14,384 research outputs found
Vast planes of satellites in a high resolution simulation of the Local Group: comparison to Andromeda
We search for vast planes of satellites (VPoS) in a high resolution
simulation of the Local Group performed by the CLUES project, which improves
significantly the resolution of former similar studies. We use a simple method
for detecting planar configurations of satellites, and validate it on the known
plane of M31. We implement a range of prescriptions for modelling the satellite
populations, roughly reproducing the variety of recipes used in the literature,
and investigate the occurence and properties of planar structures in these
populations. The structure of the simulated satellite systems is strongly
non-random and contains planes of satellites, predominantly co-rotating, with,
in some cases, sizes comparable to the plane observed in M31 by Ibata et al..
However the latter is slightly richer in satellites, slightly thinner and has
stronger co-rotation, which makes it stand out as overall more exceptional than
the simulated planes, when compared to a random population. Although the
simulated planes we find are generally dominated by one real structure, forming
its backbone, they are also partly fortuitous and are thus not kinematically
coherent structures as a whole. Provided that the simulated and observed planes
of satellites are indeed of the same nature, our results suggest that the VPoS
of M31 is not a coherent disc and that one third to one half of its satellites
must have large proper motions perpendicular to the plane
High resolution simulations of the reionization of an isolated Milky Way - M31 galaxy pair
We present the results of a set of numerical simulations aimed at studying
reionization at galactic scale. We use a high resolution simulation of the
formation of the Milky Way-M31 system to simulate the reionization of the local
group. The reionization calculation was performed with the post-processing
radiative transfer code ATON and the underlying cosmological simulation was
performed as part of the CLUES project. We vary the source models to bracket
the range of source properties used in the literature. We investigate the
structure and propagation of the galatic ionization fronts by a visual
examination of our reionization maps. Within the progenitors we find that
reionization is patchy, and proceeds locally inside out. The process becomes
patchier with decreasing source photon output. It is generally dominated by one
major HII region and 1-4 additional isolated smaller bubbles, which eventually
overlap. Higher emissivity results in faster and earlier local reionization. In
all models, the reionization of the Milky Way and M31 are similar in duration,
i.e. between 203 Myr and 22 Myr depending on the source model, placing their
zreion between 8.4 and 13.7. In all models except the most extreme, the MW and
M31 progenitors reionize internally, ignoring each other, despite being
relatively close to each other even during the epoch of reionization. Only in
the case of strong supernova feedback suppressing star formation in haloes less
massive than 10^9 M_sun, and using our highest emissivity, we find that the MW
is reionized by M31.Comment: Accepted for publication in ApJ. 14 pages, 4 figures, 1 tabl
Basement-cover relations and internal structure of the Cape Smith klippe: A 1.9 Ga greenstone belt in northern Quebec, Canada
The Cape Smith Belt is a 380x60 km tectonic klippe composed of greenschistto amphibolite-grade mafic and komatiitic lava flows and fine-grained quartzose sediment, intruded by minor syn- to post-tectonic granitoids. Previously studied transects in areas of relatively high structural level show that the belt is constructed of seven or more north-dipping thrust sheets which verge toward the Superior Province (Archean) foreland in the south and away from an Archean basement massif (Kovik Antiform) external to the Trans-Hudson Orogen (Early Proterozoic) in the north. A field project (mapping and structural-stratigraphic-metamorphic studies) directed by MRS was begun in 1985 aimed at the structurally deeper levels of the belt and underlying basement, which are superby exposed in oblique cross-section (12 km minimum structural relief) at the west-plunging eastern end of the belt. Mapping now complete of the eastern end of the belt confirms that all of the metavolcanic and most of the metasedimentary rocks are allochthonous with respect to the Archean basement, and that the thrusts must have been rooted north of Kovik Antiform. The main findings follow
Large Scale Power Spectrum from Peculiar Velocities Via Likelihood Analysis
The power spectrum (PS) of mass density fluctuations, independent of
`biasing', is estimated from the Mark III catalog of peculiar velocities using
Bayesian statistics. A parametric model is assumed for the PS, and the free
parameters are determined by maximizing the probability of the model given the
data. The method has been tested using detailed mock catalogs. It has been
applied to generalized CDM models with and without COBE normalization.
The robust result for all the models is a relatively high PS, with at . An
extrapolation to smaller scales using the different CDM models yields . The peak is weakly constrained to the range
. These results are consistent with a direct
computation of the PS (Kolatt & Dekel 1996). When compared to galaxy-density
surveys, the implied values for () are of order
unity to within 25%.
The parameters of the COBE-normalized, flat CDM model are confined by a 90%
likelihood contour of the sort , where
and for models with and without tensor
fluctuations respectively. For open CDM the powers are and (no tensor fluctuations). A -shape model free of COBE
normalization yields only a weak constraint: .Comment: 19 pages, 8 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
Effective medium theory of elastic waves in random networks of rods
We formulate an effective medium (mean field) theory of a material consisting
of randomly distributed nodes connected by straight slender rods, hinged at the
nodes. Defining novel wavelength-dependent effective elastic moduli, we
calculate both the static moduli and the dispersion relations of ultrasonic
longitudinal and transverse elastic waves. At finite wave vector the waves
are dispersive, with phase and group velocities decreasing with increasing wave
vector. These results are directly applicable to networks with empty pore
space. They also describe the solid matrix in two-component (Biot) theories of
fluid-filled porous media. We suggest the possibility of low density materials
with higher ratios of stiffness and strength to density than those of foams,
aerogels or trabecular bone.Comment: 14 pp., 3 fig
Neutral Hydrogen Mapping of Virgo Cluster Blue Compact Dwarf Galaxies
A new installment of neutral hydrogen mappings of Blue Compact Dwarf
galaxies, as defined by optical morphology, in and near the Virgo cluster is
presented. The primary motivation was to search for outlying clouds of HI as
potential interactive triggers of the enhanced star formation, and therefore
the mapped galaxies were selected for large HI} mass, large optical diameter,
and large velocity profile width. Approximately half the sample proved to have
one or more small, low column density star-free companion clouds, either
detached or appearing as an appendage in our maps, at resolution of order 4
kpc. Comparison is made to a sample of similarly mapped field BCD galaxies
drawn from the literature; however, the Virgo cluster sample of mapped BCDs is
still too small for conclusive comparisons to be made.
We found, on the one hand, little or no evidence for ram pressure stripping
nor, on the other, for extremely extended low column density HI envelopes. The
HI rotation curves in most cases rise approximately linearly, and slowly, as
far out as we can trace the gas.Comment: To appear in AJ, Dec. 200
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