127 research outputs found
Analysing surveys of our Galaxy I: basic astrometric data
We consider what is the best way to extract science from large surveys of the
Milky Way galaxy. The diversity of data gathered in these surveys, together
with our position within the Galaxy, imply that science must be extracted by
fitting dynamical models to the data in the space of the observables. Models
based on orbital tori promise to be superior for this task than traditional
types of models, such as N-body models and Schwarzschild models. A formalism
that allows such models to be fitted to data is developed and tested on
pseudodata of varying richness.Comment: 15 pages, 6 figures, MNRAS accepted, changed to reflect final versio
The uncertainty in Galactic parameters
We reanalyse the measurements of parallax, proper motion, and line-of-sight
velocity for 18 masers in high mass star-forming regions presented by Reid et
al. (2009). We use a likelihood analysis to investigate the distance of the Sun
from the Galactic centre, R_0, the rotational speed of the local standard of
rest, v_0, and the peculiar velocity of the Sun, vsol, for various models of
the rotation curve, and models which allow for a typical peculiar motion of the
high mass star-forming regions.
We find that these data are best fit by models with non-standard values for
vsol or a net peculiar motion of the high mass star-forming regions. We argue
that a correction to vsol is much more likely, and that these data support the
conclusion of Binney (2009) that V_sol should be revised upwards from 5.2 km/s
to 11 km/s. We find that the values of R_0 and v_0 that we determine are
heavily dependent on the model we use for the rotation curve, with
model-dependent estimates of R_0 ranging from 6.7 \pm 0.5kpc to 8.9 \pm 0.9kpc,
and those of v_0 ranging from 200 \pm 20 km/s to 279 \pm 33 km/s. We argue that
these data cannot be thought of as implying any particular values of R_0 or
v_0. However, we find that v_0/R_0 is better constrained, lying in the range
29.9-31.6 km/s/kpc for all models but one.Comment: 8 pages. MNRAS accepted. Revised to reflect final versio
Is Galactic Structure Compatible with Microlensing Data?
We generalize to elliptical models the argument of Kuijken (1997), which
connects the microlensing optical depth towards the Galactic bulge to the
Galactic rotation curve. When applied to the latest value from the MACHO
collaboration for the optical depth for microlensing of bulge sources, the
argument implies that the Galactic bar cannot plausibly reconcile the measured
values of the optical depth, the rotation curve and the local mass density.
Either there is a problem with the interpretation of the microlensing data, or
our line of sight to the Galactic centre is highly atypical in that it passes
through a massive structure that wraps only a small distance around the
Galactic centre.Comment: Submitted to ApJ Letters. 8 pages LaTeX, 3 figures. Corrected error
in description of microlensing observation
Entropy Evolution of the Gas in Cooling Flow Clusters
We emphasise the importance of the gas entropy in studying the evolution of
cluster gas evolving under the influence of radiative cooling. On this basis,
we develop an analytical model for this evolution. We then show that the
assumptions needed for such a model are consistent with a numerical solution of
the same equations. We postulate that the passive cooling phase ends when the
central gas temperature falls to very low values. It follows a phase during
which an unspecified mechanism heats the cluster gas. We show that in such a
scenario the small number of clusters containing gas with temperatures below
about 1 keV is simply a consequence of the radiative cooling.Comment: Contribution to Proceedings of `The Riddle of Cooling Flows in
Galaxies and Clusters of Galaxies', Charlottesville, VA, USA. May 31 -- June
4, 2003. Editors: Reiprich, T. H., Kempner, J. C., and Soker, N. Requires
included style fil
Analysing surveys of our Galaxy -- II. Determining the potential
We consider the problem of determining the Galaxy's gravitational potential
from a star catalogue. We show that orbit-based approaches to this problem
suffer from unacceptable numerical noise deriving from the use of only a finite
number of orbits. An alternative approach, which requires an ability to
determine the model's phase-space density at predetermined positions and
velocities, has a level of numerical noise that lies well below the intrinsic
uncertainty associated with the finite size of the catalogue analysed. A
catalogue of 10000 stars brighter than V=17 and distributed over the sky at
b>30 degrees enables us to determine the scaleheight of the disc that
contributes to the potential with an uncertainty below 20pc if the catalogue
gives proper motions, line-of-sight velocities and parallaxes with errors
typical of the Gaia Catalogue, rising to 36pc if only proper motions are
available. The uncertainty in the disc's scalelength is significantly smaller
than 0.25kpc.Comment: 16 pages, MNRAS accepted. Revised to reflect final versio
The dangers of deprojection of proper motions
We re-examine the method of deprojection of proper motions, which has been
used for finding the velocity ellipsoid of stars in the nearby Galaxy. This
method is only legitimate if the lines of sight to the individual stars are
uncorrelated with the stars' velocities. Very simple models are used to show
that spurious results similar to ones recently reported are obtained when
velocity dispersion decreases with galactocentric radius in the expected way. A
scheme that compensates for this bias is proposed.Comment: 5 pages, 5 figures (3 colour), MNRAS submitte
AGN effect on cooling flow dynamics
We analyzed the feedback of AGN jets on cooling flow clusters using
three-dimensional AMR hydrodynamic simulations. We studied the interaction of
the jet with the intracluster medium and creation of low X-ray emission
cavities (Bubbles) in cluster plasma. The distribution of energy input by the
jet into the system was quantified in its different forms, i.e. internal,
kinetic and potential. We find that the energy associated with the bubbles, (pV
+ gamma pV/(gamma-1)), accounts for less than 10 percent of the jet energy.Comment: "Accepted for publication in Astrophysics & Space Science
Extracting science from surveys of our Galaxy
Our knowledge of the Galaxy is being revolutionised by a series of
photometric, spectroscopic and astrometric surveys. Already an enormous body of
data is available from completed surveys, and data of ever increasing quality
and richness will accrue at least until the end of this decade. To extract
science from these surveys we need a class of models that can give probability
density functions in the space of the observables of a survey -- we should not
attempt to "invert" the data from the space of observables into the physical
space of the Galaxy. Currently just one class of model has the required
capability, so-called "torus models". A pilot application of torus models to
understanding the structure of the Galaxy's thin and thick discs has already
produced two significant results: a major revision of our best estimate of the
Sun's velocity with respect to the Local Standard of Rest, and a successful
prediction of the way in which the vertical velocity dispersion in the disc
varies with distance from the Galactic plane.Comment: 13 pages. Invited review to appear in Pramana - journal of physics
(Indian Academy of Sciences
Limitations of the Standard Gravitational Perfect Fluid Paradigm
We show that the standard perfect fluid paradigm is not necessarily a valid
description of a curved space steady state gravitational source. Simply by
virtue of not being flat, curved space geometries have to possess intrinsic
length scales, and such length scales can affect the fluid structure. For modes
of wavelength of order or greater than such scales eikonalized geometrical
optics cannot apply and rays are not geodesic. Covariantizing thus entails not
only the replacing of flat space functions by covariant ones, but also the
introduction of intrinsic scales that were absent in flat space. In principle
it is thus unreliable to construct the curved space energy-momentum tensor as
the covariant generalization of a geodesic-based flat spacetime energy-momentum
tensor. By constructing the partition function as an incoherent average over a
complete set of modes of a scalar field propagating in a curved space
background, we show that for the specific case of a static, spherically
symmetric geometry, the steady state energy-momentum tensor that ensues will in
general be of the form
where the
anisotropic is a symmetric, traceless rank two tensor which
obeys . Such a type term is absent for an
incoherently averaged steady state fluid in a spacetime where there are no
intrinsic length scales, and in principle would thus be missed in a
covariantizing of a flat spacetime . While the significance of such
type terms would need to be evaluated on a case by case basis,
through the use of kinetic theory we reassuringly find that the effect of such
type terms is small for weak gravity stars where perfect fluid
sources are commonly used.Comment: Final version to appear in General Relativity and Gravitation (the
final publication is available at http://www.springerlink.com). 29 pages, 1
figur
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