2,459 research outputs found
WIYN Imaging of the Globular Cluster Systems of the Spiral Galaxies NGC891 and NGC4013
We present results from a WIYN 3.5m telescope imaging study of the globular
cluster (GC) systems of the edge-on spiral galaxies NGC891 and NGC4013. We used
the 10' x 10' Minimosaic Imager to observe the galaxies in BVR filters to
projected radii of ~20 kpc from the galaxy centers. We combined the WIYN data
with archival and published data from WFPC2 and ACS on the Hubble Space
Telescope to assess the contamination level of the WIYN GC candidate sample and
to follow the GC systems further in toward the galaxies' centers. We
constructed radial distributions for the GC systems using both the WIYN and HST
data. The GC systems of NGC891 and NGC4013 extend to 9+/-3 kpc and 14+/-5 kpc,
respectively, before falling off to undetectable levels in our images. We use
the radial distributions to calculate global values for the total number (N_GC)
and specific frequencies (S_N and T) of GCs. NGC4013 has N_GC = 140+/-20, S_N =
1.0+/-0.2 and T = 1.9+/-0.5; our N_GC value is ~40% smaller than a previous
determination from the literature. The HST data were especially useful for
NGC891, because the GC system is concentrated toward the plane of the galaxy
and was only weakly detected in our WIYN images. Although NGC891 is thought to
resemble the Milky Way in its overall properties, it has only half as many GCs,
with N_GC = 70+/-20, S_N = 0.3+/-0.1 and T = 0.6+/-0.3. We also calculate the
galaxy-mass-normalized number of blue (metal-poor) GCs in NGC891 and NGC4013
and find that they fall along a general trend of increasing specific frequency
of blue GCs with increasing galaxy mass. Given currently available resources,
the optimal method for studying the global properties of extragalactic GC
systems is to combine HST data with wide-field, ground-based imaging with good
resolution. The results here demonstrate the advantage gained by using both
methods when possible.Comment: 43 pages, 11 figures 6 tables; accepted to The Astronomical Journal.
Online AJ version at http://iopscience.iop.org/1538-3881/140/2/430
A Computer Code for Swirling Turbulent Axisymmetric Recirculating Flows in Practical Isothermal Combustor Geometries
A primitive pressure-velocity variable finite difference computer code was developed to predict swirling recirculating inert turbulent flows in axisymmetric combustors in general, and for application to a specific idealized combustion chamber with sudden or gradual expansion. The technique involves a staggered grid system for axial and radial velocities, a line relaxation procedure for efficient solution of the equations, a two-equation k-epsilon turbulence model, a stairstep boundary representation of the expansion flow, and realistic accommodation of swirl effects. A user's manual, dealing with the computational problem, showing how the mathematical basis and computational scheme may be translated into a computer program is presented. A flow chart, FORTRAN IV listing, notes about various subroutines and a user's guide are supplied as an aid to prospective users of the code
Are HI Supershells the Remnants of Gamma-Ray Bursts?
Gamma-Ray Bursts (GRBs) are thought to originate at cosmological distances
from the most powerful explosions in the Universe. If GRBs are not beamed then
the distribution of their number as a function of Gamma-ray flux implies that
they occur once per (0.3-40) million years per bright galaxy and that they
deposit >10^{53} ergs into their surrounding interstellar medium. The blast
wave generated by a GRB explosion would be washed out by interstellar
turbulence only after tens of millions of years when it finally slows down to a
velocity of 10 km/s. This rather long lifetime implies that there could be up
to several tens of active GRB remnants in each galaxy at any given time. For
many years, radio observations have revealed the enigmatic presence of
expanding neutral-hydrogen (HI) supershells of kpc radius in the Milky Way and
in other nearby galaxies. The properties of some supershells cannot be easily
explained in terms of conventional sources such as stellar winds or supernova
explosions. However, the inferred energy and frequency of the explosions
required to produce most of the observed supershells agree with the above GRB
parameters. More careful observations and analysis might reveal which fraction
of these supershells are GRB remnants. We show that if this link is
established, the data on HI supershells can be used to constrain the energy
output, the rate per galaxy, the beaming factor, and the environment of GRB
sources in the Universe.Comment: 8 pages, final version, ApJ Letters, in pres
The Effect of Substructure on Mass Estimates of Galaxies
Large galaxies are thought to form hierarchically, from the accretion and
disruption of many smaller galaxies. Such a scenario should naturally lead to
galactic phase-space distributions containing some degree of substructure. We
examine the errors in mass estimates of galaxies and their dark halos made
using the projected phase-space distribution of a tracer population (such as a
globular cluster system or planetary nebulae) due to falsely assuming that the
tracers are distributed randomly. The level of this uncertainty is assessed by
applying a standard mass estimator to samples drawn from 11 random realizations
of galaxy halos containing levels of substructure consistent with current
models of structure formation. We find that substructure will distort our mass
estimates by up to ~20% - a negligible error compared to statistical and
measurement errors in current derivations of masses for our own and other
galaxies. However, this represents a fundamental limit to the accuracy of any
future mass estimates made under the assumption that the tracer population is
distributed randomly, regardless of the size of the sample or the accuracy of
the measurements.Comment: 9 pages, 8 figures, Astrophysical Journal, in pres
The Active Traveling Wave in the Cochlea
A sound stimulus entering the inner ear excites a deformation of the basilar
membrane which travels along the cochlea towards the apex. It is well
established that this wave-like disturbance is amplified by an active system.
Recently, it has been proposed that the active system consists of a set of
self-tuned critical oscillators which automatically operate at an oscillatory
instability. Here, we show how the concepts of a traveling wave and of
self-tuned critical oscillators can be combined to describe the nonlinear wave
in the cochlea.Comment: 5 pages, 2 figure
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