3,835 research outputs found
Active primate simulator Final report
Systems engineering data and design specifications for Biosatellite active primate simulato
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.
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
The origin of the ionization of the diffuse interstellar medium in spiral galaxies. II. Modelling the distribution of ionizing radiation in NGC 157
In this paper we make a quantitative study of the hypothesis that the diffuse
H-alpha emitted from the discs of spiral galaxies owes its origin to the
ionizing photons escaping from HII regions. We use the H-alpha measurements of
the complete set of HII regions in the spiral NGC 157, for which an HI density
map was available, to derive a family of models which predict the ionizing
photon distribution in the disc of this galaxy. The predicted diffuse H-alpha
surface brightness distributions from our models were compared with the
observed distributions showing that, in general terms, the hypothesis of
density bounding for the HII regions allows us to predict well the spatial
distribution of the diffuse ionized gas. In the model yielding the best fit to
the data, the regions of lower luminosity lose a constant fraction of their
ionizing flux to their surroundings, while for HII region luminosities above a
specific transition value the ionizing escape fraction is a rising function of
the Ha luminosity.Comment: 16 pages, 10 figures (Postscript level 2), accepted for publication
in A&
Introduction to the Special Issue: The Role of Seed Dispersal in Plant Populations: Perspectives and Advances in a Changing World
Despite the importance of seed dispersal as a driving process behind plant community assembly, our understanding of the role of seed dispersal in plant population persistence and spread remains incomplete. As a result, our ability to predict the effects of global change on plant populations is hampered. We need to better understand the fundamental link between seed dispersal and population dynamics in order to make predictive generalizations across species and systems, to better understand plant community structure and function, and to make appropriate conservation and management responses related to seed dispersal. To tackle these important knowledge gaps, we established the CoDisperse Network and convened an interdisciplinary, NSF-sponsored Seed Dispersal Workshop in 2016, during which we explored the role of seed dispersal in plant population dynamics (NSF DEB Award # 1548194). In this Special Issue, we consider the current state of seed dispersal ecology and identify the following collaborative research needs: (i) the development of a mechanistic understanding of the movement process influencing dispersal of seeds; (ii) improved quantification of the relative influence of seed dispersal on plant fitness compared to processes occurring at other life history stages; (iii) an ability to scale from individual plants to ecosystems to quantify the influence of dispersal on ecosystem function; and (iv) the incorporation of seed dispersal ecology into conservation and management strategies
Mixed Early and Late-Type Properties in the Bar of NGC 6221: Evidence for Evolution along the Hubble Sequence?
Rotation curves and velocity dispersion profiles are presented for both the
stellar and gaseous components along five different position angles (P.A.=5,
50, 95, 125 and 155 degrees) of the nearby barred spiral NGC 6221. The observed
kinematics extends out to about 80" from the nucleus. Narrow and broad-band
imaging is also presented. The radial profiles of the fluxes ratio [NII]/Halpha
reveal the presence of a ring-like structure of ionized gas, with a radius of
about 9" and a deprojected circular velocity of about 280 km/s. The analysis of
the dynamics of the bar indicates this ring is related to the presence of an
inner Lindblad resonance (ILR) at 1.3 kpc. NGC6221 is found to exhibit
intermediate properties between those of the early-type barred galaxies: the
presence of a gaseous ring at an ILR, the bar edge located between the ILR's
and the corotation radius beyond the steep rising portion of the rotation
curve, the dust-lane pattern, and those of the late-type galaxies: an almost
exponential surface brightness profile, the presence of Halpha regions along
all the bar, the spiral-arm pattern. It is consistent with scenarios of
bar-induced evolution from later to earlier-type galaxies.Comment: 1 File ds7406.tar.gz which contains: one latex file (ds7406.tex), and
10 encsulated postscript figures (ds7406f**.eps). To be compiled with aa-l
latex2e macro style. To be published in A&A Sup. Serie
A composite HII region luminosity function in H alpha of unprecedented statistical weight
Context. Statistical properties of HII region populations in disk galaxies
yield important clues to the physics of massive star formation.
Aims. We present a set of HII region catalogues and luminosity functions for
a sample of 56 spiral galaxies in order to derive the most general form of
their luminosity function.
Methods. HII region luminosity functions are derived for individual galaxies
which, after photometric calibration, are summed to form a total luminosity
function comprising 17,797 HII regions from 53 galaxies.
Results. The total luminosity function, above its lower limit of
completeness, is clearly best fitted by a double power law with a significantly
steeper slope for the high luminosity portion of the function. This change of
slope has been reported in the literature for individual galaxies, and occurs
at a luminosity of log L = 38.6\pm0.1 (L in erg/s) which has been termed the
Stromgren luminosity. A steep fall off in the luminosity function above log L =
40 is also noted, and is related to an upper limit to the luminosities of
underlying massive stellar clusters. Detailed data are presented for the
individual sample galaxies.
Conclusions. The luminosity functions of HII regions in spiral galaxies show
a two slope power law behaviour, with a significantly steeper slope for the
high luminosity branch. This can be modelled by assuming that the high
luminosity regions are density bounded, though the scenario is complicated by
the inhomogeneity of the ionized interstellar medium. The break, irrespective
of its origin, is of potential use as a distance indicator for disc galaxies.Comment: Accepted for publication as a Letter in Astronomy & Astrophysics.
Latex with postscript figures. Online-only tables and figures are included in
this preprint. The HII region catalogues for 56 galaxies will be published
electronically on the CDS but are available also on request from the author
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