1,304 research outputs found
SALT Long-slit Spectroscopy of Luminous Obscured Quasars: An Upper Limit on the Size of the Narrow-Line Region?
We present spatially resolved long-slit spectroscopy from the Southern
African Large Telescope (SALT) to examine the spatial extent of the narrow-line
regions (NLRs) of a sample of 8 luminous obscured quasars at 0.10 < z < 0.43.
Our results are consistent with an observed shallow slope in the relationship
between NLR size and L_[OIII], which has been interpreted to indicate that NLR
size is limited by the density and ionization state of the NLR gas rather than
the availability of ionizing photons. We also explore how the NLR size scales
with a more direct measure of instantaneous AGN power using mid-IR photometry
from WISE, which probes warm to hot dust near the central black hole and so,
unlike [OIII], does not depend on the properties of the NLR. Using our results
as well as samples from the literature, we obtain a power-law relationship
between NLR size and L_8micron that is significantly steeper than that observed
for NLR size and L_[OIII]. We find that the size of the NLR goes approximately
as L^(1/2)_8micron, as expected from the simple scenario of constant-density
clouds illuminated by a central ionizing source. We further see tentative
evidence for a flattening of the relationship between NLR size and L_8micron at
the high luminosity end, and propose that we are seeing a limiting NLR size of
10 - 20 kpc, beyond which the availability of gas to ionize becomes too low. We
find that L_[OIII] ~ L_8micron^(1.4), consistent with a picture in which the
L_[OIII] is dependent on the volume of the NLR. These results indicate that
high-luminosity quasars have a strong effect in ionizing the available gas in a
galaxy.Comment: 9 Pages, 5 figures, accepted to Ap
Composite Spectral Energy Distributions and Infrared-Optical Colors of Type 1 and Type 2 Quasars
We present observed mid-infrared and optical colors and composite spectral
energy distributions (SEDs) of type 1 (broad-line) and 2 (narrow-line) quasars
selected from Sloan Digital Sky Survey (SDSS) spectroscopy. A significant
fraction of powerful quasars are obscured by dust, and are difficult to detect
in optical photometric or spectroscopic surveys. However these may be more
easily identified on the basis of mid-infrared (MIR) colors and SEDs. Using
samples of SDSS type 1 type 2 matched in redshift and [OIII] luminosity, we
produce composite rest-frame 0.2-15 micron SEDs based on SDSS, UKIDSS, and
Wide-Field Infrared Survey Explorer (WISE) photometry and perform model fits
using simple galaxy and quasar SED templates. The SEDs of type 1 and 2 quasars
are remarkably similar, with the differences explained primarily by the
extinction of the quasar component in the type 2 systems. For both types of
quasar, the flux of the AGN relative to the host galaxy increases with AGN
luminosity (L_[OIII]) and redder observed MIR color, but we find only weak
dependencies of the composite SEDs on mechanical jet power as determined
through radio luminosity. We conclude that luminous quasars can be effectively
selected using simple MIR color criteria similar to those identified previously
(W1-W2 > 0.7 [Vega]), although these criteria miss many heavily obscured
objects. Obscured quasars can be further identified based on optical-IR colors
(for example, (u-W3 [AB]) > 1.4(W1-W2 [Vega])+3.2). These results illustrate
the power of large statistical studies of obscured quasars selected on the
basis of mid-IR and optical photometry.Comment: Accepted for publication in ApJ; 14 pages, 9 figures, 2 tables;
composite Type 1 and Type 2 quasar SEDs available at
http://www.dartmouth.edu/~hickox/Hickox2017_QSO_SED_Table1.tx
Gemini Long-slit Observations of Luminous Obscured Quasars: Further Evidence for an Upper Limit on the Size of the Narrow-Line Region
We examine the spatial extent of the narrow-line regions (NLRs) of a sample
of 30 luminous obscured quasars at observed with spatially
resolved Gemini-N GMOS long-slit spectroscopy. Using the [OIII]
emission feature, we estimate the size of the NLR using a cosmology-independent
measurement: the radius where the surface brightness falls to 10 erg
s cm arcsec. We then explore the effects of atmospheric
seeing on NLR size measurements and conclude that direct measurements of the
NLR size from observed profiles are too large by 0.1 - 0.2 dex on average, as
compared to measurements made to best-fit S\'{e}rsic or Voigt profiles
convolved with the seeing. These data, which span a full order of magnitude in
IR luminosity () also provide strong evidence that there is a flattening of the
relationship between NLR size and AGN luminosity at a seeing-corrected size of
kpc. The objects in this sample have high luminosities which place
them in a previously under-explored portion of the size-luminosity
relationship. These results support the existence of a maximal size of the
narrow-line region around luminous quasars; beyond this size either there is
not enough gas, or the gas is over-ionized and does not produce enough
[OIII] emission.Comment: 10 pages, 6 figures, accepted for publication in the Astrophysical
Journa
A Tale of Two Narrow-Line Regions: Ionization, Kinematics, and Spectral Energy Distributions for a Local Pair of Merging Obscured Active Galaxies
We explore the gas ionization and kinematics, as well as the optical--IR
spectral energy distributions for UGC 11185, a nearby pair of merging galaxies
hosting obscured active galactic nuclei (AGNs), also known as SDSS
J181611.72+423941.6 and J181609.37+423923.0 (J1816NE and J1816SW, ). Due to the wide separation between these interacting galaxies ( kpc), observations of these objects provide a rare glimpse of the
concurrent growth of supermassive black holes at an early merger stage. We use
BPT line diagnostics to show that the full extent of the narrow line emission
in both galaxies is photoionized by an AGN and confirm the existence of a
10-kpc-scale ionization cone in J1816NE, while in J1816SW the AGN narrow-line
region is much more compact (1--2 kpc) and relatively undisturbed. Our
observations also reveal the presence of ionized gas that nearly spans the
entire distance between the galaxies which is likely in a merger-induced tidal
stream. In addition, we carry out a spectral analysis of the X-ray emission
using data from {\em XMM-Newton}. These galaxies represent a useful pair to
explore how the [\ion{O}{3}] luminosity of an AGN is dependent on the size of
the region used to explore the extended emission. Given the growing evidence
for AGN "flickering" over short timescales, we speculate that the appearances
and impact of these AGNs may change multiple times over the course of the
galaxy merger, which is especially important given that these objects are
likely the progenitors of the types of systems commonly classified as "dual
AGNs."Comment: 15 pages, 10 figures, accepted by the Astrophysical Journa
Universally Sloppy Parameter Sensitivities in Systems Biology
Quantitative computational models play an increasingly important role in
modern biology. Such models typically involve many free parameters, and
assigning their values is often a substantial obstacle to model development.
Directly measuring \emph{in vivo} biochemical parameters is difficult, and
collectively fitting them to other data often yields large parameter
uncertainties. Nevertheless, in earlier work we showed in a
growth-factor-signaling model that collective fitting could yield
well-constrained predictions, even when it left individual parameters very
poorly constrained. We also showed that the model had a `sloppy' spectrum of
parameter sensitivities, with eigenvalues roughly evenly distributed over many
decades. Here we use a collection of models from the literature to test whether
such sloppy spectra are common in systems biology. Strikingly, we find that
every model we examine has a sloppy spectrum of sensitivities. We also test
several consequences of this sloppiness for building predictive models. In
particular, sloppiness suggests that collective fits to even large amounts of
ideal time-series data will often leave many parameters poorly constrained.
Tests over our model collection are consistent with this suggestion. This
difficulty with collective fits may seem to argue for direct parameter
measurements, but sloppiness also implies that such measurements must be
formidably precise and complete to usefully constrain many model predictions.
We confirm this implication in our signaling model. Our results suggest that
sloppy sensitivity spectra are universal in systems biology models. The
prevalence of sloppiness highlights the power of collective fits and suggests
that modelers should focus on predictions rather than on parameters.Comment: Submitted to PLoS Computational Biology. Supplementary Information
available in "Other Formats" bundle. Discussion slightly revised to add
historical contex
Gemini Long-Slit Observations of Luminous Obscured Quasars: Further Evidence for an Upper Limit on the Size of the Narrow-Line Region
We examine the spatial extent of the narrow-line regions (NLRs) of a sample of 30 luminous obscured quasars at 0.4 \u3c z \u3c 0.7 observed with spatially resolved Gemini-N GMOS long-slit spectroscopy. Using the [O III] λ5007 emission feature, we estimate the size of the NLR using a cosmology-independent measurement: the radius where the surface brightness falls to 10â15 erg sâ1 cmâ2 arcsecâ2. We then explore the effects of atmospheric seeing on NLR size measurements and conclude that direct measurements of the NLR size from observed profiles are too large by 0.1-0.2 dex on average, as compared to measurements made to best-fit SĂ©rsic or Voigt profiles convolved with the seeing. These data, which span a full order of magnitude in IR luminosity (log (L 8 ÎŒm/erg sâ1) = 44.4-45.4), also provide strong evidence that there is a flattening of the relationship between NLR size and active galactic nucleus luminosity at a seeing-corrected size of ~7 kpc. The objects in this sample have high luminosities which place them in a previously under-explored portion of the size-luminosity relationship. These results support the existence of a maximal size of the NLR around luminous quasars; beyond this size, there is either not enough gas or the gas is over-ionized and does not produce enough [O III] λ5007 emission
A Spectroscopic Survey of Wise -Selected Obscured Quasars with the Southern African Large Telescope
We present the results of an optical spectroscopic survey of a sample of 40 candidate obscured quasars identified on the basis of their mid-infrared emission detected by the Wide-Field Infrared Survey Explorer (WISE). Optical spectra for this survey were obtained using the Robert Stobie Spectrograph on the Southern African Large Telescope. Our sample was selected with WISE colors characteristic of active galactic nuclei (AGNs), as well as red optical to mid-IR colors indicating that the optical/UV AGN continuum is obscured by dust. We obtain secure redshifts for the majority of the objects that comprise our sample (35/40), and find that sources that are bright in the WISE W4 (22 ÎŒm) band are typically at moderate redshift (z = 0.35) while sources fainter in W4 are at higher redshifts (z = 0.73). The majority of the sources have narrow emission lines with optical colors and emission line ratios of our WISE-selected sources that are consistent with the locus of AGN on the rest-frame g â z color versus [Ne III] λ3869/[O II] λλ3726+3729 line ratio diagnostic diagram. We also use empirical AGN and galaxy templates to model the spectral energy distributions (SEDs) for the objects in our sample, and find that while there is significant variation in the observed SEDs for these objects, the majority require a strong AGN component. Finally, we use the results from our analysis of the optical spectra and the SEDs to compare our selection criteria to alternate criteria presented in the literature. These results verify the efficacy of selecting luminous obscured AGNs based on their WISE colors
There are no multiply-perfect Fibonacci numbers
Here, we show that no Fibonacci number (larger than 1) divides the sum of its divisors
Effects of Short-Term Cattle Exclusion on Plant Community Composition: Prairie Dog and Ecological Site Influences
On the Ground Maintaining cattle and prairie dogs on rangelands is important ecologically, economically, and culturally. However, competition between these species, both actual and perceived, has led to conflict. We explored the effects of short-term (2-year) cattle exclusion on plant communities both on and off prairie dog towns and among three common ecological sites. Plant communities were different between on-town and off-town plots and among ecological sites but were similar between cattle-excluded and nonexcluded plots. Plant community composition did not differ between rangeland targeted for moderate forage utilization and that in which cattle had been excluded for 2 years
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