5,650 research outputs found
The Second-Generation Guide Star Catalog: Description and Properties
The GSC-II is an all-sky database of objects derived from the uncompressed
DSS that the STScI has created from the Palomar and UK Schmidt survey plates
and made available to the community. Like its predecessor (GSC-I), the GSC-II
was primarily created to provide guide star information and observation
planning support for HST. This version, however, is already employed at some of
the ground-based new-technology telescopes such as GEMINI, VLT, and TNG, and
will also be used to provide support for the JWST and Gaia space missions as
well as LAMOST, one of the major ongoing scientific projects in China. Two
catalogs have already been extracted from the GSC-II database and released to
the astronomical community. A magnitude-limited (R=18.0) version, GSC2.2, was
distributed soon after its production in 2001, while the GSC2.3 release has
been available for general access since 2007.
The GSC2.3 catalog described in this paper contains astrometry, photometry,
and classification for 945,592,683 objects down to the magnitude limit of the
plates. Positions are tied to the ICRS; for stellar sources, the all-sky
average absolute error per coordinate ranges from 0.2" to 0.28" depending on
magnitude. When dealing with extended objects, astrometric errors are 20% worse
in the case of galaxies and approximately a factor of 2 worse for blended
images. Stellar photometry is determined to 0.13-0.22 mag as a function of
magnitude and photographic passbands (B,R,I). Outside of the galactic plane,
stellar classification is reliable to at least 90% confidence for magnitudes
brighter than R=19.5, and the catalog is complete to R=20.Comment: 52 pages, 33 figures, to be published in AJ August 200
A QSO survey via optical variability and zero proper motion in the M92 field.III. Narrow emission line galaxies
We study a sample of 23 narrow-emission line galaxies (NELGs) which were
selected by their strong variability as QSO candidates in the framework of a
variability-and-proper motion QSO survey on digitised Schmidt plates. In
previous work, we have shown that variability is an efficient method to find
AGNs. The variability properties of the NELGs are however significantly
different from those of the QSOs. The main aim of this paper is to clarify the
nature of this variability and to estimate the fraction of AGN-dominated NELGs
in this sample. New photometric and spectroscopic observations are presented,
along with revised data from the photographic photometry. The originally
measured high variability indices could not be confirmed. The diagnostic
line-ratios of the NELG spectra are consistent with HII region-like spectra. No
AGN could be proved, yet we cannot rule out the existence of faint
low-luminosity AGNs masked by HII regions from intense star formation.Comment: 10 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Decision Tree Classifiers for Star/Galaxy Separation
We study the star/galaxy classification efficiency of 13 different decision
tree algorithms applied to photometric objects in the Sloan Digital Sky Survey
Data Release Seven (SDSS DR7). Each algorithm is defined by a set of parameters
which, when varied, produce different final classification trees. We
extensively explore the parameter space of each algorithm, using the set of
SDSS objects with spectroscopic data as the training set. The
efficiency of star-galaxy separation is measured using the completeness
function. We find that the Functional Tree algorithm (FT) yields the best
results as measured by the mean completeness in two magnitude intervals: () and (). We compare the performance of the
tree generated with the optimal FT configuration to the classifications
provided by the SDSS parametric classifier, 2DPHOT and Ball et al. (2006). We
find that our FT classifier is comparable or better in completeness over the
full magnitude range , with much lower contamination than all but
the Ball et al. classifier. At the faintest magnitudes (), our classifier
is the only one able to maintain high completeness (80%) while still
achieving low contamination (). Finally, we apply our FT classifier
to separate stars from galaxies in the full set of SDSS
photometric objects in the magnitude range .Comment: Submitted to A
A QSO survey via optical variability and zero proper motion in the M92 field. IV. More QSOs due to improved photometry
We continue the QSO search in the 10 square degrees Schmidt field around M92
based on variability and proper motion (VPM) constraints. We have re-reduced
162 digitised B plates with a time-baseline of more than three decades and have
considerably improved both the photometric accuracy and the star-galaxy
separation at B>19. QSO candidates are selected and marked with one out of
three degrees of priority based on the statistical significance of their
measured variability and zero proper motion. Spectroscopic follow-up
observations of 84 new candidates with B>19 revealed an additional 37 QSOs and
7 Seyfert1s. In particular, all 92 high-priority candidates are
spectroscopically classified now; among them are 70 QSOs and 9 Seyfert1s
(success rate 86%). We expect that 87% (55%) of all QSOs with B<19.0 (19.8) are
contained in this high-priority subsample. For the combined sample of
high-priority and medium-priority objects, a completeness of 89% is estimated
up to B_lim=19.5. The sample of all AGNs detected in the framework of the VPM
search in the M92 field contains now 95 QSOs and 14 Seyfert1s with B<19.9.
Although the VPM QSOs were selected by completely different criteria, their
properties do not significantly differ from those of QSOs found by more
traditional optical survey techniques. In particular, the spectra and the
optical broad band colours do not provide any hints on a substantial population
of red QSOs up to the present survey limit.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Wide Field Imaging. I. Applications of Neural Networks to object detection and star/galaxy classification
[Abriged] Astronomical Wide Field Imaging performed with new large format CCD
detectors poses data reduction problems of unprecedented scale which are
difficult to deal with traditional interactive tools. We present here NExt
(Neural Extractor): a new Neural Network (NN) based package capable to detect
objects and to perform both deblending and star/galaxy classification in an
automatic way. Traditionally, in astronomical images, objects are first
discriminated from the noisy background by searching for sets of connected
pixels having brightnesses above a given threshold and then they are classified
as stars or as galaxies through diagnostic diagrams having variables choosen
accordingly to the astronomer's taste and experience. In the extraction step,
assuming that images are well sampled, NExt requires only the simplest a priori
definition of "what an object is" (id est, it keeps all structures composed by
more than one pixels) and performs the detection via an unsupervised NN
approaching detection as a clustering problem which has been thoroughly studied
in the artificial intelligence literature. In order to obtain an objective and
reliable classification, instead of using an arbitrarily defined set of
features, we use a NN to select the most significant features among the large
number of measured ones, and then we use their selected features to perform the
classification task. In order to optimise the performances of the system we
implemented and tested several different models of NN. The comparison of the
NExt performances with those of the best detection and classification package
known to the authors (SExtractor) shows that NExt is at least as effective as
the best traditional packages.Comment: MNRAS, in press. Paper with higher resolution images is available at
http://www.na.astro.it/~andreon/listapub.htm
A New Wolf-Rayet Star and its Ring Nebula: PCG11
In a search for new Galactic planetary nebulae from our systematic scans of
the Anglo-Australian Observatory/United Kingdom Schmidt Telescope (AAO/UKST)
Halpha survey of the Southern Galactic Plane, we have identified a Pop I
Wolf-Rayet star of type WN7h associated with an unusual ring nebula that has a
fractured rim. We present imagery in Halpha, the 843-MHz continuum from the
Molonglo Observatory Synthesis Telescope (MOST), the mid-infrared from the
Midcourse Space Experiment (MSX), and confirmatory optical spectroscopy of the
character of the nebula and of its central star. The inner edge of the Halpha
shell shows gravitational instabilities with a well-defined wavelength around
its complete circumference.Comment: 13 pages, LaTeX (mn2e.cls), incl. 9 PostScript (ps or eps) figures
and 3 tables. Accepted by MNRA
The Fornax Spectroscopic Survey I. Survey Strategy and Preliminary Results on the Redshift Distribution of a Complete Sample of Stars and Galaxies
The Fornax Spectroscopic Survey will use the Two degree Field spectrograph
(2dF) of the Anglo-Australian Telescope to obtain spectra for a complete sample
of all 14000 objects with 16.5<=Bj<=19.7 in a 12 square degree area centred on
the Fornax Cluster. By selecting all objects---both stars and
galaxies---independent of morphology, we cover a much larger range of surface
brightness and scale size than previous surveys. In this paper we present
results from the first 2dF field. Redshift distributions and velocity
structures are shown for all observed objects in the direction of Fornax,
including Galactic stars, galaxies in and around the Fornax Cluster, and for
the background galaxy population. The velocity data for the stars show the
contributions from the different Galactic components, plus a small tail to high
velocities. We find no galaxies in the foreground to the cluster in our 2dF
field. The Fornax Cluster is clearly defined kinematically. The mean velocity
from the 26 cluster members having reliable redshifts is 1560+/-80 km/s. They
show a velocity dispersion of 380+/-50 km/s. Large-scale structure can be
traced behind the cluster to a redshift beyond z=0.3. Background compact
galaxies and low surface brightness galaxies are found to follow the general
galaxy distribution.Comment: LaTeX format; uses aa.cls (included). Accepted for publication in
Astronomy and Astrophysic
Perceptual Image Similarity Metrics and Applications.
This dissertation presents research in perceptual image similarity metrics and applications, e.g., content-based image retrieval, perceptual image compression, image similarity assessment and texture analysis.
The first part aims to design texture similarity metrics consistent with human perception. A new family of statistical texture similarity features, called Local Radius Index (LRI), and corresponding similarity metrics are proposed. Compared to state-of-the-art metrics in the STSIM family, LRI-based metrics achieve better texture retrieval performance with much less computation. When applied to the recently developed perceptual image coder, Matched Texture Coding (MTC), they enable similar performance while significantly accelerating encoding. Additionally, in photographic paper classification, LRI-based metrics also outperform pre-existing metrics. To fulfill the needs of texture classification and other applications, a rotation-invariant version of LRI, called Rotation-Invariant Local Radius Index (RI-LRI), is proposed. RI-LRI is also grayscale and illuminance insensitive. The corresponding similarity metric achieves texture classification accuracy comparable to state-of-the-art metrics. Moreover, its much lower dimensional feature vector requires substantially less computation and storage than other state-of-the-art texture features.
The second part of the dissertation focuses on bilevel images, which are images whose pixels are either black or white. The contributions include new objective similarity metrics intended to quantify similarity consistent with human perception, and a subjective experiment to obtain ground truth for judging the performance of objective metrics. Several similarity metrics are proposed that outperform existing ones in the sense of attaining significantly higher Pearson and Spearman-rank correlations with the ground truth. The new metrics include Adjusted Percentage Error, Bilevel Gradient Histogram, Connected Components Comparison and combinations of such.
Another portion of the dissertation focuses on the aforementioned MTC, which is a block-based image coder that uses texture similarity metrics to decide if blocks of the image can be encoded by pointing to perceptually similar ones in the already coded region. The key to its success is an effective texture similarity metric, such as an LRI-based metric, and an effective search strategy. Compared to traditional image compression algorithms, e.g., JPEG, MTC achieves similar coding rate with higher reconstruction quality. And the advantage of MTC becomes larger as coding rate decreases.PhDElectrical Engineering: SystemsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113586/1/yhzhai_1.pd
A tidal disruption flare in a massive galaxy? Implications for the fuelling mechanisms of nuclear black holes
We argue that the `changing look' AGN recently reported by LaMassa et al.
could be a luminous flare produced by the tidal disruption of a super-solar
mass star passing just a few gravitational radii outside the event horizon of a
nuclear black hole. This flare occurred in a massive,
star forming galaxy at redshift , robustly characterized thanks to
repeated late-time photometric and spectroscopic observations. By taking
difference-photometry of the well sampled multi-year SDSS Stripe-82
light-curve, we are able to probe the evolution of the nuclear spectrum over
the course of the outburst. The tidal disruption event (TDE) interpretation is
consistent with the very rapid rise and the decay time of the flare, which
displays an evolution consistent with the well-known behaviour (with
a clear superimposed re-brightening flare). Our analysis places constraints on
the physical properties of the TDE, such as the putative disrupted star's mass
and orbital parameters, as well as the size and temperature of the emitting
material. The properties of the broad and narrow emission lines observed in two
epochs of SDSS spectra provide further constraints on the circum-nuclear
structure, and could be indicative that the system hosted a moderate-luminosity
AGN as recently as a few years ago, and is likely undergoing residual
accretion as late as ten years after peak, as seen from the broad H
emission line. We discuss the complex interplay between tidal disruption events
and gas accretion episodes in galactic nuclei, highlighting the implications
for future TDE searches and for estimates of their intrinsic rates.Comment: 20 pages, 9 figures, 3 tables. Accepted for publication in MNRA
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