6,137 research outputs found
Dependence of Galaxy Shape on Environment in the Sloan Digital Sky Survey
Using a sample of galaxies from the Sloan Digital Sky Survey (SDSS) Data
Release 4, we study the trends relating surface brightness profile type and
apparent axis ratio to the local galaxy environment. We use the SDSS parameter
`fracDeV' to quantify the profile type. We find that galaxies with M_r > -18
are mostly described by exponential profiles in all environments. Galaxies with
-21 < M_r < -18 mainly have exponential profiles in low density environments
and de Vaucouleurs profiles in high density environments. The most luminous
galaxies, with M_r < -21, are mostly described by de Vaucouleurs profiles in
all environments. For galaxies with M_r < -19, the fraction of de Vaucouleurs
galaxies is a monotonically increasing function of local density, while the
fraction of exponential galaxies is monotonically decreasing. For a fixed
surface brightness profile type, apparent axis ratio is frequently correlated
with environment. As the local density of galaxies increases, we find that for
-20 < M_r < -18, galaxies of all profile types become slightly rounder, on
average; for -22 < M_r < -20, galaxies with exponential profiles tend to become
flatter, while galaxies with de Vaucouleurs profiles become rounder; for M_r <
-22, galaxies with exponential profiles become flatter, while the de
Vaucouleurs galaxies become rounder in their inner regions, yet exhibit no
change in their outer regions. We comment on how the observed trends relate to
the merger history of galaxies.Comment: 23 pages, 7 figures, accepted by Ap
The Nature of the H2-Emitting Gas in the Crab Nebula
Understanding how molecules and dust might have formed within a rapidly
expanding young supernova remnant is important because of the obvious
application to vigorous supernova activity at very high redshift. In previous
papers, we found that the H2 emission is often quite strong, correlates with
optical low-ionization emission lines, and has a surprisingly high excitation
temperature. Here we study Knot 51, a representative, bright example, for which
we have available long slit optical and NIR spectra covering emission lines
from ionized, neutral, and molecular gas, as well as HST visible and SOAR
Telescope NIR narrow-band images. We present a series of CLOUDY simulations to
probe the excitation mechanisms, formation processes and dust content in
environments that can produce the observed H2 emission. We do not try for an
exact match between model and observations given Knot 51's ambiguous geometry.
Rather, we aim to explain how the bright H2 emission lines can be formed from
within the volume of Knot 51 that also produces the observed optical emission
from ionized and neutral gas. Our models that are powered only by the Crab's
synchrotron radiation are ruled out because they cannot reproduce the strong,
thermal H2 emission. The simulations that come closest to fitting the
observations have the core of Knot 51 almost entirely atomic with the H2
emission coming from just a trace molecular component, and in which there is
extra heating. In this unusual environment, H2 forms primarily by associative
detachment rather than grain catalysis. In this picture, the 55 H2-emitting
cores that we have previously catalogued in the Crab have a total mass of about
0.1 M_sun, which is about 5% of the total mass of the system of filaments. We
also explore the effect of varying the dust abundance. We discuss possible
future observations that could further elucidate the nature of these H2 knots.Comment: 51 pages, 15 figures, accepted for publication in MNRAS, revised
Figure 12 results unchange
Singularity in the boundary resistance between superfluid He and a solid surface
We report new measurements in four cells of the thermal boundary resistance
between copper and He below but near the superfluid-transition
temperature . For fits of to the data yielded ,
whereas a fit to theoretical values based on the renormalization-group theory
yielded . Alternatively, a good fit of the theory to the data could
be obtained if the {\it amplitude} of the prediction was reduced by a factor
close to two. The results raise the question whether the boundary conditions
used in the theory should be modified.Comment: 4 pages, 4 figures, revte
Albedo and Reflection Spectra of Extrasolar Giant Planets
We generate theoretical albedo and reflection spectra for a full range of
extrasolar giant planet (EGP) models, from Jovian to 51-Pegasi class objects.
Our albedo modeling utilizes the latest atomic and molecular cross sections, a
Mie theory treatment of extinction by condensates, a variety of particle size
distributions, and an extension of the Feautrier radiative transfer method
which allows for a general treatment of the scattering phase function. We find
that due to qualitative similarities in the compositions and spectra of objects
within each of five broad effective temperature ranges, it is natural to
establish five representative EGP albedo classes: a ``Jovian'' class (T K; Class I) with tropospheric ammonia clouds, a ``water
cloud'' class (T K; Class II) primarily affected by
condensed HO, a ``clear'' class (T K; Class III)
which lacks clouds, and two high-temperature classes: Class IV (900 K
T 1500 K) for which alkali metal absorption
predominates, and Class V (T 1500 K and/or low surface
gravity ( 10 cm s)) for which a high silicate layer
shields a significant fraction of the incident radiation from alkali metal and
molecular absorption. The resonance lines of sodium and potassium are expected
to be salient features in the reflection spectra of Class III, IV, and V
objects. We derive Bond albedos and effective temperatures for the full set of
known EGPs and explore the possible effects of non-equilibrium condensed
products of photolysis above or within principal cloud decks. As in Jupiter,
such species can lower the UV/blue albedo substantially, even if present in
relatively small mixing ratios.Comment: revised LaTeX manuscript accepted to Ap.J.; also available at
http://jupiter.as.arizona.edu/~burrows/paper
The Mass of the Black Hole in the Seyfert 1 Galaxy NGC 4593 from Reverberation Mapping
We present new observations leading to an improved black hole mass estimate
for the Seyfert 1 galaxy NGC 4593 as part of a reverberation-mapping campaign
conducted at the MDM Observatory. Cross-correlation analysis of the H_beta
emission-line light curve with the optical continuum light curve reveals an
emission-line time delay of 3.73 (+-0.75) days. By combining this time delay
with the H_beta line width, we derive a central black hole mass of M_BH =
9.8(+-2.1)x10^6 M_sun, an improvement in precision of a factor of several over
past results.Comment: 22 pages, 3 tables, 5 figures, accepted for publication in Ap
Cloud type comparisons of AIRS, CloudSat, and CALIPSO cloud height and amount
The precision of the two-layer cloud height fields derived from the Atmospheric Infrared Sounder (AIRS) is explored and quantified for a five-day set of observations. Coincident profiles of vertical cloud structure by CloudSat, a 94 GHz profiling radar, and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), are compared to AIRS for a wide range of cloud types. Bias and variability in cloud height differences are shown to have dependence on cloud type, height, and amount, as well as whether CloudSat or CALIPSO is used as the comparison standard. The CloudSat-AIRS biases and variability range from &minus;4.3 to 0.5&plusmn;1.2&ndash;3.6 km for all cloud types. Likewise, the CALIPSO-AIRS biases range from 0.6&ndash;3.0&plusmn;1.2&ndash;3.6 km (&minus;5.8 to &minus;0.2&plusmn;0.5&ndash;2.7 km) for clouds &ge;7 km (&lt;7 km). The upper layer of AIRS has the greatest sensitivity to Altocumulus, Altostratus, Cirrus, Cumulonimbus, and Nimbostratus, whereas the lower layer has the greatest sensitivity to Cumulus and Stratocumulus. Although the bias and variability generally decrease with increasing cloud amount, the ability of AIRS to constrain cloud occurrence, height, and amount is demonstrated across all cloud types for many geophysical conditions. In particular, skill is demonstrated for thin Cirrus, as well as some Cumulus and Stratocumulus, cloud types infrared sounders typically struggle to quantify. Furthermore, some improvements in the AIRS Version 5 operational retrieval algorithm are demonstrated. However, limitations in AIRS cloud retrievals are also revealed, including the existence of spurious Cirrus near the tropopause and low cloud layers within Cumulonimbus and Nimbostratus clouds. Likely causes of spurious clouds are identified and the potential for further improvement is discussed
Disciplining Skepticism Through Kant’s Critique, Fichte’s Idealism, and Hegel’s Negations
This chapter considers the encounter of skepticism with the Kantian and post-Kantian philosophical enterprise and focuses on the intriguing feature whereby it is assimilated into this enterprise. In this period, skepticism becomes interchangeable with its other, which helps understand the proliferation of many kinds of views under its name and which forms the background for transforming skepticism into an anonymous, routine practice of raising objections and counter-objections to one’s own view. German philosophers of this era counterpose skepticism to dogmatism and criticism, ancient to modern skepticism, and, importantly, conceptualize the transitions from one form to another, which forms the conceptual matrix in which new disciplinary forms, such as psychology, anthropology, and historicism contend for cultural-intellectual standing beside philosophy. I present this assimilationist trajectory by reviewing three well-known moments of this encounter of skepticism and idealism: (1) Kant’s idealization of skepticism as a floating position amidst various philosophical positions through the dialectic, polemics, systematics, and history of pure reason; (2) Fichte’s schematic conception of skepticism as a dispute of systems in the early Wissenschaftslehre following his review of the skeptic G. E. Schulze’s attacks on Critical philosophy; (3) Hegel’s historicizing conception of skepticism in the context of differences between subjective idealism and speculative thought and his early Jena review of another work by the same skeptic Schulze
NGC 5548 in a Low-Luminosity State: Implications for the Broad-Line Region
We describe results from a new ground-based monitoring campaign on NGC 5548,
the best studied reverberation-mapped AGN. We find that it was in the lowest
luminosity state yet recorded during a monitoring program, namely L(5100) = 4.7
x 10^42 ergs s^-1. We determine a rest-frame time lag between flux variations
in the continuum and the Hbeta line of 6.3 (+2.6/-2.3) days. Combining our
measurements with those of previous campaigns, we determine a weighted black
hole mass of M_BH = 6.54 (+0.26/-0.25) x 10^7 M_sun based on all broad emission
lines with suitable variability data. We confirm the previously-discovered
virial relationship between the time lag of emission lines relative to the
continuum and the width of the emission lines in NGC 5548, which is the
expected signature of a gravity-dominated broad-line region. Using this lowest
luminosity state, we extend the range of the relationship between the
luminosity and the time lag in NGC 5548 and measure a slope that is consistent
with alpha = 0.5, the naive expectation for the broad line region for an
assumed form of r ~ L^alpha. This value is also consistent with the slope
recently determined by Bentz et al. for the population of reverberation-mapped
AGNs as a whole.Comment: 24 pages, 3 tables, 7 figures, accepted for publication in Ap
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