2,034 research outputs found
A Search for Optical Variability of Type 2 Quasars in SDSS Stripe 82
Hundreds of Type 2 quasars have been identified in Sloan Digital Sky Survey
(SDSS) data, and there is substantial evidence that they are generally galaxies
with highly obscured central engines, in accord with unified models for active
galactic nuclei (AGNs). A straightforward expectation of unified models is that
highly obscured Type 2 AGNs should show little or no optical variability on
timescales of days to years. As a test of this prediction, we have carried out
a search for variability in Type 2 quasars in SDSS Stripe 82 using
difference-imaging photometry. Starting with the Type 2 AGN catalogs of
Zakamska et al. (2003) and Reyes et al. (2008), we find evidence of significant
g-band variability in 17 out of 173 objects for which light curves could be
measured from the Stripe 82 data. To determine the nature of this variability,
we obtained new Keck spectropolarimetry observations for seven of these
variable AGNs. The Keck data show that these objects have low continuum
polarizations (p<~1% in most cases) and all seven have broad H-alpha and/or
MgII emission lines in their total (unpolarized) spectra, indicating that they
should actually be classified as Type 1 AGNs. We conclude that the primary
reason variability is found in the SDSS-selected Type 2 AGN samples is that
these samples contain a small fraction of Type 1 AGNs as contaminants, and it
is not necessary to invoke more exotic possible explanations such as a
population of "naked" or unobscured Type 2 quasars. Aside from misclassified
Type 1 objects, the Type 2 quasars do not generally show detectable optical
variability over the duration of the Stripe 82 survey.Comment: 14 pages, 8 figures. Accepted for publication in A
Inertia compensation while scanning screw threads on coordinate-measuring machines
Usage of scanning coordinate-measuring machines for inspection of screw
threads has become a common practice nowadays. Compared to touch trigger
probing, scanning capabilities allow to speed up measuring process while still
maintaining high accuracy. However, in some cases accuracy drasticaly depends
on the scanning speed. In this paper a compensation method is proposed allowing
to reduce the influence of some dynamic effects while scanning screw threads on
coordinate-measuring machines
Early foetal development of the articular disc in the human temporomandibular joint
In foetuses of 9 and 10 weeks the articular disc presents a more cellular structure
with bands of connective tissue fibres. It is connected with the articular
capsule and lateral pterygoid muscle. During weeks 11 and 12 there is an increase
in collagenous fibres and fusiform cells are located mainly close to the
surface of the disc
Water and Fire – The History of a Mound of Ash: Water Management in Hellenistic Berenike, Its Environmental and Logistical Setting
Archaeological work since 2014 in the Hellenistic areas of Berenike, a key port on Egypt’s Red Sea coast, founded by Ptolemy II Philadelphus in c. 275 BC, has brought extensive evidence of water-related structures: a rock-cut well located inside a rebuilt early-Hellenistic gate and a nearby cistern with an associated rainwater-collection system, that has changed the way in which the sources and uses of water in Hellenistic Berenike is understood today. The research started with the excavation of an ash mound, a characteristic landmark in the western part of the site, which is now believed to be the rubbish dump from the furnace that heated a Hellenistic bathhouse. At this stage in the research, it can be argued that Hellenistic Berenike had sufficient water available on site not only for drinking (hence potable), and agricultural and industrial uses, but also for public bathing. This article summarises the current state of research, presenting recent discoveries of a Hellenistic date at the site within their archaeological, architectural and environmental contexts
Differentiation of the nuclear groups in the posterior horn of the human embryonic spinal cord
The formation of nuclear groups in the posterior horns of the human embryonic
spinal cord was traced in serial sections of embryos of developmental stages
13 to 23 (32 to 56 postovulatory days). The following observations, new for
the human, are presented: 1. The differentiation of the neural tube into
3 zones (germinal, mantle and marginal) is detected in the middle of the
5th week. 2. The primordia of the posterior horns are marked at stage
14 (33 days). 3. In the middle of the 7th week the nucleus proprius and substantia
gelatinosa are discerned. 4. Differentiation of the nuclei within the posterior
horns proceeds in the ventrodorsal and rostrocaudal gradients. (Folia Morphol
2011; 70, 4: 245–251
Skeletal units of the human embryonic mandible
The development of the mandible was traced on serial sections of 20 human
embryos aged 5-8 weeks (developmental stages 13-23). Special consideration
was given to the differentiation of skeletal units proposed by Sperber. The first
skeletal units, namely the mandibular body, the alveolar unit and the condylar
unit, may be distinguished in the 7th week. The primordia of all units are identified
by the end of the embryonic period (8 weeks)
Schwann units in the human foetal phrenic nerve
In three human foetuses aged 15, 17, and 23 weeks the number of axons
surrounded by single Schwann cells was counted. These Schwann cell/axon complexes
form the Schwann units. The largest Schwann units in the foetus aged
15 weeks contained 232 axons, in the foetus of 17 weeks the number was
140 and in the foetus of 23 weeks the largest units contained 65 axons
Early development of the human palate in stages 16 and 17
A study was performed on 12 human embryos at developmental stages 16 and
17 (6th week). In the investigated embryos the primary palate is formed from
medial nasal, lateral nasal, and maxillary processes. The medial and lateral nasal
processes merge and form the nasal fin at stage 16. This fin regresses and at
stage 17 and persists as the oronasal membrane. The primordia of the secondary
palate appear at stage 17. (Folia Morphol 2011; 70, 1: 29-32
Evidence that the caudal portion of the neural tube develops by cavitation of a neural cord in the caudal eminence of human embryos
The formation of the secondary neural tube was traced in serial sections of human
embryos of developmental stages 13 to 17 (32-41 days after fertilisation). It was
found that the secondary neural tube formation begins with cavitation of the
neural cord. The minute cavities are seen in embryos at stages 13 and 15. At
stages 16 and 17 the numerous cavities coalesce to form a single central canal
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