2,385 research outputs found
The Infrared Diameter -- Velocity Dispersion Relation for Elliptical Galaxies
Using single channel infrared photometry from the literature, a provisional
K-band diameter--velocity dispersion relation for elliptical galaxies in the
Coma and Virgo clusters is derived. The Coma cluster relation has \~1.5 times
lower scatter at K than in B or V. Excluding 4 outliers, the RMS scatter at K
for 24 galaxies in Coma is only 4.8% in distance, close to the limit implied by
the observational errors. Distance estimates based on the IR \Dsig relation
will be more accurate than those derived from optical data. The improvement in
the infrared is attributed to a decrease in sensitivity to stellar population
parameters (age, metallicity, and slope of the IMF) as well as lower internal
extinction from dust compared to the optical. That the \Dsig relation has a
larger scatter in the optical indicates that there are detectable, but small,
stellar population or dust content differences among the Coma ellipticals.
Since the \Dsig relations are based on the fundamental plane, this result
promises that the fundamental plane is thinner in the infrared than it is in
the optical.
Infrared photometric data available for Virgo are limited to just 13 objects;
the spread in distance due to the depth of the cluster precludes any
significant improvement over B and V. A relative Coma-Virgo distance of 5.56 is
derived from the K band data, in agreement with estimates in other colors and
using other techniques, indicating that there is no significant age difference
between Virgo and Coma ellipticals.Comment: 10 pages, 3 Postscript figures, uuencoded and Z-compressed with csh
script uufiles; to appear in the September Astronomical Journa
Content and action: The guidance theory of representation
The current essay introduces the guidance theory of representation, according to which the content and intentionality of representations can be accounted for in terms of the way they provide guidance for action. We offer a brief account of the biological origins of representation, a formal characterization of the guidance theory, some examples of its use, and show how the guidance theory handles some traditional problem cases for representation: the problems of error and of representation of fictional and abstract entities
A brief introduction to the guidance theory of representation
Recent trends in the philosophy of mind and cognitive science can be fruitfully characterized as part of the ongoing attempt to come to grips with the very idea of homo sapiens--an intelligent, evolved, biological agent--and its signature contribution is the emergence of a philosophical anthropology which, contra Descartes and his thinking thing, instead puts doing at the center of human being. Applying this agency-oriented line of thinking to the problem of representation, this paper introduces the Guidance Theory, according to which the content and intentionality of representations can be accounted for in terms of the way they provide guidance for action. We offer a brief account of the motivation for the theory, and a formal characterization
Optical Detection Properties of Silicon-Germanium Quantum Well Structures
A study has been carried out on Si/SiGe multi quantum well structures to determine their applicability as normal incidence infrared detectors in the spectral range of 2-12 micrometers. The research effort was primarily experimental; however, extensive calculations were performed to initially explain the experimental data and then used to design subsequent structures. Multiple quantum well structures grown on both Si[001] and Si[110] substrates via molecular beam epitaxy were studied by photoluminescence, absorption, and photoresponse measurements over a wide parameter space. Variables included quantum well depth and width, well doping, number of wells and growth temperature. Well widths were varied from 20Å to 50Å, Ge composition from 10% to 60%, boron doping from 1 x 1018 cm-3 to 8 x 1019 cm-3, number of wells from 5 to 30 and growth temperature from 550 to 710 °C. Calculations using k.p theory and the envelope function approximation were performed to determine the position of the bound states in the wells, the amount of band mixing and the transition strengths for bound-to-bound transitions for Si[001]/Si1-xGe sub x, Si[110]/Si1-xGex and GaAs/AlGaAs quantum well structures. The Si[110] structures have more allowed energy bands which are significantly mixed. A comparison was made between Si[001]/Si1-xGex, Si[110]/Si1-xGex and GaAs/AlGaAs quantum well structures designed to operate in the 8-12 µm region, and all three showed comparable momentum matrix elements
The full spectral radiative properties of Proxima Centauri
The discovery of Proxima b, a terrestrial temperate planet, presents the
opportunity of studying a potentially habitable world in optimal conditions. A
key aspect to model its habitability is to understand the radiation environment
of the planet in the full spectral domain. We characterize the X-rays to mid-IR
radiative properties of Proxima with the goal of providing the
top-of-atmosphere fluxes on the planet. We also aim at constraining the
fundamental properties of the star. We employ observations from a large number
of facilities and make use of different methodologies to piece together the
full spectral energy distribution of Proxima. In the high-energy domain, we pay
particular attention to the contribution by rotational modulation, activity
cycle, and flares so that the data provided are representative of the overall
radiation dose received by the atmosphere of the planet. We present the full
spectrum of Proxima covering 0.7 to 30000 nm. The integration of the data shows
that the top-of-atmosphere average XUV irradiance on Proxima b is 0.293 W m^-2,
i.e., nearly 60 times higher than Earth, and that the total irradiance is
877+/-44 W m^-2, or 64+/-3% of the solar constant but with a significantly
redder spectrum. We also provide laws for the XUV evolution of Proxima
corresponding to two scenarios. Regarding the fundamental properties of
Proxima, we find M=0.120+/-0.003 Msun, R=0.146+/-0.007 Rsun, Teff=2980+/-80 K,
and L=0.00151+/-0.00008 Lsun. In addition, our analysis reveals a ~20% excess
in the 3-30 micron flux of the star that is best interpreted as arising from
warm dust in the system. The data provided here should be useful to further
investigate the current atmospheric properties of Proxima b as well as its past
history, with the overall aim of firmly establishing the habitability of the
planet.Comment: 12 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
First Detection of Thermal Radio Emission from Solar-Type Stars with the Karl G. Jansky Very Large Array
We present the first detections of thermal radio emission from the
atmospheres of solar-type stars {\tau} Cet, {\eta} Cas A, and 40 Eri A. These
stars all resemble the Sun in age and level of magnetic activity, as indicated
by X-ray luminosity and chromospheric emission in calcium-II H and K lines. We
observed these stars with the Karl G. Jansky Very Large Array with
sensitivities of a few {\mu}Jy at combinations of 10.0, 15.0, and 34.5 GHz.
{\tau} Cet, {\eta} Cas A, and 40 Eri A are all detected at 34.5 GHz with
signal-to-noise ratios of 6.5, 5.2, and 4.5, respectively. 15.0-GHz upper
limits imply a rising spectral index greater than 1.0 for {\tau} Cet and 1.6
for {\eta} Cas A, at the 95% confidence level. The measured 34.5-GHz flux
densities correspond to stellar disk-averaged brightness temperatures of
roughly 10,000 K, similar to the solar brightness temperature at the same
frequency. We explain this emission as optically- thick thermal free-free
emission from the chromosphere, with possible contributions from coronal
gyroresonance emission above active regions and coronal free-free emission.
These and similar quality data on other nearby solar-type stars, when combined
with ALMA observations, will enable the construction of temperature profiles of
their chromospheres and lower transition regions.Comment: 9 pages, 2 figures. Accepted for publication in Ap
An investigation of the Eigenvalue Calibration Method (ECM) using GASP for non-imaging and imaging detectors
Polarised light from astronomical targets can yield a wealth of information
about their source radiation mechanisms, and about the geometry of the
scattered light regions. Optical observations, of both the linear and circular
polarisation components, have been impeded due to non-optimised
instrumentation. The need for suitable observing conditions and the
availability of luminous targets are also limiting factors. GASP uses division
of amplitude polarimeter (DOAP) (Compain and Drevillon) to measure the four
components of the Stokes vector simultaneously, which eliminates the
constraints placed upon the need for moving parts during observation, and
offers a real-time complete measurement of polarisation. Results from the GASP
calibration are presented in this work for both a 1D detector system, and a
pixel-by-pixel analysis on a 2D detector system. Following Compain et al. we
use the Eigenvalue Calibration Method (ECM) to measure the polarimetric
limitations of the instrument for each of the two systems. Consequently, the
ECM is able to compensate for systematic errors introduced by the calibration
optics, and it also accounts for all optical elements of the polarimeter in the
output. Initial laboratory results of the ECM are presented, using APD
detectors, where errors of 0.2% and 0.1{\deg} were measured for the degree of
linear polarisation and polarisation angle respectively. Channel-to-channel
image registration is an important aspect of 2-D polarimetry. We present our
calibration results of the measured Mueller matrix of each sample, used by the
ECM. A set of Zenith flat-field images were recorded during an observing
campaign at the Palomar 200 inch telescope in November 2012. From these we show
the polarimetric errors from the spatial polarimetry indicating both the
stability and absolute accuracy of GASP.Comment: Accepted for publication in Experimental Astronom
Virgo Intergalactic Globulars from the Sloan Survey
We have identified a new sample of Virgo intergalactic globular clusters (IGCs) and ultra compact dwarfs (UCDs) which have been serendipitously observed to date in Sloan Survey spectroscopy. There are 23 new objects with secure redshifts, all relatively red point sources with reliable velocities placing them at Virgo distances. They are spread widely across Virgo, significantly extending the spatial distribution of Virgo IGCs and UCDs to regions outside the well-studied M87 core region. The new sample are generally fainter, bluer, and probably more metal poor on average than the more centrally located, previously known objects. This systematic change carries information about the formation and continued evolution by accretion of the Virgo cluster, indicating a transition to less massive and less luminous objects being tidally disrupted in the outskirts now and in the recent past, compared to conditions in the inner cluster at early epochs
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