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Search Results: 1 - 10 of 213523 matches for " Robert W. O'Connell "
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Super Star Clusters in M82
Robert W. O'Connell
Physics , 2001,
Abstract: M82 is the nearest starburst galaxy. It contains two large systems of super star clusters, one being spawned today in the active starforming core, and one produced by an earlier starburst event which coincided with the last orbital passage of its neighbor, M81. The proximity of M82 makes it uniquely valuable for a wide range of studies of massive young clusters and their environments.
The Ultraviolet Morphology of Galaxies
Robert W. O'Connell
Physics , 1997, DOI: 10.1063/1.53740
Abstract: The vacuum ultraviolet offers a unique perspective on galaxy morphology, stellar populations, and interstellar material which is of particular relevance to interpreting high redshift galaxies and the history of cosmic star formation. Here we review UV imaging studies of galaxies since 1990.
Far-Ultraviolet Radiation from Elliptical Galaxies
Robert W. O'Connell
Physics , 1999, DOI: 10.1146/annurev.astro.37.1.603
Abstract: Far-ultraviolet radiation is a ubiquitous, if unanticipated, phenomenon in elliptical galaxies and early-type spiral bulges. It is the most variable photometric feature associated with old stellar populations. Recent observational and theoretical evidence shows that it is produced mainly by low-mass, small-envelope, helium-burning stars in extreme horizontal branch and subsequent phases of evolution. These are probably descendents of the dominant, metal rich population of the galaxies. Their lifetime UV outputs are remarkably sensitive to their physical properties and hence to the age and the helium and metal abundances of their parents. UV spectra are therefore exceptionally promising diagnostics of old stellar populations, although their calibration requires a much improved understanding of giant branch mass loss, helium enrichment, and atmospheric diffusion.
Stellar Populations at Large Redshifts
Robert W. O'Connell
Physics , 1999, DOI: 10.1023/A:1002766328850
Abstract: Rapid progress is now being made in the study of stellar populations of galaxies at large lookback times, both in dense clusters and the field. Dramatic transformations in star formation histories (even morphologies) appear to prevail among all types of galaxies and in all environments, and these can be traced to relatively recent times (z ~ 0.2). This article updates to December 1998 a review made in 1994 to cover the recent watershed of observational results emerging from the Hubble Deep Field and deep surveys made with large ground-based telescopes.
Energy Distributions and the Formation Times of Spheroidal Populations
Robert W. O'Connell
Physics , 1999,
Abstract: I review recent progress in exploring the formation times of spheroidal stellar populations (elliptical galaxies and large spiral bulges) using spectrophotometric techniques. A quickly growing body of evidence shows that although massive spheroids can form at early times, there are strong environmental dependencies, and major transitions in star formation histories and even morphologies are detectable to surprisingly small redshifts (z ~ 0.2). These features are consistent with neither the strict monolithic collapse nor hierarchical merging scenarios. Restframe UV observations are a promising means of improving our understanding of spheroid evolution.
Constraints on the Stellar Populations of Elliptical Galaxies from Ultraviolet Spectra
Ben Dorman,Robert W. O'Connell
Physics , 1996,
Abstract: We present preliminary results from spectral synthesis models of old stellar populations for the spectral range 912-4000 A with ~ 10A resolution, which can be used to investigate the UVX phenomenon and to assess ages and abundances. Model spectra incorporating extreme horizontal branch (EHB) and Post-Asymptotic Giant Branch (P-AGB) populations give good matches to the far-UV spectra of galaxies. These models indicate an EHB fraction which is < 10% of the total HB population in all but the most extreme examples of the UVX phenomenon, where the EHB fraction is still <= 20. Once the hot component that gives rise to the UVX phenomenon is accounted for, the mid-UV wavelength range (2200 < lambda < 3300 A) provides information about the age and metallicity of the underlying stellar population. The flux in this spectral range arises mainly from stars close to the main sequence turnoff. We compare models with the spectrum of M31 and discuss UV features which should be useful as population diagnostics.
The Ultraviolet Morphology of Galaxies
Robert W. O'Connell,Pamela Marcum
Physics , 1996,
Abstract: Optical band images of distant (z > 0.5) galaxies, such as those of the Hubble Deep Field, record light from the rest-frame vacuum ultraviolet (< 3000 A). Because the appearance of a galaxy is a very strong function of wavelength, and especially so in the UV, evolutionary studies of distant galaxies can be seriously influenced by a "morphological k-correction" effect. We use images obtained by the Ultraviolet Imaging Telescope during the Astro missions to explore the extent of this effect and intercompare far-UV with optical morphologies for various types of galaxies.
Constraining the Abundances and Ages of Early-type Galaxies
Ben Dorman,Robert W. O'Connell
Physics , 1997, DOI: 10.1063/1.53790
Abstract: We consider the extent to which broadband mid-UV (2000 - 3000 Angstrom) colors can give superior constraints on the ages and abundances of old stellar populations than can their optical counterparts. The ultraviolet colors directly measure the turnoff component of old populations. They vary much more strongly with t and Z and give much tighter constraints for a given degree of observational precision than do optical-band indices. We present an analysis of the permitted range of (t,Z) arising from observational uncertainties in observations of early-type galaxies.
Ultraviolet Radiation from Evolved Stellar Populations: II. The Ultraviolet Upturn Phenomenon in Elliptical Galaxies
Ben Dorman,Robert W. O'Connell,Robert T. Rood
Physics , 1994, DOI: 10.1086/175428
Abstract: We present an analysis of the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is that the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. We re-derive the broad-band UV colors $1500-V$ and $2500-V$ for globular clusters and elliptical galaxies from the available satellite data and investigate color-color and color-line strength correlations. We also provide the ingredients necessary for constructing models with arbitrary HB morphologies.
Age and Abundance Discrimination in Old Stellar Populations Using Mid-Ultraviolet Colors
Ben Dorman,Robert W. O'Connell,Robert T. Rood
Physics , 2003, DOI: 10.1086/375413
Abstract: The restframe mid-ultraviolet spectral region (2000-3200 A) is important in analyzing the stellar populations of the "red envelope" systems observed at high redshifts. Here, we explore the usefulness of the mid-UV for determining ages and abundances of old populations. A mid-UV to optical/IR wavelength baseline provides good separation of population components because the main sequence turnoff dominates the integrated light between 2500 and 4000 A. We find a six magnitude difference in the mid-UV continuum level over the metallicity range -1.5 < log Z/Z_o < +0.5 and a comparable difference (per unit log t) for ages in the range 4-16 Gyr. Logarithmic derivatives of mid-UV colors with respect to age or metal abundance are 3-10 times larger than for the UBV region. Most of the spectral information on old populations therefore resides below 4000 A. We investigate the capability of UBV and mid-UV broad-band colors to separately determine age and abundance, taking into account precision in the color measurements. We find that the mid-UV improves resolution in (log t,log Z) space by about a factor of 3 for a given observational precision. Contamination by hot horizontal branch phases can seriously affect mid-UV spectra, reaching over 80% in some cases. However, this is straightforward to remove as long as far-UV measurements are available. Finally, we show that a 4 Gyr, solar abundance model based on empirical spectra provides an excellent fit to the mid-UV spectrum of the E galaxy M32. This indicates that the poorer results obtained from theoretical spectra arise from limitations of the synthesis models for individual stars. [Condensed]
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