11,687 research outputs found
A dynamical and kinematical model of the Galactic stellar halo and possible implications for galaxy formation scenarios
We re-analyse the kinematics of the system of blue horizontal branch field
(BHBF) stars in the Galactic halo (in particular the outer halo), fitting the
kinematics with the model of radial and tangential velocity dispersions in the
halo as a function of galactocentric distance r proposed by Sommer-Larsen,
Flynn & Christensen (1994), using a much larger sample (almost 700) of BHBF
stars. The basic result is that the character of the stellar halo velocity
ellipsoid changes markedly from radial anisotropy at the sun to tangential
anisotropy in the outer parts of the Galactic halo (r greater than approx 20
kpc). Specifically, the radial component of the stellar halo's velocity
ellipsoid decreases fairly rapidly beyond the solar circle, from approx 140 +/-
10 km/s at the sun, to an asymptotic value of 89 +/- 19 km/s at large r. The
rapid decrease in the radial velocity dispersion is matched by an increase in
the tangential velocity dispersion, with increasing r.
Our results may indicate that the Galaxy formed hierarchically (partly or
fully) through merging of smaller subsystems - the 'bottom-up' galaxy formation
scenario, which for quite a while has been favoured by most theorists and
recently also has been given some observational credibility by HST observations
of a potential group of small galaxies, at high redshift, possibly in the
process of merging to a larger galaxy (Pascarelle et al 1996).Comment: Latex, 16 pages. 2 postscript figures. Submitted to the Astrophysical
Journal. also available at http://astro.utu.fi/~cflynn/outerhalo.htm
Non-perturbative renormalization of the axial current with improved Wilson quarks
We present a new normalization condition for the axial current, which is
derived from the PCAC relation with non-vanishing mass.
Using this condition reduces the O(r_0 m) corrections to the axial current
normalization constant Z_A for an easier chiral extrapolation in the cases,
where simulations at zero quark-mass are not possible. The method described
here also serves as a preparation for a determination of Z_A in the full
two-flavor theory.Comment: 3 pages, 3 figures, Lattice2003(improve
CDM, Feedback and the Hubble Sequence
We have performed TreeSPH simulations of galaxy formation in a standard LCDM
cosmology, including effects of star formation, energetic stellar feedback
processes and a meta-galactic UV field, and obtain a mix of disk, lenticular
and elliptical galaxies. The disk galaxies are deficient in angular momentum by
only about a factor of two compared to observed disk galaxies. The stellar
disks have approximately exponential surface density profiles, and those of the
bulges range from exponential to r^{1/4}, as observed. The bulge-to-disk ratios
of the disk galaxies are consistent with observations and likewise are their
integrated B-V colours, which have been calculated using stellar population
synthesis techniques. Furthermore, we can match the observed I-band
Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk
galaxies, (M/L_I), is about 0.8. The ellipticals and lenticulars have
approximately r^{1/4} stellar surface density profiles, are dominated by
non-disklike kinematics and flattened due to non-isotropic stellar velocity
distributions, again consistent with observations.Comment: 6 pages, incl. 4 figs. To appear in the proceedings of the
EuroConference "The Evolution of Galaxies: II - Basic Building Blocks", Ile
de La Reunion (France), 16-21 October 2001 (Slightly updated version). A much
more comprehensive paper about this work with links to pictures of some of
the galaxies can be found at http://babbage.sissa.it/abs/astro-ph/020436
Surveying the Inner Halo of the Galaxy with 2MASS-Selected Horizontal Branch Candidates
We use 2MASS photometry to select blue horizontal branch (BHB) candidates
covering the sky |b|>15 deg. A 12.5<J<15.5 sample of BHB stars traces the thick
disk and inner halo to d<9 kpc, with a density comparable to that of M giant
stars. We base our sample selection strategy on the Century Survey Galactic
Halo Project, a survey that provides a complete, spectroscopically-identified
sample of blue stars to a similar depth as the 2MASS catalog. We show that a
-0.20<(J-H)_0<0.10, -0.10<(H-K)_0<0.10 color-selected sample of stars is 65%
complete for BHB stars, and is composed of 47% BHB stars. We apply this
photometric selection to the full 2MASS catalog, and see no spatial
overdensities of BHB candidates at high Galactic latitude |b|>50 deg. We insert
simulated star streams into the data and conclude that the high Galactic
latitude BHB candidates are consistent with having no ~5 deg wide star stream
with density greater than 0.33 objects deg^-2 at the 95% confidence level. The
absence of structure suggests there have been no major accretion events in the
inner halo in the last few Gyr. However, at low Galactic latitudes a two-point
angular correlation analysis reveals structure on angular scales <1 deg. This
structure is apparently associated with stars in the thick disk, and has a
physical scale of 10-100 pc. Interestingly, such structures are expected by
cosmological simulations that predict the majority of the thick disk may arise
from accretion and disruption of satellite mergers.Comment: 11 pages, including figures. Accepted by AJ with minor revision
A Stimulus Timing Device for Capturing Fast Physiologic Events by Quick-Freezing
A timing device was designed that, in conjunction with an impact type of quick-freezing apparatus and an externally-triggerable stimulus generator, allows the application of an electrical stimulus to a muscle preparation at a selected time interval before quick-freezing and the measurement of the interval with submillisecond precision. This is needed for stopping fast physiological events in calcium release and excitation-contraction coupling and allows studying the morphological parameters (by freeze-fracture and freeze-substitution) and elemental distributions (by x-ray microanalysis) as a function of time after stimulation. The device should be adaptable for use with most equipment designed for quick-freezing electrically excitable tissue by impact on a cold solid surface
Running quark mass in two flavor QCD
We present first results for the step scaling function sigma_P of the
renormalization factor Z_P of the pseudoscalar density. The simulations are
performed within the framework of the Schroedinger functional with two flavors
of O(a) improved Wilson fermions. The knowledge of sigma_P is required to
compute the renormalization group invariant quark masses. We also study the
performance of a variant of the HMC algorithm using two pseudofermion fields.Comment: 3 pages, 2 figures, Lattice2002(spectrum
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