Determination of ball bearing dynamic stiffness

The dynamic radial stiffness characteristics of rolling element bearings are currently determined by analytical methods that have not been experimentally verified. These bearing data are vital to rotating machinery design integrity because accurate critical speeds and rotor stability predictions are highly dependent on the bearing stiffness. A tester was designed capable of controlling the bearing axial preload, speed, and rotor unbalance. The rotor and support structures were constructed to permit critical speeds that are predominantly determined by a 57 mm test bearing. A curve of calculated critical speed versus stiffness was used to determine the actual bearing stiffness from the empirical data. The results of extensive testing are used to verify analytical predictions, increase confidence in existing bearing computer programs, and to serve as a data base for efforts to correct these programs

The star-formation history of the universe - an infrared perspective

A simple and versatile parameterized approach to the star formation history allows a quantitative investigation of the constraints from far infrared and submillimetre counts and background intensity measurements. The models include four spectral components: infrared cirrus (emission from interstellar dust), an M82-like starburst, an Arp220-like starburst and an AGN dust torus. The 60 $\mu$m luminosity function is determined for each chosen rate of evolution using the PSCz redshift data for 15000 galaxies. The proportions of each spectral type as a function of 60 $\mu$m luminosity are chosen for consistency with IRAS and SCUBA colour-luminosity relations, and with the fraction of AGN as a function of luminosity found in 12 $\mu$m samples. The luminosity function for each component at any wavelength can then be calculated from the assumed spectral energy distributions. With assumptions about the optical seds corresponding to each component and, for the AGN component, the optical and near infrared counts can be accurately modelled. A good fit to the observed counts at 0.44, 2.2, 15, 60, 90, 175 and 850 $\mu$m can be found with pure luminosity evolution in all 3 cosmological models investigated: $\Omega_o$ = 1, $\Omega_o$ = 0.3 ($\Lambda$ = 0), and $\Omega_o$ = 0.3, $\Lambda$ = 0.7. All 3 models also give an acceptable fit to the integrated background spectrum. Selected predictions of the models, for example redshift distributions for each component at selected wavelengths and fluxes, are shown. The total mass-density of stars generated is consistent with that observed, in all 3 cosmological models.Comment: 20 pages, 25 figures. Accepted for publication in ApJ. Full details of models can be found at http://astro.ic.ac.uk/~mrr/countmodel

Faint Radio Sources and Star Formation History

Faint extragalactic radio sources provide important information about the global history of star formation. Sensitive radio observations of the Hubble Deep Field and other fields have found that sub-mJy radio sources are predominantly associated with star formation activity rather than AGN. Radio observations of star forming galaxies have the advantage of being independent of extinction by dust. We use the FIR-radio correlation to compare the radio and FIR backgrounds, and make several conclusions about the star forming galaxies producing the FIR background. We then use the redshift distribution of faint radio sources to determine the evolution of the radio luminosity function, and thus estimate the star formation density as a function of redshift.Comment: 12 pages, 9 figures, latex using texas.sty, to appear in the CD-ROM Proceedings of the 19th Texas Symposium on Relativistic Astrophysics and Cosmology, held in Paris, France, Dec. 14-18, 1998. Eds.: J. Paul, T. Montmerle, and E. Aubourg (CEA Saclay). No changes to paper, just updated publication info in this commen

(Non)Parallel Evolution

Parallel evolution across replicate populations has provided evolutionary biologists with iconic examples of adaptation. When multiple populations colonize seemingly similar habitats, they may evolve similar genes, traits, or functions. Yet, replicated evolution in nature or in the laboratory often yields inconsistent outcomes: Some replicate populations evolve along highly similar trajectories, whereas other replicate populations evolve to different extents or in distinct directions. To understand these heterogeneous outcomes, biologists are increasingly treating parallel evolution not as a binary phenomenon but rather as a quantitative continuum ranging from parallel to nonparallel. By measuring replicate populations’ positions along this (non)parallel continuum, we can test hypotheses about evolutionary and ecological factors that influence the extent of repeatable evolution. We review evidence regarding the manifestation of (non)parallel evolution in the laboratory, in natural populations, and in applied contexts such as cancer. We enumerate the many genetic, ecological, and evolutionary processes that contribute to variation in the extent of parallel evolution

New Results from a Near-Infrared Search for Hidden Broad-Line Regions in Ultraluminous Infrared Galaxies

This paper reports the latest results from a near-infrared search for hidden broad-line regions (BLRs: FWHM >~ 2,000 km/s) in ultraluminous infrared galaxies (ULIGs). The new sample contains thirty-nine ULIGs from the 1-Jy sample selected for their lack of BLRs at optical wavelengths. The results from this new study are combined with those from our previous optical and near-infrared surveys to derive the fraction of all ULIGs with optical or near-infrared signs of genuine AGN activity (either a BLR or [Si VI] emission). Comparisons of the dereddened emission-line luminosities of the optical or obscured BLRs detected in the ULIGs of the 1-Jy sample with those of optical quasars indicate that the obscured AGN/quasar in ULIGs is the main source of energy in at least 15 -- 25% of all ULIGs in the 1-Jy sample. This fraction is 30 -- 50% among ULIGs with L_ir > 10^{12.3} L_sun. These results are compatible with those from recent mid-infrared spectroscopic surveys carried out with ISO. (abridged)Comment: 40 pages including 10 figures and 3 tables (Table 3 should be printed in landscape mode

Surface Brightness of Starbursts at Low and High Redshifts

Observations in the rest frame ultraviolet from various space missions are used to define the nearby starburst regions having the highest surface brightness on scales of several hundred pc. The bright limit is found to be 6x10^-16 ergs/cm^2-s-A-arcsec^2 for rest frame wavelength of 1830 A. Surface brightness in the brightest pixel is measured for 18 galaxies in the Hubble Deep Field having z > 2.2. After correcting for cosmological dimming, we find that the high redshift starbursts have intrinsic ultraviolet surface brightness that is typically four times brighter than low redshift starbursts. It is not possible to conclude whether this difference is caused by decreased dust obscuration in the high redshift starburst regions or by intrinsically more intense star formation. Surface brightness enhancement of starburst regions may be the primary factor for explaining the observed increase with redshift of the ultraviolet luminosity arising from star formation.Comment: accepted for publication in AJ; 11 pages text, 3 tables, 3 figures (embedded

On the Determination of Star Formation Rates in Evolving Galaxy Populations

The redshift dependence of the luminosity density in certain wavebands (e.g. UV and H-alpha) can be used to infer the history of star formation in the populations of galaxies producing this luminosity. This history is a useful datum in studies of galaxy evolution. It is therefore important to understand the errors that attend the inference of star formation rate densities from luminosity densities. This paper explores the self-consistency of star formation rate diagnostics by reproducing commonly used observational procedures in a model with known galaxy populations, evolutionary histories and spectral emission properties. The study reveals a number of potential sources of error in the diagnostic processes arising from the differential evolution of different galaxy types. We argue that multi-wavelength observations can help to reduce these errors.Comment: 13 pages (including 5 encapsulated postscript figures), aastex, accepted for publication in Ap

The Relationship Between Molecular Gas Tracers and Kennicutt-Schmidt Laws

We provide a model for how Kennicutt-Schmidt (KS) laws, which describe the correlation between star formation rate and gas surface or volume density, depend on the molecular line chosen to trace the gas. We show that, for lines that can be excited at low temperatures, the KS law depends on how the line critical density compares to the median density in a galaxy's star-forming molecular clouds. High critical density lines trace regions with similar physical properties across galaxy types, and this produces a linear correlation between line luminosity and star formation rate. Low critical density lines probe regions whose properties vary across galaxies, leading to a star formation rate that varies superlinearly with line luminosity. We show that a simple model in which molecular clouds are treated as isothermal and homogenous can quantitatively reproduce the observed correlations between galactic luminosities in far infrared and in the CO(1->0) and HCN(1->0) lines, and naturally explains why these correlations have different slopes. We predict that IR-line luminosity correlations should change slope for galaxies in which the median density is close to the line critical density. This prediction may be tested by observations of lines such as HCO^+(1->0) with intermediate critical densities, or by HCN(1->0) observations of intensely star-forming high redshift galaxies with very high densities. Recent observations by Gao et al. hint at just such a change in slope. We argue that deviations from linearity in the HCN(1->0)-IR correlation at high luminosity are consistent with the assumption of a constant star formation efficiency.Comment: Accepted to ApJ. 11 pages, 4 figures, emulateapj format. This version has some additional models exploring the effects of varying metallicity and temperature. The conclusions are unchange

Type Ia supernova counts at high z: signatures of cosmological models and progenitors

Determination of the rates at which supernovae of Type Ia (SNe Ia) occur in the early Universe can give signatures of the time spent by the binary progenitor systems to reach explosion and of the geometry of the Universe. Observations made within the Supernova Cosmology Project are already providing the first numbers. Here it is shown that, for any assumed SNe Ia progenitor, SNe Ia counts up to $m_{R}\simeq 23-26$ are useful tests of the SNe Ia progenitor systems and cosmological tracers of a possible non-zero value of the cosmological constant, $\Lambda$. The SNe Ia counts at high redshifts compare differently with those at lower redshifts depending on the cosmological model. Flat $\Omega_{\Lambda}$--dominated universes would show a more significant increase of the SNe Ia counts at $z \sim 1$ than a flat, $\Omega_{M} = 1$ universe. Here we consider three sorts of universes: a flat universe with $H_{0} = 65 km s^{-1} Mpc^{-1}$, $\Omega_{M} = 1.0$, $\Omega_{\Lambda} = 0.0$; an open universe with $H_{0} = 65 km s^{-1} Mpc^{-1}$, $\Omega_{M} = 0.3$, $\Omega_{\Lambda} = 0.0$; and a flat, $\Lambda$--dominated universe with $H_{0} = 65 km s^{-1} Mpc^{-1}$, $\Omega_{M} = 0.3$, $\Omega_{\Lambda} = 0.7$). On the other hand, the SNe Ia counts from one class of binary progenitors (double degenerate systems) should not increase steeply in the $z= 0$ to $z= 1$ range, contrary to what should be seen for other binary progenitors. A measurement of the SNe Ia counts up to $z \sim 1$ is within reach of ongoing SNe Ia searches at high redshifts.Comment: 16 pages, incl. 2 figures. To appear in ApJ (Letters