491 research outputs found
The Impact of Stellar Migration on Disk Outskirts
Stellar migration, whether due to trapping by transient spirals (churning),
or to scattering by non-axisymmetric perturbations, has been proposed to
explain the presence of stars in outer disks. After a review of the basic
theory, we present compelling, but not yet conclusive, evidence that churning
has been important in the outer disks of galaxies with type II (down-bending)
profiles, while scattering has produced the outer disks of type III
(up-bending) galaxies. In contrast, field galaxies with type I (pure
exponential) profiles appear to not have experienced substantial migration. We
conclude by suggesting work that would improve our understanding of the origin
of outer disks.Comment: Invited review, Book chapter in "Outskirts of Galaxies", Eds. J. H.
Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and Space Science Library,
Springer, in press 39 pages, 15 figure
Transient backbending behavior in the Ising model with fixed magnetization
The physical origin of the backbendings in the equations of state of finite
but not necessarily small systems is studied in the Ising model with fixed
magnetization (IMFM) by means of the topological properties of the observable
distributions and the analysis of the largest cluster with increasing lattice
size. Looking at the convexity anomalies of the IMFM thermodynamic potential,
it is shown that the order of the transition at the thermodynamic limit can be
recognized in finite systems independently of the lattice size. General
statistical mechanics arguments and analytical calculations suggest that the
backbending in the caloric curve is a transient behaviour which should not
converge to a plateau in the thermodynamic limit, while the first order
transition is signalled by a discontinuity in other observables.Comment: 24 pages, 11 figure
Modeling galactic halos with predominantly quintessential matter
This paper discusses a new model for galactic dark matter by combining an
anisotropic pressure field corresponding to normal matter and a quintessence
dark energy field having a characteristic parameter such that
. Stable stellar orbits together with an attractive
gravity exist only if is extremely close to , a result
consistent with the special case studied by Guzman et al. (2003). Less
exceptional forms of quintessence dark energy do not yield the desired stable
orbits and are therefore unsuitable for modeling dark matter.Comment: 12 pages, 1 figur
A Large Catalog of Accurate Distances to Molecular Clouds from PS1 Photometry
Distance measurements to molecular clouds are important but are often made separately for each cloud of interest, employing very different data and techniques. We present a large, homogeneous catalog of distances to molecular clouds, most of which are of unprecedented accuracy. We determine distances using optical photometry of stars along lines of sight toward these clouds, obtained from PanSTARRS-1. We simultaneously infer the reddenings and distances to these stars, tracking the full probability distribution function using a technique presented in Green et al. We fit these star-by-star measurements using a simple dust screen model to find the distance to each cloud. We thus estimate the distances to almost all of the clouds in the Magnani et al. catalog, as well as many other well-studied clouds, including Orion, Perseus, Taurus, Cepheus, Polaris, California, and Monoceros R2, avoiding only the inner Galaxy. Typical statistical uncertainties in the distances are 5%, though the systematic uncertainty stemming from the quality of our stellar models is about 10%. The resulting catalog is the largest catalog of accurate, directly measured distances to molecular clouds. Our distance estimates are generally consistent with available distance estimates from the literature, though in some cases the literature estimates are off by a factor of more than two
Galactic rotation curves inspired by a noncommutative-geometry background
This paper discusses the observed at rotation curves of galaxies in the
context of noncommutative geometry. The energy density of such a geometry is
diffused throughout a region due to the uncertainty encoded in the coordinate
commutator. This intrinsic property appears to be sufficient for producing
stable circular orbits, as well as attractive gravity, without the need for
dark matter.Comment: 12 pages, 3 figures. Published in Gen.Rel.Grav. 44 (2012) 905-91
Large-scale collective motion of RFGC galaxies in curved space-time
We consider large-scale collective motion of flat edge-on spiral galaxies
from the Revised Flat Galaxy Catalogue (RFGC) taking into account the curvature
of space-time in the Local Universe at the scale 100 Mpc/h. We analyse how the
relativistic model of collective motion should be modified to provide the best
possible values of parameters, the effects that impact these parameters and
ways to mitigate them. Evolution of galactic diameters, selection effects, and
difference between isophotal and angular diameter distances are inadequate to
explain this impact. At the same time, measurement error in HI line widths and
angular diameters can easily provide such an impact. This is illustrated in a
toy model, which allows analytical consideration, and then in the full model
using Monte Carlo simulations. The resulting velocity field is very close to
that provided by the non-relativistic model of motion. The obtained bulk flow
velocity is consistent with {\Lambda}CDM cosmology.Comment: 10 pages, 3 figures, 2 table
Modified gravity without dark matter
On an empirical level, the most successful alternative to dark matter in
bound gravitational systems is the modified Newtonian dynamics, or MOND,
proposed by Milgrom. Here I discuss the attempts to formulate MOND as a
modification of General Relativity. I begin with a summary of the
phenomenological successes of MOND and then discuss the various covariant
theories that have been proposed as a basis for the idea. I show why these
proposals have led inevitably to a multi-field theory. I describe in some
detail TeVeS, the tensor-vector-scalar theory proposed by Bekenstein, and
discuss its successes and shortcomings. This lecture is primarily pedagogical
and directed to those with some, but not a deep, background in General
RelativityComment: 28 pages, 10 figures, lecture given at Third Aegean Summer School,
The Invisible Universe: Dark Matter and Dark Energy, minor errors corrected,
references update
The Optical-infrared Extinction Curve and Its Variation in the Milky Way
The dust extinction curve is a critical component of many observational programs and an important diagnostic of the physics of the interstellar medium. Here we present new measurements of the dust extinction curve and its variation toward tens of thousands of stars, a hundred-fold larger sample than in existing detailed studies. We use data from the APOGEE spectroscopic survey in combination with ten-band photometry from Pan-STARRS1, the Two Micron All-Sky Survey, and Wide-field Infrared Survey Explorer. We find that the extinction curve in the optical through infrared is well characterized by a one-parameter family of curves described by R(V). The extinction curve is more uniform than suggested in past works, with , and with less than one percent of sight lines having . Our data and analysis have revealed two new aspects of Galactic extinction: first, we find significant, wide-area variations in R(V) throughout the Galactic plane. These variations are on scales much larger than individual molecular clouds, indicating that R(V) variations must trace much more than just grain growth in dense molecular environments. Indeed, we find no correlation between R(V) and dust column density up to . Second, we discover a strong relationship between R(V) and the far-infrared dust emissivity
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