2,117 research outputs found
Lensing and the Centers of Distant Early-Type Galaxies
Gravitational lensing provides a unique probe of the inner 10-1000 pc of
distant galaxies (z=0.2-1). Lens theory predicts that every strong lens system
should have a faint image near the center of the lens galaxy, which should be
visible in radio lenses but have not been observed. We study these ``core''
images using models derived from the stellar distributions in nearby early-type
galaxies. We find that realistic galaxies predict a remarkably wide range of
core images, with lensing magnifications spanning some six orders of magnitude.
More concentrated galaxies produce fainter core images, although not with any
simple, quantitative, model independent relation. Some real galaxies have
diffuse cores and predict bright core images (magnification mu>~0.1), but more
common are galaxies that predict faint core images (mu<~0.001). Thus, stellar
mass distributions alone are probably concentrated enough to explain the lack
of observed core images, and may require observational sensitivity to improve
by an order of magnitude before detections of core images become common.
Two-image lenses will tend to have brighter core images than four-image lenses,
so they will be the better targets for finding core images and exploiting these
tools for studying the central mass distributions of distant galaxies.Comment: 13 pages, emulateapj; submitted to Ap
Dark matter in elliptical galaxies
We present measurements of the shape of the stellar line-of-sight velocity
distribution out to two effective radii along the major axes of the four
elliptical galaxies NGC 2434, 2663, 3706, and 5018. The velocity dispersion
profiles are flat or decline gently with radius. We compare the data to the
predictions of f=f(E,L_z) axisymmetric models with and without dark matter.
Strong tangential anisotropy is ruled out at large radii. We conclude from our
measurements that massive dark halos must be present in three of the four
galaxies, while for the fourth galaxy (NGC 2663) the case is inconclusive.Comment: 15 pages, uuencoded compressed PostScript, includes 3 figure
The ESO Spectroscopic facility
We present the concept of a novel facility dedicated to massively-multiplexed
spectroscopy. The telescope has a very wide field Cassegrain focus optimised
for fibre feeding. With a Field of View (FoV) of 2.5 degrees diameter and a
11.4m pupil, it will be the largest etendue telescope. The large focal plane
can easily host up to 16.000 fibres. In addition, a gravity invariant focus for
the central 10 arc-minutes is available to host a giant integral field unit
(IFU). The 3 lenses corrector includes an ADC, and has good performance in the
360-1300 nm wavelength range. The top level science requirements were developed
by a dedicated ESO working group, and one of the primary cases is high
resolution spectroscopy of GAIA stars and, in general, how our Galaxy formed
and evolves. The facility will therefore be equipped with both, high and low
resolution spectrographs. We stress the importance of developing the telescope
and instrument designs simultaneously. The most relevant R\&D aspect is also
briefly discussed.Comment: 6 pages 4 figures , presented at IAU Symposium 334 "rediscovering our
galaxy
The dynamical distance and intrinsic structure of the globular cluster omega Centauri
We determine the dynamical distance D, inclination i, mass-to-light ratio M/L
and the intrinsic orbital structure of the globular cluster omega Cen, by
fitting axisymmetric dynamical models to the ground-based proper motions of van
Leeuwen et al. and line-of-sight velocities from four independent data-sets. We
correct the observed velocities for perspective rotation caused by the space
motion of the cluster, and show that the residual solid-body rotation component
in the proper motions can be taken out without any modelling other than
assuming axisymmetry. This also provides a tight constraint on D tan i.
Application of our axisymmetric implementation of Schwarzschild's orbit
superposition method to omega Cen reveals no dynamical evidence for a
significant radial dependence of M/L. The best-fit dynamical model has a
stellar V-band mass-to-light ratio M/L_V = 2.5 +/- 0.1 M_sun/L_sun and an
inclination i = 50 +/- 4 degrees, which corresponds to an average intrinsic
axial ratio of 0.78 +/- 0.03. The best-fit dynamical distance D = 4.8 +/- 0.3
kpc (distance modulus 13.75 +/- 0.13 mag) is significantly larger than obtained
by means of simple spherical or constant-anisotropy axisymmetric dynamical
models, and is consistent with the canonical value 5.0 +/- 0.2 kpc obtained by
photometric methods. The total mass of the cluster is (2.5 +/- 0.3) x 10^6
M_sun. The best-fit model is close to isotropic inside a radius of about 10
arcmin and becomes increasingly tangentially anisotropic in the outer region,
which displays significant mean rotation. This phase-space structure may well
be caused by the effects of the tidal field of the Milky Way. The cluster
contains a separate disk-like component in the radial range between 1 and 3
arcmin, contributing about 4% to the total mass.Comment: 37 pages (23 figures), accepted for publication in A&A, abstract
abridged, for PS and PDF file with full resolution figures, see
http://www.strw.leidenuniv.nl/~vdven/oc
Triaxial orbit-based modelling of the Milky Way Nuclear Star Cluster
We construct triaxial dynamical models for the Milky Way nuclear star cluster
using Schwarzschild's orbit superposition technique. We fit the stellar
kinematic maps presented in Feldmeier et al. (2014). The models are used to
constrain the supermassive black hole mass M_BH, dynamical mass-to-light ratio
M/L, and the intrinsic shape of the cluster. Our best-fitting model has M_BH =
(3.0 +1.1 -1.3)x10^6 M_sun, M/L = (0.90 +0.76 -0.08) M_sun/L_{sun,4.5micron},
and a compression of the cluster along the line-of-sight. Our results are in
agreement with the direct measurement of the supermassive black hole mass using
the motion of stars on Keplerian orbits. The mass-to-light ratio is consistent
with stellar population studies of other galaxies in the mid-infrared. It is
possible that we underestimate M_BH and overestimate the cluster's triaxiality
due to observational effects. The spatially semi-resolved kinematic data and
extinction within the nuclear star cluster bias the observations to the near
side of the cluster, and may appear as a compression of the nuclear star
cluster along the line-of-sight. We derive a total dynamical mass for the Milky
Way nuclear star cluster of M_MWNSC = (2.1 +-0.7)x10^7 M_sun within a sphere
with radius r = 2 x r_eff = 8.4 pc. The best-fitting model is tangentially
anisotropic in the central r = 0.5-2 pc of the nuclear star cluster, but close
to isotropic at larger radii. Our triaxial models are able to recover complex
kinematic substructures in the velocity map.Comment: 14 pages, 10 figures. Accepted for publication in MNRA
A Hipparcos census of the nearby OB associations
A comprehensive census of the stellar content of the nearby OB associations
is presented, based on Hipparcos positions, proper motions, and parallaxes.
Moving groups are identified by combining de Bruijne's refurbished convergent
point method with the `Spaghetti method' of Hoogerwerf & Aguilar. Monte Carlo
simulations are used to estimate the expected number of interloper field stars.
Astrometric members are listed for 12 young stellar groups, out to a distance
of ~650 pc. These are the 3 subgroups Upper Scorpius, Upper Centaurus Lupus and
Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2,
alpha Persei (Per OB3), Cas-Tau, Lac OB1, Cep OB2, and a new group designated
as Cep OB6. The selection procedure corrects the list of previously known
astrometric and photometric B- and A-type members, and identifies many new
members, including a significant number of F stars, as well as evolved stars,
e.g., the Wolf-Rayet stars gamma^2 Vel (Vel OB2) and EZ CMa (Col 121), and the
classical Cepheid delta Cep in Cep OB6. In the nearest associations the
later-type members include T Tauri objects and other pre-main sequence stars.
Astrometric evidence for moving groups in the fields of R CrA, CMa OB1, Mon
OB1, Ori OB1, Cam OB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is
inconclusive, due to their large distance or unfavorable kinematics.
The mean distances of the well-established groups are systematically smaller
than previous estimates. The mean motions display a systematic pattern, which
is discussed in relation to the Gould Belt. Six of the 12 detected moving
groups do not appear in the classical list of nearby OB associations. The
number of unbound young stellar groups in the Solar neighbourhood may be
significantly larger than thought previously.Comment: 51 pages, 30 PostScript figures, 6 tables in PostScript format,
default LaTeX using psfig.sty; accepted for publication in the Astronomical
Journal, scheduled for January 1999 issue. Abbreviated abstrac
OB Associations
Since the previous (1990) edition of this meeting enormous progress in the
field of OB associations has been made. Data from X-ray satellites have greatly
advanced the study of the low-mass stellar content of associations, while
astrometric data from the Hipparcos satellite allow for a characterization of
the higher-mass content of associations with unprecedented accuracy. We review
recent work on the OB associations located within 1.5 kpc from the Sun, discuss
the Hipparcos results at length, and point out directions for future research.Comment: To appear in The Physics of Star Formation and Early Stellar
Evolution II, eds C.J. Lada & N. Kylafis (Kluwer Academic), 30 pages, 9
EPS-figures, LaTeX using crckapb.sty, epsfig.sty, amssymb.st
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