280 research outputs found
Effect of two-particle correlations on x-ray coherent diffractive imaging studies performed with continuum models
Coherent diffraction imaging (CDI) of single molecules at atomic resolution
is a major goal for the x-ray free electron lasers (XFELs). However, during an
imaging pulse, the fast laser-induced ionization may strongly affect the
recorded diffraction pattern of the irradiated sample. The radiation tolerance
of the imaged molecule should then be investigated 'a priori' with a dedicated
simulation tool. The continuum approach is a powerful tool for modeling the
evolution of irradiated large systems consisting of more than a few hundred
thousand atoms. However, this method follows the evolution of average
single-particle densities, and the experimentally recorded intensities reflect
the spatial two-particle correlations. The information on these correlations is
then inherently not accessible within the continuum approach. In this paper we
analyze this limitation of continuum models and discuss the applicability of
continuum models for imaging studies. We propose a formula to calculate
scattered intensities (including both elastic and inelastic scattering) from
the estimates obtained with a single-particle continuum model. We derive this
formula for systems under conditions typical for CDI studies with XFELs
Discovery of Ultra-Compact Dwarf Galaxies in the Virgo Cluster
We have discovered nine ultra-compact dwarf galaxies (UCDs) in the Virgo
Cluster, extending samples of these objects outside the Fornax Cluster. Using
the 2dF multi-fiber spectrograph on the Anglo-Australian Telescope, the new
Virgo members were found among 1500 color-selected, star-like targets with 16.0
< b_j < 20.2 in a two-degree diameter field centered on M87 (NGC4486). The
newly-found UCDs are comparable to the UCDs in the Fornax Cluster, with sizes
<~ 100 pc, -12.9 < M_B < -10.7, and exhibiting red, absorption-line spectra,
indicative of an older stellar population. The properties of these objects
remain consistent with the tidal threshing model for the origin of UCDs from
the surviving nuclei of nucleated dwarf ellipticals disrupted in the cluster
core, but can also be explained as objects that were formed by mergers of star
clusters created in galaxy interactions. The discovery that UCDs exist in Virgo
shows that this galaxy type is probably a ubiquitous phenomenon in clusters of
galaxies; coupled with their possible origin by tidal threshing, the UCD
population is a potential indicator and probe of the formation history of a
given cluster.
We also describe one additional bright UCD with M_B = -12.0 in the core of
the Fornax Cluster. We find no further UCDs in our Fornax Cluster Spectroscopic
Survey down to b_j = 19.5 in two additional 2dF fields extending as far as 3
degrees from the center of the cluster. All six Fornax bright UCDs identified
with 2dF lie within 0.5 degree (projected distance of 170 kpc) of the central
elliptical galaxy NGC1399.Comment: 26 pages; 12 figures; accepted for publication in the Astronomical
Journa
A Strategy for Finding Near Earth Objects with the SDSS Telescope
We present a detailed observational strategy for finding Near Earth Objects
(NEOs) with the Sloan Digital Sky Survey (SDSS) telescope. We investigate
strategies in normal, unbinned mode as well as binning the CCDs 2x2 or 3x3,
which affects the sky coverage rate and the limiting apparent magnitude. We
present results from 1 month, 3 year and 10 year simulations of such surveys.
For each cadence and binning mode, we evaluate the possibility of achieving the
Spaceguard goal of detecting 90% of 1 km NEOs (absolute magnitude H <= 18 for
an albedo of 0.1). We find that an unbinned survey is most effective at
detecting H <= 20 NEOs in our sample. However, a 3x3 binned survey reaches the
Spaceguard Goal after only seven years of operation. As the proposed large
survey telescopes (PanStarss; LSST) are at least 5-10 years from operation, an
SDSS NEO survey could make a significant contribution to the detection and
photometric characterization of the NEO population.Comment: Accepted by AJ -- 12 pages, 11 figure
An Improved Photometric Calibration of the Sloan Digital Sky Survey Imaging Data
We present an algorithm to photometrically calibrate wide field optical
imaging surveys, that simultaneously solves for the calibration parameters and
relative stellar fluxes using overlapping observations. The algorithm decouples
the problem of "relative" calibrations, from that of "absolute" calibrations;
the absolute calibration is reduced to determining a few numbers for the entire
survey. We pay special attention to the spatial structure of the calibration
errors, allowing one to isolate particular error modes in downstream analyses.
Applying this to the Sloan Digital Sky Survey imaging data, we achieve ~1%
relative calibration errors across 8500 sq.deg. in griz; the errors are ~2% for
the u band. These errors are dominated by unmodelled atmospheric variations at
Apache Point Observatory. These calibrations, dubbed "ubercalibration", are now
public with SDSS Data Release 6, and will be a part of subsequent SDSS data
releases.Comment: 16 pages, 17 figures, matches version accepted in ApJ. These
calibrations are available at http://www.sdss.org/dr
The Sloan Digital Sky Survey Quasar Catalog IV. Fifth Data Release
We present the fourth edition of the Sloan Digital Sky Survey (SDSS) Quasar
Catalog. The catalog contains 77,429 objects; this is an increase of over
30,000 entries since the previous edition. The catalog consists of the objects
in the SDSS Fifth Data Release that have luminosities larger than M_i = -22.0
(in a cosmology with H_0 = 70 km/s/Mpc, Omega_M = 0.3, and Omega_Lambda = 0.7)
have at least one emission line with FWHM larger than 1000 km/s, or have
interesting/complex absorption features, are fainter than i=15.0, and have
highly reliable redshifts. The area covered by the catalog is 5740 sq. deg. The
quasar redshifts range from 0.08 to 5.41, with a median value of 1.48; the
catalog includes 891 quasars at redshifts greater than four, of which 36 are at
redshifts greater than five. Approximately half of the catalog quasars have i <
19; nearly all have i < 21. For each object the catalog presents positions
accurate to better than 0.2 arcsec. rms per coordinate, five-band (ugriz)
CCD-based photometry with typical accuracy of 0.03 mag, and information on the
morphology and selection method. The catalog also contains basic radio,
near-infrared, and X-ray emission properties of the quasars, when available,
from other large-area surveys. The calibrated digital spectra cover the
wavelength region 3800--9200A at a spectral resolution of ~2000. The spectra
can be retrieved from the public database using the information provided in the
catalog. The average SDSS colors of quasars as a function of redshift, derived
from the catalog entries, are presented in tabular form. Approximately 96% of
the objects in the catalog were discovered by the SDSS.Comment: 37 pages, Accepted for publication in A
Inversions of Levy Measures and the Relation Between Long and Short Time Behavior of Levy Processes
The inversion of a Levy measure was first introduced (under a different name)
in Sato 2007. We generalize the definition and give some properties. We then
use inversions to derive a relationship between weak convergence of a Levy
process to an infinite variance stable distribution when time approaches zero
and weak convergence of a different Levy process as time approaches infinity.
This allows us to get self contained conditions for a Levy process to converge
to an infinite variance stable distribution as time approaches zero. We
formulate our results both for general Levy processes and for the important
class of tempered stable Levy processes. For this latter class, we give
detailed results in terms of their Rosinski measures
A New Milky Way Dwarf Galaxy in Ursa Major
In this Letter, we report the discovery of a new dwarf satellite to the Milky
Way, located at () (158.72,51.92) in the
constellation of Ursa Major. This object was detected as an overdensity of red,
resolved stars in Sloan Digital Sky Survey data. The color-magnitude diagram of
the Ursa Major dwarf looks remarkably similar to that of Sextans, the lowest
surface brightness Milky Way companion known, but with approximately an order
of magnitude fewer stars. Deeper follow-up imaging confirms this object has an
old and metal-poor stellar population and is 100 kpc away. We roughly
estimate M -6.75 and 250 pc for this dwarf. Its luminosity is
several times fainter than the faintest known Milky Way dwarf. However, its
physical size is typical for dSphs. Even though its absolute magnitude and size
are presently quite uncertain, Ursa Major is likely the lowest luminosity and
lowest surface brightness galaxy yet known.Comment: Replaced with ApJL accepted version. Includes some additional
details, corrected references, and minor changes to Figure
All naturally occurring autoantibodies against the NMDA receptor subunit NR1 have pathogenic potential irrespective of epitope and immunoglobulin class
An Initial Survey of White Dwarfs in the Sloan Digital Sky Survey
An initial assessment is made of white dwarf and hot subdwarf stars observed
in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees),
observed much like the full survey will be, 269 white dwarfs and 56 hot
subdwarfs are identified spectroscopically where only 44 white dwarfs and 5 hot
subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and
DB (helium) types. In addition, in the full survey to date, a number of WDs
have been found with uncommon spectral types. Among these are blue DQ stars
displaying lines of atomic carbon; red DQ stars showing molecular bands of C_2
with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na,
and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic
field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry
alone allows identification of stars hotter than 12000 K, and the density of
these stars for 15<g<20 is found to be ~2.2 deg^{-2} at Galactic latitudes
29-62 deg. Spectra are obtained for roughly half of these hot stars. The
spectra show that, for 15<g<17, 40% of hot stars are WDs and the fraction of
WDs rises to ~90% at g=20. The remainder are hot sdB and sdO stars.Comment: Accepted for AJ; 43 pages, including 12 figures and 5 table
GAMA/WiggleZ: The 1.4GHz radio luminosity functions of high- and low-excitation radio galaxies and their redshift evolution to z=0.75
We present radio Active Galactic Nuclei (AGN) luminosity functions over the redshift range 0.005 < z < 0.75. The sample from which the luminosity functions are constructed is an optical spectroscopic survey of radio galaxies, identified from matched Faint Images of the Radio Sky at Twenty-cm survey (FIRST) sources and Sloan Digital Sky Survey (SDSS) images.The radio AGN are separated into Low Excitation Radio Galaxies (LERGs) and High Excitation Radio Galaxies (HERGs) using the optical spectra. We derive radio luminosity functions for LERGs and HERGs separately in the three redshift bins (0.005 < z < 0.3, 0.3 < z < 0.5 and 0.5 < z <0.75). The radio luminosity functions can be well described by a double power-law. Assuming this double power-law shape the LERG population displays little or no evolution over this redshift range evolving as ~ assuming pure density evolution or ~ assuming pure luminosity evolution. In contrast, the HERG population evolves more rapidly, best fitted by ~ assuming a double power-law shape and pure density evolution. If a pure luminosity model is assumed the best fitting HERG evolution is parameterised by ~. The characteristic break in the radio luminosity function occurs at a significantly higher power (~1 dex) for the HERG population in comparison to the LERGs. This is consistent with the two populations representing fundamentally different accretion modes
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