5,197 research outputs found
The internal dynamical equilibrium of HII regions: a statistical study
We present an analysis of the integrated Halpha emission line profiles for
the HII region population of the spiral galaxies NGC 1530, NGC 6951 and NGC
3359. We show that 70% of the line profiles show two or three Gaussian
components. The relations between the Halpha luminosity and non-thermal line
width for the HII regions of the three galaxies are studied and compared with
the relation found taken all the HII regions of the three galaxies as a single
distribution. A clearer envelope in non-thermal line width is found when only
those HII regions with non-thermal line width bigger than 13kms are considered.
The linear fit for the envelope is logL=36.8+2.0*log(sigma). The masses of the
HII regions on the envelope using the virial theorem and the mass estimates
from the Halpha luminosity are comparable, which offers evidence that the HII
regions on the envelope are virialized systems, while the remaining regions,
the majority, are not in virial equilibrium.Comment: 19 pages, 10 figures,accepted for publication in A&
HIIphot: Automated Photometry of HII Regions Applied to M51
We have developed a robust, automated method, hereafter designated HIIphot,
which enables accurate photometric characterization of HII regions while
permitting genuine adaptivity to irregular source morphology. HIIphot utilizes
object-recognition techniques to make a first guess at the shapes of all
sources then allows for departure from such idealized ``seeds'' through an
iterative growing procedure. Photometric corrections for spatially coincident
diffuse emission are derived from a low-order surface fit to the background
after exclusion of all detected sources. We present results for the
well-studied, nearby spiral M51 in which 1229 HII regions are detected above
the 5-sigma level. A simple, weighted power-law fit to the measured H-alpha
luminosity function (HII LF) above log L_H-alpha = 37.6 gives alpha =
-1.75+/-0.06, despite a conspicuous break in the HII LF observed near L_H-alpha
= 10^38.9. Our best- fit slope is marginally steeper than measured by Rand
(1992), perhaps reflecting our increased sensitivity at low luminosities and to
notably diffuse objects. HII regions located in interarm gaps are
preferentially less luminous than counterparts which constitute M51's
grand-design spiral arms and are best fit with a power-law slope of alpha =
-1.96+/-0.15. We assign arm/interarm status for HII regions based upon the
varying surface brightness of diffuse emission as a function of position
throughout the image. Using our measurement of the integrated flux contributed
by resolved HII regions in M51, we estimate the diffuse fraction to be
approximately 0.45 -- in agreement with the determination of Greenawalt et al.
(1998). Automated processing of degraded datasets is undertaken to gauge
systematic effects associated with limiting spatial resolution and sensitivity.Comment: 41 pages, 14 figures, Postscript version with high-resolution figures
at ftp://ftp.aoc.nrao.edu/staff/dthilker/preprint
Star Forming Objects in the Tidal Tails of Compact Groups
A search for star forming objects belonging to tidal tails has been carried
out in a sample of deep Halpha images of 16 compact groups of galaxies. A total
of 36 objects with Halpha luminosity larger than 10^38 erg s-1 have been
detected in five groups. The fraction of the total Halpha luminosity of their
respective parent galaxies shown by the tidal objects is always below 5% except
for the tidal features of HCG95, whose Halpha luminosity amounts to 65% of the
total luminosity. Out of this 36 objects, 9 star forming tidal dwarf galaxy
candidates have been finally identified on the basis of their projected
distances to the nuclei of the parent galaxies and their total Halpha
luminosities. Overall, the observed properties of the candidates resemble those
previously reported for the so-called tidal dwarf galaxies.Comment: 5 gif figures. Accepted for publication in Astrophysical Journa
Tension and stiffness of the hard sphere crystal-fluid interface
A combination of fundamental measure density functional theory and Monte
Carlo computer simulation is used to determine the orientation-resolved
interfacial tension and stiffness for the equilibrium hard-sphere crystal-fluid
interface. Microscopic density functional theory is in quantitative agreement
with simulations and predicts a tension of 0.66 kT/\sigma^2 with a small
anisotropy of about 0.025 kT and stiffnesses with e.g. 0.53 kT/\sigma^2 for the
(001) orientation and 1.03 kT/\sigma^2 for the (111) orientation. Here kT is
denoting the thermal energy and \sigma the hard sphere diameter. We compare our
results with existing experimental findings
An improved method for statistical studies of the internal kinematics of HII regions: the case of M 83
We present the integrated Halpha emission line profile for 157 HII regions in
the central 3.4' x 3.4' of the galaxy M 83 (NGC 5236). Using the Fabry-Perot
interferometer GHaFaS, on the 4.2 m William Herschel on La Palma, we show the
importance of a good characterization of the instrumental response function for
the study of line profile shapes. The luminosity-velocity dispersion relation
is also studied, and in the log(L)-log(sigma) plane we do not find a linear
relation, but an upper envelope with equation log(L)=0.9 *log(sigma)+38.1. For
the adopted distance of 4.5 Mpc, the upper envelope appears at the luminosity
L=10^38.5 ergs, in full agreement with previous studies of other galaxies,
reinforcing the idea of using HII regions as standard candles.Comment: 13 pages, 9 figures, accepted for publication in MNRA
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&
Interacting holographic tachyon model of dark energy
We propose a holographic tachyon model of dark energy with interaction
between the components of the dark sector. The correspondence between the
tachyon field and the holographic dark energy densities allows the
reconstruction of the potential and the dynamics of the tachyon scalar field in
a flat Friedmann-Robertson-Walker universe. We show that this model can
describe the observed accelerated expansion of our universe with a parameter
space given by the most recent observational results.Comment: 7 pages, 8 figures, accepted for publication in IJMP
Propagation of ionizing radiation in HII regions: the effects of optically thick density fluctuations
The accepted explanation of the observed dichotomy of two orders of magnitude
between in situ measurements of electron density in HII regions, derived from
emission line ratios, and average measurements based on integrated emission
measure, is the inhomogeneity of the ionized medium. This is expressed as a
"filling factor", the volume ratio of dense to tenuous gas, measured with
values of order 10^-3. Implicit in the filling factor model as normally used,
is the assumption that the clumps of dense gas are optically thin to ionizing
radiation. Here we explore implications of assuming the contrary: that the
clumps are optically thick. A first consequence is the presence within HII
regions of a major fraction of neutral hydrogen. We estimate the mean H^o/H^+
ratio for a population of HII regions in the spiral galaxy NGC 1530 to be the
order of 10, and support this inference using dynamical arguments. The
optically thick clumpy models allow a significant fraction of the photons
generated by the ionizing stars to escape from their HII region. We show, by
comparing model predictions with observations, that these models give an
account at least as good as, and probably better than that of conventional
models, of the radial surface brightness distribution and of selected spectral
line diagnostics for physical conditions within HII regions. These models
explain how an HII region can appear, from its line ratios, to be ionization
bounded, yet permit a major fraction of its ionizing photons to escape.Comment: 9 pages, 7 figures (2 of them in colours), accepted for publication
in A&
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