88 research outputs found
Rest-frame optical continua of L ~ L*, z>3 quasars: probing the faint end of the high z quasar luminosity function
Near-IR photometry for 20 radio-loud z>3 quasars, 16 of which are radio-
selected, are presented. These data sample the rest-frame optical/UV continuum,
which is commonly interpreted as emission from an accretion disk. In a previous
study, we compared the rest-frame optical/UV continuum shapes of 15 optically
bright (V3 quasars with those of 27 low redshift (z~0.1) ones that
were matched to the high redshift sample in evolved luminosity (i.e. having
luminosities ranging from 1-7 times the characteristic luminosity, L*, where
L*~(1+z)^{~3}) to look for signs of evolution in the central engines. We found
the continuum shapes at z~0.1 and z>3 similar, consistent with no significant
change in the ratio mdot/M, where mdot is the accretion rate with respect to
the Eddington rate and M is the black hole mass. This study expands our earlier
high redshift sample to lower luminosity, away from extreme objects and towards
a luminosity overlap with lower redshift samples. The distribution of
rest-frame optical/UV continuum shapes for this fainter sample is broader,
extending further to the red than that of the brighter z>3 one. Three quasars
from this fainter sample, two radio-selected and one optically-selected, have
optical continuum slopes alpha<-1 (F_{nu}~nu^{alpha}). The optically-selected
one, LBQS0056+0125, appears to be reddened by dust along the line of sight or
in the host galaxy, whereas the radio-selected ones, PKS2215+02 and
TXS2358+189, could derive their red continua from the contribution of a
relatively strong synchrotron component to the rest-frame optical. These
objects may represent a bridge to a population of very red high redshift
quasars to which ongoing or future near-IR, optical and deep X-ray surveys will
be sensitive.Comment: 16 pages, 6 figures; accepted for publication in MNRA
Archaeomagnetic and rock magnetic study of six kilns from North Africa (Tunisia and Morocco)
International audienceNew full-vector archaeomagnetic data for North Africa recovered from the study of six kilns, five from Tunisia and one from Morocco, are presented. Archaeological and historical considerations, along with three radiocarbon dates, indicate that the age of the kilns ranges between the 9th and 15th centuries AD. Rock magnetic analyses showed that the principal magnetic carriers are magnetite and low Ti titanomagnetite, along with variable contributions of thermally stable maghemite and a high coercivity phase with low unblocking temperatures. The magnetic mineralogy of the studied material is thermally stable and behaves ideally during archaeointensity experiments. Stepwise alternating field demagnetization isolated a single, stable, characteristic remanence component with very well defined directions at both specimen and structure levels. Mean archaeointensities have been obtained from successful classical Thellier experiments conducted on between five and eight independent samples per kiln. Thermoremanent magnetization (TRM) anisotropy and cooling rate effects upon TRM intensity have been investigated. The results showed that these effects are low for four of the six studied kilns, with differences between the uncorrected and corrected means of less than 3 per cent. For the other two structures differences between the uncorrected and corrected mean site intensities are 4.4 per cent and 5.8 per cent. These results highlight the necessity for TRM anisotropy and cooling rate corrections in archaeomagnetic studies if accurate archaeointensities are to be obtained. The new results suggest that high intensities occurred in Northwest Africa during the 9th century. Although more data are clearly needed to define this period of high intensity, the results are in agreement with the available European archaeointensity data. Acomparison between the newdata, other available archaeomagnetic determinations in nearby locations, and palaeosecular variation (PSV) curves derived from the regional SCHA.DIF.3k and global ARCH3K.1 geomagnetic field models shows good agreement between the new data and directional results derived from the models. However, some differences are observed between geomagnetic field models intensity results and available archaeointensity data for the studied regions. This highlights the need for new data for unexplored regions such as North Africa. The new data presented here better constrains the evolution of the geomagnetic field during historical times in this region. They represent a new step towards the construction of a reference PSV curve for Northwest Africa. Once established, this curve will represent a new dating method for this region
Late ron Age occupation at Povoado de Crestelos (Meirinhos, Mogadouro, Portugal): combustion structures and reoccupation of ditch 1 for food processing activities
The archaeological excavation undertaken at Quinta de Crestelos (Merinhos, Mogadouro, Portugal), revealed a large occupation site dated between Late Iron Age and Early Roman periods. On the ridge top a small fortified site with elaborated defensive structures, consisting of large stone walls and ditches, showed several different phases of construction and large modifications.
The different phases of the site evidenced the ‘Romanization’ Process of this area and the large changes occurring in the Late Iron Age sites, with defensive structures, such as the original ditches, reused as functional and production areas.
Paleomagnetism studies and analysis of the combustion structures of the later phases of the site intended to clarify nature, main components and changes during the time when they were used
End of the Kiaman Superchron in the Permian of SW England: Magnetostratigraphy of the Aylesbeare Mudstone and Exeter groups
The chronology of Permian strata in SW England is fragmentary and largely based on radiometric dating of associated volcanic units. Magnetostratigraphy from the c. 2 km of sediments in the Exeter and Aylesbeare Mudstone groups was undertaken to define a detailed chronology, using the end of the Kiaman Superchron and the overlying reverse and normal polarity in the Middle and Upper Permian as age constraints. The palaeomagnetic directions are consistent with other European Permian palaeopoles, with data passing fold and reversal tests. The end of the Kiaman Superchron (in the Wordian) occurs in the uppermost part of the Exeter Group. The overlying Aylesbeare Mudstone Group is early Capitanian to latest Wuchiapingian in age. The Changhsingian and most of the Lower Triassic sequence is absent. Magnetostratigraphic comparison with the Southern Permian Basin shows that the Exeter and Aylesbeare Mudstone groups are closely comparable in age with the Havel and Elbe Subgroups of the Rotliegend II succession. The Altmark unconformities in these successions appear similar in age to the sequence boundaries in SW England, indicating that both may be climate controlled. Clasts in the Exeter Group, from unroofing of the Dartmoor Granite, first occurred at a minimum of c. 8 myr after formation of the granite
New archaeomagnetic data recovered from the study of Roman and Visigothic remains from central Spain (3rd–7th centuries)
New archaeomagnetic results from four heated/combustion structures recovered from two archaeological sites in central Spain are reported. They have been dated by archaeological evidence and in two cases by radiocarbon dating. Rock magnetic experiments indicate low coercivity magnetic phases, such as magnetite and thermally stable maghaemite, as the main carriers of the remanent magnetization. Haematite has been observed in poorly heated baked clays. Archaeomagnetic directions have been obtained from either alternating field or thermal demagnetization experiments performed on 57 specimens coming from 46 independently oriented samples. The four well-defined archaeomagnetic directions obtained are in good agreement with previous archaeomagnetic data and with recent regional and global field models. They define the beginning of easterly declination drift that was initiated around 350–400 AD and culminated around 800–850 AD, and delineate the maximum in inclination that took place around 600–650 AD. In addition, classical Thellier–Thellier experiments including thermal remanent magnetization anisotropy and cooling rate corrections were conducted on 23 specimens. Only 13 specimens, corresponding to well-defined single component behaviour, gave reliable results. New mean archaeointensities have been obtained for two of the four studied structures (VBK1, 64.2 ± 5.0 μT and VBT1, 62.4 ± 2.6 μT). The new data suggest that two relative intensity maxima occurred in Western Europe around 320 and 630 AD, being of lower magnitude that observed in Eastern Europe.Peer reviewe
VLT/ISAAC Spectra of the H-beta Region in Intermediate-Redshift Quasars II. Black Hole Mass and Eddington Ratio
We derive black hole masses for a sample of about 300 AGNs in the redshift
range 0 < z < 2.5. We use the same virial velocity measure (FWHM Hbeta broad
component) for all sources which represents a significant improvement over
previous studies. We review methods and caveats for determining AGN black hole
masses via the virial assumption for motions in the gas producing low
ionization broad emission lines. We derive a corrected FWHM measure for the
broad component of H-beta that better estimates the virialized line emitting
component by comparing our FWHM measures with a sample of reverberated sources
with H-beta radial velocity dispersion measures. We also consider the FWHM of
the FeII 4570 blend as a potential alternative velocity estimator. We find a
range of black hole mass between log M ~ 6 - 10, where the black hole mass M is
in solar masses. Estimates using corrected FWHM, as well as FWHM(Fe II)
measures, reduce the number of sources with log M > 9.5 and suggest that
extremely large M values (log M >~ 10) may not be realistic. Derived Eddington
ratio values values show no evidence for a significant population of
super-Eddington radiators especially after correction is made for sources with
extreme orientation to our line of sight. Sources with FWHM(Hbeta broad
component) <~ 4000 km/s show systematically higher Eddington ratio and lower M
values than broader lined AGNs (including almost all radio-loud sources).Comment: Accepted for publication in Astronomy & Astrophysic
Are red 2MASS QSOs young?
We use photometric data from Spitzer to explore the mid- and far-IR
properties of 10 red QSOs (J-K>2, R-K>5) selected by combining the 2MASS in the
NIR with the SDSS at optical wavelengths. Optical and/or near-infrared spectra
are available for 8/10 sources. Modeling the SED from UV to far-IR shows that
moderate dust reddening (A_V=1.3-3.2) can explain the red optical and near-IR
colours of the sources in the sample. There is also evidence that red QSOs have
60/12micron luminosity ratio higher than PG QSOs (97% significance). This can
be interpreted as a higher level of star-formation in these systems (measured
by the 60micron luminosity) for a given AGN power (approximated by the 12micron
luminosity). This is consistent with a picture where red QSOs represent an
early phase of AGN evolution, when the supermassive black hole is enshrouded in
dust and gas clouds, which will eventually be blown out (possibly by AGN driven
outflows) and the system will appear as typical optically luminous QSO. There
is also tentative evidence significant at the 96% level that red 2MASS QSOs are
more often associated with radio emission than optically selected SDSS QSOs.
This may indicate outflows, also consistent with the young AGN interpretation.
We also estimate the space density of red QSOs relative to optically selected
SDSS QSOs, taking into account the effect of dust extinction and the intrinsic
luminosity of the sources. We estimate that the fraction of red QSOs in the
overall population increases from 3% at M_K=-27.5mag to 12% at M_K=-29.5mag.
This suggests that reddened QSOs become more important at the bright end of the
Luminosity Function. If red QSOs are transition objects on the way to becoming
typical optically luminous QSOs, the low fractions above suggest that these
systems spent <12% of their lifetime at the "reddened" stage.Comment: Accepted by MNRA
A putative functional role for oligodendrocytes in mood regulation
Altered glial structure and function is implicated in several major mental illnesses and increasing evidence specifically links changes in oligodendrocytes with disrupted mood regulation. Low density and reduced expression of oligodendrocyte-specific gene transcripts in postmortem human subjects points toward decreased oligodendrocyte function in most of the major mental illnesses. Similar features are observed in rodent models of stress-induced depressive-like phenotypes, such as the unpredictable chronic mild stress and chronic corticosterone exposure, suggesting an effect downstream from stress. However, whether oligodendrocyte changes are a causal component of psychiatric phenotypes is not known. Traditional views that identify oligodendrocytes solely as nonfunctional support cells are being challenged, and recent studies suggest a more dynamic role for oligodendrocytes in neuronal functioning than previously considered, with the region adjacent to the node of Ranvier (i.e., paranode) considered a critical region of glial–neuronal interaction. Here, we briefly review the current knowledge regarding oligodendrocyte disruptions in psychiatric disorders and related animal models, with a focus on major depression. We then highlight several rodent studies, which suggest that alterations in oligodendrocyte structure and function can produce behavioral changes that are informative of mood regulatory mechanisms. Together, these studies suggest a model, whereby impaired oligodendrocyte and possibly paranode structure and function can impact neural circuitry, leading to downstream effects related to emotionality in rodents, and potentially to mood regulation in human psychiatric disorders
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