51 research outputs found
ПОБУДОВА ПОВЕРХОНЬ ЗА ДОПОМОГОЮ ТЕХНОЛОГІЇ T-СПЛАЙНІВ
It was maked a review and comparative analysis of modern methods of building of surfaces, namely, T-Splines and NURBS. Also was considered the main features, characteristics, advantages and disadvantages of T-Splines.Проведен обзор и сравнительный анализ современных методов построения поверхностей, а именно Т-сплайнов и NURBS. Рассмотрены основные черты Т-сплайнов, их особенности, преимущества и недостатки.Проведено огляд та порівняльний аналіз сучасних методів побудови поверхонь, а саме Т-сплайнів і NURBS. Розглянуто основні риси Т-сплайнів, їхні особливості, переваги та недоліки
Environment, Ram Pressure, and Shell Formation in HoII
Neutral hydrogen VLA D-array observations of the dwarf irregular galaxy HoII,
a prototype galaxy for studies of shell formation, are presented. HI is
detected to radii over 16' or 4 R_25, and M_HI=6.44x10^8 M_sun. The total HI
map has a comet-like appearance suggesting that HoII is affected by ram
pressure from an intragroup medium (IGM). A rotation curve corrected for
asymmetric drift was derived and an analysis of the mass distribution yields a
total mass 6.3x10^9 M_sun, of which about 80% is dark. HoII lies northeast of
the M81 group's core, along with Kar52 (M81dwA) and UGC4483. No signs of
interaction are observed and it is argued that HoII is part of the NGC2403
subgroup, infalling towards M81. A case is made for ram pressure stripping and
an IGM in the M81 group. Stripping of the disk outer parts would require an IGM
density n_IGM>=4.0x10^-6 atoms/cm^3 at the location of HoII. This corresponds
to 1% of the virial mass of the group uniformly distributed over a volume just
enclosing HoII and is consistent with the X-ray properties of small groups. It
is argued that existing observations of HoII do not support self-propagating
star formation scenarios, whereby the HI holes and shells are created by
supernova explosions and stellar winds. Many HI holes are located in low
surface density regions of the disk, where no star formation is expected or
observed. Ram pressure has the capacity to enlarge preexisting holes and lower
their creation energies, helping to bridge the gap between the observed star
formation rate and that required to create the holes. (abridged)Comment: 43 pages, including 7 figures. 4 figures available as JPEG only.
Complete manuscript including full resolution figures available at
http://www.strw.leidenuniv.nl/~bureau/pub_list.html . Accepted for
publication in The Astronomical Journa
Dark Matter and the CACTUS Gamma-Ray Excess from Draco
The CACTUS atmospheric Cherenkov telescope collaboration recently reported a
gamma-ray excess from the Draco dwarf spheroidal galaxy. Draco features a very
low gas content and a large mass-to-light ratio, suggesting as a possible
explanation annihilation of weakly interacting massive particles (WIMPs) in the
Draco dark-matter halo. We show that with improved angular resolution, future
measurements can determine whether the halo is cored or cuspy, as well as its
scale radius. We find the relevant WIMP masses and annihilation cross sections
and show that supersymmetric models can account for the required gamma-ray
flux. The annihilation cross section range is found to be not compatible with a
standard thermal relic dark-matter production. We compute for these
supersymmetric models the resulting Draco gamma-ray flux in the GLAST energy
range and the rates for direct neutralino detection and for the flux of
neutrinos from neutralino annihilation in the Sun. We also discuss the
possibility that the bulk of the signal detected by CACTUS comes from direct
WIMP annihilation to two photons and point out that a decaying-dark-matter
scenario for Draco is not compatible with the gamma-ray flux from the Galactic
center and in the diffuse gamma-ray background.Comment: 24 pages, 10 figures; version accepted for publication in JCA
Observational Manifestations of the First Protogalaxies in the 21 cm Line
The absorption properties of the first low-mass protogalaxies (mini-halos)
forming at high redshifts in the 21-cm line of atomic hydrogen are considered.
The absorption properties of these protogalaxies are shown to depend strongly
on both their mass and evolutionary status. The optical depths in the line
reach 0.1-0.2 for small impact parameters of the line of sight. When a
protogalaxy being compressed, the influence of gas accretion can be seen
manifested in a non-monotonic frequency dependence of the optical depth. The
absorption characteristics in the 21-cm line are determined by the thermal and
dynamical evolution of the gas in protogalaxies. Since the theoretical line
width in the observer's reference frame is 1-6 kHz and the expected separation
between lines 8.4 kHz, the lines from low mass protogalaxies can be resolved
using ongoing and future low frequency interferometers.Comment: 12 pages, 5 figure
Dark Matter signals from Draco and Willman 1: Prospects for MAGIC II and CTA
The next generation of ground-based Imaging Air Cherenkov Telescopes (IACTs)
will play an important role in indirect dark matter searches. In this article,
we consider two particularly promising candidate sources for dark matter
annihilation signals, the nearby dwarf galaxies Draco and Willman 1, and study
the prospects of detecting such a signal for the soon-operating MAGIC II
telescope system as well as for the planned installation of CTA, taking special
care of describing the experimental features that affect the detectional
prospects. For the first time in such a study, we fully take into account the
effect of internal bremsstrahlung, which has recently been shown to
considerably enhance, in some cases, the gamma-ray flux at the high energies
where Atmospheric Cherenkov Telescopes operate, thus leading to significantly
harder annihilation spectra than traditionally considered. While the detection
of the spectral features introduced by internal bremsstrahlung would constitute
a smoking gun signature for dark matter annihilation, we find that for most
models the overall flux still remains at a level that will be challenging to
detect unless one adopts rather (though by no means overly) optimistic
astrophysical assumptions about the distribution of dark matter in the dwarfs.Comment: 10 pages, 4 figures, minor changes, matches the published version
(JCAP
The Star Formation & Chemical Evolution History of the Fornax Dwarf Spheroidal Galaxy
We present deep photometry in the B,V and I filters from CTIO/MOSAIC for
about 270.000 stars in the Fornax dwarf Spheroidal galaxy, out to a radius of
r_ell\sim0.8 degrees. By combining the accurately calibrated photometry with
the spectroscopic metallicity distributions of individual Red Giant Branch
stars we obtain the detailed star formation and chemical evolution history of
Fornax. Fornax is dominated by intermediate age (1-10 Gyr) stellar populations,
but also includes ancient (10-14 Gyr), and young (<1 Gyr) stars. We show that
Fornax displays a radial age gradient, with younger, more metal-rich
populations dominating the central region. This confirms results from previous
works. Within an elliptical radius of 0.8 degrees, or 1.9 kpc from the centre,
a total mass in stars of 4.3x10^7 Msun was formed, from the earliest times
until 250 Myr ago. Using the detailed star formation history, age estimates are
determined for individual stars on the upper RGB, for which spectroscopic
abundances are available, giving an age-metallicity relation of the Fornax dSph
from individual stars. This shows that the average metallicity of Fornax went
up rapidly from [Fe/H]<-2.5 dex to [Fe/H]=-1.5 dex between 8-12 Gyr ago, after
which a more gradual enrichment resulted in a narrow, well-defined sequence
which reaches [Fe/H]\sim-0.8 dex, \sim3 Gyr ago. These ages also allow us to
measure the build-up of chemical elements as a function of time, and thus
determine detailed timescales for the evolution of individual chemical
elements. A rapid decrease in [Mg/Fe] is seen for the stars with [Fe/H]>-1.5
dex, with a clear trend in age.Comment: 18 pages, 20 figure
Catalogue of far-infrared loops in the Galaxy
Aims: An all-sky survey of loop- and arc-like intensity enhancements has been
performed in order to investigate the large-scale structure of the diffuse
far-infrared emission. Methods: We used maps made of 60 and 100 micrometer
processed IRAS data (Sky Survey Atlas and dust infrared emission maps) to
identify large-scale structures: loops, arcs or cavities, in the far-infrared
emission in the Galaxy. Distances were attributed to a subsample of loops using
associated objects. Results: We identified 462 far-infrared loops, analyzed
their individual FIR properties and their distribution. This data forms the
Catalogue of Far-Infrared Loops in the Galaxy. We obtained observational
estimates of f_in~30% and f_out~5% for the hot gas volume filling factor of the
inward and outward Galactic neighbourhood of the Solar System. We obtained a
slope of the power law size luminosity function {beta}=1.37 for low Galactic
latitudes in the outer Milky Way. Conclusions: Deviations in the celestial
distribution of far-infrared loops clearly indicate, that violent events
frequently overwrite the structure of the interstellar matter in the inner
Galaxy. Our objects trace out the spiral arm structure of the Galaxy in the
neighbourhood of the Sun and their distribution clearly suggests that there is
an efficient process that can generate loop-like features at high Galactic
latitudes. Power law indices of size luminosity distributions suggest, that the
structure of the ISM is ruled by supernovae and stellar winds at low Galactic
latitudes while it is governed by supersonic turbulence above the Galactic
plane.Comment: 11 pages - 10 figures (together with Appendix A), accepted for
publication in Astronomy & Astrophysics. Appendix B, C and D are available at
http://kisag.konkoly.hu/CFIRLG
Dark Matter in the Milky Way's Dwarf Spheroidal Satellites
The Milky Way's dwarf spheroidal satellites include the nearest, smallest and
least luminous galaxies known. They also exhibit the largest discrepancies
between dynamical and luminous masses. This article reviews the development of
empirical constraints on the structure and kinematics of dSph stellar
populations and discusses how this phenomenology translates into constraints on
the amount and distribution of dark matter within dSphs. Some implications for
cosmology and the particle nature of dark matter are discussed, and some
topics/questions for future study are identified.Comment: A version with full-resolution figures is available at
http://www.cfa.harvard.edu/~mwalker/mwdsph_review.pdf; 70 pages, 22 figures;
invited review article to be published in Vol. 5 of the book "Planets, Stars,
and Stellar Systems", published by Springe
Dynamics of Disks and Warps
This chapter reviews theoretical work on the stellar dynamics of galaxy
disks. All the known collective global instabilities are identified, and their
mechanisms described in terms of local wave mechanics. A detailed discussion of
warps and other bending waves is also given. The structure of bars in galaxies,
and their effect on galaxy evolution, is now reasonably well understood, but
there is still no convincing explanation for their origin and frequency. Spiral
patterns have long presented a special challenge, and ideas and recent
developments are reviewed. Other topics include scattering of disk stars and
the survival of thin disks.Comment: Chapter accepted to appear in Planets, Stars and Stellar Systems, vol
5, ed G. Gilmore. 32 pages, 17 figures. Includes minor corrections made in
proofs. Uses emulateapj.st
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