5,880 research outputs found
Statistical evaluation of the composition, physical properties, and surface configuration of terrestrial test sites and their correlation with remotely sensed data Final report
Utilizing spatial filtering for analyzing structural configuration of Michigan Basin for application to remote sensin
Density wave triggered star formation in grand design spirals
In normal spiral galaxies the arms are the main sites for star formation. This is the cause of their optical contrast compared with the rest of the disc. The spiral structure can be observed as a higher concentration of H2 regions, neutral gas (both atomic and molecular via CO), dust and stars than in the interarm disc. It seens generally accepted that, at least in grand design spirals, there are density waves in the discs. However, several questions are not clear yet and still under discussion. An important question could be termed the triggering dilemma (by analogy with the 'winding dilemma' raised in the forties): Is the enhanced star formation in the spiral arms triggered by the passage of a system of density waves or is it simply due to the presence of a higher column density of gas there? In the present work, we use triggering in the same sense as the moderate to strong triggering defined by Elmegreen (1992), that is to say that star formation in the arms occurs at a rate faster than that in the interarm zone, relative to the available placental gas. Our group has designed several tests to elucidate whether or not star formation is triggered in the arms with respect to the interarm region and we summarize one of them, that of the ratio of the star formation efficiency in the arms divided by that of the interarm zone at the same galactocentric distance which we may call the relative massive star formation efficiency, where the efficiency is defined using the ratio of the mass of stars (evaluated via the H alpha flux) to the mass of neutral gas, atomic plus molecular (which must be measured with the adequate angular resolution). If the relative efficiency is of order unity, the star formation is proportional to the mass of gas, if some kind of induced star formation is present, the relative efficiency should be considerably larger than unity
The Central Region in M100: Observations and Modeling
We present new high-resolution observations of the center of the late-type
spiral M100 (NGC 4321) supplemented by 3D numerical modeling of stellar and gas
dynamics, including star formation (SF). NIR imaging has revealed a stellar
bar, previously inferred from optical and 21 cm observations, and an
ovally-shaped ring-like structure in the plane of the disk. The K isophotes
become progressively elongated and skewed to the position angle of the bar
(outside and inside the `ring') forming an inner bar-like region. The galaxy
exhibits a circumnuclear starburst in the inner part of the K `ring'. Two
maxima of the K emission have been observed to lie symmetrically with respect
to the nucleus and equidistant from it slightly leading the stellar bar. We
interpret the twists in the K isophotes as being indicative of the presence of
a double inner Lindblad resonance (ILR) and test this hypothesis by modeling
the gas flow in a self-consistent gas + stars disk embedded in a halo, with an
overall NGC4321-like mass distribution. We have reproduced the basic morphology
of the region (the bar, the large scale trailing shocks, two symmetric K peaks
corresponding to gas compression maxima which lie at the caustic formed by the
interaction of a pair of trailing and leading shocks in the vicinity of the
inner ILR, both peaks being sites of SF, and two additional zones of SF
corresponding to the gas compression maxima, referred usually as `twin peaks').Comment: 31 pages, postscript, compressed, uuencoded. 21 figures available in
postscript, compressed form by anonymous ftp from
ftp://asta.pa.uky.edu/shlosman/main100 , mget *.ps.Z. To appear in Ap.
The structure, energy, and electronic states of vacancies in Ge nanocrystals
The atomic structure, energy of formation, and electronic states of vacancies
in H-passivated Ge nanocrystals are studied by density functional theory (DFT)
methods. The competition between quantum self-purification and the free surface
relaxations is investigated. The free surfaces of crystals smaller than 2 nm
distort the Jahn-Teller relaxation and enhance the reconstruction bonds. This
increases the energy splitting of the quantum states and reduces the energy of
formation to as low as 1 eV per defect in the smallest nanocrystals. In
crystals larger than 2 nm the observed symmetry of the Jahn-Teller distortion
matches the symmetry expected for bulk Ge crystals. Near the nanocrystal's
surface the vacancy is found to have an energy of formation no larger than 0.5
to 1.4 eV per defect, but a vacancy more than 0.7 nm inside the surface has an
energy of formation that is the same as in bulk Ge. No evidence of the
self-purification effect is observed; the dominant effect is the free surface
relaxations, which allow for the enhanced reconstruction. From the evidence in
this paper, it is predicted that for moderate sized Ge nanocrystals a vacancy
inside the crystal will behave bulk-like and not interact strongly with the
surface, except when it is within 0.7 nm of the surface.Comment: In Press at Phys. Rev.
A surface-fitting program for areally- distributed data from the earth sciences and remote sensing
Fortran II program for analysis of data from earth sciences and remote sensin
Recommended from our members
Observations of the J = 2→1 transitions of <sup>12</sup>C<sup>16</sup>O and <sup>12</sup>C<sup>18</sup>O towards galactic H II regions
Observations are reported of the J = 2→1 transitions of CO and 12C18O at 230 and 219 GHz respectively from a number of galactic sources. A map of the central 1/2° × 1/2° of the Orion A molecular cloud is presented. The spectra are interpreted to derive molecular densities and abundance ratios in the molecular clouds observed
An Empirical Relation Between The Large-Scale Magnetic Field And The Dynamical Mass In Galaxies
The origin and evolution of cosmic magnetic fields as well as the influence
of the magnetic fields on the evolution of galaxies are unknown. Though not
without challenges, the dynamo theory can explain the large-scale coherent
magnetic fields which govern galaxies, but observational evidence for the
theory is so far very scarce. Putting together the available data of
non-interacting, non-cluster galaxies with known large-scale magnetic fields,
we find a tight correlation between the integrated polarized flux density,
S(PI), and the rotation speed, v(rot), of galaxies. This leads to an almost
linear correlation between the large-scale magnetic field B and v(rot),
assuming that the number of cosmic ray electrons is proportional to the star
formation rate, and a super-linear correlation assuming equipartition between
magnetic fields and cosmic rays. This correlation cannot be attributed to an
active linear alpha-Omega dynamo, as no correlation holds with global shear or
angular speed. It indicates instead a coupling between the large-scale magnetic
field and the dynamical mass of the galaxies, B ~ M^(0.25-0.4). Hence, faster
rotating and/or more massive galaxies have stronger large-scale magnetic
fields. The observed B-v(rot) correlation shows that the anisotropic turbulent
magnetic field dominates B in fast rotating galaxies as the turbulent magnetic
field, coupled with gas, is enhanced and ordered due to the strong gas
compression and/or local shear in these systems. This study supports an
stationary condition for the large-scale magnetic field as long as the
dynamical mass of galaxies is constant.Comment: 23 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
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