439 research outputs found
Fast high-efficiency integrated waveguide photodetectors using novel hybrid vertical/butt coupling geometry
We report a novel coupling geometry for integrated waveguide photodetectorsâa hybrid vertical coupling/butt coupling scheme that allows the integration of fast, efficient, photodetectors with conventional double heterostructure waveguides. It can be employed to yield a planar, or pseudo-planar, surface that supports further levels of integration. The approach is demonstrated with a 25-”m-long p-i-n detector integrated with an InP/InGaAsP/InP waveguide, which displays a high (~90%) efficiency and large (~15 GHz) bandwidth. This is the fastest high-efficiency integrated waveguide photodetector reported to date
Cosmic shear analysis of archival HST/ACS data: I. Comparison of early ACS pure parallel data to the HST/GEMS Survey
This is the first paper of a series describing our measurement of weak
lensing by large-scale structure using archival observations from the Advanced
Camera for Surveys (ACS) on board the Hubble Space Telescope (HST).
In this work we present results from a pilot study testing the capabilities
of the ACS for cosmic shear measurements with early parallel observations and
presenting a re-analysis of HST/ACS data from the GEMS survey and the GOODS
observations of the Chandra Deep Field South (CDFS). We describe our new
correction scheme for the time-dependent ACS PSF based on observations of
stellar fields. This is currently the only technique which takes the full time
variation of the PSF between individual ACS exposures into account. We estimate
that our PSF correction scheme reduces the systematic contribution to the shear
correlation functions due to PSF distortions to < 2*10^{-6} for galaxy fields
containing at least 10 stars. We perform a number of diagnostic tests
indicating that the remaining level of systematics is consistent with zero for
the GEMS and GOODS data confirming the success of our PSF correction scheme.
For the parallel data we detect a low level of remaining systematics which we
interpret to be caused by a lack of sufficient dithering of the data.
Combining the shear estimate of the GEMS and GOODS observations using 96
galaxies arcmin^{-2} with the photometric redshift catalogue of the GOODS-MUSIC
sample, we determine a local single field estimate for the mass power spectrum
normalisation sigma_{8,CDFS}=0.52^{+0.11}_{-0.15} (stat) +/- 0.07 (sys) (68%
confidence assuming Gaussian cosmic variance) at fixed Omega_m=0.3 for a
LambdaCDM cosmology. We interpret this exceptionally low estimate to be due to
a local under-density of the foreground structures in the CDFS.Comment: Version accepted for publication in Astronomy & Astrophysics with 28
pages, 25 figures. A version with full resolution figures can be downloaded
from http://www.astro.uni-bonn.de/~schrabba/papers/cosmic_shear_acs1_v2.pd
The star formation rate history in the FORS Deep and GOODS South Fields
We measure the star formation rate (SFR) as a function of redshift z up to z
\~4.5, based on B, I and (I+B) selected galaxy catalogues from the FORS Deep
Field (FDF) and the K-selected catalogue from the GOODS-South field. Distances
are computed from spectroscopically calibrated photometric redshifts accurate
to (Delta_z / (z_spec+1)) ~0.03 for the FDF and ~0.056 for the GOODS-South
field. The SFRs are derived from the luminosities at 1500 Angstroem. We find
that the total SFR estimates derived from B, I and I+B catalogues agree very
well (\lsim 0.1 dex) while the SFR from the K catalogue is lower by ~0.2 dex.
We show that the latter is solely due to the lower star-forming activity of
K-selected intermediate and low luminosity (L<L_*) galaxies. The SFR of bright
(L>L_*) galaxies is independent of the selection band, i.e. the same for B, I,
(I+B), and K-selected galaxy samples. At all redshifts, luminous galaxies
(L>L_*) contribute only ~1/3 to the total SFR. There is no evidence for
significant cosmic variance between the SFRs in the FDF and GOODs-South field,
~0.1 dex, consistent with theoretical expectations. The SFRs derived here are
in excellent agreement with previous measurements provided we assume the same
faint-end slope of the luminosity function as previous works (alpha ~ -1.6).
However, our deep FDF data indicate a shallower slope of alpha=-1.07, implying
a SFR lower by ~0.3 dex. We find the SFR to be roughly constant up to z ~4 and
then to decline slowly beyond, if dust extinctions are assumed to be constant
with redshift.Comment: 6 pages, 2 figures, Accepted for publication in ApJ
KiDS-i-800: Comparing weak gravitational lensing measurements in same-sky surveys
We present a weak gravitational lensing analysis of 815 square degree of
-band imaging from the Kilo-Degree Survey (KiDS--800). In contrast to the
deep -band observations, which take priority during excellent seeing
conditions and form the primary KiDS dataset (KiDS--450), the complementary
yet shallower KiDS--800 spans a wide range of observing conditions. The
overlapping KiDS--800 and KiDS--450 imaging therefore provides a unique
opportunity to assess the robustness of weak lensing measurements. In our
analysis, we introduce two new `null' tests. The `nulled' two-point shear
correlation function uses a matched catalogue to show that the calibrated
KiDS--800 and KiDS--450 shear measurements agree at the level of \%. We use five galaxy lens samples to determine a `nulled' galaxy-galaxy
lensing signal from the full KiDS--800 and KiDS--450 surveys and find
that the measurements agree to \% when the KiDS--800 source
redshift distribution is calibrated using either spectroscopic redshifts, or
the 30-band photometric redshifts from the COSMOS survey.Comment: 24 pages, 20 figures. Submitted to MNRAS. Comments welcom
A conspicuous tangential alignment of galaxies in a STIS Parallel Shear Survey field: A new dark-lens candidate ?
We report the serendipituous discovery of a conspicuous alignment of galaxies
in a field obtained through the STIS Parallel Shear Survey. This project
collects randomly distributed 50'' x 50'' fields to investigate the cosmic
shear effect on this scale. Analyzing the parallel observations having the
Seyfert galaxy NGC625 as primary target, we recognized over the whole field of
view a strong apparent tangential alignment of galaxy ellipticities towards the
image center. The field shows several arclet-like features typical for images
of massive galaxy clusters, but no obvious over-density of bright foreground
galaxies. We also find a multiple image candidate. On the basis of the possible
strong and weak lensing effect within the data, we discuss whether this could
be compatible with a massive halo with no clear optical counterpart.Comment: 6 pages, 2 figures, submitted to A&
Photometric Redshifts for Galaxies in the GOODS Southern Field
We use extensive multi-wavelength photometric data from the Great
Observatories Origins Deep Survey (GOODS) to estimate photometric redshifts for
a sample of 434 galaxies with spectroscopic redshifts in the Chandra Deep Field
South. Using the Bayesian method, which incorporates redshift/magnitude priors,
we estimate photometric redshifts for galaxies in the range 18 < R (AB) < 25.5,
giving an rms scatter of 0.11. The outlier fraction is < 10%, with the
outlier-clipped rms being 0.047. We examine the accuracy of photometric
redshifts for several, special sub--classes of objects. The results for
extremely red objects are more accurate than those for the sample as a whole,
with rms of 0.051 and very few outliers (3%). Photometric redshifts for active
galaxies, identified from their X-ray emission, have a dispersion of 0.104,
with 10% outlier fraction, similar to that for normal galaxies. Employing a
redshift/magnitude prior in this process seems to be crucial in improving the
agreement between photometric and spectroscopic redshifts.Comment: Accepted for publication in ApJ
The Cluster Mass Function from Weak Gravitational Lensing
We present the first measurement of the mass function of galaxy clusters
based directly on cluster masses derived from observations of weak
gravitational lensing. To investigate the degree of sample incompleteness
resulting from the X-ray based selection of the target clusters, we use a
sample of 50 clusters with weak lensing mass measurements to empirically
determine the relation between lensing mass and X-ray luminosity and the
scatter about this relation. We use a complete, volume-limited sub-sample of 35
X-ray luminous clusters of galaxies at 0.15<z<0.3 to constrain the abundance of
very massive (M >~ 10^15/h M_sun) clusters. From this, we constrain
sigma_8(Omega_m/0.3)^{0.37} = 0.67^{+0.04}_{-0.05} (68% confidence limits),
agreeing well with constraints from the 3-year WMAP CMB measurements and
estimates of cluster abundances based on X-ray observations, but somewhat lower
than constraints from ``cosmic shear'' weak lensing measurements in random
fields.Comment: 10 pages, 3 figures. Accepted for publication in Ap
Chandra Observations of the QSO Pair Q2345+007: Binary Quasar or Massive Dark Lens?
The components of the wide (7.3") separation quasar pair Q2345+007A,B
(z=2.15) have the most strikingly similar optical spectra seen to date (Steidel
& Sargent 1991) yet no detected lensing mass, making this system the best
candidate known for a massive (1e14 Msun) dark matter lens system. Here we
present results from a 65ksec Chandra observation designed to investigate
whether it is a binary quasar or a gravitational lens. We find no X-ray
evidence for a lensing cluster to a (0.5-2keV) flux limit of 2e-15 cgs, which
is consistent with lensing only for a reduced baryon fraction. Using the
Chandra X-ray observations of the quasars themselves, together with new and
published optical measurements, we use the observed emission properties of the
quasars for further tests between the lens and binary hypotheses. Assuming
similar line-of-sight absorption to the images, we find that their X-ray
continuum slopes are inconsistent (Gamma_A=2.30 and Gamma_B=0.83) as are their
X-ray to optical flux ratios. The probability that B suffers absorption
sufficient to account for these spectral differences is negligible. We present
new optical evidence that the flux ratio of the pair is variable, so the
time-delay in a lens scenario could cause some of the discrepancies. However,
adequately large variations in overall spectral energy distribution are rare in
individual QSOs. All new evidence here weighs strongly toward the binary
interpretation. Q2345+007 thus may represent the highest redshift example known
of interaction-triggered but as-yet unmerged luminous AGN.Comment: 15 pages, Latex, emulateapj style, including 3 tables and 5 figures.
Accepted Feb 1, 2002 for publication in ApJ Main Journal. See also
http://hea-www.harvard.edu/~pgreen/Papers.htm
- âŠ