27,561 research outputs found
Estimating Black Hole Masses in Active Galaxies Using the Halpha Emission Line
It has been established that virial masses for black holes in low-redshift
active galaxies can be estimated from measurements of the optical continuum
strength and the width of the broad Hbeta line. Under various circumstances,
however, both of these quantities can be challenging to measure or can be
subject to large systematic uncertainties. To mitigate these difficulties, we
present a new method for estimating black hole masses. From analysis of a new
sample of broad-line active galactic nuclei, we find that Halpha luminosity
scales almost linearly with optical continuum luminosity and that a strong
correlation exists between Halpha and Hbeta line widths. These two empirical
correlations allow us to translate the standard virial mass system to a new one
based solely on observations of the broad Halpha emission line.Comment: to appear in Apj; 8 pages; 5 figures; uses emulateapj5.st
Near threshold rotational excitation of molecular ions by electron-impact
New cross sections for the rotational excitation of H by electrons are
calculated {\it ab initio} at low impact energies. The validity of the
adiabatic-nuclei-rotation (ANR) approximation, combined with -matrix
wavefunctions, is assessed by comparison with rovibrational quantum defect
theory calculations based on the treatment of Kokoouline and Greene ({\it Phys.
Rev. A} {\bf 68} 012703 2003). Pure ANR excitation cross sections are shown to
be accurate down to threshold, except in the presence of large oscillating
Rydberg resonances. These resonances occur for transitions with
and are caused by closed channel effects. A simple analytic formula is derived
for averaging the rotational probabilities over such resonances in a 3-channel
problem. In accord with the Wigner law for an attractive Coulomb field,
rotational excitation cross sections are shown to be large and finite at
threshold, with a significant but moderate contribution from closed channels.Comment: 3 figures, a5 page
A General Precipitation-Limited L_X-T-R Relation Among Early-Type Galaxies
The relation between X-ray luminosity (L_X) and ambient gas temperature (T)
among massive galactic systems is an important cornerstone of both
observational cosmology and galaxy-evolution modeling. In the most massive
galaxy clusters, the relation is determined primarily by cosmological structure
formation. In less massive systems, it primarily reflects the feedback response
to radiative cooling of circumgalactic gas. Here we present a simple but
powerful model for the L_X-T relation as a function of physical aperture R
within which those measurements are made. The model is based on the
precipitation framework for AGN feedback and assumes that the circumgalactic
medium is precipitation-regulated at small radii and limited by cosmological
structure formation at large radii. We compare this model with many different
data sets and show that it successfully reproduces the slope and upper envelope
of the L_X-T-R relation over the temperature range from ~0.2 keV through >10
keV. Our findings strongly suggest that the feedback mechanisms responsible for
regulating star formation in individual massive galaxies have much in common
with the precipitation-triggered feedback that appears to regulate
galaxy-cluster cores.Comment: Submitted to ApJ, 9 pages, 3 figures (v2 fixes a few small typos
Infrared Signature of the Superconducting State in Pr(2-x)Ce(x)CuO(4)
We measured the far infrared reflectivity of two superconducting
Pr(2-x)Ce(x)CuO(4) films above and below Tc. The reflectivity in the
superconducting state increases and the optical conductivity drops at low
energies, in agreement with the opening of a (possibly) anisotropic
superconducting gap. The maximum energy of the gap scales roughly with Tc as 2
Delta_{max} / kB Tc ~ 4.7. We determined absolute values of the penetration
depth at 5 K as lambda_{ab} = (3300 +/- 700) A for x = 0.15 and lambda_{ab} =
(2000 +/- 300) A for x = 0.17. A spectral weight analysis shows that the
Ferrell-Glover-Tinkham sum rule is satisfied at conventional low energy scales
\~ 4 Delta_{max}.Comment: 4 pages, 4 figure
Steep-Spectrum Radio Emission from the Low-Mass Active Galactic Nucleus GH 10
GH 10 is a broad-lined active galactic nucleus (AGN) energized by a black
hole of mass 800,000 Solar masses. It was the only object detected by Greene et
al. in their Very Large Array (VLA) survey of 19 low-mass AGNs discovered by
Greene & Ho. New VLA imaging at 1.4, 4.9, and 8.5 GHz reveals that GH 10's
emission has an extent of less than 320 pc, has an optically-thin synchrotron
spectrum with a spectral index -0.76+/-0.05, is less than 11 percent linearly
polarized, and is steady - although poorly sampled - on timescales of weeks and
years. Circumnuclear star formation cannot dominate the radio emission, because
the high inferred star formation rate, 18 Solar masses per year, is
inconsistent with the rate of less than 2 Solar masses per year derived from
narrow Halpha and [OII] 3727 emission. Instead, the radio emission must be
mainly energized by the low-mass black hole. GH 10's radio properties match
those of the steep-spectrum cores of Palomar Seyfert galaxies, suggesting that,
like those Seyferts, the emission is outflow-driven. Because GH 10 is radiating
close to its Eddington limit, it may be a local analog of the starting
conditions, or seeds, for supermassive black holes. Future imaging of GH 10 at
higher resolution thus offers an opportunity to study the relative roles of
radiative versus kinetic feedback during black-hole growth.Comment: 7 pages; 2 figures; emulateapj; to appear in Ap
Design and construction of a point-contact spectroscopy rig with lateral scanning capability
The design and realization of a cryogenic rig for point-contact spectroscopy
measurements in the needle-anvil configuration is presented. Thanks to the use
of two piezoelectric nano-positioners, the tip can move along the vertical
() and horizontal () direction and thus the rig is suitable to probe
different regions of a sample \textit{in situ}. Moreover, it can also form
double point-contacts on different facets of a single crystal for achieving,
e.g., an interferometer configuration for phase-sensitive measurements. For the
latter purpose, the sample holder can also host a Helmholtz coil for applying a
small transverse magnetic field to the junction. A semi-rigid coaxial cable can
be easily added for studying the behavior of Josephson junctions under
microwave irradiation. The rig can be detached from the probe and thus used
with different cryostats. The performance of this new probe has been tested in
a Quantum Design PPMS system by conducting point-contact Andreev reflection
measurements on Nb thin films over large areas as a function of temperature and
magnetic field.Comment: 7 pages, 7 figures, published in Rev. Sci. Instru
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