4,895 research outputs found
The estimation of black-hole masses in distant radio galaxies
We have estimated the masses of the central supermassive black holes of 2442
radio galaxies froma catalog compiled using data from the NED, SDSS, and CATS
databases. Mass estimates based on optical photometry and radio data are
compared. Relationships between the mass of the central black hole
and the redshift are constructed for both wavelength ranges. The
distribution of the galaxies in these diagrams and systematic effects
influencing estimation of the black-hole parameters are discussed.
Upperenvelope cubic regression fits are obtained using the maximum estimates of
the black-hole masses. The optical and radio upper envelopes show similar
behavior, and have very similar peaks in position, , and
amplitude, = 9.4. This is consistent with a model in which the
growth of the supermassive black holes is self-regulating, with this redshift
corresponding to the epoch when the accretion-flow phase begins to end and the
nuclear activity falls off.Comment: 8 pages, 6 figure
LBT and Spitzer Spectroscopy of Star-Forming Galaxies at 1 < z < 3: Extinction and Star Formation Rate Indicators
We present spectroscopic observations in the rest-frame optical and near- to
mid-infrared wavelengths of four gravitationally lensed infrared (IR) luminous
star-forming galaxies at redshift 1 < z < 3 from the LUCIFER instrument on the
Large Binocular Telescope and the Infrared Spectrograph on Spitzer. The sample
was selected to represent pure, actively star-forming systems, absent of active
galactic nuclei. The large lensing magnifications result in high
signal-to-noise spectra that can probe faint IR recombination lines, including
Pa-alpha and Br-alpha at high redshifts. The sample was augmented by three
lensed galaxies with similar suites of unpublished data and observations from
the literature, resulting in the final sample of seven galaxies. We use the IR
recombination lines in conjunction with H-alpha observations to probe the
extinction, Av, of these systems, as well as testing star formation rate (SFR)
indicators against the SFR measured by fitting spectral energy distributions to
far-IR photometry. Our galaxies occupy a range of Av from ~0 to 5.9 mag, larger
than previously known for a similar range of IR luminosities at these
redshifts. Thus, estimates of SFR even at z ~ 2 must take careful count of
extinction in the most IR luminous galaxies. We also measure extinction by
comparing SFR estimates from optical emission lines with those from far-IR
measurements. The comparison of results from these two independent methods
indicates a large variety of dust distribution scenarios at 1 < z < 3. Without
correcting for dust extinction, the H-alpha SFR indicator underestimates the
SFR; the size of the necessary correction depends on the IR luminosity and dust
distribution scenario. Individual SFR estimates based on the 6.2 micron PAH
emission line luminosity do not show a systematic discrepancy with extinction,
although a considerable, ~0.2 dex scatter is observed.Comment: Accepted for publication in The Astrophysical Journal; 14 pages, 8
figure
Large Binocular Telescope and Sptizer Spectroscopy of Star-forming Galaxies at 1 < z < 3: Extinction and Star Formation Rate Indicators
We present spectroscopic observations in the rest-frame optical and near- to mid-infrared wavelengths of four gravitationally lensed infrared (IR) luminous star-forming galaxies at redshift 1 < z < 3 from the LUCIFER instrument on the Large Binocular Telescope and the Infrared Spectrograph on Spitzer. The sample was selected to represent pure, actively star-forming systems, absent of active galactic nuclei. The large lensing magnifications result in high signal-to-noise spectra that can probe faint IR recombination lines, including Pa and Br at high redshifts. The sample was augmented by three lensed galaxies with similar suites of unpublished data and observations from the literature, resulting in the final sample of seven galaxies. We use the IR recombination lines in conjunction with H observations to probe the extinction, Av, of these systems, as well as testing star formation rate (SFR) indicators against the SFR measured by fitting spectral energy distributions to far-IR photometry. Our galaxies occupy a range of Av from 0 to 5.9 mag, larger than previously known for a similar range of IR luminosities at these redshifts. Thus, estimates of SFR even at z 2 must take careful count of extinction in the most IR luminous galaxies.We also measure extinction by comparing SFR estimates from optical emission lines with those from far- IR measurements. The comparison of results from these two independent methods indicates a large variety of dust distribution scenarios at 1 < z < 3. Without correcting for dust extinction, the H SFR indicator underestimates the SFR; the size of the necessary correction depends on the IR luminosity and dust distribution scenario. Individual SFR estimates based on the 6.2m polycyclic aromatic hydrocarbon emission line luminosity do not show a systematic discrepancy with extinction, although a considerable, 0.2 dex, scatter is observed
Investigation of Sparse Data Mouse Imaging Using Micro-CT with a Carbon-Nanotube-Based X-ray Source
There has been a renewed interest in algorithm development for image reconstruction from highly incomplete data in computed tomography (CT). Such algorithms may lead to reduced imaging dose and time, and to the design of innovative configurations tailored to specific imaging tasks. In recent years, a carbon-nanotube (CNT)-based field-emission x-ray source has been developed, which offers easy electronic control of radiation and thus can be an ideal candidate for gated imaging. We have recently proposed algorithms for image reconstruction from fan-and cone-beam data collected at highly sparse angular views through minimization of the total-variation (TV) of the image subject to the condition that the estimated data are consistent with the measured data. In this work, we investigate and demonstrate the application of the TV-minimization algorithm to reconstructing images from mouse data acquired with a CNT-based CT scanner at a number of views much lower than what is used in conventional CT imaging. The results demonstrate that the TV-minimization algorithm can yield images with quality comparable to those obtained from a large number of views by use of the conventional algorithms. The significance of the work may lie in that the substantial reduction of projection views promised by the TV-minimization algorithm can be exploited for reducing imaging dose and time or for improving temporal resolution in tasks such as dynamic imaging
Quantum states and linear response in dc and electromagnetic fields for charge current and spin polarization of electrons at Bi/Si interface with giant spin-orbit coupling
An expansion of the nearly free-electron model constructed by Frantzeskakis,
Pons and Grioni [Phys. Rev. B {\bf 82}, 085440 (2010)] describing quantum
states at Bi/Si(111) interface with giant spin-orbit coupling is developed and
applied for the band structure and spin polarization calculation, as well as
for the linear response analysis for charge current and induced spin caused by
dc field and by electromagnetic radiation. It is found that the large
spin-orbit coupling in this system may allow resolving the spin-dependent
properties even at room temperature and at realistic collision rate. The
geometry of the atomic lattice combined with spin-orbit coupling leads to an
anisotropic response both for current and spin components related to the
orientation of the external field. The in-plane dc electric field produces only
the in-plane components of spin in the sample while both the in-plane and
out-of-plane spin components can be excited by normally propagating
electromagnetic wave with different polarizations.Comment: 10 pages, 9 figure
Host model uncertainties in aerosol radiative forcing estimates: results from the AeroCom Prescribed intercomparison study
Simulated multi-model âdiversityâ in aerosol direct
radiative forcing estimates is often perceived as a measure
of aerosol uncertainty. However, current models used
for aerosol radiative forcing calculations vary considerably
in model components relevant for forcing calculations and
the associated âhost-model uncertaintiesâ are generally convoluted
with the actual aerosol uncertainty. In this AeroCom
Prescribed intercomparison study we systematically isolate
and quantify host model uncertainties on aerosol forcing experiments
through prescription of identical aerosol radiative
properties in twelve participating models.
Even with prescribed aerosol radiative properties, simulated
clear-sky and all-sky aerosol radiative forcings show
significant diversity. For a purely scattering case with globally
constant optical depth of 0.2, the global-mean all-sky
top-of-atmosphere radiative forcing is â4.47Wmâ2 and the
inter-model standard deviation is 0.55Wmâ2, corresponding
to a relative standard deviation of 12 %. For a case
with partially absorbing aerosol with an aerosol optical
depth of 0.2 and single scattering albedo of 0.8, the forcing
changes to 1.04Wmâ2, and the standard deviation increases
to 1.01Wâ2, corresponding to a significant relative standard
deviation of 97 %. However, the top-of-atmosphere forcing
variability owing to absorption (subtracting the scattering
case from the case with scattering and absorption) is low,
with absolute (relative) standard deviations of 0.45Wmâ2
(8 %) clear-sky and 0.62Wmâ2 (11 %) all-sky.
Scaling the forcing standard deviation for a purely scattering
case to match the sulfate radiative forcing in the Aero-
Com Direct Effect experiment demonstrates that host model
uncertainties could explain about 36% of the overall sulfate
forcing diversity of 0.11Wmâ2 in the AeroCom Direct Radiative
Effect experiment
Phase-Coherent Charge Transport through a Porphyrin Nanoribbon
Quantum interference in nano-electronic devices could lead to reduced-energy
computing and efficient thermoelectric energy harvesting. When devices are
shrunk down to the molecular level it is still unclear to what extent electron
transmission is phase coherent, as molecules usually act as scattering centres,
without the possibility of showing particle-wave duality. Here we show electron
transmission remains phase coherent in molecular porphyrin nanoribbons,
synthesized with perfectly defined geometry, connected to graphene electrodes.
The device acts as a graphene Fabry-P\'erot interferometer, allowing direct
probing of the transport mechanisms throughout several regimes, including the
Kondo one. Electrostatic gating allows measurement of the molecular conductance
in multiple molecular oxidation states, demonstrating a thousand-fold increase
of the current by interference, and unravelling molecular and graphene
transport pathways. These results demonstrate a platform for the use of
interferometric effects in single-molecule junctions, opening up new avenues
for studying quantum coherence in molecular electronic and spintronic devices.Comment: 14 pages, 3 figure
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