203 research outputs found
Observing Magnetic Fields on Large Scales
Observations of magnetic fields on scales up to several Mpc are important for
understanding cluster and large-scale structure evolution. Our current census
of such structures is heavily biased -- towards fields of several microG,
towards fields in deep potential wells, and towards high inferred field
strengths in cooling flow and other clusters from improper analysis of rotation
measure data. After reviewing these biases, I show some recent results on two
relics that are powered in very different ways. I describe new investigations
that are now uncovering weak diffuse fields in the outskirts of clusters and
other low density environments, and the good prospects for further progress.Comment: To be published in JKAS, from proceedings of the 3rd Korean
Astrophysics Workshop, 16-20 Aug., 2004, "International Conference on Cosmic
Rays and Magnetic Fields in Large Scale Structure." 7 pages, 7 color figure
The distribution of polarized radio sources 15Jy in GOODS-N
We present deep VLA observations of the polarization of radio sources in the
GOODS-N field at 1.4 GHz at resolutions of 1.6" and 10". At 1.6", we find that
the peak flux cumulative number count distribution is N(p) 45 *
(p/30Jy) per square degree above a detection threshold of 14.5
Jy. This represents a break from the steeper slopes at higher flux
densities, resulting in fewer sources predicted for future surveys with the SKA
and its precursors. It provides a significant challenge for using background
RMs to study clusters of galaxies or individual galaxies. Most of the polarized
sources are well above our detection limit, and are radio galaxies which are
well-resolved even at 10", with redshifts from 0.2 - 1.9. We determined a
total polarized flux for each source by integrating the 10" polarized intensity
maps, as will be done by upcoming surveys such as POSSUM. These total polarized
fluxes are a factor of 2 higher, on average, than the peak polarized flux at
1.6"; this would increase the number counts by 50% at a fixed flux level.
The detected sources have rotation measures (RMs) with a characteristic rms
scatter of 11 around the local Galactic value, after
eliminating likely outliers. The median fractional polarization from all total
intensity sources does not continue the trend of increasing at lower flux
densities, as seen for stronger sources. The changes in the polarization
characteristics seen at these low fluxes likely represent the increasing
dominance of star-forming galaxies.Comment: Published in ApJ; this version contains corrections which will appear
as an Erratum to the published version; 18 pages, 15 figure
Multi-Resolution Imaging and Spectra of Extended Sources
I introduce a straightforward technique for the filtering of extended
astronomical images into components of different spatial scales. For a positive
original image, each component is positive definite, and the sum of all
components equals the original image. In this way, the components are each
individually suitable for flux measurements and broadband spectra calculations.
I present an illustration of this technique on the radio galaxy Cygnus~A.Comment: 4 pages, 1 figure, proceedings from 1999 'Life Cycles of Radio
Galaxies' workshop at STScI in Baltimore, M
The First Measurement of Cassiopeia A's Forward Shock Expansion Rate
We have obtained a second epoch observation of the Cassiopeia A supernova
remnant (SNR) with the Chandra X-ray Observatory to measure detailed X-ray
proper motions for the first time. Both epoch observations are 50 ks exposures
of the ACIS-S3 chip and they are separated by 2 years. Measurements of the thin
X-ray continuum dominated filaments located around the edge of the remnant
(that are identified with the forward shock) show expansion rates from 0.02%/yr
to 0.33%/yr. Many of these filaments are therefore significantly decelerated.
Their median value of 0.21%/yr is equal to the median expansion of the bright
ring (0.21%/yr) as measured with Einstein and ROSAT. This presents a conundrum
if the motion of the bright ring is indicative of the reverse shock speed. We
have also re-evaluated the motion of the radio bright ring with emphasis on
angle-averaged emissivity profiles. Our new measurement of the expansion of the
angle-averaged radio bright ring is 0.07 plus or minus 0.03%/yr, somewhat
slower than the previous radio measurements of 0.11%/yr which were sensitive to
the motions of small-scale features. We propose that the expansion of the
small-scale bright ring features in the optical, X-ray, and radio do not
represent the expansion of the reverse shock, but rather represent a
brightness-weighted average of ejecta passing through and being decelerated by
the reverse shock. The motion of the reverse shock, itself, is then represented
by the motion of the angle-averaged emissivity profile of the radio bright
ring.Comment: accepted to Ap
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