9,221 research outputs found
Speckle Statistics in Adaptively Corrected Images
(abridged) Imaging observations are generally affected by a fluctuating
background of speckles, a particular problem when detecting faint stellar
companions at small angular separations. Knowing the distribution of the
speckle intensities at a given location in the image plane is important for
understanding the noise limits of companion detection. The speckle noise limit
in a long-exposure image is characterized by the intensity variance and the
speckle lifetime. In this paper we address the former quantity through the
distribution function of speckle intensity. Previous theoretical work has
predicted a form for this distribution function at a single location in the
image plane. We developed a fast readout mode to take short exposures of
stellar images corrected by adaptive optics at the ground-based UCO/Lick
Observatory, with integration times of 5 ms and a time between successive
frames of 14.5 ms ( m). These observations temporally
oversample and spatially Nyquist sample the observed speckle patterns. We show,
for various locations in the image plane, the observed distribution of speckle
intensities is consistent with the predicted form. Additionally, we demonstrate
a method by which and can be mapped over the image plane. As the
quantity is proportional to the PSF of the telescope free of random
atmospheric aberrations, this method can be used for PSF calibration and
reconstruction.Comment: 7 pages, 4 figures, ApJ accepte
X-ray Isophote Shapes and the Mass of NGC 3923
We present analysis of the shape and radial mass distribution of the E4
galaxy NGC 3923 using archival X-ray data from the ROSAT PSPC and HRI. The
X-ray isophotes are significantly elongated with ellipticity e_x=0.15
(0.09-0.21) (90% confidence) for semi-major axis a\sim 10h^{-1}_70 kpc and have
position angles aligned with the optical isophotes within the estimated
uncertainties. Applying the Geometric Test for dark matter, which is
independent of the gas temperature profile, we find that the ellipticities of
the PSPC isophotes exceed those predicted if M propto L at a marginal
significance level of 85% (80%) for oblate (prolate) symmetry. Detailed
hydrostatic models of an isothermal gas yield ellipticities for the gravitating
matter, e_mass=0.35-0.66 (90% confidence), which exceed the intensity weighted
ellipticity of the R-band optical light, = 0.30 (e_R^max=0.39).
We conclude that mass density profiles with rho\sim r^{-2} are favored over
steeper profiles if the gas is essentially isothermal (which is suggested by
the PSPC spectrum) and the surface brightness in the central regions (r<~15")
is not modified substantially by a multi-phase cooling flow, magnetic fields,
or discrete sources. We argue that these effects are unlikely to be important
for NGC 3923. (The derived e_{mass} range is very insensitive to these issues.)
Our spatial analysis also indicates that the allowed contribution to the ROSAT
emission from a population of discrete sources with Sigma_x propto Sigma_R is
significantly less than that indicated by the hard spectral component measured
by ASCA.Comment: 14 pages (6 figures), To Appear in MNRA
Diamonds on the Hat: Globular Clusters in The Sombrero Galaxy (M104)
Images from the HST ACS are used to carry out a new photometric study of the
globular clusters (GCs) in M104, the Sombrero galaxy. The primary focus of our
study is the characteristic distribution function of linear sizes (SDF) of the
GCs. We measure the effective radii for 652 clusters with PSF-convolved King
and Wilson dynamical model fits. The SDF is remarkably similar to those
measured for other large galaxies of all types, adding strong support to the
view that it is a "universal" feature of globular cluster systems.
We develop a more general interpretation of the size distribution function
for globular clusters, proposing that the shape of the SDF that we see today
for GCs is strongly influenced by the early rapid mass loss during their star
forming stage, coupled with stochastic differences from cluster to cluster in
the star formation efficiency (SFE) and their initial sizes. We find that the
observed SDF shape can be accurately predicted by a simple model in which the
protocluster clouds had characteristic sizes of pc and SFEs of
. The colors and luminosities of the M104 clusters show the
clearly defined classic bimodal form. The blue sequence exhibits a
mass/metallicity relation (MMR), following a scaling of heavy-element abundance
with luminosity of very similar to what has been found in most
giant elliptical galaxies. A quantitative self-enrichment model provides a good
first-order match to the data for the same initial SFE and protocluster size
that were required to explain the SDF. We also discuss various forms of the
globular cluster Fundamental Plane (FP) of structural parameters, and show that
useful tests of it can be extended to galaxies beyond the Local Group.Comment: In press for MNRA
The Origin of the Intrinsic Scatter in the Relation Between Black Hole Mass and Bulge Luminosity for Nearby Active Galaxies
We investigate the origin of the intrinsic scatter in the correlation between
black hole mass (MBH) and bulge luminosity [L(bulge)] in a sample of 45
massive, local (z < 0.35) type~1 active galactic nuclei (AGNs). We derive MBH
from published optical spectra assuming a spherical broad-line region, and
L(bulge) from detailed two-dimensional decomposition of archival optical Hubble
Space Telescope images. AGNs follow the MBH-L(bulge) relation of inactive
galaxies, but the zero point is shifted by an average of \Delta log MBH ~ -0.3
dex. We show that the magnitude of the zero point offset, which is responsible
for the intrinsic scatter in the MBH-L(bulge) relation, is correlated with
several AGN and host galaxy properties, all of which are ultimately related to,
or directly impact, the BH mass accretion rate. At a given bulge luminosity,
sources with higher Eddington ratios have lower MBH. The zero point offset can
be explained by a change in the normalization of the virial product used to
estimate MBH, in conjunction with modest BH growth (~ 10%--40%) during the AGN
phase. Galaxy mergers and tidal interactions appear to play an important role
in regulating AGN fueling in low-redshift AGNs.Comment: To appear in ApJ; 67 pages, 56 figures, 4 tables, version with full
resolution figures at http://users.ociw.edu/mjkim/papers/scatter.pd
PILOT: a balloon-borne experiment to measure the polarized FIR emission of dust grains in the interstellar medium
Future cosmology space missions will concentrate on measuring the
polarization of the Cosmic Microwave Background, which potentially carries
invaluable information about the earliest phases of the evolution of our
universe. Such ambitious projects will ultimately be limited by the sensitivity
of the instrument and by the accuracy at which polarized foreground emission
from our own Galaxy can be subtracted out. We present the PILOT balloon project
which will aim at characterizing one of these foreground sources, the
polarization of the dust continuum emission in the diffuse interstellar medium.
The PILOT experiment will also constitute a test-bed for using multiplexed
bolometer arrays for polarization measurements. We present the results of
ground tests obtained just before the first flight of the instrument.Comment: 17 pages, 13 figures. Presented at SPIE, Millimeter, Submillimeter,
and Far-Infrared Detectors and Instrumentation for Astronomy VII. To be
published in Proc. SPIE volume 915
Hubble and Keck Telescope Observations of Active Asteroid 288P/300163 (2006 VW139)
We present Hubble Space Telescope and Keck 10 meter telescope observations of
active asteroid 288P/300163 (2006 VW139) taken to examine ejected dust. The
nucleus is a C-type object with absolute magnitude = 17.00.1 and
estimated diameter 2.6 km (for assumed visual geometric albedo =
0.04). Variations in the brightness of the nucleus at the 10% to 15% level are
significant in both 2011 December and 2012 October but we possess too few data
to distinguish variations caused by activity from those caused by rotation. The
dust scattering cross-section in 2011 December is 40 km,
corresponding to a dust mass 910 kg (88 m mean particle
radius assumed). The full width at half maximum of the debris sheet varies from
100 km near the nucleus to 1000 km 30arcsec (40,000 km) east of it.
Dust dynamical models indicate ejection speeds between 0.06 and 0.3 m s,
particle sizes between 10 and 300 m and an inverse square-root relation
between particle size and velocity. Overall, the data are most simply explained
by prolonged, low velocity ejection of dust, starting in or before 2011 July
and continuing until at least 2011 October. These properties are consistent
with the sublimation of near-surface ice aided by centrifugal forces. The high
spatial resolution of our HST images (52 km per pixel) reveals details that
remained hidden in previous ground-based observations, such as the
extraordinarily small vertical extent of the dust sheet, ejection speeds well
below the nucleus escape speed, and the possibility of a binary nucleus.Comment: Accepted for publication by A
Quasi-equilibrium optical nonlinearities in spin-polarized GaAs
Semiconductor Bloch equations, which microscopically describe the dynamics of
a Coulomb interacting, spin-unpolarized electron-hole plasma, can be solved in
two limits: the coherent and the quasi-equilibrium regime. These equations have
been recently extended to include the spin degree of freedom, and used to
explain spin dynamics in the coherent regime. In the quasi-equilibrium limit,
one solves the Bethe-Salpeter equation in a two-band model to describe how
optical absorption is affected by Coulomb interactions within a
spin-unpolarized plasma of arbitrary density. In this work, we modified the
solution of the Bethe-Salpeter equation to include spin-polarization and light
holes in a three-band model, which allowed us to account for spin-polarized
versions of many-body effects in absorption. The calculated absorption
reproduced the spin-dependent, density-dependent and spectral trends observed
in bulk GaAs at room temperature, in a recent pump-probe experiment with
circularly polarized light. Hence our results may be useful in the microscopic
modelling of density-dependent optical nonlinearities in spin-polarized
semiconductors.Comment: 7 pages, 6 figure
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