation, we compute a representative library of orbits in a given
potential. We then project each orbit onto the space of observables, consisting
of position on the sky and line-of-sight velocity, while properly taking into
account seeing convolution and pixel binning. We find the combination of orbits
that produces a dynamical model that best fits the observed photometry and
kinematics of the galaxy. A key new element of this work is the ability to
predict and match to the data the full line-of-sight velocity profile shapes. A
dark component (such as a black hole and/or a dark halo) can easily be included
in the models.
We have tested our method, by using it to reconstruct the properties of a
two-integral model built with independent software. The test model is
reproduced satisfactorily, either with the regular orbits, or with the
two-integral components. This paper mainly deals with the technical aspects of
the method, while applications to the galaxies M32 and NGC 4342 are described
elsewhere (van der Marel et al., Cretton & van den Bosch). (abridged)Comment: minor changes, accepted for publication in the Astrophysical Journal
Supplement
Regularized orbit models unveiling the stellar structure and dark matter halo of the Coma elliptical NGC 4807
This is the second in a series of papers dedicated to unveil the mass
structure and orbital content of a sample of flattened early-type galaxies in
the Coma cluster. The ability of our orbit libraries to reconstruct internal
stellar motions and the mass composition of a typical elliptical in the sample
is investigated by means of Monte-Carlo simulations of isotropic rotator
models. The simulations allow a determination of the optimal amount of
regularization needed in the orbit superpositions. It is shown that under
realistic observational conditions and with the appropriate regularization
internal velocity moments can be reconstructed to an accuracy of about 15 per
cent; the same accuracy can be achieved for the circular velocity and dark
matter fraction. In contrast, the flattening of the halo remains unconstrained.
Regularized orbit superpositions are applied to a first galaxy in our sample,
NGC 4807, for which stellar kinematical observations extend to 3 Reff. The
galaxy seems dark matter dominated outside 2 Reff. Logarithmic dark matter
potentials are consistent with the data, as well as NFW-profiles, mimicking
logarithmic potentials over the observationally sampled radial range. In both
cases, the derived stellar mass-to-light ratio agrees well with independently
obtained mass-to-light ratios from stellar population analysis. Kinematically,
NGC 4807 is characterized by mild radial anisotropy outside r>0.5 Reff,
becoming isotropic towards the center. Our orbit models hint at either a
distinct stellar component or weak triaxiality in the outer parts of the
galaxy.Comment: 20 pages, 25 figures, accepted for publication in MNRA
The flattening and the orbital structure of early-type galaxies and collisionless N-body binary disk mergers
We use oblate axisymmetric dynamical models including dark halos to determine
the orbital structure of intermediate mass to massive Coma early-type galaxies.
We find a large variety of orbital compositions. Averaged over all sample
galaxies the unordered stellar kinetic energy in the azimuthal and the radial
direction are of the same order, but they can differ by up to 40 percent in
individual systems. In contrast, both for rotating and non-rotating galaxies
the vertical kinetic energy is on average smaller than in the other two
directions. This implies that even most of the rotating ellipticals are
flattened by an anisotropy in the stellar velocity dispersions. Using
three-integral axisymmetric toy models we show that flattening by stellar
anisotropy maximises the entropy for a given density distribution.
Collisionless disk merger remnants are radially anisotropic. The apparent lack
of strong radial anisotropy in observed early-type galaxies implies that they
may not have formed from mergers of disks unless the influence of dissipational
processes was significant.Comment: 14 pages, 8 figures; accepted for publication in MNRA
Supermassive Black Holes and the Evolution of Galaxies
Black holes, an extreme consequence of the mathematics of General Relativity,
have long been suspected of being the prime movers of quasars, which emit more
energy than any other objects in the Universe. Recent evidence indicates that
supermassive black holes, which are probably quasar remnants, reside at the
centers of most galaxies. As our knowledge of the demographics of these relics
of a violent earlier Universe improve, we see tantalizing clues that they
participated intimately in the formation of galaxies and have strongly
influenced their present-day structure.Comment: 20 pages, - This is a near-duplicate of the paper in Nature 395, A14,
1998 (Oct 1
The velocity and mass distribution of clusters of galaxies from the CNOC1 cluster redshift survey
In the context of the CNOC1 cluster survey, redshifts were obtained for
galaxies in 16 clusters. The resulting sample is ideally suited for an analysis
of the internal velocity and mass distribution of clusters. Previous analyses
of this dataset used the Jeans equation to model the projected velocity
dispersion profile. However, the results of such an analysis always yield a
strong degeneracy between the mass density profile and the velocity dispersion
anisotropy profile. Here we analyze the full (R,v) dataset of galaxy positions
and velocities in an attempt to break this degeneracy. We build an `ensemble
cluster' from the individual clusters under the assumption that they form a
homologous sequence. To interpret the data we study a one-parameter family of
spherical models with different constant velocity dispersion anisotropy. The
best-fit model is sought using a variety of statistics, including the overall
likelihood of the dataset. Although the results of our analysis depend slightly
on which statistic is used to judge the models, all statistics agree that the
best-fit model is close to isotropic. This result derives primarily from the
fact that the observed grand-total velocity histogram is close to Gaussian,
which is not expected to be the case for a strongly anisotropic model. The
best-fitting models have a mass-to-number-density ratio that is approximately
independent of radius over the range constrained by the data. They also have a
mass-density profile that is consistent with the dark matter halo profile
advocated by Navarro, Frenk & White, in terms of both the profile shape and the
characteristic scale length. This adds important new weight to the evidence
that clusters do indeed follow this proposed universal mass density profile.
[Abridged]Comment: 37 pages, LaTeX, with 11 PostScript figures. Accepted by the
Astronomical Journal, to appear in the May 2000 issue. This replacement
version contains an additional Appendix and one additional Figure with
respect to the version submitted to astro-ph originall
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