Magnetic field observations in high beta regions of the magnetosphere

OGO for magnetic field observations in high beta regions of magnetospher

The Evolutionary Status of Isolated Dwarf Irregular Galaxies II. Star Formation Histories and Gas Depletion

The results of UBV and H alpha imaging of a large sample of isolated dwarf irregular galaxies are interpreted in the context of composite stellar population models. The observed optical colors are best fit by composite stellar populations which have had approximately constant star formation rates for at least 10 Gyr. The galaxies span a range of central surface brightness, from 20.5 to 25.0 mag arcsec^{-2}; there is no correlation between surface brightness and star formation history. Although the current star formation rates are low, it is possible to reproduce the observed luminosities without a major starburst episode. The derived gas depletion timescales are long, typically ~20 Gyr. These results indicate that dwarf irregular galaxies will be able to continue with their slow, but constant, star formation activity for at least another Hubble time. The sample of isolated dIs is compared to a sample of star bursting dwarf galaxies taken from the literature. The star bursting dwarf galaxies have many similar properties; the main difference between these two types of gas-rich dwarf galaxies is that the current star formation is concentrated in the center of the star bursting systems while it is much more distributed in the quiescent dIs. This results in pronounced color gradients for the starbursting dwarf galaxies, while the majority of the quiescent dwarf irregular galaxies have minor or non-existent color gradients. Thus, the combination of low current star formation rates, blue colors, and the lack of significant color gradients indicates that star formation percolates slowly across the disk of normal dwarf galaxies in a quasi-continuous manner.Comment: 16 pages, uses emulateapj, to appear in The Astronomical Journal (April 2001

Radiative Transfer Effects in He I Emission Lines

We consider the effect of optical depth of the 2 ^{3}S level on the nebular recombination spectrum of He I for a spherically symmetric nebula with no systematic velocity gradients. These calculations, using many improvements in atomic data, can be used in place of the earlier calculations of Robbins. We give representative Case B line fluxes for UV, optical, and IR emission lines over a range of physical conditions: T=5000-20000 K, n_{e}=1-10^{8} cm^{-3}, and tau_{3889}=0-100. A FORTRAN program for calculating emissivities for all lines arising from quantum levels with n < 11 is also available from the authors. We present a special set of fitting formulae for the physical conditions relevant to low metallicity extragalactic H II regions: T=12,000-20,000 K, n_{e}=1-300 cm^{-3}, and tau_{3889} < 2.0. For this range of physical conditions, the Case B line fluxes of the bright optical lines 4471 A, 5876 A, and 6678 A, are changed less than 1%, in agreement with previous studies. However, the 7065 A corrections are much smaller than those calculated by Izotov & Thuan based on the earlier calculations by Robbins. This means that the 7065 A line is a better density diagnostic than previously thought. Two corrections to the fitting functions calculated in our previous work are also given.Comment: To be published in 10 April 2002 ApJ; relevant code available at ftp://wisp.physics.wisc.edu/pub/benjamin/Heliu

Evaluation of the synoptic and mesoscale predictive capabilities of a mesoscale atmospheric simulation system

The overall performance characteristics of a limited area, hydrostatic, fine (52 km) mesh, primitive equation, numerical weather prediction model are determined in anticipation of satellite data assimilations with the model. The synoptic and mesoscale predictive capabilities of version 2.0 of this model, the Mesoscale Atmospheric Simulation System (MASS 2.0), were evaluated. The two part study is based on a sample of approximately thirty 12h and 24h forecasts of atmospheric flow patterns during spring and early summer. The synoptic scale evaluation results benchmark the performance of MASS 2.0 against that of an operational, synoptic scale weather prediction model, the Limited area Fine Mesh (LFM). The large sample allows for the calculation of statistically significant measures of forecast accuracy and the determination of systematic model errors. The synoptic scale benchmark is required before unsmoothed mesoscale forecast fields can be seriously considered

The Nature of Radio Continuum Emission in the Dwarf Starburst Galaxy NGC 625

We present new multi-frequency radio continuum imaging of the dwarf starburst galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm reveal global continuum emission dominated by free-free emission, with only mild synchrotron components. Each of the major HII regions is detected; the individual spectral indices are thermal for the youngest regions (showing strongest H Alpha emission) and nonthermal for the oldest. We do not detect any sources that appear to be associated with deeply embedded, dense, young clusters, though we have discovered one low-luminosity, obscured source that has no luminous optical counterpart and which resides in the region of highest optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent star formation, these radio properties suggest that the youngest star formation complexes have not yet evolved to the point where their thermal spectra are significantly contaminated by synchrotron emission. The nonthermal components are associated with regions of older star formation that have smaller ionized gas components. These results imply a range of ages of the HII regions and radio components that agrees with our previous resolved stellar population analysis, where an extended burst of star formation has pervaded the disk of NGC 625 over the last ~ 50 Myr. We compare the nature of radio continuum emission in selected nearby dwarf starburst and Wolf-Rayet galaxies, demonstrating that thermal radio continuum emission appears to be more common in these systems than in typical HII galaxies with less recent star formation and more evolved stellar clusters.Comment: ApJ, in press; 27 pages, 5 figures. Full-resolution version may be obtained at http://www.astro.umn.edu/~cannon/n625.vla.p

Comparison of bacterial biofilm communities using barcoded pyrosequencing and analysis to determine origin of biofilm fouling of reverse osmosis membranes in a full scale desalination system

Biofouling is the single most important issue in reverse osmosis sea water desalination worldwide (Ridgway et al., 1999) and may account for up to 50% of energy use. Which species are responsible and their origin is unclear. With the advent of next generation sequencing, species diversity and transience can be examined at orders of magnitude greater detail than was previously possible. We found many similarities in bacterial families across source water, prefiltration units and membranes in this study and in the few other studies available, despite disparate locations and seasons. Key groups included members of the Bacteroidetes (e. Flavobacteriaceae), Planctomycetes, Alphaproteobacteria (eg. Rhodobacteraceae, Sphingomonadales), Betaproteobacteria (eg. Burkholderia) and Gammaproteobacteria (eg. Oceanospirillales, Xanthomonadaceae). Despite similarities in families, the predominant fouling species on reverse osmosis (RO) membranes appear to differ between studies. This seems likely to reflect a common origin (seawater) but subsequent adaptation or selective pressures in different niches, particularly on RO membranes under high pressure and salt concentration. We can now select environmental isolates from our culture collection representing key bacterial groups responsible for biofouling in seawater systems. This will enable more accurate evaluation of the effectiveness of anti-fouling strategies

CO Emission in Low Luminosity, HI Rich Galaxies

We present 12CO 1-0 observations of eleven low luminosity M_B > -18), HI--rich dwarf galaxies. Only the three most metal-rich galaxies, with 12+log(O/H) ~ 8.2, are detected. Very deep CO spectra of six extremely metal-poor systems (12+log(O/H) < 7.5) yield only low upper limits on the CO surface brightness, I_CO < 0.1 K km/s. Three of these six have never before been observed in a CO line, while the others now have much more stringent upper limits. For the very low metallicity galaxy Leo A, we do not confirm a previously reported detection in CO, and the limits are consistent with another recent nondetection. We combine these new observations with data from the literature to form a sample of dwarf galaxies which all have CO observations and measured oxygen abundances. No known galaxies with 12+log(O/H) < 7.9 (Z < 0.1 solar) have been detected in CO. Most of the star-forming galaxies with higher (12+log(O/H) > 8.1) metallicities are detected at similar or higher I_CO surface brightnesses. The data are consistent with a strong dependence of the I_CO/M_H_2 = X_CO conversion factor on ambient metallicity. The strikingly low upper limits on some metal-poor galaxies lead us to predict that the conversion factor is non-linear, increasing sharply below approximately 1/10 of the solar metallicity (12+log(O/H) < 7.9).Comment: 25 pages, 4 figures, 3 tables. Accepted for publication in AJ Tables replaced -- now formated for landscape orientatio