347 research outputs found
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
New statistical goodness of fit techniques in noisy inhomogeneous inverse problems - With application to the recovering of the luminosity distribution of the Milky Way
The assumption that a parametric class of functions fits the data structure sufficiently well is common in fitting curves and surfaces to regression data. One then derives a parameter estimate resulting from a least squares fit, say, and in a second step various kinds of chi^2 goodness of fit measures, to assess whether the deviation between data and estimated surface is due to random noise and not to systematic departures from the model. In this paper we show that commonly-used chi^2-measures are invalid in regression models, particularly when inhomogeneous noise is present. Instead we present a bootstrap algorithm which is applicable in problems described by noisy versions of Fredholm integral equations. of the first kind. We apply the suggested method to the problem of recovering the luminosity density in the Milky Way from data of the DIRBE experiment on board the COBE satellite
Testing for lack of fit in inverse regression - with applications to photonic imaging.
Regression; Problems; Lack-of-fit; Applications;
3D Distribution of Molecular Gas in the Barred Milky Way
We present a new model of the three-dimensional distribution of molecular gas
in the Milky Way Galaxy, based on CO line data. Our analysis is based on a
gas-flow simulation of the inner Galaxy using smoothed-particle hydrodynamics
(SPH) using a realistic barred gravitional potential derived from the observed
COBE/DIRBE near-IR light distribution. The gas model prescribes the gas orbits
much better than a simple circular rotation model and is highly constrained by
observations, but it cannot predict local details. In this study, we provide a
3D map of the observed molecular gas distribution using the velocity field from
the SPH model. A comparison with studies of the Galactic Center region suggests
that the main structures are reproduced but somewhat stretched along the
line-of-sight, probably on account of limited resolution of the underlying SPH
simulation. The gas model will be publicly available and may prove useful in a
number of applications, among them the analysis of diffuse gamma-ray emission
as measured with GLAST.Comment: ApJ in pres
Convergence rates of general regularization methods for statistical inverse problems and applications
During the past the convergence analysis for linear statistical inverse problems has mainly focused
on spectral cut-off and Tikhonov type estimators. Spectral cut-off estimators achieve minimax rates for a broad
range of smoothness classes and operators, but their practical usefulness is limited by the fact that they require
a complete spectral decomposition of the operator. Tikhonov estimators are simpler to compute, but still involve
the inversion of an operator and achieve minimax rates only in restricted smoothness classes. In this paper we
introduce a unifying technique to study the mean square error of a large class of regularization methods (spectral
methods) including the aforementioned estimators as well as many iterative methods, such as ν-methods and the
Landweber iteration. The latter estimators converge at the same rate as spectral cut-off, but only require matrixvector
products. Our results are applied to various problems, in particular we obtain precise convergence rates for
satellite gradiometry, L2-boosting, and errors in variable problems.
AMS subject classifications: 62G05, 62J05, 62P35, 65J10, 35R3
Microlensing Optical Depth of the COBE Bulge
We examine the left-right asymmetry in the cleaned COBE/DIRBE near-infrared
data of the inner Galaxy and show (i) that the Galactic bar is probably not
seen very nearly end-on, and (ii) that even if it is, it is not highly
elongated. The assumption of constant mass-to-light ratio is used to derive
simulated terminal-velocity plots for the ISM from our model luminosity
distributions. By comparing these plots with observed terminal velocities we
determine the mass-to-light ratio of the near-IR bulge and disk.
Assuming that all this mass contributes to gravitational microlensing we
compute optical depths for microlensing in Galactic-centre fields. For
three models with bar major axis between from the Sun-Galactic
Center line, the resulting optical depths in Baade's window lie in the range
0.83\times10^{-6} \lta \tau \lta 0.89\times10^{-6} for main-sequence stars
and 1.2\times10^{-6} \lta \tau \lta 1.3\times10^{-6} for red-clump giants. We
discuss a number of uncertainties including possible variations of the
near-infrared mass-to-light ratio. We conclude that, although the values
predicted from analyzing the COBE and gas velocity data are inconsistent at the
level with recent observational determinations of , we
believe they should be taken seriously.Comment: 9 pages, TeX. 7 figures (gif). Submitted to MNRAS. Also available
with full resolution figures as ps-file at
http://www.astro.unibas.ch/dynamics/papers.htm
Effect of Binary Source Companions on the Microlensing Optical Depth Determination toward the Galactic Bulge Field
Currently, gravitational microlensing survey experiments toward the Galactic
bulge field utilize two different methods of minimizing blending effect for the
accurate determination of the optical depth \tau. One is measuring \tau based
on clump giant (CG) source stars and the other is using `Difference Image
Analysis (DIA)' photometry to measure the unblended source flux variation.
Despite the expectation that the two estimates should be the same assuming that
blending is properly considered, the estimates based on CG stars systematically
fall below the DIA results based on all events with source stars down to the
detection limit. Prompted by the gap, we investigate the previously
unconsidered effect of companion-associated events on determination.
Although the image of a companion is blended with that of its primary star and
thus not resolved, the event associated with the companion can be detected if
the companion flux is highly magnified. Therefore, companions work effectively
as source stars to microlensing and thus neglect of them in the source star
count could result in wrong \tau estimation. By carrying out simulations based
on the assumption that companions follow the same luminosity function of
primary stars, we estimate that the contribution of the companion-associated
events to the total event rate is ~5f_{bi}% for current surveys and can reach
up to ~6f_{bi}% for future surveys monitoring fainter stars, where f_{bi} is
the binary frequency. Therefore, we conclude that the companion-associated
events comprise a non-negligible fraction of all events. However, their
contribution to the optical depth is not large enough to explain the systematic
difference between the optical depth estimates based on the two different
methods.Comment: 4 pages, 1 figure, 1 table, ApJ, submitte
Scalable N-body code for the modelling of early-type galaxies
Early-type galaxies exhibit a wealth of photometric and dynamical structures.
These signatures are fossil records of their formation and evolution processes.
In order to examine these structures in detail, we build models aimed at
reproducing the observed photometry and kinematics. The developed method is a
generalization of the one introduced by Syer and Tremaine (1996), consisting in
an N-body representation, in which the weights of the particles are changing
with time. Our code is adapted for integral-field spectroscopic data, and is
able to reproduce the photometric as well as stellar kinematic data of observed
galaxies. We apply this technique on SAURON data of early-type galaxies, and
present preliminary results on NGC 3377.Comment: 6 pages, 2 figures. Original version printed in the Proceedings of
"Science perspective for 3D spectroscopy", 2005, Eds Kissler-Patig, Walsh,
Roth, ES0, Springe
Analysing observed star cluster SEDs with evolutionary synthesis models: systematic uncertainties
The definitive version is available at www.blackwell-synergy.com. Copyright Blackwell Publishing DOI : 10.1111/j.1365-2966.2004.07197.xWe discuss the systematic uncertainties inherent to analyses of observed (broad-band) Spectral Energy Distributions (SEDs) of star clusters with evolutionary synthesis models. We investigate the effects caused by restricting oneself to a limited number of available passbands, choices of various passband combinations, finite observational errors, non-continuous model input parameter values, and restrictions in parameter space allowed during analysis. Starting from a complete set of UBVRIJH passbands (respectively their Hubble Space Telescope/WFPC2 equivalents) we investigate to which extent clusters with different combinations of age, metallicity, internal extinction and mass can or cannot be disentangled in the various evolutionary stages throughout their lifetimes and what are the most useful passbands required to resolve the ambi- guities. We find the U and B bands to be of the highest significance, while the V band and near-infrared data provide additional constraints. A code is presented that makes use of luminosities of a star cluster system in all of the possibly available passbands, and tries to find ranges of allowed age-metallicity-extinction-mass combinations for individual members of star cluster systems. Numerous tests and examples are pre- sented. We show the importance of good photometric accuracies and of determining the cluster parameters independently without any prior assumptions.Peer reviewe
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