1,592 research outputs found
On Optimal Detection of Point Sources in CMB Maps
Point-source contamination in high-precision Cosmic Microwave Background
(CMB) maps severely affects the precision of cosmological parameter estimates.
Among the methods that have been proposed for source detection, wavelet
techniques based on ``optimal'' filters have been proposed.In this paper we
show that these filters are in fact only restrictive cases of a more general
class of matched filters that optimize signal-to-noise ratio and that have, in
general, better source detection capabilities, especially for lower amplitude
sources. These conclusions are confirmed by some numerical experiments.
\keywords{Methods: data analysis -- Methods: statisticalComment: 6 pages, 3 figure
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 pressure confined wind of the massive and compact superstar cluster M82-A1
The observed parameters of the young superstar cluster M82-A1 and its
associated compact HII region are here shown to indicate a low heating
efficiency or immediate loss, through radiative cooling, of a large fraction of
the energy inserted by stellar winds and supernovae during the early evolution
of the cluster. This implies a bimodal hydrodynamic solution which leads to a
reduced mass deposition rate into the ISM, with a much reduced outflow
velocity. Furthermore, to match the observed parameters of the HII region
associated to M82-A1, the resultant star cluster wind is here shown to ought to
be confined by a high pressure interstellar medium. The cluster wind
parameters, as well as the location of the reverse shock, its cooling length
and the radius of the standing outer HII region are derived analytically. All
of these properties are then confirmed with a semi-analytical integration of
the flow equations, which provides us also with the run of the hydrodynamic
variables as a function of radius. The impact of the results is discussed and
extended to other massive and young superstar clusters surrounded by a compact
HII region.Comment: 19 pages, 4 figures, accepted for publication in Ap
On the Extreme Positive Feedback Star-Forming Mode from Massive and Compact Superstar Clusters
The force of gravity acting within the volume occupied by young, compact and
massive superstar clusters, is here shown to drive in situ all the matter
deposited by winds and supernovae into several generations of star formation.
These events are promoted by radiative cooling which drains the thermal energy
of the ejected gas causing its accumulation to then rapidly exceed the
gravitational instability criterion. A detailed account of the integrated
ionizing radiation and mechanical luminosity as a function of time is here
shown to lead to a new stationary solution. In this, the mass deposition rate
, instead of causing a wind as in the adiabatic solution, turns into a
positive feedback star-forming mode equal to the star formation rate. Some of
the implications of this extreme positive feedback mode are discussed.Comment: 4 pages, 4 figures, accepted for publication in The Astrophysical
Journal Letter
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
On the X-ray Emission from Massive Star Clusters and their Evolving Superbubbles
The X-ray emission properties from the hot thermalized plasma that results
from the collisions of individual stellar winds and supernovae ejecta within
rich and compact star clusters are discussed. We propose a simple analytical
way of estimating the X-ray emission generated by super star clusters and
derive an expression that indicates how this X-ray emission depends on the main
cluster parameters. Our model predicts that the X-ray luminosity from the star
cluster region is highly dependent on the star cluster wind terminal speed, a
quantity related to the temperature of the thermalized ejecta.We have also
compared the X-ray luminosity from the SSC plasma with the luminosity of the
interstellar bubbles generated from the mechanical interaction of the high
velocity star cluster winds with the ISM.We found that the hard (2.0 keV - 8.0
keV) X-ray emission is usually dominated by the hotter SSC plasma whereas the
soft (0.3 keV - 2.0 keV) component is dominated by the bubble plasma. This
implies that compact and massive star clusters should be detected as point-like
hard X-ray sources embedded into extended regions of soft diffuse X-ray
emission. We also compared our results with predictions from the population
synthesis models that take into consideration binary systems and found that in
the case of young,massive and compact super star clusters the X-ray emission
from the thermalized star cluster plasma may be comparable or even larger than
that expected from the HMXB population.Comment: 24 pages, 8 figures, Accepted for publication in The Astrophysical
Journa
- âŠ