11,156 research outputs found
Dissecting the quasar main sequence: insight from host galaxy properties
The diverse properties of broad-line quasars appear to follow a well-defined
main sequence along which the optical FeII strength increases. It has been
suggested that this sequence is mainly driven by the Eddington ratio (L/L_Edd)
of the black hole (BH) accretion. Shen & Ho demonstrated with quasar clustering
analysis that the average BH mass decreases with increasing FeII strength when
quasar luminosity is fixed, consistent with this suggestion. Here we perform an
independent test by measuring the stellar velocity dispersion sigma* (hence the
BH mass via the M-sigma* relation) from decomposed host spectra in low-redshift
Sloan Digital Sky Survey quasars. We found that at fixed quasar luminosity,
sigma* systematically decreases with increasing FeII strength, confirming that
Eddington ratio increases with FeII strength. We also found that at fixed
luminosity and FeII strength, there is little dependence of sigma* on the broad
Hbeta FWHM. These new results reinforce the framework put forward by Shen & Ho
that Eddington ratio and orientation govern most of the diversity seen in
broad-line quasar properties.Comment: ApJL in press; 5 pages and 4 figure
On the Eccentricity Distribution of Exoplanets from Radial Velocity Surveys
We investigate the estimation of orbital parameters by least-
Keplerian fits to radial velocity (RV) data using synthetic data sets. We find
that while the fitted period is fairly accurate, the best-fit eccentricity and
are systematically biased upward from the true values for low
signal-to-noise ratio and moderate number of observations
, leading to a suppression of the number of nearly
circular orbits. Assuming intrinsic distributions of orbital parameters, we
generate a large number of mock RV data sets and study the selection effect on
the eccentricity distribution. We find the overall detection efficiency only
mildly decreases with eccentricity. This is because although high eccentricity
orbits are more difficult to sample, they also have larger RV amplitudes for
fixed planet mass and orbital semi-major axis. Thus the primary source of
uncertainties in the eccentricity distribution comes from biases in Keplerian
fits to detections with low-amplitude and/or small , rather than
from selection effects. Our results suggest that the abundance of
low-eccentricity exoplanets may be underestimated in the current sample and we
urge caution in interpreting the eccentricity distributions of low-amplitude
detections in future RV samples.Comment: Accepted for publication in Ap
The diversity of quasars unified by accretion and orientation
Quasars are rapidly accreting supermassive black holes at the center of
massive galaxies. They display a broad range of properties across all
wavelengths, reflecting the diversity in the physical conditions of the regions
close to the central engine. These properties, however, are not random, but
form well-defined trends. The dominant trend is known as Eigenvector 1, where
many properties correlate with the strength of optical iron and [OIII]
emission. The main physical driver of Eigenvector 1 has long been suspected to
be the quasar luminosity normalized by the mass of the hole (the Eddington
ratio), an important quantity of the black hole accretion process. But a
definitive proof has been missing. Here we report an analysis of archival data
that reveals that Eddington ratio indeed drives Eigenvector 1. We also find
that orientation plays a significant role in determining the observed
kinematics of the gas, implying a flattened, disklike geometry for the
fast-moving clouds close to the hole. Our results show that most of the
diversity of quasar phenomenology can be unified with two simple quantities,
Eddington ratio and orientation.Comment: This is the author's version of the work; 18 pages including
Supplementary Information; to appear in the 11 September 2014 issue of Nature
at http://dx.doi.org/10.1038/nature1371
Optimal regularity of minimal graphs in the hyperbolic space
We discuss the global regularity of solutions to the Dirichlet problem
for minimal graphs in the hyperbolic space when the boundary of the domain
has a nonnegative mean curvature and prove an
optimal regularity . We can improve the
H\"older exponent for if certain combinations of principal curvatures of
the boundary do not vanish, a phenomenon observed by F.-H. Lin.Comment: Accepted by Calc. Var. Partial Differential Equation
Fluctuation Induced First Order Phase Transitions
We study a symmetric scalar field model in four and three
dimensions. First, using our data in four dimensions in the weak coupling
region, we demonstrate explicitly that the observed first order phase
transition is induced by quantum fluctuations. Next, based on the
renormalization group and our new simulation results in three dimensions we
argue that even if the symmetry is restored below the critical
temperature the QCD finite temperature chiral phase transition for two flavor
could be extremely weak first order. Contribution to Lattice '93 proceedings.
Needs espcrc2.sty file (included). Search Figure1.ps, Figure2.ps, ... for
postscript files.Comment: 3 pages, 4 postscript figures attached. Preprint BUHEP-93-2
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