738 research outputs found
Supermassive Black Holes in Active Galactic Nuclei. I. The Consistency of Black Hole Masses in Quiescent and Active Galaxies
We report the first results of a program to measure accurate stellar velocity
dispersions in the bulges of the host galaxies of active galactic nuclei (AGNs)
for which accurate black hole (BH) masses have been determined via
reverberation mapping. We find good agreement between BH masses obtained from
reverberation mapping, and from the M(BH) - sigma relation as defined by
quiescent galaxies, indicating a common relationship between active and
quiescent black holes and their large-scale environments.Comment: Submitted to ApJ
Don't Thrash: How to Cache Your Hash on Flash
This paper presents new alternatives to the well-known Bloom filter data
structure. The Bloom filter, a compact data structure supporting set insertion
and membership queries, has found wide application in databases, storage
systems, and networks. Because the Bloom filter performs frequent random reads
and writes, it is used almost exclusively in RAM, limiting the size of the sets
it can represent. This paper first describes the quotient filter, which
supports the basic operations of the Bloom filter, achieving roughly comparable
performance in terms of space and time, but with better data locality.
Operations on the quotient filter require only a small number of contiguous
accesses. The quotient filter has other advantages over the Bloom filter: it
supports deletions, it can be dynamically resized, and two quotient filters can
be efficiently merged. The paper then gives two data structures, the buffered
quotient filter and the cascade filter, which exploit the quotient filter
advantages and thus serve as SSD-optimized alternatives to the Bloom filter.
The cascade filter has better asymptotic I/O performance than the buffered
quotient filter, but the buffered quotient filter outperforms the cascade
filter on small to medium data sets. Both data structures significantly
outperform recently-proposed SSD-optimized Bloom filter variants, such as the
elevator Bloom filter, buffered Bloom filter, and forest-structured Bloom
filter. In experiments, the cascade filter and buffered quotient filter
performed insertions 8.6-11 times faster than the fastest Bloom filter variant
and performed lookups 0.94-2.56 times faster.Comment: VLDB201
The Demography of Massive Dark Objects in Galaxy Centres
We construct dynamical models for a sample of 36 nearby galaxies with Hubble
Space Telescope photometry and ground-based kinematics. The models assume that
each galaxy is axisymmetric, with a two-integral distribution function,
arbitrary inclination angle, a position-independent stellar mass-to-light ratio
Upsilon, and a central massive dark object (MDO) of arbitrary mass M_bh. They
provide acceptable fits to 32 of the galaxies for some value of M_bh and
Upsilon; the four galaxies that cannot be fit have kinematically decoupled
cores. The mass-to-light ratios inferred for the 32 well-fit galaxies are
consistent with the fundamental plane correlation Upsilon \propto L^0.2, where
L is galaxy luminosity. In all but six galaxies the models require at the 95%
confidence level an MDO of mass M_bh ~ 0.006 M_bulge = 0.006 Upsilon L. Five of
the six galaxies consistent with M_bh=0 are also consistent with this
correlation. The other (NGC 7332) has a much stronger upper limit on M_bh. We
consider various parameterizations for the probability distribution describing
the correlation of the masses of these MDOs with other galaxy properties. One
of the best models can be summarized thus: a fraction f ~0.97 of galaxies have
MDOs, whose masses are well described by a Gaussian distribution in log
(M_bh/M_bulge) of mean -2.27 and width ~0.07.Comment: 28 pages including 13 figures and 4 tables. Submitted to A
Increasing vertical resolution in US models to improve track forecasts of Hurricane Joaquin with HWRF as an example
The atmosphereâocean coupled Hurricane Weather Research and Forecast model (HWRF) developed at the National Centers for Environmental Prediction (NCEP) is used as an example to illustrate the impact of model vertical resolution on track forecasts of tropical cyclones. A number of HWRF forecasting experiments were carried out at different vertical resolutions for Hurricane Joaquin, which occurred from September 27 to October 8, 2015, in the Atlantic Basin. The results show that the track prediction for Hurricane Joaquin is much more accurate with higher vertical resolution. The positive impacts of higher vertical resolution on hurricane track forecasts suggest that National Oceanic and Atmospheric Administration/NCEP should upgrade both HWRF and the Global Forecast System to have more vertical levels
The Centers of Early-Type Galaxies with HST. VI. Bimodal Central Surface Brightness Profiles
We combine the results from several HST investigations of the central
structure of early-type galaxies to generate a large sample of parameterized
surface photometry. The studies included were those that used the "Nuker law"
to characterize the inner light distributions of the galaxies. The sample
comprises WFPC1 and WFPC2 V band observations published earlier by our group, R
band WFPC2 photometry of Rest et al., NICMOS H band photometry by Ravindranath
et al. and Quillen et al., and the BCG WFPC2 I band photometry of Laine et al.
The distribution of the logarithmic slopes of the central profiles strongly
affirms that the central structure of elliptical galaxies with Mv < -19 is
bimodal, based on both parametric and non-parametric analysis. At the HST
resolution limit, most galaxies are either power-law systems, which have steep
cusps in surface brightness, or core systems, which have shallow cusps interior
to a steeper envelope brightness distribution. A rapid transition between the
two forms occurs over the luminosity range -22 < Mv < -20, with cores
dominating at the highest luminosities, and power-laws at the lowest. There are
a few "intermediate" systems that have both cusp slopes and total luminosities
that fall within the core/power-law transition, but they are rare and do not
fill in the overall bimodal distribution of cusp slopes. These results are
inconsistent with the Ferrarese et al. Virgo Cluster Survey (VCS) analysis.
However, using galaxies common to the VCS samples, we demonstrate that the VCS
models of the cusps are either a poor match to the observations or consist of
forms fitted to the galaxy envelopes and extrapolated inward to the HST
resolution limit.Comment: Submitted to the Astrophysical Journal; 60 pages and 20 postscript
figure
Excited TBA Equations II: Massless Flow from Tricritical to Critical Ising Model
We consider the massless tricritical Ising model M(4,5) perturbed by the
thermal operator in a cylindrical geometry and apply integrable boundary
conditions, labelled by the Kac labels (r,s), that are natural off-critical
perturbations of known conformal boundary conditions. We derive massless
thermodynamic Bethe ansatz (TBA) equations for all excitations by solving, in
the continuum scaling limit, the TBA functional equation satisfied by the
double-row transfer matrices of the A_4 lattice model of Andrews, Baxter and
Forrester (ABF) in Regime IV. The resulting TBA equations describe the massless
renormalization group flow from the tricritical to critical Ising model. As in
the massive case of Part I, the excitations are completely classified in terms
of (m,n) systems but the string content changes by one of three mechanisms
along the flow. Using generalized q-Vandemonde identities, we show that this
leads to a flow from tricritical to critical Ising characters. The excited TBA
equations are solved numerically to follow the continuous flows from the UV to
the IR conformal fixed points.Comment: 26 pages, 9 figure
Imaging of the protoelliptical NGC 1700 and its globular cluster system
An excellent candidate for a young elliptical, or `protoelliptical' galaxy is
NGC 1700. Here we present new B, V and I band imaging using the Keck telescope
and reanalyse existing V and I band images from the Hubble Space Telescope.
After subtracting a model of the galaxy from the Keck images NGC 1700 reveals
two symmetric tidal tail-like structures. If this interpretation is correct, it
suggests a past merger event involving two spiral galaxies. These tails are
largely responsible for the `boxiness' of the galaxy isophotes observed at a
radius of about 13 kpc. We also show that the B-I colour distribution of the
globular cluster system is bimodal. The mean colour of the blue population is
consistent with those of old Galactic globular clusters. Relative to this old,
metal poor population, we find that the red population is younger and more
metal rich. This young population has a similar age and metallicity as that
inferred for the central stars, suggesting that they are both associated with
an episode of star formation triggered by the merger that may have formed the
galaxy. Although possessing large errors, we find that the majority of the age
estimates of NGC 1700 are reasonably consistent and we adopt a `best estimate'
for the age of 3.0 +/-1.0 Gyr. This relatively young age places NGC 1700 within
the age range where there is a notable lack of obvious candidates for
protoellipticals. The total globular cluster specific frequency is rather low
for a typical elliptical, even after taking into account fading of the galaxy
over the next 10 Gyr. We speculate that NGC 1700 will eventually form a
relatively `globular cluster poor' elliptical galaxy.Comment: 15 pages, 15 figures, accepted for publication in MNRA
The Relationship Between Black Hole Mass and Velocity Dispersion in Seyfert 1 Galaxies
Black hole masses in active galactic nuclei (AGN) are difficult to measure
using conventional dynamical methods, but can be determined using the technique
of reverberation mapping. However, it is important to verify that the results
of these different methods are equivalent. This can be done indirectly, using
scaling relations between the black hole and the host galaxy spheroid. For this
purpose, we have obtained new measurements of the bulge stellar velocity
dispersion, sigma, in Seyfert 1 galaxies. These are used in conjunction with
the M_bh -- sigma relation to validate nuclear black hole masses, M_bh, in
active galaxies determined through reverberation mapping. We find that Seyfert
galaxies follow the same M_bh -- sigma relation as non-active galaxies,
indicating that reverberation mapping measurements of M_bh are consistent with
those obtained using other methods. We also reconsider the relationship between
bulge absolute magnitude, M_bulge, and black hole mass. We find that Seyfert
galaxies are offset from non-active galaxies, but that the deviation can be
entirely understood as a difference in bulge luminosity, not black hole mass;
Seyfert hosts are brighter than normal galaxies for a given value of their
velocity dispersion, perhaps as a result of younger stellar populations.Comment: 12 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Policy Recommendations for Meeting the Grand Challenge to Ensure Healthy Development for All Youth
This brief was created forSocial Innovation for Americaâs Renewal, a policy conference organized by the Center for Social Development in collaboration with the American Academy of Social Work & Social Welfare, which is leading theGrand Challenges for Social Work initiative to champion social progress. The conference site includes links to speeches, presentations, and a full list of the policy briefs
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