92 research outputs found
Surveys of Galaxy Clusters with the Sunyaev Zel'dovich Effect
We have created mock Sunyaev-Zel'dovich effect (SZE) surveys of galaxy
clusters using high resolution N-body simulations. To the pure surveys we add
`noise' contributions appropriate to instrument and primary CMB anisotropies.
Applying various cluster finding strategies to these mock surveys we generate
catalogues which can be compared to the known positions and masses of the
clusters in the simulations. We thus show that the completeness and efficiency
that can be achieved depend strongly on the frequency coverage, noise and beam
characteristics of the instruments, as well as on the candidate threshold. We
study the effects of matched filtering techniques on completeness, and bias. We
suggest a gentler filtering method than matched filtering in single frequency
analyses. We summarize the complications that arise when analyzing the SZE
signal at a single frequency, and assess the limitations of such an analysis.
Our results suggest that some sophistication is required when searching for
`clusters' within an SZE map.Comment: 8 pages, 7 figure
Bow Shocks from Neutron Stars: Scaling Laws and HST Observations of the Guitar Nebula
The interaction of high-velocity neutron stars with the interstellar medium
produces bow shock nebulae, where the relativistic neutron star wind is
confined by ram pressure. We present multi-wavelength observations of the
Guitar Nebula, including narrow-band H-alpha imaging with HST/WFPC2, which
resolves the head of the bow shock. The HST observations are used to fit for
the inclination of the pulsar velocity vector to the line of sight, and to
determine the combination of spindown energy loss, velocity, and ambient
density that sets the scale of the bow shock. We find that the velocity vector
is most likely in the plane of the sky. We use the Guitar Nebula and other
observed neutron star bow shocks to test scaling laws for their size and
H-alpha emission, discuss their prevalence, and present criteria for their
detectability in targeted searches. The set of H-alpha bow shocks shows
remarkable consistency, in spite of the expected variation in ambient densities
and orientations. Together, they support the assumption that a pulsar's
spindown energy losses are carried away by a relativistic wind that is
indistinguishable from being isotropic. Comparison of H-alpha bow shocks with
X-ray and nonthermal, radio-synchrotron bow shocks produced by neutron stars
indicates that the overall shape and scaling is consistent with the same
physics. It also appears that nonthermal radio emission and H-alpha emission
are mutually exclusive in the known objects and perhaps in all objects.Comment: 12 pages, 7 figures (3 degraded), submitted to ApJ; minor revisions
and updates in response to referee report. (AASTeX, includes emulateapj5 and
onecolfloat5.
The Origin of X-shaped Radio Galaxies: Clues from the Z-symmetric Secondary Lobes
Existing radio images of a few X-shaped radio galaxies reveal Z-symmetric
morphologies in their weaker secondary lobes which cannot be naturally
explained by either the galactic merger or radio-lobe backflow scenarios, the
two dominant models for these X-shaped radio sources. We show that the merger
picture can explain these morphologies provided one takes into account that,
prior to the coalescence of their supermassive black holes, the smaller galaxy
releases significant amounts of gas into the ISM of the dominant active galaxy.
This rotating gas, whose angular momentum axis will typically not be aligned
with the original jets, is likely to provide sufficient ram pressure at a
distance ~10 kpc from the nucleus to bend the extant jets emerging from the
central engine, thus producing a Z-symmetry in the pair of radio lobes. Once
the two black holes have coalesced some 10^7 yr later, a rapid reorientation of
the jets along a direction close to that of the orbital angular momentum of the
swallowed galaxy relative to the primary galaxy would create the younger
primary lobes of the X-shaped radio galaxy. This picture naturally explains why
such sources typically have powers close to the FR I/II break. We suggest that
purely Z-symmetric radio sources are often en route to coalescence and the
concomitant emission of substantial gravitational radiation, while X-shaped
ones have already merged and radiated.Comment: 12 pages, 1 compressed figure; accepted for publication in ApJ
Letter
The Radio Quiescence of Active Galaxies with High Accretion Rates
We present 6 cm Very Large Array observations of the Greene & Ho (2004)
sample of 19 low-mass active galaxies with high accretion rates. This is one of
the only studies of a uniform sample of narrow-line Seyfert 1 (NLS1) galaxies
with such high sensitivity and resolution. Although we detect only one source,
the entire sample is very radio-quiet down to strong limits. GH10 was found to
have a radio power of 8.5 x 10^21 W/Hz, and a ratio R = f(6 cm)/f(4400 A) of
2.8. The 3 sigma upper limits for the remaining nondetections correspond to
radio powers from 3 x 10^20 to 8 x 10^21 W/Hz and 0.47 < R <9.9. Stacking all
nondetections yields an even stronger upper limit of R < 0.27. An assessment of
existing observations in the literature confirms our finding that NLS1s are
consistently radio-quiet, with a radio-loud fraction of 0%-6%, which is
significantly lower than the 10%-20% observed in the general quasar population.
By analogy with stellar-mass black holes, we argue that AGNs undergo a state
transition at L_bol/L_Edd~0.01. Below this value a radiatively inefficient
accretion flow effectively drives an outflow, which disappears when the flow
turns into an optically thick, geometrically thin disk, or a radiation
pressure-dominated slim disk at still higher L_bol/L_Edd.Comment: To appear in ApJ; 8 pages, 3 figures; uses emulateapj5.st
Luminosity Evolution of Early-type Galaxies to z=0.83: Constraints on Formation Epoch and Omega
We present deep spectroscopy with the Keck telescope of eight galaxies in the
luminous X-ray cluster MS1054-03 at z=0.83. The data are combined with imaging
observations from the Hubble Space Telescope (HST). The spectroscopic data are
used to measure the internal kinematics of the galaxies, and the HST data to
measure their structural parameters. Six galaxies have early-type spectra, and
two have "E+A" spectra. The galaxies with early-type spectra define a tight
Fundamental Plane (FP) relation. The evolution of the mass-to-light ratio is
derived from the FP. The M/L ratio evolves as \Delta log M/L_B \propto -0.40 z
(Omega_m=0.3, Omega_Lambda=0). The observed evolution of the M/L ratio provides
a combined constraint on the formation redshift of the stars, the IMF, and
cosmological parameters. For a Salpeter IMF (x=2.35) we find that z_form>2.8
and Omega_m<0.86 with 95% confidence. The constraint on the formation redshift
is weaker if Omega_Lambda>0: z_form>1.7 if Omega_m=0.3 and Omega_Lambda=0.7. At
present the limiting factor in constraining z_form and Omega from the observed
luminosity evolution of early-type galaxies is the poor understanding of the
IMF. We find that if Omega_m=1 the IMF must be significantly steeper than the
Salpeter IMF (x>2.6).Comment: To be published in ApJ Letters, Volume 504, September 1, 1998. 5
pages, 4 figure
Shape Reconstruction and Weak Lensing Measurement with Interferometers: A Shapelet Approach
We present a new approach for image reconstruction and weak lensing
measurements with interferometers. Based on the shapelet formalism presented in
Refregier (2001), object images are decomposed into orthonormal Hermite basis
functions. The shapelet coefficients of a collection of sources are
simultaneously fit on the uv plane, the Fourier transform of the sky brightness
distribution observed by interferometers. The resulting chi-square fit is
linear in its parameters and can thus be performed efficiently by simple matrix
multiplications. We show how the complex effects of bandwidth smearing, time
averaging and non-coplanarity of the array can be easily and fully corrected
for in our method. Optimal image reconstruction, co-addition, astrometry, and
photometry can all be achieved using weighted sums of the derived coefficients.
As an example we consider the observing conditions of the FIRST radio survey
(Becker et al. 1995; White et al. 1997). We find that our method accurately
recovers the shapes of simulated images even for the sparse uv sampling of this
snapshot survey. Using one of the FIRST pointings, we find our method compares
well with CLEAN, the commonly used method for interferometric imaging. Our
method has the advantage of being linear in the fit parameters, of fitting all
sources simultaneously, and of providing the full covariance matrix of the
coefficients, which allows us to quantify the errors and cross-talk in image
shapes. It is therefore well-suited for quantitative shape measurements which
require high-precision. In particular, we show how our method can be combined
with the results of Refregier & Bacon (2001) to provide an accurate measurement
of weak lensing from interferometric data.Comment: 9 pages, 5 figures. To appear in ApJ; minor changes to match accepted
versio
Optimal Image Reconstruction in Radio Interferometry
We introduce a method for analyzing radio interferometry data which produces
maps which are optimal in the Bayesian sense of maximum posterior probability
density, given certain prior assumptions. It is similar to maximum entropy
techniques, but with an exact accounting of the multiplicity instead of the
usual approximation involving Stirling's formula. It also incorporates an Occam
factor, automatically limiting the effective amount of detail in the map to
that justified by the data. We use Gibbs sampling to determine, to any desired
degree of accuracy, the multi-dimensional posterior density distribution. From
this we can construct a mean posterior map and other measures of the posterior
density, including confidence limits on any well-defined function of the
posterior map.Comment: 41 pages, 11 figures. High resolution figures 8 and 9 available at
http://www.astro.uiuc.edu/~bwandelt/SuttonWandelt200
A One-sided, Highly Relativistic Jet from Cygnus X-3
Very Long Baseline Array images of the X-ray binary, Cygnus X-3, were
obtained 2, 4 and 7 days after the peak of a 10 Jy flare on 4 February 1997.
The first two images show a curved one-sided jet, the third a scatter-broadened
disc, presumably at the position of the core. The jet curvature changes from
the first to the second epoch, which strongly suggests a precessing jet. The
ratio of the flux density in the approaching to that in the (undetected)
receding jet is > 330; if this asymmetry is due to Doppler boosting, the
implied jet speed is > 0.81c. Precessing jet model fits, together with the
assumptions that the jet is intrinsically symmetric and was ejected during or
after the major flare, yield the following constraints: the jet inclination to
the line of sight must be < 14 degrees; the cone opening angle must be < 12
degrees; and the precession period must be > 60 days.Comment: 12 pages 7 figures, accepted by Ap
Further Investigation of the Time Delay, Magnification Ratios, and Variability in the Gravitational Lens 0218+357
High precision VLA flux density measurements for the lensed images of
0218+357 yield a time delay of 10.1(+1.5-1.6)days (95% confidence). This is
consistent with independent measurements carried out at the same epoch (Biggs
et al. 1999), lending confidence in the robustness of the time delay
measurement. However, since both measurements make use of the same features in
the light curves, it is possible that the effects of unmodelled processes, such
as scintillation or microlensing, are biasing both time delay measurements in
the same way. Our time delay estimates result in confidence intervals that are
somewhat larger than those of Biggs et al., probably because we adopt a more
general model of the source variability, allowing for constant and variable
components. When considered in relation to the lens mass model of Biggs et al.,
our best-fit time delay implies a Hubble constant of H_o = 71(+17-23) km/s-Mpc
for Omega_o=1 and lambda_o=0 (95% confidence; filled beam). This confidence
interval for H_o does not reflect systematic error, which may be substantial,
due to uncertainty in the position of the lens galaxy. We also measure the flux
ratio of the variable components of 0218+357, a measurement of a small region
that should more closely represent the true lens magnification ratio. We find
ratios of 3.2(+0.3-0.4) (95% confidence; 8 GHz) and 4.3(+0.5-0.8) (15 GHz).
Unlike the reported flux ratios on scales of 0.1", these ratios are not
strongly significantly different. We investigate the significance of apparent
differences in the variability properties of the two images of the background
active galactic nucleus. We conclude that the differences are not significant,
and that time series much longer than our 100-day time series will be required
to investigate propagation effects in this way.Comment: 33 pages, 9 figures. Accepted for publication in ApJ. Light curve
data may be found at http://space.mit.edu/RADIO/papers.htm
Bootstrap resampling as a tool for radio-interferometric imaging fidelity assessment
We report on a numerical evaluation of the statistical bootstrap as a
technique for radio-interferometric imaging fidelity assessment. The
development of a fidelity assessment technique is an important scientific
prerequisite for automated pipeline reduction of data from modern radio
interferometers. We evaluate the statistical performance of two bootstrap
methods, the model-based bootstrap and subsample bootstrap, against a Monte
Carlo parametric simulation, using interferometric polarization calibration and
imaging as the representative problem under study. We find both statistical
resampling techniques to be viable approaches to radio-interferometric imaging
fidelity assessment which merit further investigation. We also report on the
development and implementation of a new self-calibration algorithm for
radio-interferometric polarimetry which makes no approximations for the
polarization source model.Comment: Accepted by AJ; 41 pages, 13 figure
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