256 research outputs found
ANNz: estimating photometric redshifts using artificial neural networks
We introduce ANNz, a freely available software package for photometric
redshift estimation using Artificial Neural Networks. ANNz learns the relation
between photometry and redshift from an appropriate training set of galaxies
for which the redshift is already known. Where a large and representative
training set is available ANNz is a highly competitive tool when compared with
traditional template-fitting methods.
The ANNz package is demonstrated on the Sloan Digital Sky Survey Data Release
1, and for this particular data set the r.m.s. redshift error in the range 0 <
z < 0.7 is 0.023. Non-ideal conditions (spectroscopic sets which are small, or
which are brighter than the photometric set for which redshifts are required)
are simulated and the impact on the photometric redshift accuracy assessed.Comment: 6 pages, 6 figures. Replaced to match version accepted by PASP (minor
changes to original submission). The ANNz package may be obtained from
http://www.ast.cam.ac.uk/~aa
Isotope shifts in francium isotopes Fr206-213 and Fr-221
We present the isotope shifts of the 7s(1/2) to 7p(1/2) transition for francium isotopes Fr206-213 with reference to Fr-221 collected from two experimental periods. The shifts are measured on a sample of atoms prepared within a magneto-optical trap by a fast sweep of radio-frequency sidebands applied to a carrier laser. King plot analysis, which includes literature values for 7s(1/2) to 7p(3/2) isotope shifts, provides a field shift constant ratio of 1.0520(10) and a difference between the specific mass shift constants of 170(100) GHz amu between the D-1 and D-2 transitions, of sufficient precision to differentiate between ab initio calculations
Altmetrics and Library Publishing
Altmetrics are a valuable offering that can enhance the services provided by a library publishing program and attract potential publishing partners. This presentation describes the use of altmetrics in the 38 journals published by the University Library System, University of Pittsburgh, as part of its library publishing program. By using a widget from Plum Analytics, altmetrics from each journal article are displayed on abstract pages; furthermore, journal editors have access to a robust dashboard of metrics that allows editors, authors, and readers to access full information about the journal’s impact. Librarians who are part of a library publishing operation have a valuable role to play in training and supporting journal staff and users in the meaning and potential applications of altmetrics, which transforms altmetrics from a component of a publishing program to a service
The Influence of Particle Concentration and Bulk Characteristics on Polarized Oceanographic Lidar Measurements
Oceanographic lidar measurements of the linear depolarization ratio, δ, contain information on the bulk characteristics of marine particles that could improve our ability to study ocean biogeochemistry. However, a scarcity of information on the polarized light-scattering properties of marine particles and the lack of a framework for separating single and multiple scattering effects on δ have hindered the development of polarization-based retrievals of bulk particle properties. To address these knowledge gaps, we made single scattering measurements of δ for several compositionally and morphologically distinct marine particle assemblages. We then used a bio-optical model to explore the influence of multiple scattering and particle characteristics on lidar measurements of δ made during an expedition to sample a mesoscale coccolithophore bloom. Laboratory measurements of linear depolarization revealed a complex dependency on particle shape, size, and composition that were consistent with scattering simulations for idealized nonspherical particles. Model results suggested that the variability in δ measured during the field expedition was driven predominantly by shifts in particle concentration rather than their bulk characteristics. However, model estimates of δ improved when calcite particles were represented by a distinct particle class, highlighting the influence of bulk particle properties on δ. To advance polarized lidar retrievals of bulk particle properties and to constrain the uncertainty in satellite lidar retrievals of particulate backscattering, these results point to the need for future efforts to characterize the variability of particulate depolarization in the ocean and to quantify the sensitivity of operational ocean lidar systems to multiple scattering
Accuracy of photometric redshifts for future weak lensing surveys from space
Photometric redshifts are a key tool to extract as much information as
possible from planned cosmic shear experiments. In this work we aim to test the
performances that can be achieved with observations in the near-infrared from
space and in the optical from the ground. This is done by performing realistic
simulations of multi-band observations of a patch of the sky, and submitting
these mock images to software usually applied to real images to extract the
photometry and then a redshift estimate for each galaxy. In this way we mimic
the most relevant sources of uncertainty present in real data analysis,
including blending and light pollution between galaxies. As an example we adopt
the infrared setup of the ESA-proposed Euclid mission, while we simulate
different observations in the optical, modifying filters, exposure times and
seeing values. Finally, we consider directly some future ground-based
experiments, such as LSST, Pan-Starrs and DES. The results highlight the
importance of u-band observations, especially to discriminate between low (z <
0.5) and high (z ~ 3) redshifts, and the need for good observing sites, with
seeing FWHM < 1. arcsec. The former of these indications clearly favours the
LSST experiment as a counterpart for space observations, while for the other
experiments we need to exclude at least 15 % of the galaxies to reach a
precision in the photo-zs equal to < 0.05.Comment: 11 pages, to be published in MNRAS. Minor changes to match the
published versio
Cosmic shear requirements on the wavelength-dependence of telescope point spread functions
Cosmic shear requires high precision measurement of galaxy shapes in the
presence of the observational Point Spread Function (PSF) that smears out the
image. The PSF must therefore be known for each galaxy to a high accuracy.
However, for several reasons, the PSF is usually wavelength dependent,
therefore the differences between the spectral energy distribution of the
observed objects introduces further complexity. In this paper we investigate
the effect of the wavelength-dependence of the PSF, focusing on instruments in
which the PSF size is dominated by the diffraction-limit of the telescope and
which use broad-band filters for shape measurement.
We first calculate biases on cosmological parameter estimation from cosmic
shear when the stellar PSF is used uncorrected. Using realistic galaxy and star
spectral energy distributions and populations and a simple three-component
circular PSF we find that the colour-dependence must be taken into account for
the next generation of telescopes. We then consider two different methods for
removing the effect (i) the use of stars of the same colour as the galaxies and
(ii) estimation of the galaxy spectral energy distribution using multiple
colours and using a telescope model for the PSF. We find that both of these
methods correct the effect to levels below the tolerances required for per-cent
level measurements of dark energy parameters. Comparison of the two methods
favours the template-fitting method because its efficiency is less dependent on
galaxy redshift than the broad-band colour method and takes full advantage of
deeper photometry.Comment: 10 pages, 8 figures, version accepted for publication in MNRA
Discovery of optically faint obscured quasars with Virtual Observatory tools
We use Virtual Observatory (VO) tools to identify optically faint, obscured
(i.e., type 2) active galactic nuclei (AGN) in the two Great Observatories
Origins Deep Survey (GOODS) fields. By employing publicly available X-ray and
optical data and catalogues we discover 68 type 2 AGN candidates. The X-ray
powers of these sources are estimated by using a previously known correlation
between X-ray luminosity and X-ray-to-optical flux ratio. Thirty-one of our
candidates have high estimated powers (Lx > 10^44 erg/s) and therefore qualify
as optically obscured quasars, the so-called ``QSO 2''. Based on the derived
X-ray powers, our candidates are likely to be at relatively high redshifts, z ~
3, with the QSO 2 at z ~ 4. By going ~ 3 magnitudes fainter than previously
known type 2 AGN in the two GOODS fields we are sampling a region of redshift -
power space which was previously unreachable with classical methods. Our method
brings to 40 the number of QSO 2 in the GOODS fields, an improvement of a
factor ~ 4 when compared to the only 9 such sources previously known. We derive
a QSO 2 surface density down to 10^-15 erg/cm^2/s in the 0.5 - 8 keV band of >~
330/deg^2, ~ 30% of which is made up of previously known sources. This is
larger than current estimates and some predictions and suggests that the
surface density of QSO 2 at faint flux limits has been underestimated. This
work demonstrates that VO tools are mature enough to produce cutting-edge
science results by exploiting astronomical data beyond ``classical''
identification limits (R <~ 25) with interoperable tools for statistical
identification of sources using multiwavelength information.Comment: 16 pages, 6 figures, accepted for publication in Astronomy &
Astrophysics. PDF file with higher resolution figures available at
http://www.eso.org/~ppadovan/AVO-paper.pd
Clustering of Radio Galaxies and Quasars
We compute the cross-correlation between a sample of 14,000 radio-loud AGN
(RLAGN) with redshifts between 0.4 and 0.8 selected from the Sloan Digital Sky
Survey and a reference sample of 1.2 million luminous red galaxies in the same
redshift range. We quantify how the clustering of radio-loud AGN depends on
host galaxy mass and on radio luminosity. Radio-loud AGN are clustered more
strongly on all scales than control samples of radio-quiet galaxies with the
same stellar masses and redshifts, but the differences are largest on scales
less than 1 Mpc. In addition, the clustering amplitude of the RLAGN varies
significantly with radio luminosity on scales less than 1 Mpc. This proves that
the gaseous environment of a galaxy on the scale of its dark matter halo, plays
a key role in determining not only the probability that a galaxy is radio-loud
AGN, but also the total luminosity of the radio jet. Next, we compare the
clustering of radio galaxies with that of radio-loud quasars in the same
redshift range. Unified models predict that both types of active nuclei should
cluster in the same way. Our data show that most RLAGN are clustered more
strongly than radio-loud QSOs, even when the AGN and QSO samples are matched in
both black hole mass and radio luminosity. Only the most extreme RLAGN and
RLQSOs in our sample, with radio luminosities in excess of 10^26 W/Hz, have
similar clustering properties. The majority of the strongly evolving RLAGN
population at z~0.5 are found in different environments to the quasars, and
hence must be triggered by a different physical mechanism.Comment: 12 pages, 13 Figures, submitted to MNRA
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