4,733 research outputs found
Dissecting the Power Sources of Low-Luminosity Emission-Line Galaxy Nuclei via Comparison of HST-STIS and Ground-Based Spectra
Using a sample of ~100 nearby line-emitting galaxy nuclei, we have built the
currently definitive atlas of spectroscopic measurements of H_alpha and
neighboring emission lines at subarcsecond scales. We employ these data in a
quantitative comparison of the nebular emission in Hubble Space Telescope (HST)
and ground-based apertures, which offer an order-of-magnitude difference in
contrast, and provide new statistical constraints on the degree to which
Transition Objects and low-ionization nuclear emission-line regions (LINERs)
are powered by an accreting black hole at <10 pc. We show that while the
small-aperture observations clearly resolve the nebular emission, the aperture
dependence in the line ratios is generally weak, and this can be explained by
gradients in the density of the line-emitting gas: the higher densities in the
more nuclear regions potentially flatten the excitation gradients, suppressing
the forbidden emission. The Transition Objects show a threefold increase in the
incidence of broad H_alpha emission in the high-resolution data, as well as the
strongest density gradients, supporting the composite model for these systems
as accreting sources surrounded by star-forming activity. The narrow-line
LINERs appear to be the weaker counterparts of the Type 1 LINERs, where the low
accretion rates cause the disappearance of the broad-line component. The
enhanced sensitivity of the HST observations reveals a 30% increase in the
incidence of accretion-powered systems at z~0. A comparison of the strength of
the broad-line emission detected at different epochs implies potential
broad-line variability on a decade-long timescale, with at least a factor of
three in amplitude.Comment: 27 pages, 13 figures, 4 tables, accepted for publication in Ap
The Extraordinary Abundances of QSO Broad Absorption Line Regions: A Matter of Novae?
The broad absorption lines (BALs) of QSOs indicate abundances of heavy
elements, relative to hydrogen, that are 1 to 2 orders of magnitude higher than
the solar values. In at least one QSO, an especially large enhancement of
phosphorus is observed. These abundances resemble those in Galactic novae, and
this suggests that novae may produce the BAL gas. The needed rate of nova
outbursts may come from single white dwarfs that accrete gas as they pass
through a supermassive accretion disk around a central black hole.Comment: 9 pages including 1 Postscript figure. Uses aaspp4.sty and
flushrt.sty. Uuencoded, gzipped tarfile. To appear in Astrophys. J.
(Letters), 1996 April 1
The SPICE carbon isotope excursion in Siberia: a combined study of the upper Middle Cambrian-lowermost Ordovician Kulyumbe River section, northwestern Siberian Platform
An integrated, high-resolution chemostratigraphic (C, O and Sr isotopes) and
magnetostratigraphic study through the upper Middle Cambrianālowermost Ordovician shallowmarine
carbonates of the northwestern margin of the Siberian Platform is reported. The interval was
analysed at the Kulyumbe section, which is exposed along the Kulyumbe River, an eastern tributary
of the Enisej River. It comprises the upper Ustā-Brus, Labaz, Orakta, Kulyumbe, Ujgur and lower
Iltyk formations and includes the Steptoean positive carbon isotopic excursion (SPICE) studied here
in detail from upper Cambrian carbonates of the Siberian Platform for the first time. The peak of the
excursion, showing Ī“13C positive values as high as+4.6ā°and least-altered 87Sr/86Sr ratios of 0.70909,
is reported herein from the Yurakhian Horizon of the Kulyumbe Formation. The stratigraphic position
of the SPICE excursion does not support traditional correlation of the boundary between theOrakta and
Labaz formations at the Kulyumbe River with its supposedly equivalent level in Australia, Laurentia,
South China and Kazakhstan, where the Glyptagnostus stolidotus and G. reticulatus biozones are
known to immediately precede the SPICE excursion and span the MiddleāUpper Cambrian boundary.
The CambrianāOrdovician boundary is probably situated in the middle Nyajan Horizon of the Iltyk
Formation, in which carbon isotope values show a local maximum below a decrease in the upper
part of the Nyajan Horizon, attributed herein to the Tremadocian Stage. A refined magnetic polarity
sequence confirms that the geomagnetic reversal frequency was very high during Middle Cambrian
times at 7ā10 reversals per Ma, assuming a total duration of about 10 Ma and up to 100 magnetic
intervals in the Middle Cambrian. By contrast, the sequence attributed herein to the Upper Cambrian
on chemostratigraphic grounds contains only 10ā11 magnetic intervals
Avoiding the Detector Blinding Attack on Quantum Cryptography
We show the detector blinding attack by Lydersen et al [1] will be
ineffective on most single photon avalanche photodiodes (APDs) and certainly
ineffective on any detectors that are operated correctly. The attack is only
successful if a redundant resistor is included in series with the APD, or if
the detector discrimination levels are set inappropriately
Preclinical Applications of 3'-Deoxy-3'-[18F]Fluorothymidine in Oncology - A Systematic Review
The positron emission tomography (PET) tracer 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT) has been proposed to measure cell proliferation non-invasively in vivo. Hence, it should provide valuable information for response assessment to tumor therapies. To date, [18F]FLT uptake has found limited use as a response biomarker in clinical trials in part because a better understanding is needed of the determinants of [18F]FLT uptake and therapy-induced changes of its retention in the tumor. In this systematic review of preclinical [18F]FLT studies, comprising 174 reports, we identify the factors governing [18F]FLT uptake in tumors, among which thymidine kinase 1 plays a primary role. The majority of publications (83 %) report that decreased [18F]FLT uptake reflects the effects of anticancer therapies. 144 times [18F]FLT uptake was related to changes in proliferation as determined by ex vivo analyses. Of these approaches, 77 % describe a positive relation, implying a good concordance of tracer accumulation and tumor biology. These preclinical data indicate that [18F]FLT uptake holds promise as an imaging biomarker for response assessment in clinical studies. Understanding of the parameters which influence cellular [18F]FLT uptake and retention as well as the mechanism of changes induced by therapy is essential for successful implementation of this PET tracer. Hence, our systematic review provides the background for the use of [18F]FLT in future clinical studies
The Changing Nature of Regional Poverty
Weāre witnessing a dramatic change in the social geography of greater Richmond. Itās becoming more like that of the global south, where wealth is centered in the cities and poverty rings the outskirts of the city
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