512 research outputs found
The Maximum Optical Depth Towards Bulge Stars From Axisymmetric Models of the Milky Way
It has been known that recent microlensing results towards the bulge imply
mass densities that are surprisingly high given dynamical constraints on the
Milky Way mass distribution. We derive the maximum optical depth towards the
bulge that may be generated by axisymmetric structures in the Milky Way, and
show that observations are close to surpassing these limits. This result argues
in favor of a bar as a source of significantly enhanced microlensing. Several
of the bar models in the literature are discussed.Comment: Latex, 6 pages, 4 figures, uses aas2pp4 and epsf style files.
Accepted for publication in ApJ Letter
Growth of Galactic Bulges by Mergers: I. Dense Satellites
Andredakis, Peletier & Balcells (1995) fit Sersic's law
to the bulges of the Balcells & Peletier (1994) galaxy sample, and infer that
drops with morphological type T from 4--6 for S0 to
(exponential) for Sc's. We use collisionless N body simulations to test the
assumption that initially the surface brightness profiles of all bulges were
exponential, and that the steepening of the profiles toward the early-types is
due to satellite accretion. The results are positive. After the accretion of a
satellite, bulge-disk fits show that the bulge grows and that the bulge profile
index increases proportional to the satellite mass. For a satellite as
massive as the bulge, rises from 1 to 4. We present kinematic diagnostics
on the remnants and disk thickening. The latter suggests that the bulge growth
must have occurred before the last formation of a thin disk in the galaxy. The
thick disks created by the merger are reminiscent of thick disks seen in
early-type edge-on galaxies.
The efficiency of the process suggests that present day bulges of late-type
spirals showing exponential profiles cannot have grown significantly by
collisionless mergers.Comment: 10 figures (8 poscript and 2 gif). accepted for publication in A&
A Broad Search for Counterrotating Gas and Stars: Evidence for Mergers and Accretion
We measure the frequency of bulk gas-stellar counterrotation in a sample of
67 galaxies drawn from the Nearby Field Galaxy Survey, a broadly representative
survey of the local galaxy population down to M_B-15. We detect 4
counterrotators among 17 E/S0's with extended gas emission (24% +8 -6). In
contrast, we find no clear examples of bulk counterrotation among 38 Sa-Sbc
spirals, although one Sa does show peculiar gas kinematics. This result implies
that, at 95% confidence, no more than 8% of Sa-Sbc spirals are bulk
counterrotators. Among types Sc and later, we identify only one possible
counterrotator, a Magellanic irregular. We use these results together with the
physical properties of the counterrotators to constrain possible origins for
this phenomenon.Comment: 19 pages, 4 figures, AJ, accepte
Limitations of model fitting methods for lensing shear estimation
Gravitational lensing shear has the potential to be the most powerful tool
for constraining the nature of dark energy. However, accurate measurement of
galaxy shear is crucial and has been shown to be non-trivial by the Shear
TEsting Programme. Here we demonstrate a fundamental limit to the accuracy
achievable by model-fitting techniques, if oversimplistic models are used. We
show that even if galaxies have elliptical isophotes, model-fitting methods
which assume elliptical isophotes can have significant biases if they use the
wrong profile. We use noise-free simulations to show that on allowing
sufficient flexibility in the profile the biases can be made negligible. This
is no longer the case if elliptical isophote models are used to fit galaxies
made up of a bulge plus a disk, if these two components have different
ellipticities. The limiting accuracy is dependent on the galaxy shape but we
find the most significant biases for simple spiral-like galaxies. The
implications for a given cosmic shear survey will depend on the actual
distribution of galaxy morphologies in the universe, taking into account the
survey selection function and the point spread function. However our results
suggest that the impact on cosmic shear results from current and near future
surveys may be negligible. Meanwhile, these results should encourage the
development of existing approaches which are less sensitive to morphology, as
well as methods which use priors on galaxy shapes learnt from deep surveys.Comment: 10 pages, 8 figure
The case for two-dimensional galaxy-galaxy lensing
We revisit the performance and biases of the two-dimensional approach to
galaxy-galaxy lensing. This method exploits the information for the actual
positions and ellipticities of source galaxies, rather than using only the
ensemble properties of statistically equivalent samples. We compare the
performance of this method with the traditionally used one-dimensional
tangential shear signal on a set of mock data that resemble the current
state-of-the-art weak lensing surveys. We find that under idealised
circumstances the confidence regions of joint constraints for the amplitude and
scale parameters of the NFW model in the two-dimensional analysis can be more
than three times tighter than the one-dimensional results. Moreover, this
improvement depends on the lens number density and it is larger for higher
densities. We compare the method against the results from the hydrodynamical
EAGLE simulation in order to test for possible biases that might arise due to
lens galaxies being missed, and find that the method is able to return unbiased
estimates of halo masses when compared to the true properties of the EAGLE
galaxies. Because of its advantage in high galaxy density areas, the method is
especially suitable for studying the properties of satellite galaxies in
clusters of galaxies.Comment: 9 pages, 7 figures, accepted for publication in A&
Curvature and Acoustic Instabilities in Rotating Fluid Disks
The stability of a rotating fluid disk to the formation of spiral arms is
studied in the tightwinding approximation in the linear regime. The dispersion
relation for spirals that was derived by Bertin et al. is shown to contain a
new, acoustic instability beyond the Lindblad resonances that depends only on
pressure and rotation. In this regime, pressure and gravity exchange roles as
drivers and inhibitors of spiral wave structures. Other instabilities that are
enhanced by pressure are also found in the general dispersion relation by
including higher order terms in the small parameter 1/kr for wavenumber k and
radius r. These instabilities are present even for large values of Toomre's
parameter Q. Unstable growth rates are determined in four cases: a
self-gravitating disk with a flat rotation curve, a self-gravitating disk with
solid body rotation, a non-self-gravitating disk with solid body rotation, and
a non-self-gravitating disk with Keplerian rotation. The most important
application appears to be as a source of spiral structure, possibly leading to
accretion in non-self-gravitating disks, such as some galactic nuclear disks,
disks around black holes, and proto-planetary disks. All of these examples have
short orbital times so the unstable growth time can be small.Comment: 30 pages, 5 figures, scheduled for ApJ 520, August 1, 199
Ornithologisch belang van de Belgische zeegebieden: identificatie van mariene gebieden die in aanmerking komen als Speciale Beschermingszones in uitvoering van de Europese Vogelrichtlijn = Importance ornithologique des espaces marins de la Belgique: identification des zones marines méritant le statut de Zones de Protection Spéciale en application de la Directive européenne Oiseaux
A Study of the Direct-Fitting Method for Measurement of Galaxy Velocity Dispersions
We have measured the central stellar velocity dispersions of 33 nearby spiral
and elliptical galaxies, using a straightforward template-fitting algorithm
operating in the pixel domain. The spectra, obtained with the Double
Spectrograph at Palomar Observatory, cover both the Ca triplet and the Mg b
region, and we present a comparison of the velocity dispersion measurements
from these two spectral regions. Model fits to the Ca triplet region generally
yield good results with little sensitivity to the choice of template star. In
contrast, the Mg b region is more sensitive to template mismatch and to details
of the fitting procedure such as the order of a polynomial used to match the
continuum shape of the template to the object. As a consequence of the
correlation of the [Mg/Fe] ratio with velocity dispersion, it is difficult to
obtain a satisfactory model fit to the Mg b lines and the surrounding Fe blends
simultaneously, particularly for giant elliptical galaxies with large velocity
dispersions. We demonstrate that if the metallicities of the galaxy and
template star are not well matched, then direct template-fitting results are
improved if the Mg b lines themselves are excluded from the fit and the
velocity dispersion is determined from the surrounding weaker lines.Comment: 14 pages. To appear in A
Sersiclets - A Matched Filter Extension of Shapelets for Weak Lensing Studies
The precision study of dark matter using weak lensing by large scale
structure is strongly constrained by the accuracy with which one can measure
galaxy shapes. Several methods have been devised but none have demonstrated the
ability to reach the level of precision required by future weak lensing
surveys. In this Letter we explore new avenues to the existing Shapelets
approach, combining a priori knowledge of the galaxy profile with the power of
orthogonal basis function decomposition. This Letter discusses the new issues
raised by this matched filter approach and proposes promising alternatives to
shape measurement techniques. In particular it appears that the use of a
matched filter (e.g. Sersic profile) restricted to elliptical radial fitting
functions resolves several well known Shapelet issues.Comment: 6 pages, 6 figures. MNRAS Accepte
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