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Search Results: 1 - 10 of 162356 matches for " William H. Waller "
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Starburst Anatomy: Stellar and Nebular Properties of Nearby Giant HII Regions
William H. Waller
Physics , 1996,
Abstract: Resolution of nearby giant HII regions into their stellar and nebular constituents provides fundamental insights for interpreting more distant and powerful starburst activity. The following summarizes recent advances in our understanding of the stellar populations and nebular energetics associated with giant HII regions. Photometry and spectroscopy of ionizing clusters in the Galaxy, LMC, and SMC reveal no significant relation between metallicity and the slope of the power-law initial mass function (IMF). HST/WFPC2 photometry of 3 giant HII regions in M33 also does not show any consistent trend involving metal abundance and IMF slope -- contrary to predictions based on emission-line ratios. The upper stellar mass limit appears to be constrained more by cluster age than by anything else. The ionizing luminosities from some of the resolved stellar populations are insufficient to account for the ionization rates inferred from measurements of the composite (integrated) H-alpha emission. Absorption of stellar EUV emission by nebular dust grains would only amplify these photoionizing shortfalls. Leakage of ionizing photons from the HII regions would further exacerbate the situation. What then is providing the additional ionization? Prospects for higher stellar EUV luminosities and/or alternative sources of nebular ionization (e.g. shocks) are evaluated with this question in mind.
Survey of Fine-Scale Structure in the Far-Infrared Milky Way
William H. Waller,Frank Varosi,Francois Boulanger,Seth W. Digel
Physics , 1996,
Abstract: Using the IRAS Infrared Sky Survey Atlas, we have made 60 x 60 deg mosaics of the far-infrared emission in the Milky Way. By applying a median normalizing spatial filter, we were able to eliminate the strong gradient in brightness towards the Galactic midplane. The resulting images reveal a "froth" of superposed filaments, voids, and shells. This fine-scale structure extends all the way down to the Galactic midplane. Moreover, it scales in intensity with the smoothly varying background, independent of latitude, thus indicating that the fine-scale residual emission is co-extensive with the smooth background. We conclude that the fine-scale structure is not merely of local origin, but consists of both nearby and more distant features in the disk. Although we had expected to find morphological evidence for supernova-driven "worms" or "chimneys" rooted in the Galactic plane, our processing shows the FIR fine-scale structure to be more complex (e.g. less coherent and less rooted) as viewed in projection. Analysis of the spatial statistics shows that the FIR fine-scale structure is self-similar with a spatial power-law exponent of -3 and a fractal dimension of 2.5 --- similar behavior to that found in isolated cirrus and molecular clouds. On scales larger than 1.5 deg, the power-law exponent flattens to -2.5, perhaps indicating a change in the characteristic structure. This could be due to different dynamical inputs organizing the small and large-scale structures (e.g. turbulence and diffusion on small scales vs. macroscopic winds and shock fronts on larger scales).
UV Signposts of Resonant Dynamics in Disk Galaxies
William H. Waller,Christine Winslow,Michael Fanelli,Theodore B. Stecher
Physics , 1998,
Abstract: Imaging in the restframe ultraviolet has proven to be an effective and vital means of tracing dynamical patterns of star formation in galaxies out to high redshifts. Using images from the Ultraviolet Imaging Telescope (UIT), Hubble Space Telescope (HST) and complementary groundbased telescopes, we have investigated the starburst activity and associated dynamics in nearby early-type disk galaxies. Concentrating on the starburst-ring (R)SA(r)ab galaxy M94 (NGC 4736), we find compelling evidence for bar-mediated resonances as the primary drivers of evolution at the present epoch. Similar ring-bar dynamics may prevail in the centers of early-type disk galaxies at high redshift. The gravitationally-lensed "Pretzel Galaxy" (0024+1654) at a redshift of ~1.5 provides an important precedent in this regard.
Orchestration of Starbirth Activity in Disk Galaxies: New Perspectives from Ultraviolet Imaging
William H. Waller,Theodore P. Stecher,the Ultraviolet Imaging Telescope,Science Team
Physics , 1996, DOI: 10.1063/1.52794
Abstract: Ultraviolet imaging of nearby disk galaxies reveals the star-forming activity in these systems with unprecedented clarity. UV images recently obtained with the Shuttle-borne Ultraviolet Imaging Telescope (UIT) reveal a remarkable variety of star-forming morphologies. The respective roles of tides, waves, and resonances in orchestrating the observed patterns of starbirth activity are discussed in terms of the extant UV data.
The UIT Survey of the Ultraviolet Sky Background
William H. Waller,Theodore P. Stecher,Ultraviolet Imaging Telescope Science Team
Physics , 1996,
Abstract: When viewed from above the Earth's atmosphere, the nighttime ultraviolet sky background is profoundly dark - up to 100 times fainter than the equivalent visible background as measured by groundbased telescopes. Because the UV background is so faint, its measured strength and spatial distribution remain controversial topics. Estimates range from a few hundred photons/(sec cm^2 sr^2 A) (~27 mag/arcsec^2) with no obvious spatial distribution to several thousand ``photon units'' with a strong gradient toward the Galactic midplane. Herein, we summarize recent results from an analysis of UV images obtained by the Ultraviolet Imaging Telescope - where relatively high nighttime UV intensities are found near the Galactic midplane which then diminish to levels of ~300 photon units or less at high Galactic latitude.
An Infrared Census of Star Formation in the Horsehead Nebula
Brendan P. Bowler,William H. Waller,S. Thomas Megeath,Brian M. Patten,Motohide Tamura
Physics , 2009, DOI: 10.1088/0004-6256/137/3/3685
Abstract: At ~ 400 pc, the Horsehead Nebula (B33) is the closest radiatively-sculpted pillar to the Sun, but the state and extent of star formation in this structure is not well understood. We present deep near-infrared (IRSF/SIRIUS JHKs) and mid-infrared (Spitzer/IRAC) observations of the Horsehead Nebula in order to characterize the star forming properties of this region and to assess the likelihood of triggered star formation. Infrared color-color and color-magnitude diagrams are used to identify young stars based on infrared excess emission and positions to the right of the Zero-Age Main Sequence, respectively. Of the 45 sources detected at both near- and mid-infrared wavelengths, three bona fide and five candidate young stars are identified in this 7' by 7' region. Two bona fide young stars have flat infrared SEDs and are located at the western irradiated tip of the pillar. The spatial coincidence of the protostars at the leading edge of this elephant trunk is consistent with the Radiation-Driven Implosion (RDI) model of triggered star formation. There is no evidence, however, for sequential star formation within the immediate ~ 1.5' (0.17 pc) region from the cloud/H II region interface.
L'integrazione delle pratiche di Sanitá Pubblica in Europa
H. Waller
Italian Journal of Public Health , 2003, DOI: 10.2427/6034
Abstract: Rilevanza dell’argomento: questo tema è diventato un importante problema di Sanità Pubblica anche in Europa. Studi epidemiologici mostrano che quasi tutti i problemi di salute e la prevalenza di malattie e di disabilità sono più frequenti negli strati sociali più bassi, indipendentemente dal modo in cui la classe sociale viene misurata (con qualche eccezione come ad esempio il cancro della mammella e le allergie). Inoltre, l’associazione tra svantaggio sociale e malattia è divenuta sempre più forte negli ultimi decenni (Marmot e Wilkinson, 1999). La rilevanza del tema è diventata ancora più grande dal momento che in molti Paesi candidati all’ingresso nell’Unione Europea sono frequenti povertà e disgregazione sociale (Mackenbach e Bakker, 2002). Future necessita’ di ricerca: in futuro, tutte le indagini nazionali sulla salute dovranno includere la classe sociale tra le variabili registrate regolarmente. La ricerca dovrà infatti concentrarsi sulla spiegazione della correlazione tra classe sociale e malattia usando studi longitudinali ed includendo fattori sociali, oltre a determinanti medico-sanitari. La correlazione tra classe sociale e la presenza di problemi di salute va studiata in entrambe le direzioni: un basso status sociale come determinante di malattia, come pure la presenza di problemi di salute come causa della discesa nella “scala sociale”. La pratica di Sanità Pubblica e lo sviluppo di politiche sociali devono affrontare adeguatamente entrambe le ipotesi.
Emission-Line Properties of the LMC Bubble N70
Brooke P. Skelton,William H. Waller,Richard F. Gelderman,Larry Brown,Bruce Woodgate,Adeline Caulet,Robert A. Schommer
Physics , 1999,
Abstract: We present a spectrophotometric imaging study of the emission bubble N70 (DEM 301) in the Large Magellanic Cloud. N70 is approximately 100 pc in size with a nearly circular shell-like morphology. The nebular emission is powered by an uncertain combination of EUV photons, intense winds, and supernova shock waves from the central population of high-mass stars (the OB association LH 114). We have obtained narrow-band images (FWHM~6A) of N70 in the light of H-alpha (6563A), [N II] (6584A), [S II] (6717,6731A), and [O III] (5007A) along with the corresponding red and green continua. The resulting line fluxes and flux ratios are used to derive ionization rates, nebular densities, volume filling fractions, and excitation indices. Comparison with similar spectrophotometry of the Orion nebula indicates that the outer shell of N70 is especially enhanced in [SII] emission compared to its H-alpha and [N II] emission. The measured intensity ratios in N70 more closely match the range of excitation spanned by giant and supergiant H II shells and even by some of the supernova remnants observed in the LMC. From these spectral diagnostics, we conclude that EUV photons probably contribute most of the inner nebula's ionization, whereas a combination of photoionization plus collisional ionization and excitation of sulfur atoms by low-velocity shocks is driving the emission-line luminosities and intensity ratios observed in the outer shell. Considerations of the radiative and mechanical energetics that are involved may indicate the need for one or two supernova explosions having occurred during the last ~Myr.
HST-WFPC2 Observations of the Star Clusters in the Giant HII Regions of M33
Myung Gyoon Lee,Hong Soo Park,Sang Chul Kim,William H. Waller,Joel Wm. Parker,Eliot M. Malumuth,Paul W. Hodge
Physics , 2001,
Abstract: We present a photometric study of the stars in ionizing star clusters embedded in several giant HII regions of M33 (CC93, IC 142, NGC 595, MA2, NGC 604 and NGC 588). Our photometry are based on the HST-WFPC2 images of these clusters. Color-magnitude diagrams and color-color diagrams of these clusters are obtained and are used for estimating the reddenings and ages of the clusters. The luminosity functions (LFs) and initial mass functions (IMFs) of the massive stars in these clusters are also derived. The slopes of the IMFs range from Gamma = -0.5 to -2.1. It is found interestingly that the IMFs get steeper with increasing galactocentric distance and with decreasing [O/H] abundance.
Ultraviolet Signposts of Resonant Dynamics in the Starburst-Ringed Sab Galaxy, M94 (NGC 4736)
William H. Waller,Michael N. Fanelli,William C. Keel,Ralph Bohlin,Nicholas R. Collins,Barry F. Madore,Pamela M. Marcum,Susan G. Neff,Robert W. O'Connell,Joel D. Offenberg,Morton S. Roberts,Andrew M. Smith,Theodore P. Stecher
Physics , 2000, DOI: 10.1086/319384
Abstract: M94 (NGC 4736) is investigated using images from the Ultraviolet Imaging Telescope (FUV-band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m telescope (H-alpha, R, and I bands), and Palomar 5-m telescope (B-band), along with spectra from the International Ultraviolet Explorer and Lick 1-m telescopes. The wide-field UIT image shows FUV emission from (a) an elongated nucleus, (b) a diffuse inner disk, where H-alpha is observed in absorption, (c) a bright inner ring of H II regions at the perimeter of the inner disk (R = 48 arcsec. = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the ring on diametrically opposite sides of the nucleus (R= 130 arcsec. = 2.9 kpc). The HST/FOC image resolves the NUV emission from the nuclear region into a bright core and a faint 20 arcsec. long ``mini-bar'' at a position angle of 30 deg. Optical and IUE spectroscopy of the nucleus and diffuse inner disk indicates an approximately 10^7 or 10^8 yr-old stellar population from low-level starbirth activity blended with some LINER activity. Analysis of the H-alpha, FUV, NUV, B, R, and I-band emission along with other observed tracers of stars and gas in M94 indicates that most of the star formation is being orchestrated via ring-bar dynamics involving the nuclear mini-bar, inner ring, oval disk, and outer ring. The inner starburst ring and bi-symmetric knots at intermediate radius, in particular, argue for bar-mediated resonances as the primary drivers of evolution in M94 at the present epoch. Similar processes may be governing the evolution of the ``core-dominated'' galaxies that have been observed at high redshift. The gravitationally-lensed ``Pretzel Galaxy'' (0024+1654) at a redshift of approximately 1.5 provides an important precedent in this regard.
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