Learning and Shifts in Long-Run Growth

Shifts in the long-run rate of productivity growth--such as those those experienced by the U.S. economy in the 1970s and 1990s--are difficult in real time to distinguish from transitory fluctuations. In this paper, we explore how economists' projections of trend productivity growth gradually evolved during the 1970s and 1990s, and examine the consequences of such real-time learning on the dynamic responses to shifts in trend growth in the context of a dynamic stochastic general equilibrium model. We find that a simple updating rule based on an estimated Kalman filter model using real-time data describes the evolution of economists' long-run growth expectations extremely well. We then show that incorporating learning in this fashion has profound implications for the dynamic effects of shifts in trend productivity growth, whether they are concentrated in the investment-goods sector or affect the entire economy. If immediately recognized, increases in the trend growth rate cause long-term interest rates to rise and produce a sharp decline in employment and investment. In contrast, with learning, a productivity acceleration sets off a sustained boom in employment and investment, and long-term interest rates rise only gradually in a pattern consistent with the experience of the 1990sKalman filter, real-time data, signal extraction

Welfare-maximizing monetary policy under parameter uncertainty

This paper examines welfare-maximizing monetary policy in an estimated micro-founded general equilibrium model of the U.S. economy where the policymaker faces uncertainty about model parameters. Uncertainty about parameters describing preferences and technology implies not only uncertainty about the dynamics of the economy. It also implies uncertainty about the model's utility-based welfare criterion and about the economy's natural rate measures of interest and output. We analyze the characteristics and performance of alternative monetary policy rules given the estimated uncertainty regarding parameter estimates. We find that the natural rates of interest and output are imprecisely estimated. We then show that, relative to the case of known parameters, optimal policy under parameter uncertainty responds less to natural-rate terms and more to other variables, such as price and wage inflation and measures of tightness or slack that do not depend on natural rates.Monetary policy

Evaluation of Ultrasonic Velocity Tests to Characterize Extraterrestrial Rock Masses

This thesis investigates the use of ultrasonic pulse velocity (UPV) tests for extraterrestrial rock characterization by evaluating the effects of physical sample properties on the signal attenuation of ultrasonic waves. An initial review of standard laboratory UPV testing methods and expected space conditions isolated four key disparities between the two testing environments: (1) sample dimensions, (2) regolith cover, (3) sample surface roughness, and (4) utility of a gel couplant. Laboratory tests were then performed to assess the influence of these parameters in simulated space conditions, using concrete beams to simulate large sample sizes, and ground limestone to simulate regolith cover. The ensuing data analysis identified strategies to ensure adequate signal transmission and data interpretation.;Overall, the experimental results show that the signal attenuation curves can be constructed and simple regression curves can provide estimations of the true P-wave velocity even without a gel couplant. For the case of large sample sizes, simple exponential regression of the measured P-wave velocity at various transmission lengths can be extrapolated to zero length to estimate the true value. For the case of regolith cover, the particle top size and particle size distribution affect signal attenuation; however, an exponential fit to measured velocity versus depth data can be used to accurately predict true sample velocity at zero depth. Additionally, testing with rough samples showed that compacted regolith can be used to ensure contact between the sample and the UPV transmitter, and this method produced similar attenuation curves to those determined from smooth samples. Altogether, these data indicate the reliability of UPV to characterize material in-situ; however, without a gel couplant, UPV measurements become increasingly imprecise. (Abstract shortened by ProQuest.)

Discovery of the Central Excess Brightness in Hard X-rays in the Cluster of Galaxies Abell 1795

Using the X-ray data from \ASCA, spectral and spatial properties of the intra-cluster medium (ICM) of the cD cluster Abell 1795 are studied, up to a radial distance of $\sim 12'$ ($\sim 1.3$ $h_{50}^{-1}$ kpc). The ICM temperature and abundance are spatially rather constant, although the cool emission component is reconfirmed in the central region. The azimuthally- averaged radial X-ray surface brightness profiles are very similar between soft (0.7--3 keV) and hard (3--10 keV) energy bands, and neither can be fitted with a single-$\beta$ model due to a strong data excess within $\sim5'$ of the cluster center. In contrast, double-$\beta$ models can successfully reproduce the overall brightness profiles both in the soft and hard energy bands, as well as that derived with the \ROSAT PSPC. Properties of the central excess brightness are very similar over the 0.2--10 keV energy range spanned by \ROSAT and \ASCA. Thus, the excess X-ray emission from the core region of this cluster is confirmed for the first time in hard X-rays above 3 keV. This indicates that the shape of the gravitational potential becomes deeper than the King-type one towards the cluster center. Radial profiles of the total gravitating matter, calculated using the double-$\beta$ model, reveal an excess mass of $\sim 3 \times 10^{13}~ M_{\odot}$ within $\sim 150 h^{-1}_{50}$ kpc of the cluster center. This suggests a hierarchy in the gravitational potential corresponding to the cD galaxy and the entire cluster.Comment: 27 pages, 8 figures; to appear ApJ 500 (June 20, 1998

The Properties of Poor Groups of Galaxies: II. X-ray and Optical Comparisons

We use ROSAT PSPC data to study the X-ray properties of a sample of twelve poor groups that have extensive membership information (Zabludoff and Mulchaey 1997; Paper I). Diffuse X-ray emission is detected in nine of these groups. In all but one of the X-ray detected groups, the X-ray emission is centered on a luminous elliptical galaxy. Fits to the surface brightness profiles of the X-ray emission suggest the presence of two X-ray components in these groups. The first component is centered on the central elliptical galaxy. The location and extent of this component, combined with its X-ray temperature and luminosity, favor an origin in the interstellar medium of the central galaxy. Alternatively, the central component may be the result of a large-scale cooling flow. The second X-ray component is detected out to a radius of at least 100-300 kpc. This component follows the same relationships found among the X-ray temperature, X-ray luminosity and optical velocity dispersion of rich clusters. This result suggests that the X-ray detected groups are low-mass versions of clusters and that the extended gas component can properly be called the intragroup medium, in analogy to the intracluster medium in clusters. We also find a trend for the position angle of the optical light in the central elliptical galaxy to align with the position angle of the large-scale X-ray emission. (Abridged)Comment: 38 pages, AASLaTeX with 16 PS figures. Figure 1a-1l available in gzipped postscript format at ftp://corvus.ociw.edu/pub/mulchae

Constraints on Type Ia Supernova Models from X-ray Spectra of Galaxy Clusters

We present constraints on theoretical models of Type Ia supernovae using spatially resolved ASCA X-ray spectroscopy of three galaxy clusters: Abell 496, Abell 2199 and Abell 3571. All three clusters have central iron abundance enhancements; an ensemble of abundance ratios are used to show that most of the iron in the central regions of the clusters comes from SN Ia. These observations are consistent with the suppressed galactic wind scenario proposed by Dupke and White (1999). At the center of each cluster, simultaneous analysis of spectra from all ASCA instruments shows that the nickel to iron abundance ratio (normalized by the solar ratio) is Ni/Fe ~ 4. We use the nickel to iron ratio as a discriminator between SN Ia explosion models: the Ni/Fe ratio of ejecta from the "Convective Deflagration" model W7 is consistent with the observations, while those of "delayed detonation" models are not consistent at the 90% confidence level.Comment: 20 pages, 2 figures, accepted by The Astrophysical Journa

Detection of Bulk Motions in the ICM of the Centaurus Cluster

Several recent numerical simulations of off-center cluster mergers predict that significant angular momentum with associated velocities of a few x 10^{3} km/s can be imparted to the resulting cluster. Such gas bulk velocities can be detected by the Doppler shift of X-ray spectral lines with ASCA spectrometers. Using two ASCA observations of the Centaurus cluster, we produced a velocity map for the gas in the cluster's central regions. We also detected radial and azimuthal gradients in temperature and metal abundance distributions, which seem to be associated with the infalling sub-group centered at NGC 4709 (Cen 45). More importantly, we found a significant (>99.8% confidence level) velocity gradient along a line near-perpendicular to the direction of the incoming sub-group and with a maximum velocity difference of ~3.4+-1.1 x 10^{3} km/s. It is unlikely (P < 0.002) that the observed velocity gradient is generated by gain fluctuations across the detectors. While the observed azimuthal temperature and abundance variations can be attributed to the interaction with Cen 45, we argue that the intracluster gas velocity gradient is more likely due to a previous off-center merging event in the main body of the Centaurus cluster.Comment: 13 pages in emulateapj5 style, 8 postscript figures; Accepted by ApJ; Revised version with minor change

Hydrodynamic simulations of merging clusters of galaxies

We present the results of high-resolution AP3M+SPH simulations of merging clusters of galaxies. We find that the compression and shocking of the core gas during a merger can lead to large increases in bolometric X-ray luminosities and emission-weighted temperatures of clusters. Cooling flows are completely disrupted during equal-mass mergers, with the mass deposition rate dropping to zero as the cores of the clusters collide. The large increase in the cooling time of the core gas strongly suggests that cooling flows will not recover from such a merger within a Hubble time. Mergers with subclumps having one eighth of the mass of the main cluster are also found to disrupt a cooling flow if the merger is head-on. However, in this case the entropy injected into the core gas is rapidly radiated away and the cooling flow restarts within a few Gyr of the merger. Mergers in which the subcluster has an impact parameter of 500 kpc do not disrupt the cooling flow, although the mass deposition rate is reduced by ∼30 per cent. Finally, we find that equal mass, off-centre mergers can effectively mix gas in the cores of clusters, while head on mergers lead to very little mixing. Gas stripped from the outer layers of subclumps results in parts of the outer layers of the main cluster being well mixed, although they have little effect on the gas in the core of the cluster. None of the mergers examined here resulted in the intracluster medium being well mixed globally

Investigation of the prevalence of thermophilic Campylobacter species at Lake Simcoe recreational beaches

Thermophilic Campylobacter species have been implicated in human gastrointestinal infections and can occur in agricultural run off, sewage discharges, and the feces of domestic and wild animals including birds. A 2-year study was designed to investigate the occurrence of the primary thermophilic Campylobacter species (C. jejuni, coli, and lari) associated with human disease at 5 recreational beaches on Lake Simcoe, Ontario, Canada. A biweekly sampling program involved collecting water samples across 3 depth zones (sand pore water and ankle- and chest-depth waters). To identify the potential sources of contamination, samples were also collected from 4 neighboring rivers corresponding to selected beaches, a few fresh seagull and Canada geese fecal droppings on beaches, and a stormwater outfall. Water and fecal samples were processed for Campylobacter spp. isolation and detection using a minimum probable number culture enrichment protocol. Thermophilic Campylobacter spp. generally occurred infrequently and at low concentrations (&le;30 cells L-1) at all sampling locations; they were detected in 12% of water samples from beaches (n&nbsp;=&nbsp;289) compared to 14% from rivers (n = 100). C. jejuni and C. lari were the species most commonly detected. Nine isolates identified as unknown Campylobacter spp. were further sequenced and shown to be more closely related to Arcobacter spp. At beaches, thermophilic Campylobacter spp. were generally detected more often in sand pore water than in ankle- or chest-depth water. The study suggests that sand, rivers, and bird droppings could be potential sources of Campylobacter spp. contamination at Lake Simcoe recreational beaches.&nbsp

Evolution of X-ray cluster scaling relations in simulations with radiative cooling and non-gravitational heating

We investigate the redshift dependence of X-ray cluster scaling relations drawn from three hydrodynamic simulations of the LCDM cosmology: a Radiative model that incorporates radiative cooling of the gas, a Preheating model that additionally heats the gas uniformly at high redshift, and a Feedback model that self-consistently heats cold gas in proportion to its local star-formation rate. While all three models are capable of reproducing the observed local Lx-Tx relation, they predict substantially different results at high redshift (to z=1.5), with the Radiative, Preheating and Feedback models predicting strongly positive, mildly positive and mildly negative evolution, respectively. The physical explanation for these differences lies in the structure of the intracluster medium. All three models predict significant temperature fluctuations at any given radius due to the presence of cool subclumps and, in the case of the Feedback simulation, reheated gas. The mean gas temperature lies above the dynamical temperature of the halo for all models at z=0, but differs between models at higher redshift with the Radiative model having the lowest mean gas temperature at z=1.5. We have not attempted to model the scaling relations in a manner that mimics the observational selection effects, nor has a consistent observational picture yet emerged. Nevertheless, evolution of the scaling relations promises to be a powerful probe of the physics of entropy generation in clusters. First indications are that early, widespread heating is favored over an extended period of heating that is associated with galaxy formation.Comment: Accepted for publication in ApJ. Minor changes following referee's comment