536 research outputs found
RAVE as a Gaia precursor: what to expect from the Gaia RVS?
The Radial Velocity Experiment (RAVE) is a large wide-field spectroscopic
stellar survey of the Milky Way. Over the period 2003-2013, 574,630 spectra for
483,330 stars have been amassed at a resolution of R=7500 in the Ca-triplet
region of 8410-8795\AA. Wavelength coverage and resolution are thus comparable
to that anticipated from the Gaia RVS. Derived data products of RAVE include
radial velocities, stellar parameters, chemicals abundances for Mg, Al, Si, Ca,
Ti, Fe, and Ni, and absorption measures based on the diffuse interstellar bands
(DIB) at 8620\AA. Since more than 290000 RAVE targets are drawn from the
Tycho-2 catalogue, RAVE will be an interesting prototype for the anticipated
full Gaia data releases, in particular when combined with the early Gaia data
releases, which contain astrometry but not yet stellar parameters and
abundances.Comment: 7 pages, 3 color figures. Invited contribution to the GREAT-ITN
conference "The Milky Way Unravelled by Gaia: GREAT Science from the Gaia
Data Releases", 1-5 December 2014, University of Barcelona, Spain, EAS
Publications Series, eds Nicholas Walton, Francesca Figueras, and Caroline
Soubira
On the spin parameter of dark-matter haloes
The study by White (1984) on the growth of angular momentum in dark haloes is
extended towards a more detailed investigation of the spin parameter
. Starting from the Zel'dovich
approximation to structure formation, a dark halo is approximated by a
homogeneous ellipsoid with the inertial tensor of the (highly irregular)
Lagrangian region from which the dark halo forms. Within this
approximation, an expression for the spin parameter can be derived, which
depends on the geometry of , the cosmological density parameter
, the overdensity of the dark halo, and the tidal torque exerted on
it. For Gaussian random fields, this expression can be evaluated statistically.
As a result, we derive a probability distribution of the spin parameter which
gives , consistent with numerical
investigations. This probability distribution steeply rises with increasing
spin parameter, reaching its maximum at . The 10 (50,90)
percentile values are (0.05,0.11, respectively). There is a weak
anticorrelation of the spin parameter with the peak height of the density
fluctuation field . The dependence on
and the variance of the density-contrast field is very weak; there is
only a marginal tendency for the spin parameter to be slightly larger for
late-forming objects in an open universe. Due to the weak dependence on
, our results should be quite generally applicable and independent onComment: 16 pages, preprint MPA 79
Simulating Galaxy Formation
A review on numerical simulations of galaxy formation is given. Different
numerical methods to solve collisionless and gas dynamical systems are outlined
and one particular simulation technique, Smoothed Particle Hydrodynamics, is
discussed in some detail. After a short discussion of the most relevant
physical processes which affect the dynamics of the gas, the success and
shortcomings of state of the art simulations are discussed via the example of
the formation of disk galaxies.Comment: 24 pages, uuencoded postscript file, 5 figures, 2 figures included
Proc. ``International School of Physics Enrico Fermi'', Course CXXXII: Dark
Matter in the Universe, Varenna 1995, eds.: S. Bonometto, J. Primack, A.
Provenzale, IOP, to appear; complete version available at
http://www.mpa-garching.mpg.de/Galaxien/prep.htm
A Comparison of X-ray and Strong Lensing Properties of Simulated X-ray Clusters
We use gas-dynamical simulations of galaxy clusters to compare their X-ray
and strong lensing properties. Special emphasis is laid on mass estimates. The
cluster masses range between 6 x 10^14 solar masses and 4 x 10^15 solar masses,
and they are examined at redshifts between 1 and 0. We compute the X-ray
emission of the intracluster gas by thermal bremsstrahlung, add background
contamination, and mimic imaging and spectral observations with current X-ray
telescopes. Although the beta model routinely provides excellent fits to the
X-ray emission profiles, the derived masses are typically biased low because of
the restricted range of radii within which the fit can be done. For beta values
of ~ 2/3, which is the average in our numerically simulated sample, the mass is
typically underestimated by ~ 40 per cent. The masses of clusters which exhibit
pronounced substructure are often substantially underestimated. We suggest that
the ratio between peak temperature and emission-weighted average cluster
temperature may provide a good indicator for ongoing merging and, therefore,
for unreliable mass estimates. X-ray mass estimates are substantially improved
if we fit a King density profile rather than the beta model to the X-ray
emission, thereby dropping the degree of freedom associated with beta. Clusters
selected for their strong lensing properties are typically dynamically more
active than typical clusters. Bulk flows in the intracluster gas contain a
larger than average fraction of the internal energy of the gas in such objects,
hence the measured gas temperatures are biased low. The bulk of the optical
depth for arc formation is contributed by clusters with intermediate rather
than high X-ray luminosity. Arcs occur predominantly in clusters which exhibit
substructure and are not in an equilibrium state. Finally we explain why theComment: 22 pages including figures, submitted to MNRA
- …