658 research outputs found
The Kinematics and Metallicity of the M31 Globular Cluster System
With the ultimate aim of distinguishing between various models describing the
formation of galaxy halos (e.g. radial or multi-phase collapse, random
mergers), we have completed a spectroscopic study of the globular cluster
system of M31. We present the results of deep, intermediate-resolution,
fibre-optic spectroscopy of several hundred of the M31 globular clusters using
the Wide Field Fibre Optic Spectrograph (WYFFOS) at the William Herschel
Telescope in La Palma, Canary Islands. These observations have yielded precise
radial velocities (+/-12 km/s) and metallicities (+/-0.26 dex) for over 200
members of the M31 globular cluster population out to a radius of 1.5 degrees
from the galaxy center. Many of these clusters have no previous published
radial velocity or [Fe/H] estimates, and the remainder typically represent
significant improvements over earlier determinations. We present analyses of
the spatial, kinematic and metal abundance properties of the M31 globular
clusters. We find that the abundance distribution of the cluster system is
consistent with a bimodal distribution with peaks at [Fe/H] = -1.4 and -0.5.
The metal-rich clusters demonstrate a centrally concentrated spatial
distribution with a high rotation amplitude, although this population does not
appear significantly flattened and is consistent with a bulge population. The
metal-poor clusters tend to be less spatially concentrated and are also found
to have a strong rotation signature.Comment: 33 pages, 20 figure
Large-scale study of the NGC 1399 globular cluster system in Fornax
We present a Washington C and Kron-Cousins R photometric study of the
globular cluster system of NGC 1399, the central galaxy of the Fornax cluster.
A large areal coverage of 1 square degree around NGC 1399 is achieved with
three adjoining fields of the MOSAIC II Imager at the CTIO 4-m telescope.
Working on such a large field, we can perform the first indicative
determination of the total size of the NGC 1399 globular cluster system. The
estimated angular extent, measured from the NGC 1399 centre and up to a
limiting radius where the areal density of blue globular clusters falls to 30
per cent of the background level, is 45 +/- 5 arcmin, which corresponds to 220
- 275 kpc at the Fornax distance. The bimodal colour distribution of this
globular cluster system, as well as the different radial distribution of blue
and red clusters, up to these large distances from the parent galaxy, are
confirmed. The azimuthal globular cluster distribution exhibits asymmetries
that might be understood in terms of tidal stripping of globulars from NGC
1387, a nearby galaxy. The good agreement between the areal density profile of
blue clusters and a projected dark-matter NFW density profile is emphasized.Comment: 9 pages, 9 figures. Accepted for publication in A&
From Uruk to Ur: Automated Matching of Virtual Tablet Fragments
Unlike jigsaw puzzles of thousands of pieces, which computers can now solve, fragmented tablets constitute much more complex freeform three-dimensional “puzzles” whose pieces can belong to an unknown number of complete or incomplete tablets. Computer-aided reconstruction of archaeological fragments has been an active area of research in recent years, although most published work has been specific to the joining of potsherds. Challenges in automated reassembly, aside from the difficulties of acquiring three dimensional scans, include the extremely difficult search problems, the large numbers of false-positive matches and the incorporation of surface imagery with object geometry. We have produced and refined a means of automatically joining fragments. The process of automated computer joining comprises three main stages: preprocessing, pairwise matching and match ranking. Pairwise matching is an enormously computationally intensive problem that significantly benefits from the parallel computing capacity of modern graphical processing units. The first successful pair of automated fragment joins was achieved in 2014 for a pair of fragments from Uruk. Further automated joins have since been made using fragments from Ur, currently held by the British Museum. The 3D virtual Ur fragments were acquired with high-resolution photographic texture using a novel, portable, low-cost “scannerless” rotary photogrammetric scanning system. The workflow for the method was designed such that minimal user intervention or training is required for both the 3D acquisition and the automated joining. Future ambitions include the achievement of long-distance photogrammetric matches, for example, of London and Pennsylvania virtual tablet fragments
Granular flow down a rough inclined plane: transition between thin and thick piles
The rheology of granular particles in an inclined plane geometry is studied
using molecular dynamics simulations. The flow--no-flow boundary is determined
for piles of varying heights over a range of inclination angles . Three
angles determine the phase diagram: , the angle of repose, is the
angle at which a flowing system comes to rest; , the maximum angle
of stability, is the inclination required to induce flow in a static system;
and is the maximum angle for which stable, steady state flow is
observed. In the stable flow region , three
flow regimes can be distinguished that depend on how close is to
: i) : Bagnold rheology, characterized by a
mean particle velocity in the direction of flow that scales as
, for a pile of height , ii)
: the slow flow regime, characterized by a linear
velocity profile with depth, and iii) : avalanche flow
characterized by a slow underlying creep motion combined with occasional free
surface events and large energy fluctuations. We also probe the physics of the
initiation and cessation of flow. The results are compared to several recent
experimental studies on chute flows and suggest that differences between
measured velocity profiles in these experiments may simply be a consequence of
how far the system is from jamming.Comment: 19 pages, 14 figs, submitted to Physics of Fluid
Mechanisms for slow strengthening in granular materials
Several mechanisms cause a granular material to strengthen over time at low
applied stress. The strength is determined from the maximum frictional force
F_max experienced by a shearing plate in contact with wet or dry granular
material after the layer has been at rest for a waiting time \tau. The layer
strength increases roughly logarithmically with \tau -only- if a shear stress
is applied during the waiting time. The mechanisms of strengthening are
investigated by sensitive displacement measurements and by imaging of particle
motion in the shear zone. Granular matter can strengthen due to a slow shift in
the particle arrangement under shear stress. Humidity also leads to
strengthening, but is found not to be its sole cause. In addition to these time
dependent effects, the static friction coefficient can also be increased by
compaction of the granular material under some circumstances, and by cycling of
the applied shear stress.Comment: 21 pages, 11 figures, submitted to Phys. Rev.
Discovery of a Probable CH Star in the Globular Cluster M14 and Implications for the Evolution of Binaries in Clusters
We report the discovery of a probable CH star in the core of the Galactic
globular cluster M14, identified from an integrated-light spectrum of the
cluster obtained with the MOS spectrograph on the CFHT. From a high- resolution
echelle spectrum of the same star obtained with the Hydra fiber positioner and
bench spectrograph on the WIYN telescope, we measure a radial velocity of
km s. Although this velocity is inconsistent with
published estimates of the systemic radial velocity of M14 (eg, km s), we use high-precision Hydra velocities for 20 stars
in the central 2.6 arcminutes of M14 to calculate improved values for the
cluster mean velocity and one-dimensional velocity dispersion: km
s and km s, respectively. Both the star's location
near the tip of the red giant branch in the cluster color magnitude diagram and
its radial velocity therefore argue for membership in M14. Since the
intermediate-resolution MOS spectrum shows not only enhanced CH absorption but
also strong Swan bands of C, M14 joins Omega Cen as the only globular
clusters known to contain classical CH stars. Although evidence for its
duplicity must await additional radial velocity measurements, the CH star in
M14 is probably, like all field CH stars, a spectroscopic binary with a
degenerate (white dwarf) secondary. The candidate and confirmed CH stars in M14
and Omega Cen, and in a number of Galactic dSph galaxies, may then owe their
existence to the long timescales for the shrinking and coalescence of hard
binaries in low-concentration environments.Comment: Accepted to the Astrophysical Journal Letters. 13 pages, AAS LaTeX
and three postscript figures (numbers 2,3,4). Entire paper (including Figure
1) available at http://www.hia.nrc.ca/DAO/SCIENCE/science.htm
The X-ray Faint Early-Type Galaxy NGC4697
We analyze archival ROSAT HRI, ROSAT PSPC, and ASCA data of the X-ray faint
early-type galaxy NGC4697. The joint ROSAT PSPC + ASCA spectrum is fit by a
two-component thermal model, a MEKAL model with kT_{MEKAL}=0.26^{+0.04}_{-0.03}
keV with low metallicity and a bremsstrahlung model with
kT_{BREM}=5.2^{+3.0}_{-1.6} keV. A similar model was found to fit the spectra
of another faint early-type galaxy (NGC4382) and the bulge of M31. We interpret
this soft emission as a combination of emission from a soft component of low
mass X-ray binaries (LMXBs) and from a low temperature interstellar medium,
although the relative contributions of the two components could not be
determined. Twelve point sources were identified within 4' of NGC4697, of which
11 are most likely LMXBs associated with the galaxy. The soft X-ray colors of
four of the LMXBs in NGC4697 support the claim that LMXBs possess a soft
spectral component. Finally, we present a simulation of what we believe the
Chandra data of NGC4697 will look like.Comment: 10 pages, uses emulateapj.sty, accepted by Astrophysical Journa
M87, Globular Clusters, and Galactic Winds: Issues in Giant Galaxy Formation
New VRI photometry is presented for the globular clusters in the innermost
140'' of the M87 halo. The results are used to discuss several issues
concerning the formation and evolution of globular cluster systems in
supergiant ellipticals like M87. (1) we find no significant change in the
globular cluster luminosity function (GCLF) with galactocentric radius, for
cluster masses M < 10^5 solar masses, indicating that the main effects of
dynamical evolution may be only on lower-mass clusters. (2) Within the core
radius (1') of the globular cluster system, the metallicity distribution is
uniform, but at larger radii the mean metallicity declines steadily as Z ~
r^-0.9. (3) The various options for explaining the existence of high specific
frequency galaxies like M87 are evaluated, and scaling laws for the GCSs in
these galaxies are given. Interpretations involving secondary evolution
(formation of many globular clusters during mergers, intergalactic globular
clusters, etc.) are unlikely to be the primary explanation for high-S_N
galaxies. (4) We suggest that central-supergiant E galaxies may have formed in
an exceptionally turbulent or high-density environment in which an early,
powerful galactic wind drove out a high fraction of the protogalactic gas, thus
artificially boosting the specificComment: 67 pages, 17 figures. To appear in Astronomical Journal, in press for
May 1998. Preprints also available from W.Harris; send e-mail request to
[email protected]
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