676 research outputs found
Chandra Observations of Radio-Loud Quasars at z > 4: X-rays from the Radio Beacons of the Early Universe
We present the results of Chandra observations of six radio-loud quasars
(RLQs) and one optically bright radio-quiet quasar (RQQ) at z = 4.1-4.4. These
observations cover a representative sample of RLQs with moderate radio-loudness
(R ~ 40-400), filling the X-ray observational gap between optically selected
RQQs and the five known blazars at z > 4 (R ~ 800-27000). We study the
relationship between X-ray luminosity and radio-loudness for quasars at high
redshift and constrain RLQ X-ray continuum emission and absorption. From a
joint spectral fit of nine moderate-R RLQs observed by Chandra, we find
tentative evidence for absorption above the Galactic N_H, with a best-fit
neutral intrinsic column density of N_H = 2.4^{+2.0}_{-1.8} x 10^{22} cm^{-2},
consistent with earlier claims of increased absorption toward high-redshift
RLQs. We also search for evidence of an enhanced jet-linked component in the
X-ray emission due to the increased energy density of the cosmic microwave
background (CMB) at high redshift, but we find neither spatial detections of
X-ray jets nor a significant enhancement in the X-ray emission relative to
comparable RLQs at low-to-moderate redshifts. Overall, the z ~ 4-5 RLQs have
basic X-ray properties consistent with comparable RLQs in the local universe,
suggesting that the accretion/jet mechanisms of these objects are similar as
well.Comment: 12 pages, The Astronomical Journal, in pres
A Super-Alfvenic Model of Dark Clouds
Supersonic random motions are observed in dark clouds and are traditionally
interpreted as Alfven waves, but the possibility that these motions are
super-Alfvenic has not been ruled out. In this work we report the results of
numerical experiments in two opposite regimes; M_a ~ 1 and M_a >> 1, where M_a
is the initial Alfvenic Mach number --the ratio of the rms velocity to the
Alfven speed. Our results show that models with M_a >> 1 are consistent with
the observed properties of molecular clouds that we have tested --statistics of
extinction measurements, Zeeman splitting measurements of magnetic field
strength, line width versus integrated antenna temperature of molecular
emission line spectra, statistical B-n relation, and scatter in that relation--
while models with M_a ~ 1 have properties that are in conflict with the
observations. We find that both the density and the magnetic field in molecular
clouds may be very intermittent. The statistical distributions of magnetic
field and gas density are related by a power law, with an index that decreases
with time in experiments with decaying turbulence. After about one dynamical
time it stabilizes at B ~ n^{0.4}. Magnetically dominated cores form early in
the evolution, while later on the intermittency in the density field wins out,
and also cores with weak field can be generated, by mass accretion along
magnetic field lines.Comment: 10 figures, 2 tables include
Vertical-external-cavity surface-emitting lasers and quantum dot lasers
The use of cavity to manipulate photon emission of quantum dots (QDs) has
been opening unprecedented opportunities for realizing quantum functional
nanophotonic devices and also quantum information devices. In particular, in
the field of semiconductor lasers, QDs were introduced as a superior
alternative to quantum wells to suppress the temperature dependence of the
threshold current in vertical-external-cavity surface-emitting lasers
(VECSELs). In this work, a review of properties and development of
semiconductor VECSEL devices and QD laser devices is given. Based on the
features of VECSEL devices, the main emphasis is put on the recent development
of technological approach on semiconductor QD VECSELs. Then, from the viewpoint
of both single QD nanolaser and cavity quantum electrodynamics (QED), a
single-QD-cavity system resulting from the strong coupling of QD cavity is
presented. A difference of this review from the other existing works on
semiconductor VECSEL devices is that we will cover both the fundamental aspects
and technological approaches of QD VECSEL devices. And lastly, the presented
review here has provided a deep insight into useful guideline for the
development of QD VECSEL technology and future quantum functional nanophotonic
devices and monolithic photonic integrated circuits (MPhICs).Comment: 21 pages, 4 figures. arXiv admin note: text overlap with
arXiv:0904.369
The radio luminosity, black hole mass and Eddington ratio for quasars from the Sloan Digital Sky Survey
We investigate the \mbh- \sigma_* relation for radio-loud quasars with
redshift in Data Release 3 of the Sloan Digital Sky Survey (SDSS). The
sample consists of 3772 quasars with better model of H and \oiii lines
and available radio luminosity, including 306 radio-loud quasars, 3466
radio-quiet quasars with measured radio luminosity or upper-limit of radio
luminosity (181 radio-quiet quasars with measured radio luminosity). The virial
supermassive black hole mass (\mbh) is calculated from the broad \hb line, the
host stellar velocity dispersion () is traced by the core \oiii
gaseous velocity dispersion, and the radio luminosity and the radio loudness
are derived from the FIRST catalog. Our results are follows: (1) For
radio-quiet quasars, we confirm that there is no obvious deviation from the
\mbh- \sigma_* relation defined in inactive galaxies when \mbh uncertainties
and luminosity bias are concerned. (2) We find that radio-loud quasars deviate
much from the \mbh- \sigma_* relation respect to that for radio-quiet
quasars. This deviation is only partly due to the possible cosmology evolution
of the \mbh- \sigma_* relation and the luminosity bias. (3) The radio
luminosity is proportional to
\mbh^{1.28^{+0.23}_{-0.16}}(\lb/\ledd)^{1.29^{+0.31}_{-0.24}} for radio-quiet
quasars and \mbh^{3.10^{+0.60}_{-0.70}}(\lb/\ledd)^{4.18^{+1.40}_{-1.10}} for
radio-loud quasars. The weaker correlation of the radio luminosity dependence
upon the mass and the Eddington ratio for radio-loud quasars shows that other
physical effects would account for their radio luminosities, such as the black
hole spin.Comment: 15 pages, 8 figures, 2 tables, accepted for publication in ChJA
First 230 GHz VLBI Fringes on 3C 279 using the APEX Telescope
We report about a 230 GHz very long baseline interferometry (VLBI) fringe
finder observation of blazar 3C 279 with the APEX telescope in Chile, the
phased submillimeter array (SMA), and the SMT of the Arizona Radio Observatory
(ARO). We installed VLBI equipment and measured the APEX station position to 1
cm accuracy (1 sigma). We then observed 3C 279 on 2012 May 7 in a 5 hour 230
GHz VLBI track with baseline lengths of 2800 M to 7200 M and
a finest fringe spacing of 28.6 micro-arcseconds. Fringes were detected on all
baselines with SNRs of 12 to 55 in 420 s. The correlated flux density on the
longest baseline was ~0.3 Jy/beam, out of a total flux density of 19.8 Jy.
Visibility data suggest an emission region <38 uas in size, and at least two
components, possibly polarized. We find a lower limit of the brightness
temperature of the inner jet region of about 10^10 K. Lastly, we find an upper
limit of 20% on the linear polarization fraction at a fringe spacing of ~38
uas. With APEX the angular resolution of 230 GHz VLBI improves to 28.6 uas.
This allows one to resolve the last-photon ring around the Galactic Center
black hole event horizon, expected to be 40 uas in diameter, and probe radio
jet launching at unprecedented resolution, down to a few gravitational radii in
galaxies like M 87. To probe the structure in the inner parsecs of 3C 279 in
detail, follow-up observations with APEX and five other mm-VLBI stations have
been conducted (March 2013) and are being analyzed.Comment: accepted for publication in A&
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
A molecular insight into algal-oomycete warfare : cDNA analysis of Ectocarpus siliculosus infected with the basal oomycete Eurychasma dicksonii
Peer reviewedPublisher PD
A Genome-Wide Investigation of SNPs and CNVs in Schizophrenia
We report a genome-wide assessment of single nucleotide polymorphisms (SNPs) and copy number variants (CNVs) in schizophrenia. We investigated SNPs using 871 patients and 863 controls, following up the top hits in four independent cohorts comprising 1,460 patients and 12,995 controls, all of European origin. We found no genome-wide significant associations, nor could we provide support for any previously reported candidate gene or genome-wide associations. We went on to examine CNVs using a subset of 1,013 cases and 1,084 controls of European ancestry, and a further set of 60 cases and 64 controls of African ancestry. We found that eight cases and zero controls carried deletions greater than 2 Mb, of which two, at 8p22 and 16p13.11-p12.4, are newly reported here. A further evaluation of 1,378 controls identified no deletions greater than 2 Mb, suggesting a high prior probability of disease involvement when such deletions are observed in cases. We also provide further evidence for some smaller, previously reported, schizophrenia-associated CNVs, such as those in NRXN1 and APBA2. We could not provide strong support for the hypothesis that schizophrenia patients have a significantly greater âloadâ of large (>100 kb), rare CNVs, nor could we find common CNVs that associate with schizophrenia. Finally, we did not provide support for the suggestion that schizophrenia-associated CNVs may preferentially disrupt genes in neurodevelopmental pathways. Collectively, these analyses provide the first integrated study of SNPs and CNVs in schizophrenia and support the emerging view that rare deleterious variants may be more important in schizophrenia predisposition than common polymorphisms. While our analyses do not suggest that implicated CNVs impinge on particular key pathways, we do support the contribution of specific genomic regions in schizophrenia, presumably due to recurrent mutation. On balance, these data suggest that very few schizophrenia patients share identical genomic causation, potentially complicating efforts to personalize treatment regimens
Nuclear Luminosities and Radio Loudness of Seyfert Nuclei
Historically, Seyfert nuclei have been considered to be radio-quiet active
galactic nuclei (AGNs). We question this widely held assumption by showing that
the distribution of the radio-to-optical luminosity ratio R, when properly
measured for the {\it nuclear} component, places the majority of type 1 Seyfert
nuclei in the category of radio-loud AGNs, defined here as objects with R
greater than or equal to 10. This result is further strengthened by strong
correlations found between radio power and optical continuum and emission-line
luminosities, as has been established previously for more powerful AGNs. We
also present a new calibration of the relation between optical continuum and
Balmer emission-line luminosities valid in the regime of low-luminosity AGNs.Comment: To appear in The Astrophysical Journal. Latex, 13 page
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Jet-Launching Structure Resolved Near the Supermassive Black Hole in M87
Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 Âą 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole
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