Bayesian astrostatistics: a backward look to the future

This perspective chapter briefly surveys: (1) past growth in the use of Bayesian methods in astrophysics; (2) current misconceptions about both frequentist and Bayesian statistical inference that hinder wider adoption of Bayesian methods by astronomers; and (3) multilevel (hierarchical) Bayesian modeling as a major future direction for research in Bayesian astrostatistics, exemplified in part by presentations at the first ISI invited session on astrostatistics, commemorated in this volume. It closes with an intentionally provocative recommendation for astronomical survey data reporting, motivated by the multilevel Bayesian perspective on modeling cosmic populations: that astronomers cease producing catalogs of estimated fluxes and other source properties from surveys. Instead, summaries of likelihood functions (or marginal likelihood functions) for source properties should be reported (not posterior probability density functions), including nontrivial summaries (not simply upper limits) for candidate objects that do not pass traditional detection thresholds.Comment: 27 pp, 4 figures. A lightly revised version of a chapter in "Astrostatistical Challenges for the New Astronomy" (Joseph M. Hilbe, ed., Springer, New York, forthcoming in 2012), the inaugural volume for the Springer Series in Astrostatistics. Version 2 has minor clarifications and an additional referenc

The Nature and Origin of Low-Redshift O VI Absorbers

The O VI ion observed in quasar absorption line spectra is the most accessible tracer of the cosmic metal distribution in the low redshift (z<0.5) intergalactic medium (IGM). We explore the nature and origin of O VI absorbers using cosmological hydrodynamic simulations including galactic outflows. We consider the effects of ionization background variations, non-equilibrium ionization and cooling, uniform metallicity, and small-scale (sub-resolution) turbulence. Our main results are 1) IGM O VI is predominantly photo-ionized with T= 10^(4.2+/-0.2) K. A key reason for this is that O VI absorbers preferentially trace over-enriched regions of the IGM at a given density, which enhances metal-line cooling such that absorbers can cool within a Hubble time. As such, O VI is not a good tracer of the WHIM. 2) The predicted O VI properties fit observables only if sub-resolution turbulence is added. The required turbulence increases with O VI absorber strength such that stronger absorbers arise from more recent outflows with turbulence dissipating on the order of a Hubble time. The amount of turbulence is consistent with other examples of turbulence observed in the IGM and galactic halos. 3) Metals traced by O VI and H I do not trace exactly the same baryons, but reside in the same large-scale structure. Observed alignment statistics are reproduced in our simulations. 4) Photo-ionized O VI traces gas in a variety of environments, and is not directly associated with the nearest galaxy, though is typically nearest to ~0.1L* galaxies. Weaker O VI components trace some of the oldest cosmic metals. 5) Very strong absorbers are more likely to be collisionally ionized, tracing more recent enrichment (<2 Gyr) within or near galactic halos.Comment: 33 pages, 18 figures, accepted to MNRAS. Two new figures adde

The Welfare Enhancing Effects of a Selfish Government in the Presence of Uninsurable, Idiosyncratic Risk

This paper poses the following question: Is it possible to improve welfare by increasing taxes and throwing away the revenues? This paper demonstrates that the answer to this question is “yes.” We show that there may be welfare gains from taxing capital income even when the additional capital income tax revenues are wasted or consumed by a selfish government. Previous literature has assumed that government expenditures are exogenous or productive, or allowed for redistribution of tax revenue either via lump-sum transfers, unemployment compensation or other redistributive schemes. In our model a selfish government taxes capital above a given threshold and then consumes the proceeds. This raises the before-tax real return on capital and and thereby enhances the ability of agents to self-insure when they are long-term unemployed and have low savings. Since all agents have positive probability of finding themselves in that state there are cases where all agents prefer a selfish government to no government at all.capital income tax, selfish government, welfare improvement, redistribution

Detection of an inner gaseous component in a Herbig Be star accretion disk: Near- and mid-infrared spectro-interferometry and radiative transfer modeling of MWC 147

We study the geometry and the physical conditions in the inner (AU-scale) circumstellar region around the young Herbig Be star MWC 147 using long-baseline spectro-interferometry in the near-infrared (NIR K-band, VLTI/AMBER observations and PTI archive data) as well as the mid-infrared (MIR N-band, VLTI/MIDIobservations). The emission from MWC 147 is clearly resolved and has a characteristic physical size of approx. 1.3 AU and 9 AU at 2.2 micron and 11 micron respectively (Gaussian diameter). The spectrally dispersed AMBER and MIDI interferograms both show a strong increase in the characteristic size towards longer wavelengths, much steeper than predicted by analytic disk models assuming power-law radial temperature distributions. We model the interferometric data and the spectral energy distribution of MWC 147 with 2-D, frequency-dependent radiation transfer simulations. This analysis shows that models of spherical envelopes or passive irradiated Keplerian disks (with vertical or curved puffed-up inner rim) can easily fit the SED, but predict much lower visibilities than observed; the angular size predicted by such models is 2 to 4 times larger than the size derived from the interferometric data, so these models can clearly be ruled out. Models of a Keplerian disk with optically thick gas emission from an active gaseous disk (inside the dust sublimation zone), however, yield a good fit of the SED and simultaneously reproduce the absolute level and the spectral dependence of the NIR and MIR visibilities. We conclude that the NIR continuum emission from MWC 147 is dominated by accretion luminosity emerging from an optically thick inner gaseous disk, while the MIR emission also contains contributions from the outer, irradiated dust disk.Comment: 44 pages, 15 figures, accepted for publication in The Astrophysical Journal. The quality of the figures was slightly reduced in order to comply with the astro-ph file-size restrictions. You can find a high-quality version of the paper at http://www.mpifr-bonn.mpg.de/staff/skraus/papers/mwc147.pd

Adaptive intermittent control: A computational model explaining motor intermittency observed in human behavior

It is a fundamental question how our brain performs a given motor task in a real-time fashion with the slow sensorimotor system. Computational theory proposed an influential idea of feed-forward control, but it has mainly treated the case that the movement is ballistic (such as reaching) because the motor commands should be calculated in advance of movement execution. As a possible mechanism for operating feed-forward control in continuous motor tasks (such as target tracking), we propose a control model called "adaptive intermittent control" or "segmented control," that brain adaptively divides the continuous time axis into discrete segments and executes feed-forward control in each segment. The idea of intermittent control has been proposed in the fields of control theory, biological modeling and nonlinear dynamical system. Compared with these previous models, the key of the proposed model is that the system speculatively determines the segmentation based on the future prediction and its uncertainty. The result of computer simulation showed that the proposed model realized faithful visuo-manual tracking with realistic sensorimotor delays and with less computational costs (i.e., with fewer number of segments). Furthermore, it replicated "motor intermittency", that is, intermittent discontinuities commonly observed in human movement trajectories. We discuss that the temporally segmented control is an inevitable strategy for brain which has to achieve a given task with small computational (or cognitive) cost, using a slow control system in an uncertain variable environment, and the motor intermittency is the side-effect of this strategy

Commercial Mortgage Prepayments Under Heterogeneous Prepayment Penalty Structures

Much of the literature on pricing commercial mortgages and commercial mortgage-backed securities has assumed homogeneity in prepayment penalty structure. In this paper, we provide evidence that such an assumption is inappropriate and examine the effect of penalty structures observed in actual contracts. After conducting preliminary simulations, we present hazard models estimated from data on 1,165 multifamily mortgage loans to show how empirical prepayment rates vary with alternative penalty structures. While yield maintenance and lockout provisions are relatively more effective than fixed or step down structures in reducing or postponing prepayment, none completely eliminates the risk. Our empirical results generally confirm the theoretical findings of Kelly and Slawson (2001).

Russian gas imports in Europe: how does Gazprom reliability change the game?.

Europe’s dependence on Russian gas imports has been the subject of increasing political concern after gas conflicts between Russia and Ukraine in 2006 and 2009. This paper assesses the potential impact of Russian unreliability on the European gas market, and how it affects European gas import strategy. We also study to what extent Europe should invest in strategic gas storage capacity to mitigate the effects of possible Russian unreliability. The European gas import market is described by differentiated competition between Russia and a – more reliable – competitive fringe of other exporters. The results show that Russian contract volumes and prices decline significantly as a function of unreliability, so that not only Europe but also Russia suffers if Russia’s unreliability increases. For Europe, buying gas from more reliable suppliers at a price premium turns out to be generally more attractive than building strategic gas storage capacity.

"The Welfare Enhancing Effects of a Selfish Government in the Presence of Uninsurable, Idiosyncratic Risk"

This paper poses the following question: Is it possible to improve welfare by increasing taxes and throwing away the revenues? This paper demonstrates that the answer to this question is "yes." We show that there may be welfare gains from taxing capital income even when the additional capital income tax revenues are wasted or consumed by a selfish government. Previous literature has assumed that government expenditures are exogenous or productive, or allowed for redistribution of tax revenue either via lump-sum transfers, unemployment compensation or other redistributive schemes. In our model a selfish government taxes capital above a given threshold and then consumes the proceeds. This raises the before-tax real return on capital and and thereby enhances the ability of agents to self-insure when they are long-term unemployed and have low savings. Since all agents have positive probability of finding themselves in that state there are cases where all agents prefer a selfish government to no government at all.