3,576 research outputs found
What shapes the far-infrared spectral energy distributions of galaxies?
To explore the connection between the global physical properties of galaxies
and their far-infrared (FIR) spectral energy distributions (SEDs), we study the
variation in the FIR SEDs of a set of hydrodynamically simulated galaxies that
are generated by performing dust radiative transfer in post-processing. Our
sample includes both isolated and merging systems at various stages of the
merging process and covers infrared (IR) luminosities and dust masses that are
representative of both low- and high-redshift galaxies. We study the FIR SEDs
using principle component analysis (PCA) and find that 97\% of the variance in
the sample can be explained by two principle components (PCs). The first PC
characterizes the wavelength of the peak of the FIR SED, and the second encodes
the breadth of the SED. We find that the coefficients of both PCs can be
predicted well using a double power law in terms of the IR luminosity and dust
mass, which suggests that these two physical properties are the primary
determinants of galaxies' FIR SED shapes. Incorporating galaxy sizes does not
significantly improve our ability to predict the FIR SEDs. Our results suggest
that the observed redshift evolution in the effective dust temperature at fixed
IR luminosity is not driven by geometry: the SEDs of ultraluminous
IR galaxies (ULIRGs) are cooler than those of local ULIRGs not because the
high-redshift galaxies are more extended but rather because they have higher
dust masses at fixed IR luminosity. Finally, based on our simulations, we
introduce a two-parameter set of SED templates that depend on both IR
luminosity and dust mass.Comment: Submitted to ApJ, comments welcom
Irreversible Investment, Real Options, and Competition: Evidence from Real Estate Development
We examine the extent to which uncertainty delays investment and the effect of competition on this relationship using a sample of 1,214 condominium developments in Vancouver, Canada built from 1979-1998. We find that increases in both idiosyncratic and systematic risk lead developers to delay new real estate investments. Empirically, a one-standard deviation increase in the return volatility reduces the probability of investment by 13 percent, equivalent to a 9 percent decline in real prices. Increases in the number of potential competitors located near a project negate the negative relationship between idiosyncratic risk and development. These results support models in which competition erodes option values and provide clear evidence for the real options framework over alternatives such as simple risk aversion.
Extragalactic Background Light and Gamma-Ray Attenuation
Data from (non-) attenuation of gamma rays from active galactic nuclei (AGN)
and gamma ray bursts (GRBs) give upper limits on the extragalactic background
light (EBL) from the UV to the mid-IR that are only a little above the lower
limits from observed galaxies. These upper limits now rule out some EBL models
and purported observations, with improved data likely to provide even stronger
constraints. We present EBL calculations both based on multiwavelength
observations of thousands of galaxies and also based on semi-analytic models,
and show that they are consistent with these lower limits from observed
galaxies and with the gamma-ray upper limit constraints. Such comparisons
"close the loop" on cosmological galaxy formation models, since they account
for all the light, including that from galaxies too faint to see. We compare
our results with those of other recent works, and discuss the implications of
these new EBL calculations for gamma ray attenuation. Catching a few GRBs with
groundbased atmospheric Cherenkov Telescope (ACT) arrays or water Cherenkov
detectors could provide important new constraints on the high-redshift star
formation history of the universe.Comment: 12 pages, 8 multi-panel figures, Invited talk at the 25th Texas
Symposium on Relativistic Astrophysics, Heidelberg December 6-10, 201
Pattern recognition of satellite cloud imagery for improved weather prediction
The major accomplishment was the successful development of a method for extracting time derivative information from geostationary meteorological satellite imagery. This research is a proof-of-concept study which demonstrates the feasibility of using pattern recognition techniques and a statistical cloud classification method to estimate time rate of change of large-scale meteorological fields from remote sensing data. The cloud classification methodology is based on typical shape function analysis of parameter sets characterizing the cloud fields. The three specific technical objectives, all of which were successfully achieved, are as follows: develop and test a cloud classification technique based on pattern recognition methods, suitable for the analysis of visible and infrared geostationary satellite VISSR imagery; develop and test a methodology for intercomparing successive images using the cloud classification technique, so as to obtain estimates of the time rate of change of meteorological fields; and implement this technique in a testbed system incorporating an interactive graphics terminal to determine the feasibility of extracting time derivative information suitable for comparison with numerical weather prediction products
Numerical investigation of the Rayleigh hypothesis for electromagnetic scattering by a particle
The validity of the Rayleigh hypothesis has been a long-standing issue in the applicability of the T-matrix method to near-field calculations, and despite numerous theoretical works, the practical consequences for numerical simulations have remained unclear. Such calculations are increasingly important in the field of nanooptics, for which accurate and efficient modeling tools are in high demand. We here tackle this challenge by investigating numerically the convergence behavior of series expansions of the electric field around spheroidal particles, which provides us with unambiguous examples to clarify the conditions of convergence. This study is made possible by the combination of alternative methods to compute near-fields accurately, and crucially, the recent improvements in the calculation of T-matrix elements free from numerical instabilities, as such errors would otherwise obfuscate the intrinsic convergence properties of the field series. The resulting numerical confirmation for the range of validity of the Rayleigh hypothesis, complemented by a better understanding of the convergence behavior of the field expansions, is a crucial step toward future developments
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