4,374 research outputs found
The Nature of Radio Continuum Emission in the Dwarf Starburst Galaxy NGC 625
We present new multi-frequency radio continuum imaging of the dwarf starburst
galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm
reveal global continuum emission dominated by free-free emission, with only
mild synchrotron components. Each of the major HII regions is detected; the
individual spectral indices are thermal for the youngest regions (showing
strongest H Alpha emission) and nonthermal for the oldest. We do not detect any
sources that appear to be associated with deeply embedded, dense, young
clusters, though we have discovered one low-luminosity, obscured source that
has no luminous optical counterpart and which resides in the region of highest
optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent
star formation, these radio properties suggest that the youngest star formation
complexes have not yet evolved to the point where their thermal spectra are
significantly contaminated by synchrotron emission. The nonthermal components
are associated with regions of older star formation that have smaller ionized
gas components. These results imply a range of ages of the HII regions and
radio components that agrees with our previous resolved stellar population
analysis, where an extended burst of star formation has pervaded the disk of
NGC 625 over the last ~ 50 Myr. We compare the nature of radio continuum
emission in selected nearby dwarf starburst and Wolf-Rayet galaxies,
demonstrating that thermal radio continuum emission appears to be more common
in these systems than in typical HII galaxies with less recent star formation
and more evolved stellar clusters.Comment: ApJ, in press; 27 pages, 5 figures. Full-resolution version may be
obtained at http://www.astro.umn.edu/~cannon/n625.vla.p
Validation of an electrogoniometry system as a measure of knee kinematics during activities of daily living
Purpose: The increasing use of electrogoniometry (ELG) in clinical research requires the validation of different instrumentation. The purpose of this investigation was to examine the concurrent validity of an ELG system during activities of daily living.
Methods: Ten asymptomatic participants gave informed consent to participate. A Biometrics SG150 electrogoniometer was directly compared to a 12 camera three dimensional motion analysis system during walking, stair ascent, stair descent, sit to stand, and stand to sit activities for the measurement of the right knee angle. Analysis of validity was undertaken by linear regression. Standard error of estimate (SEE), standardised SEE (SSEE), and Pearson’s correlation coefficient r were computed for paired trials between systems for each functional activity.
Results: The 95% confidence interval of SEE was reasonable between systems across walking (LCI = 2.43 °; UCI = 2.91 °), stair ascent (LCI = 2.09 °; UCI = 2.42 °), stair descent (LCI = 1.79 °; UCI = 2.10 °), sit to stand (LCI = 1.22 °; UCI = 1.41 °), and stand to sit (LCI = 1.17 °; UCI = 1.34 °). Pearson’s correlation coefficient r across walking (LCI = 0.983; UCI = 0.990), stair ascent (LCI = 0.995; UCI = 0.997), stair descent (LCI = 0.995; UCI = 0.997), sit to stand (LCI = 0.998; UCI = 0.999), and stand to sit (LCI = 0.996; UCI = 0.997) was indicative of a strong linear relationship between systems.
Conclusion: ELG is a valid method of measuring the knee angle during activities representative of daily living. The range is within that suggested to be acceptable for the clinical evaluation of patients with musculoskeletal conditions
A Simple Model for the Absorption of Starlight by Dust in Galaxies
We present a new model to compute the effects of dust on the integrated
spectral properties of galaxies, based on an idealized prescription of the main
features of the interstellar medium (ISM). The model includes the ionization of
HII regions in the interiors of the dense clouds in which stars form and the
influence of the finite lifetime of these clouds on the absorption of
radiation. We compute the production of emission lines and the absorption of
continuum radiation in the HII regions and the subsequent transfer of line and
continuum radiation in the surrounding HI regions and the ambient ISM. This
enables us to interpret simultaneously all the observations of a homogeneous
sample of nearby UV-selected starburst galaxies, including the ratio of far-IR
to UV luminosities, the ratio of Halpha to Hbeta luminosities, the Halpha
equivalent width, and the UV spectral slope. We show that the finite lifetime
of stellar birth clouds is a key ingredient to resolve an apparent discrepancy
between the attenuation of line and continuum photons in starburst galaxies. In
addition, we find that an effective absorption curve proportional to
lambda^-0.7 reproduces the observed relation between the ratio of far-IR to UV
luminosities and the UV spectral slope. We interpret this relation most simply
as a sequence in the overall dust content of the galaxies. The shallow
wavelength dependence of the effective absorption curve is compatible with the
steepness of known extinction curves if the dust has a patchy distribution. In
particular, we find that a random distribution of discrete clouds with optical
depths similar to those in the Milky Way provides a consistent interpretation
of all the observations. Our model for absorption can be incorporated easily
into any population synthesis model. (abridged)Comment: To appear in the 2000 July 20 issue of the Astrophysical Journal; 19
pages with 13 embedded PS figures (emulateapj5.sty
Constraints on a Universal IMF from UV to Near-IR Galaxy Luminosity Densities
We obtain constraints on the slope of a universal stellar initial mass
function (IMF) over a range of cosmic star-formation histories (SFH) using
z=0.1 luminosity densities in the range from 0.2 to 2.2 microns. The age-IMF
degeneracy of integrated spectra of stellar populations can be broken for the
Universe as a whole by using direct measurements of (relative) cosmic SFH from
high-redshift observations. These have only marginal dependence on
uncertainties in the IMF, whereas, fitting to local luminosity densities
depends strongly on both cosmic SFH and the IMF. We fit to these measurements
using population synthesis and find the best-fit IMF power-law slope to be
Gamma=1.15+-0.2 (0.5 < M/M_solar < 120). This slope is in good agreement with
the Salpeter IMF slope (Gamma=1.35). A strong upper limit of Gamma<1.7 is
obtained which effectively rules out the Scalo IMF due to its too low fraction
of high-mass stars. This upper limit is at the 99.7% confidence level if we
assume a closed-box chemical evolution scenario and 95% if we assume constant
solar metallicity. Fitting to the H-alpha line luminosity density, we obtain a
best-fit IMF slope in good agreement with that derived from broadband
measurements. Marginalizing over cosmic SFH and IMF slope, we obtain (95% conf.
ranges, h=1): omega_stars = 1.1-2.0 E-3 for the stellar mass density; rho_sfr =
0.7-4.1 E-2 M_solar/yr/Mpc^3 for the star-formation rate density, and; rho_L =
1.2-1.7 E+35 W/Mpc^3 for the bolometric, attenuated, stellar, luminosity
density (0.09-5 microns). Comparing this total stellar emission with an
estimate of the total dust emission implies a relatively modest average
attenuation in the UV (<=1 magnitude at 0.2 microns).Comment: 16 pages, accepted by Ap
Continuous macroscopic limit of a discrete stochastic model for interaction of living cells
In the development of multiscale biological models it is crucial to establish
a connection between discrete microscopic or mesoscopic stochastic models and
macroscopic continuous descriptions based on cellular density. In this paper a
continuous limit of a two-dimensional Cellular Potts Model (CPM) with excluded
volume is derived, describing cells moving in a medium and reacting to each
other through both direct contact and long range chemotaxis. The continuous
macroscopic model is obtained as a Fokker-Planck equation describing evolution
of the cell probability density function. All coefficients of the general
macroscopic model are derived from parameters of the CPM and a very good
agreement is demonstrated between CPM Monte Carlo simulations and numerical
solution of the macroscopic model. It is also shown that in the absence of
contact cell-cell interactions, the obtained model reduces to the classical
macroscopic Keller-Segel model. General multiscale approach is demonstrated by
simulating spongy bone formation from loosely packed mesenchyme via the
intramembranous route suggesting that self-organizing physical mechanisms can
account for this developmental process.Comment: 4 pages, 3 figure
The Radio Continuum of the Metal-Deficient Blue Compact Dwarf Galaxy SBS0335-052
We present new Very Large Array observations at five frequencies, from 1.4 to
22GHz, of the extremely low-metallicity blue compact dwarf SBS0335-052. The
radio spectrum shows considerable absorption at 1.49GHz, and a composite
thermal+non-thermal slope. After fitting the data with a variety of models, we
find the best-fitting geometry to be one with free-free absorption
homogeneously intermixed with the emission of both thermal and non-thermal
components. The best-fitting model gives an an emission measure EM ~ 8x10^7pc
cm^{-6} and a diameter of the radio-emitting region D ~17pc. The inferred
density is n_e ~ 2000 cm^{-3}. The thermal emission comes from an ensemble of
\~9000 O7 stars, with a massive star-formation rate (>=5Msun) of 0.13-0.15
yr^{-1}, and a supernova rate of 0.006 yr^{-1}. We find evidence for ionized
gas emission from stellar winds, since the observed Bralpha line flux
significantly exceeds that inferred from the thermal radio emission. The
non-thermal fraction at 5GHz is ~0.7, corresponding to a non-thermal luminosity
of ~2x10^{20} W Hz^{-1}. We attribute the non-thermal radio emission to an
ensemble of compact SN remnants expanding in a dense interstellar medium, and
derive an equipartition magnetic field of ~0.6-1 mG, and a pressure of
\~3x10^{-8}-1x10^{-7} dyne cm^{-2}. If the radio properties of SBS0335-052 are
representative of star formation in extremely low-metallicity environments,
derivations of the star formation rate from the radio continuum in high
redshift primordial galaxies need to be reconsidered. Moreover, photometric
redshifts inferred from ``standard'' spectral energy distributions could be
incorrect.Comment: 25 pages, including 3 figures, accepted for publication in Ap
Scattering and leapfrogging of vortex rings in a superfluid
The dynamics of vortex ring pairs in the homogeneous nonlinear Schr\"odinger
equation is studied. The generation of numerically-exact solutions of traveling
vortex rings is described and their translational velocity compared to revised
analytic approximations. The scattering behavior of co-axial vortex rings with
opposite charge undergoing collision is numerically investigated for different
scattering angles yielding a surprisingly simple result for its dependence as a
function of the initial vortex ring parameters. We also study the leapfrogging
behavior of co-axial rings with equal charge and compare it with the dynamics
stemming from a modified version of the reduced equations of motion from a
classical fluid model derived using the Biot-Savart law.Comment: 12 pages, 11 figure
Diffuse Ionized Gas in the Dwarf Irregular Galaxy DDO 53
The spectral characteristics throughout the dwarf irregular galaxy DDO 53 are
studied. The results are very similar to those for other irregular galaxies:
high excitation and low values of the [SII]/Halpha ratio. The most likely
ionization source is photon leakage from the classical HII regions, without any
other source, although the interstellar medium of the galaxy is quite
perturbed. Moreover, the physical conditions throughout the galaxy do not
change very much because both the photon leakage percentage and the ionization
temperature are very similar. In addition, the determined metal content for two
HII regions indicates that DDO 53 is a low-metallicity galaxy.Comment: 32 pages, 9 figures, 7 tables. AJ, in pres
Explosive Processes during the 2015 Eruption of Axial Seamount, as Recorded by Seafloor Hydrophones
Following the installation of the Ocean Observatories Initiative cabled array, the 2015 eruption of Axial Seamount, Juan de Fuca ridge, became the first submarine eruption to be captured in real time by seafloor seismic and acoustic instruments. This eruption also marked the first instance where the entire eruption cycle of a submarine volcano, from the previous eruption in 2011 to the end of the month-long 2015 event, was monitored continuously using autonomous ocean bottom hydrophones. Impulsive sounds associated with explosive lava-water interactions are identified within hydrophone records during both eruptions. Explosions within the caldera are acoustically distinguishable from those occurring in association with north rift lava flows erupting in 2015. Acoustic data also record a series of broadband diffuse events, occurring in the waning phase of the eruption, and are interpreted as submarine Hawaiian explosions. This transition from gas-poor to gas-rich eruptive activity coincides with an increase in water temperature within the caldera and with a decrease in the rate of deflation. The last recorded diffuse events coincide with the end of the eruption, represented by the onset of inflation. All the observed explosion signals couple strongly into the water column, and only weakly into the solid Earth, demonstrating the importance of hydroacoustic observations as a complement to seismic and geodetic studies of submarine eruptions.
Plain Language Summary: Axial Seamount, a submarine volcano on the Juan de Fuca ridge, erupted in 2015. This eruption was recorded in real-time by an array of seafloor seismometers and hydrophones located on the volcano, and connected to shore by a power and data cable. Hydrophones recording the sounds generated by the eruption reveal several different types of explosions, including short bursts interpreted as lava-water interactions, and prolonged signals thought to be due to explosive ejection of gas and ash. These signals provide a window into the dynamics of the undersea eruption and are an excellent complement to other types of data including earthquakes and ground deformation
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