6,468 research outputs found
The Impact of the Uncertainty in Single-Epoch Virial Black Hole Mass Estimates on the Observed Evolution of the Black Hole - Bulge Scaling Relations
Recent observations of the black hole (BH) - bulge scaling relations usually
report positive redshift evolution, with higher redshift galaxies harboring
more massive BHs than expected from the local relations. All of these studies
focus on broad line quasars with BH mass estimated from virial estimators based
on single-epoch spectra. Since the sample selection is largely based on quasar
luminosity, the cosmic scatter in the BH-bulge relation introduces a
statistical bias leading to on average more massive BHs given galaxy properties
at high redshift (Lauer et al. 2007). We here emphasize a previously
under-appreciated statistical bias resulting from the uncertainty of
single-epoch virial BH mass estimators and the shape of the underlying (true)
BH mass function, which leads to on average overestimation of the true BH
masses at the high-mass end (Shen et al. 2008). We demonstrate that the latter
virial mass bias can contribute a substantial amount to the observed excess in
BH mass at fixed bulge properties, comparable to the Lauer et al. bias. The
virial mass bias is independent of the Lauer et al. bias, hence if both biases
are at work, they can largely (or even fully) account for the observed BH mass
excess at high redshift.Comment: Replaced with the accepted version
Cryogenic Microwave Imaging of Metal-Insulator Transition in Doped Silicon
We report the instrumentation and experimental results of a cryogenic
scanning microwave impedance microscope. The microwave probe and the scanning
stage are located inside the variable temperature insert of a helium cryostat.
Microwave signals in the distance modulation mode are used for monitoring the
tip-sample distance and adjusting the phase of the two output channels. The
ability to spatially resolve the metal-insulator transition in a doped silicon
sample is demonstrated. The data agree with a semi-quantitative finite-element
simulation. Effects of the thermal energy and electric fields on local charge
carriers can be seen in the images taken at different temperatures and DC
biases.Comment: 10 pages, 5 Figures, Accepted to Review of Scientific Instrumen
Calibration of shielded microwave probes using bulk dielectrics
A stripline-type near-field microwave probe is microfabricated for microwave
impedance microscopy. Unlike the poorly shielded coplanar probe that senses the
sample tens of microns away, the stripline structure removes the stray fields
from the cantilever body and localizes the interaction only around the
focused-ion beam deposited Pt tip. The approaching curve of an oscillating tip
toward bulk dielectrics can be quantitatively simulated and fitted to the
finite-element analysis result. The peak signal of the approaching curve is a
measure of the sample dielectric constant and can be used to study unknown bulk
materials.Comment: 10 pages, 3 figure
The Rise of Monopolistic Rideshare Companies in Asia: How Ride Hailing Companies’ Market Control Impacts Drivers
Having undergone mergers with Uber China and Uber Southeast Asia in the past few years, DiDi and Grab now hold over 90% and 80% of the market shares in China and Singapore. This market control has allowed them to dominate the rideshare industry in the two countries, and this paper examines the impact of Grab and DiDi’s monopolistic power on rideshare drivers. Specifically, this research considers changes to both licensure requirements, pricing policies, commission rates, insurance coverage, and CSR programs as well as long-term corporate objectives and strategies following the merger. In order to understand the implications of DiDi and Grab’s decrease in competition and greater market control, this research draws from existing scholarly research, DiDi and Grab’s policies and terms of use, news articles, reports, government documents, and most importantly, qualitative interviews with employees at DiDi and Grab. The paper concludes that despite changes in pricing, insurance, and benefits, monopolistic ridesharing companies are able to leverage their resources and market power for diversification, which creates synergies, and greater social impact
Comparing Single-Epoch Virial Black Hole Mass Estimators for Luminous Quasars
Single-epoch virial black hole (BH) mass estimators utilizing broad emission
lines have been routinely applied to high-redshift quasars to estimate their BH
masses. Depending on the redshift, different line estimators (Halpha, Hbeta,
MgII, CIV) are often used with optical/near-infrared spectroscopy. Here we use
a homogeneous sample of 60 intermediate-redshift (z~1.5-2.2) SDSS quasars with
optical and near-infrared spectra covering CIV through Halpha to investigate
the consistency between different line estimators. We critically compare
restframe UV line estimators (CIV, CIII], and MgII) with optical estimators
(Hbeta and Halpha) in terms of correlations between line widths and between
continuum/line luminosities, for the high-luminosity regime (L_5100>10^45.4
erg/s) probed by our sample. The continuum luminosities of L_1350 and L_3000,
and the broad line luminosities are well correlated with L_5100. We found that
the MgII FWHM correlates well with the FWHMs of the Balmer lines, and that the
MgII line estimator can be calibrated to yield consistent virial mass estimates
with those based on the Hbeta/Halpha estimators, thus extending earlier results
on less luminous objects. The CIV FWHM is poorly correlated with the Balmer
line FWHMs, and the scatter between the CIV and Hbeta FWHMs consists of an
irreducible part (~0.12 dex), and a part that correlates with the blueshift of
the CIV centroid relative to that of Hbeta. The CIII] FWHM is found to
correlate with the CIV FWHM, and hence is also poorly correlated with the Hbeta
FWHM. While the CIV and CIII] lines can be calibrated to yield consistent
virial mass estimates as Hbeta on average, the scatter is substantially larger
than MgII, and the usage of CIV/CIII] FWHM in the mass estimators does not
improve the agreement with the Hbeta estimator. (Abridged)Comment: 17 emulateapj pages; submitted to Ap
Modeling of a Cantilever-Based Near-Field Scanning Microwave Microscope
We present a detailed modeling and characterization of our scalable microwave
nanoprobe, which is a micro-fabricated cantilever-based scanning microwave
probe with separated excitation and sensing electrodes. Using finite-element
analysis, the tip-sample interaction is modeled as small impedance changes
between the tip electrode and the ground at our working frequencies near 1GHz.
The equivalent lumped elements of the cantilever can be determined by
transmission line simulation of the matching network, which routes the
cantilever signals to 50 Ohm feed lines. In the microwave electronics, the
background common-mode signal is cancelled before the amplifier stage so that
high sensitivity (below 1 atto-Farad capacitance changes) is obtained.
Experimental characterization of the microwave probes was performed on
ion-implanted Si wafers and patterned semiconductor samples. Pure electrical or
topographical signals can be realized using different reflection modes of the
probe.Comment: 7 figure
The Demographics of Broad Line Quasars in the Mass-Luminosity Plane II. Black Hole Mass and Eddington Ratio Functions
We employ a flexible Bayesian technique to estimate the black hole mass and
Eddington ratio functions for Type 1 (i.e., broad line) quasars from a
uniformly-selected data set of ~58,000 quasars from the SDSS DR7. We find that
the SDSS becomes significantly incomplete at M_{BH} < 3 x 10^8 M_{Sun} or L /
L_{Edd} < 0.07, and that the number densities of Type 1 quasars continue to
increase down to these limits. Both the mass and Eddington ratio functions show
evidence of downsizing, with the most massive and highest Eddington ratio black
holes experiencing Type 1 quasar phases first, although the Eddington ratio
number densities are flat at z < 2. We estimate the maximum Eddington ratio of
Type 1 quasars in the observable Universe to be L / L_{Edd} ~ 3. Consistent
with our results in Paper I, we do not find statistical evidence for a
so-called "sub-Eddington boundary" in the mass-luminosity plane of broad line
quasars, and demonstrate that such an apparent boundary in the observed
distribution can be caused by selection effect and errors in virial BH mass
estimates. Based on the typical Eddington ratio in a given mass bin, we
estimate typical growth times for the black holes in Type 1 quasars and find
that they are typically comparable to or longer than the age of the universe,
implying an earlier phase of accelerated (i.e., with higher Eddington ratios)
and possibly obscured growth. The large masses probed by our sample imply that
most of our black holes reside in what are locally early type galaxies, and we
interpret our results within the context of models of self-regulated black hole
growth.Comment: Submitted to ApJ, 25 pages (emulateapj), 15 figures; revised to match
accepted version with primary changes to the introduction and discussion,
replaced Fig 1
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