1,002 research outputs found
Applications of Wavelets to the Analysis of Cosmic Microwave Background Maps
We consider wavelets as a tool to perform a variety of tasks in the context
of analyzing cosmic microwave background (CMB) maps. Using Spherical Haar
Wavelets we define a position and angular-scale-dependent measure of power that
can be used to assess the existence of spatial structure. We apply planar
Daubechies wavelets for the identification and removal of points sources from
small sections of sky maps. Our technique can successfully identify virtually
all point sources which are above 3 sigma and more than 80% of those above 1
sigma. We discuss the trade-offs between the levels of correct and false
detections. We denoise and compress a 100,000 pixel CMB map by a factor of
about 10 in 5 seconds achieving a noise reduction of about 35%. In contrast to
Wiener filtering the compression process is model independent and very fast. We
discuss the usefulness of wavelets for power spectrum and cosmological
parameter estimation. We conclude that at present wavelet functions are most
suitable for identifying localized sources.Comment: 10 pages, 6 figures. Submitted to MNRA
On the feedback from super stellar clusters. I. The structure of giant HII regions and HII galaxies
We review the structural properties of giant extragalactic HII regions and
HII galaxies based on 2D hydrodynamic calculations, and propose an evolutionary
sequence that accounts for their observed detailed structure. The model assumes
a massive and young stellar cluster surrounded by a large collection of clouds.
These are thus exposed to the most important star-formation feedback
mechanisms: photoionization and the cluster wind. The models show how the two
feedback mechanisms compete in the disruption of clouds and lead to two
different hydrodynamic solutions: The storage of clouds into a long lasting
ragged shell that inhibits the expansion of the thermalized wind, and the
steady filtering of the shocked wind gas through channels carved within the
cloud stratum. Both solutions are claimed to be concurrently at work in giant
HII regions and HII galaxies, causing their detailed inner structure. This
includes multiple large-scale shells, filled with an X-ray emitting gas, that
evolve to finally merge with each other, giving the appearance of shells within
shells. The models also show how the inner filamentary structure of the giant
superbubbles is largely enhanced with matter ablated from clouds and how cloud
ablation proceeds within the original cloud stratum. The calculations point at
the initial contrast density between the cloud and the intercloud media as the
factor that defines which of the two feedback mechanisms becomes dominant
throughout the evolution. Animated version of the models can be found at
http://www.iaa.csic.es/\~{}eperez/ssc/ssc.html.Comment: 28 pages, 10 figures, accepted for publication in the ApJ. Animated
version of the models can be found at
http://www.iaa.csic.es/\~{}eperez/ssc/ssc.htm
The Dipole Observed in the COBE DMR Four-Year Data
The largest anisotropy in the cosmic microwave background (CMB) is the
mK dipole assumed to be due to our velocity with respect to the
CMB. Using the four year data set from all six channels of the COBE
Differential Microwave Radiometers (DMR), we obtain a best-fit dipole amplitude
mK in the direction , where the first
uncertainties are statistical and the second include calibration and combined
systematic uncertainties. This measurement is consistent with previous DMR and
FIRAS resultsComment: New and improved version; to be published in ApJ next mont
Initial Ionization of Compressible Turbulence
We study the effects of the initial conditions of turbulent molecular clouds
on the ionization structure in newly formed H_{ii} regions, using
three-dimensional, photon-conserving radiative transfer in a pre-computed
density field from three-dimensional compressible turbulence. Our results show
that the initial density structure of the gas cloud can play an important role
in the resulting structure of the H_{ii} region. The propagation of the
ionization fronts, the shape of the resulting H_{ii} region, and the total mass
ionized depend on the properties of the turbulent density field. Cuts through
the ionized regions generally show ``butterfly'' shapes rather than spherical
ones, while emission measure maps are more spherical if the turbulence is
driven on scales small compared to the size of the H_{ii} region. The
ionization structure can be described by an effective clumping factor , where is number density of the gas. The larger
the value of , the less mass is ionized, and the more irregular the
H_{ii} region shapes. Because we do not follow dynamics, our results apply only
to the early stage of ionization when the speed of the ionization fronts
remains much larger than the sound speed of the ionized gas, or Alfv\'en speed
in magnetized clouds if it is larger, so that the dynamical effects can be
negligible.Comment: 9 pages, 10 figures, version with high quality color images can be
found in http://research.amnh.org/~yuexing/astro-ph/0407249.pd
Simple Models for Turbulent Self-Regulation in Galaxy Disks
We propose that turbulent heating, wave pressure and gas exchanges between
different regions of disks play a dominant role in determining the preferred,
quasi-equilibrium, self-similar states of gas disks on large-scales. We present
simple families of analytic, thermohydrodynamic models for these global states,
which include terms for turbulent pressure and Reynolds stresses. Star
formation rates, phase balances, and hydrodynamic forces are all tightly
coupled and balanced. The models have stratified radial flows, with the cold
gas slowly flowing inward in the midplane of the disk, and with the warm/hot
phases that surround the midplane flowing outward.
The models suggest a number of results that are in accord with observation,
as well as some novel predictions, including the following. 1) The large-scale
gas density and thermal phase distributions in galaxy disks can be explained as
the result of turbulent heating and spatial couplings. 2) The turbulent
pressures and stresses that drive radial outflows in the warm gas also allow a
reduced circular velocity there. This effect was observed by Swaters, Sancisi
and van der Hulst in NGC 891, a particularly turbulent edge-on disk. The models
predict that the effect should be universal in such disks. 3) They suggest that
a star formation rate like the phenomenological Schmidt Law is the natural
result of global thermohydrodynamical balance, and may not obtain in disks far
from equilibrium. (Abridged)Comment: 37 pages, 1 gif figure, accepted for publication in the Astrophysical
Journa
Roche Lobe Overflow from Dwarf Stellar Systems
We use both analytical analyses and numerical simulations to examine the
evolution of residual gas within tidally-limited dwarf galaxies and globular
clusters. If the gas sound speed exceeds about 10% of the central velocity
dispersion, as is the case for ionized gas within small stellar systems, the
gas shall have significant density at the tidal radius, and the gas may be lost
on timescales as short as a few times the sound crossing time of the system. In
colder systems, the density at the tidal radius is much lower, greatly reducing
the mass loss rate, and the system may retain its gas for a Hubble time. The
tidally removed gas shall follow an orbit close to that of the original host
system, forming an extended stream of ionized, gaseous debris. Tidal mass loss
severely limits the ability of dwarf systems to continuously form stars. The
ordinary gas content in many dwarf galaxies is fully ionized during high
red-shift epochs, possibly preventing star formation in some systems, leading
to the formation of starless, dark-matter concentrations. In either the field
or in the center of galaxy clusters, ionized gas may be retained by dwarf
galaxies, even though its sound speed may be comparable to or even exceed the
velocity dispersion. These processes may help to explain some observed
differences among dwarf galaxy types, as well as observations of the haloes of
massive galaxies.Comment: 28 pages, LaTeX, AASTex macro
The LBT Panoramic View on the Recent Star-Formation Activity in IC2574
We present deep imaging of the star-forming dwarf galaxy IC2574 in the M81
group taken with the Large Binocular Telescope in order to study in detail the
recent star-formation history of this galaxy and to constrain the stellar
feedback on its HI gas. We identify the star-forming areas in the galaxy by
removing a smooth disk component from the optical images. We construct
pixel-by-pixel maps of stellar age and stellar mass surface density in these
regions by comparing their observed colors with simple stellar populations
synthesized with STARBURST99. We find that an older burst occurred about 100
Myr ago within the inner 4 kpc and that a younger burst happened in the last 10
Myr mostly at galactocentric radii between 4 and 8 kpc. We analyze the stellar
populations residing in the known HI holes of IC2574. Our results indicate
that, even at the remarkable photometric depth of the LBT data, there is no
clear one-to-one association between the observed HI holes and the most recent
bursts of star formation in IC2574. The stellar populations formed during the
younger burst are usually located at the periphery of the HI holes and are seen
to be younger than the holes dynamical age. The kinetic energy of the holes
expansion is found to be on average 10% of the total stellar energy released by
the stellar winds and supernova explosions of the young stellar populations
within the holes. With the help of control apertures distributed across the
galaxy we estimate that the kinetic energy stored in the HI gas in the form of
its local velocity dispersion is about 35% of the total stellar energy.Comment: 16 pages, 14 figures, accepted for publication in Ap
Recent expansion of dengue virus serotype 3 in West Africa
Due to non-existing or limited surveillance in Africa, little is known about the epidemiology of dengue illness in the continent. Serological and virological data obtained from returning European travellers is a key complement to this often flawed information. In the past years, dengue 3 virus has emerged in West Africa and has been detected in travellers returning to Europe. The first dengue epidemic in Cape Verde with more than 17,000 cases from September to December 2009 demonstrated that dengue virus is still expanding worldwide to new territories.S
Expansion of W 3(OH)
A direct measurement of the expansion of W 3(OH) is made by comparing Very
Large Array images taken about 10 yr apart. The expansion is anisotropic with a
typical speed of 3 to 5 km/s, indicating a dynamical age of only 2300 yr. These
observations are inconsistent with either the freely expanding shell model or a
simple bow shock model. The most favored model is a slowly expanding shell-like
HII region, with either a fast rarefied flow or another less massive diffuse
ionized region moving towards the observer. There is also a rapidly evolving
source near the projected center of emission, perhaps related to the central
star.Comment: LaTeX file, 28 pages, includes 8 figures. To appear in ApJ in
December 10 (1998) issue. Also available at
http://www.submm.caltech.edu/~kawamura/w3oh_pp.p
Correlated Errors in the COBE DMR Sky Maps
The {\it COBE} DMR sky maps contain low-level correlated noise. We obtain
estimates of the amplitude and pattern of the correlated noise from three
techniques: angular averages of the covariance matrix, Monte Carlo simulations
of two-point correlation functions, and direct analysis of the DMR maps. The
results from the three methods are mutually consistent. The noise covariance
matrix of a DMR sky map is diagonal to an accuracy of better than 1\%. For a
given sky pixel, the dominant noise covariance occurs with the ring of pixels
at an angular separation of due to the separation of the
DMR horns. The mean covariance at is of the
mean variance. Additionally, the variance in a given pixel is greater
than would be expected from a single beam experiment with the same noise
properties. Auto-correlation functions suffer from a
positive bias at while cross-correlations have no bias. Published
{\it COBE} DMR results are not significantly affected by correlated noise. COBE
pre-print 94-Comment: 11 pages + 3 figures, post-script fil
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