1,004 research outputs found
The Relationship Between Luminosity and Broad-Line Region Size in Active Galactic Nuclei
We reinvestigate the relationship between the characteristic broad-line
region size (R_blr) and the Balmer emission-line, X-ray, UV, and optical
continuum luminosities. Our study makes use of the best available
determinations of R_blr for a large number of active galactic nuclei (AGNs)
from Peterson et al. Using their determinations of R_blr for a large sample of
AGNs and two different regression methods, we investigate the robustness of our
correlation results as a function of data sub-sample and regression technique.
Though small systematic differences were found depending on the method of
analysis, our results are generally consistent. Assuming a power-law relation
R_blr \propto L^\alpha, we find the mean best-fitting \alpha is about
0.67+/-0.05 for the optical continuum and the broad H\beta luminosity, about
0.56+/-0.05 for the UV continuum luminosity, and about 0.70+/-0.14 for the
X-ray luminosity. We also find an intrinsic scatter of about 40% in these
relations. The disagreement of our results with the theoretical expected slope
of 0.5 indicates that the simple assumption of all AGNs having on average same
ionization parameter, BLR density, column density, and ionizing spectral energy
distribution, is not valid and there is likely some evolution of a few of these
characteristics along the luminosity scale.Comment: 11 pages, 2 figures, emulateapj, accepted for publication in The
Astrophysical Journa
Symbiosis-specific changes in dimethylsulphoniopropionate concentrations in Stylophora pistillata along a depth gradient
Scleractinian corals are prolific producers of dimethylsulphoniopropionate (DMSP), but ecophysiological mechanisms influencing cellular concentrations are uncertain. While DMSP is often proposed to function as an antioxidant, interactions between specific hostâsymbiont genotype associations, plasticity in DMSP concentrations and environmental conditions that can either exert or alleviate oxidative stress are unclear. We used long-term (6Â months) reciprocal transplantation of Stylophora pistillata hosting two distinct symbiont phylotypes along a depth gradient, clades A (20Â m), to assess the effect of change in depth (light intensity) on DMSP concentrations in relation to symbiont genotype and photoacclimation in corals between 3 and 50Â m in the Gulf of Aqaba. Bathymetric distribution of total DMSP (DMSPt) per cell varied significantly while particulate DMSP (DMSPp) appeared to be unaffected by depth. Highest DMSPt concentrations in control corals occurred at 20Â m. While 3-m transplants showed a significant increase in DMSPt concentration at 20Â m and became affiliated with an additional genotype (C72), 50-m transplants largely persisted with their original genotype and exhibited no significant changes in DMSPt concentrations. DMSPt concentrations in transplants at both 3 and 50Â m, on the other hand, increased significantly while all corals maintained their original symbiont genotypes. Photoacclimation differed significantly with transplantation direction relative to the controls. Symbionts in 3-m transplants at 20Â m exhibited no changes in chlorophyll a (chl a) concentration, cell density or cell diameter while symbiont densities decreased and chl a concentrations increased significantly at 50Â m. In contrast, symbiont densities in 50-m transplants remained unaffected across depths while symbiont diameters decreased. Chl a concentrations decreased at 20Â m and increased at 3Â m. Our results indicate that DMSPt concentrations following changes in depth are not only a function of symbiont genotype but result from different acclimation abilities of both symbiotic partners
Periodic optical variability and debris accretion in white dwarfs: a test for a causal connection
Recent Kepler photometry has revealed that about half of white dwarfs (WDs)
have periodic, low-level (~ 1e-4 - 1e-3), optical variations. Hubble Space
Telescope (HST) ultraviolet spectroscopy has shown that up to about one half of
WDs are actively accreting rocky planetary debris, as evidenced by the presence
of photospheric metal absorption lines. We have obtained HST ultraviolet
spectra of seven WDs that have been monitored for periodic variations, to test
the hypothesis that these two phenomena are causally connected, i.e. that the
optical periodic modulation is caused by WD rotation coupled with an
inhomogeneous surface distribution of accreted metals. We detect photospheric
metals in four out of the seven WDs. However, we find no significant
correspondence between the existence of optical periodic variability and the
detection of photospheric ultraviolet absorption lines. Thus the null
hypothesis stands, that the two phenomena are not directly related. Some other
source of WD surface inhomogeneity, perhaps related to magnetic field strength,
combined with the WD rotation, or alternatively effects due to close binary
companions, may be behind the observed optical modulation. We report the
marginal detection of molecular hydrogen in WD J1949+4734, only the fourth
known WD with detected H_2 lines. We also re-classify J1926+4219 as a
carbon-rich He-sdO subdwarf.Comment: MNRAS, in pres
The Narrow-Line Regions of LINERs as Resolved with the Hubble Space Telescope
LINERs exist in the nuclei of a large fraction of luminous galaxies, but
their connection with the AGN phenomenon has remained elusive. We present
Hubble Space Telescope narrowband [O III]5007 and H-alpha+[N II] emission-line
images of the central regions of 14 galaxies with LINER nuclei. The compact, ~1
arcsec-scale, unresolved emission that dominates the line flux in ground-based
observations is mostly resolved by HST. The bulk of this emission comes from
regions with sizes of tens to hundreds of parsecs that are resolved into knots,
filaments, and diffuse gas whose morphology differs from galaxy to galaxy. Most
of the galaxies do not show clear linear structures or ionization cones
analogous to those often seen in Seyfert galaxies. An exception is NGC 1052,
the prototypical LINER, in which we find a 3 arcsec-long (~ 250 pc) biconical
structure that is oriented on the sky along the galaxy's radio jet axis. Seven
of the galaxies have been shown in previously published HST images to have a
bright compact ultraviolet nuclear source, while the other seven do not have a
central UV source. Our images find evidence of dust in the nuclear regions of
all 14 galaxies, with clear indications of nuclear obscuration in most of the
"UV-dark" cases. The data suggest that the line-emitting gas in most LINERs is
photoionized by a central source (which may be stellar, nonstellar, or a
combination thereof) but that this source is often hidden from direct view. We
find no obvious morphological differences between LINERs with detected weak
broad H-alpha wings in their spectra and those with only narrow lines.
Likewise, there is no clear morphological distinction between objects whose UV
spectra are dominated by hot stars (e.g., NGC 4569) and those that are AGN-like
(e.g., NGC 4579).Comment: Accepted for publication in the ApJ. 25 pages, 3 tables, 9 JPEG
Figure
The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies
LINER nuclei, which are present in many nearby galactic bulges, may be the
manifestation of low-rate or low-radiative-efficiency accretion onto
supermassive central black holes. However, it has been unclear whether the
compact UV nuclear sources present in many LINERs are clusters of massive
stars, rather than being directly related to the accretion process. We have
used HST to monitor the UV variability of a sample of 17 galaxies with LINER
nuclei and compact nuclear UV sources. Fifteen of the 17 galaxies were observed
more than once, with two to five epochs per galaxy, spanning up to a year. We
detect significant variability in most of the sample, with peak-to-peak
amplitudes from a few percent to 50%. In most cases, correlated variations are
seen in two independent bands (F250W and F330W). Comparison to previous UV
measurements indicates, for many objects, long-term variations by factors of a
few over decade timescales. Variability is detected in LINERs with and without
detected compact radio cores, in LINERs that have broad H-alpha wings detected
in their optical spectra (``LINER 1's''), and in those that do not (``LINER
2s''). This variability demonstrates the existence of a non-stellar component
in the UV continuum of all types, and sets a lower limit to the luminosity of
this component. We note a trend in the UV color (F250W/F330W) with spectral
type - LINER 1s tend to be bluer than LINER 2s. This trend may indicate a link
between the shape of the nonstellar continuum and the presence or the
visibility of a broad-line region. In one target, the post-starburst galaxy NGC
4736, we detect variability in a previously noted UV source that is offset by
2.5" (60 pc in projection) from the nucleus. This may be the nearest example of
a binary active nucleus, and of the process leading to black hole merging.Comment: accepted to Ap
The Lensed Arc Production Efficiency of Galaxy Clusters: A Comparison of Matched Observed and Simulated Samples
We compare the statistical properties of giant gravitationally lensed arcs
produced in matched simulated and observed cluster samples. The observed sample
consists of 10 X-ray selected clusters at redshifts z ~ 0.2 imaged with HST by
Smith et al. The simulated dataset is produced by lensing the Hubble Deep
Field, which serves as a background source image, with 150 realizations
(different projections and shifts) of five simulated z = 0.2 clusters from a
LambdaCDM N-body simulation. The real and simulated clusters have similar
masses, the real photometric redshift is used for each background source, and
all the observational effects influencing arc detection in the real dataset,
including light from cluster galaxies, are simulated in the artificial dataset.
We develop, and apply to both datasets, an objective automatic arc-finding
algorithm. We find consistent arc statistics in the real and in the simulated
sample, with an average of ~ 1 detected giant (length to width ratio >= 10) arc
per cluster and ~ 0.2 giant luminous (R<22.3 mag) arc per cluster. Thus, taking
into account a realistic source population and observational effects, the
clusters predicted by LambdaCDM have the same arc-production efficiency as the
observed clusters. If, as suggested by other studies, there is a discrepancy
between the predicted and the observed total number of arcs on the sky, it must
be the result of differences between the redshift dependent cluster mass
functions, and not due to differences in the lensing efficiency of the most
massive clusters.Comment: 13 pages, Accepted by ApJ, High resolution version of the paper can
be found at: ftp://wise3.tau.ac.il/pub/assafh/horesh_arcs_stat_2005.ps.gz,
Arc-finding algorithm available at: http://wise-obs.tau.ac.il/~assafh/ ; A
comment was added ; A missing x-axis label in Fig. 7 was adde
Measurement of the Broad Line Region Size in Two Bright Quasars
We present 4 years of spectrophotometric monitoring data for two radio-quiet
quasars, PG 0804+762 and PG 0953+414, with typical sampling intervals of
several months. Both sources show continuum and emission line variations. The
variations of the H line follow those of the continuum with a time lag,
as derived from a cross-correlation analysis, of 9330 days for PG 0804+762
and 11155 days for PG 0953+414. This is the first reliable measurement of
such a lag in active galactic nuclei with luminosity erg s.
The broad line region (BLR) size that is implied is almost an order of
magnitude larger than that measured in several Seyfert 1 galaxies and is
consistent with the hypothesis that the BLR size grows as .Comment: 5 pages, LaTeX (including aas2pp4 and epsf), including 4 EPS figures.
Accepted for publication in Astrophysical Journal Letter
A NWB-based dataset and processing pipeline of human single-neuron activity during a declarative memory task
A challenge for data sharing in systems neuroscience is the multitude of different data formats used. Neurodata Without Borders: Neurophysiology 2.0 (NWB:N) has emerged as a standardized data format for the storage of cellular-level data together with meta-data, stimulus information, and behavior. A key next step to facilitate NWB:N adoption is to provide easy to use processing pipelines to import/export data from/to NWB:N. Here, we present a NWB-formatted dataset of 1863 single neurons recorded from the medial temporal lobes of 59 human subjects undergoing intracranial monitoring while they performed a recognition memory task. We provide code to analyze and export/import stimuli, behavior, and electrophysiological recordings to/from NWB in both MATLAB and Python. The data files are NWB:N compliant, which affords interoperability between programming languages and operating systems. This combined data and code release is a case study for how to utilize NWB:N for human single-neuron recordings and enables easy re-use of this hard-to-obtain data for both teaching and research on the mechanisms of human memory
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