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- Galaxies (62)
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Articles 601 - 614 of 614
Full-Text Articles in Physical Sciences and Mathematics
Bowen Fluoresence And He Ii Lines In Active Galaxies And Gaseous Nebulae, Hagai Netzer, Moshe Elitzur, Gary J. Ferland
Bowen Fluoresence And He Ii Lines In Active Galaxies And Gaseous Nebulae, Hagai Netzer, Moshe Elitzur, Gary J. Ferland
Physics and Astronomy Faculty Publications
We have calculated the intensity of He II and the O III and N III Bowen lines under a variety of astrophysical conditions. Our line-transfer method is based on the escape probability formalism and is especially suitable for combining line fluorescence and photoionization calculations in a simple, straightforward way. It reproduces the results of the more sophisticated line-transfer calculations quite accurately and is much better for producing realistic ionization and thermal structures. An extensive grid of models is presented and discussed, with special emphasis on the case of active galactic nuclei (AGNs). Many O III and N III Bowen lines …
The Hydrogen Line Spectra Of Narrow-Line Radio Galaxies, Gary J. Ferland, Donald E. Osterbrock
The Hydrogen Line Spectra Of Narrow-Line Radio Galaxies, Gary J. Ferland, Donald E. Osterbrock
Physics and Astronomy Faculty Publications
We report the results of the first detection of Lyα in a narrow-line radio galaxy. Nearly simultaneous optical and UV observations of 3C 192 and 3C 223 allow the measurement of both Balmer and Lyman decrements. These line ratios are approximate functions of the interstellar reddening and of a parameter which is proportional to the amount of H I collisional excitation present. The reddening of 3C 192 is slightly larger than that due to our Galaxy, although 3C 223 may have a larger value. Both galaxies have intrinsic Balmer and Lyman decrements which are significantly steeper than case B, …
Observations Of Shadow Bands At The Total Solar Eclipse Of 16 February 1980, Laurence A. Marschall, Rita Mahon, Richard C. Henry
Observations Of Shadow Bands At The Total Solar Eclipse Of 16 February 1980, Laurence A. Marschall, Rita Mahon, Richard C. Henry
Physics and Astronomy Faculty Publications
Photoelectric observations of short term light variations (shadow bands) at the 16 Feb. 1980 total solar eclipse have been made using a set of spatially separated PIN diodes. Light variations in a bandpass of 1-500 Hz were detected during the half-minutes preceding and following the total phase. Fourier analysis of the noise spectrum of the variations reveals a sharp drop-off for frequencies about 50 Hz and an overall spectrum quite similar to previously reported power spectra of stellar scintillation. This is consistent with an atmospheric origin for the shadow bands. Cross-correlations between the detector outputs are low, suggesting a short …
Cosmic Rays And The Emission-Line Regions Of Active Galactic Nuclei, Gary J. Ferland, R. F. Mushotzky
Cosmic Rays And The Emission-Line Regions Of Active Galactic Nuclei, Gary J. Ferland, R. F. Mushotzky
Physics and Astronomy Faculty Publications
We discuss the effects that the synchrotron emitting relativistic electrons, which are known to exist throughout many distance scales in active nuclei, could have on the emission-line regions that characterize these objects. Detailed models of both the inner, dense, broad-line region and the outer, lower density, narrow-line region are presented, together with the first models of the optically emitting gas often found within extended radio lobes. We show that, in all cases, if the relativistic gas that produces the synchrotron radio emission is mixed with the gas in the emission-line region, then significant changes in the emission-line spectrum will result. …
Radiation Pressure And The Stability Of Broad-Line Region Clouds, Gary J. Ferland, Moshe Elitzur
Radiation Pressure And The Stability Of Broad-Line Region Clouds, Gary J. Ferland, Moshe Elitzur
Physics and Astronomy Faculty Publications
We discuss two constraints on the existence and stability of quasar emission-line gas. The case of a constant pressure photoionized cloud in pressure equilibrium with a surrounding hot gas is considered. We show that the radiation pressure of the internally generated line emission makes a significant contribution to the total pressure in standard broad-line region models, and that models with either a higher ionization parameter or lower gas density are dominated by radiation pressure and hence unstable. Further, clouds with ionization parameters lower than the standard value cannot be supported by any hot gas which is heated only by the …
Iue Observations Of Dq Herculis And Its Nebula, And The Nature Of The Cold Nova Shells, Gary J. Ferland, R. E. Williams, D. L. Lambert, G. A. Shields, M. Slovak, P. M. Gondhalekar, J. W. Truran
Iue Observations Of Dq Herculis And Its Nebula, And The Nature Of The Cold Nova Shells, Gary J. Ferland, R. E. Williams, D. L. Lambert, G. A. Shields, M. Slovak, P. M. Gondhalekar, J. W. Truran
Physics and Astronomy Faculty Publications
The nebula ejected in the 1934 outburst of the classical nova DQ Her is remarkable for its unprecedentedly low temperature of Te 500 K as measured by Williams et al. (1978). In this paper, IUE observations are combined with Steward optical spectra. It is confirmed that the gas is quite cold. It is further shown that the gas is ionized by the radiation field of the central object. X-ray, ultraviolet, optical and infrared observations of the underlying binary are combined with the extreme-ultraviolet continuum deduced from the level of ionization of the nebula to obtain a composite energy …
Stimulated Emission And The Flat Balmer Decrements Of Cataclysmic Variable Stars, Moshe Elitzur, Gary J. Ferland, W. G. Mathews, G. A. Shields
Stimulated Emission And The Flat Balmer Decrements Of Cataclysmic Variable Stars, Moshe Elitzur, Gary J. Ferland, W. G. Mathews, G. A. Shields
Physics and Astronomy Faculty Publications
Balmer emission lines from cataclysmic variables often have nearly equal intensities rather than the rapid decrement predicted by simple nebular theory. Traditionally, this has been interpreted in terms of local thermodynamic equilibrium emission from a dense gas with small volume located just above the accretion disk. In this Letter we show that the intense radiation field within a close binary system can affect excited-state populations and optical emission in ways which allow a relatively low density gas to closely mimic the high-density situation. In at least one case, the old nova V603 Aql, the emitting gas has a low density …
Are There Any Shock-Heated Galaxies?, Gary J. Ferland, Hagai Netzer
Are There Any Shock-Heated Galaxies?, Gary J. Ferland, Hagai Netzer
Physics and Astronomy Faculty Publications
We study the spectroscopic characteristics of low ionization nuclear emission-line regions (Liners) in galaxies, and compare them with other active galactic nuclei. We show the results of new, extensive photoionization model calculations and use them to investigate line ratio diagrams, such as [O III] λ5007/Hβ versus [O II] λ3727/[O III] λ5007, and [O I] λ6300/[O III] λ5007 versus [O III] λ3727/[O III] λ5007. On the diagrams broad line objects, Seyfert 2 galaxies, and Liners form a continuous smooth sequence of decreasing ionization parameter. This suggests a common origin for line excitation in …
The Mystery Of The Missing Boundary Layer, Gary J. Ferland, S. S. Langer, J. Macdonald, G. H. Pepper, G. Shaviv, J. W. Truran
The Mystery Of The Missing Boundary Layer, Gary J. Ferland, S. S. Langer, J. Macdonald, G. H. Pepper, G. Shaviv, J. W. Truran
Physics and Astronomy Faculty Publications
The question of the nature of the ultraviolet and X-ray radiation field of cataclysmic binaries is addressed. The spectrum and luminosity of this radiation are important in determining the mass transfer rate and energy budget of the system and in studies of the ejecta surrounding novae. In many systems, the soft X-ray luminosity is ~ 102-104 times weaker than predicted by simple accretion models. We discuss several possible solutions to this discrepancy. The most likely are either that the optical luminosity of a typical old nova is produced partly by reprocessed ultraviolet light from the white dwarf, …
Broad Line Region Clouds And The Absorbing Material In Ngc 4151, Gary J. Ferland, R. F. Mushotzky
Broad Line Region Clouds And The Absorbing Material In Ngc 4151, Gary J. Ferland, R. F. Mushotzky
Physics and Astronomy Faculty Publications
The constraints imposed by X-ray, ultraviolet, and optical observations on the absorbing material in the Seyfert 1.5 galaxy NGC 4151 are discussed. These observations are compared to the predictions of photoionization models of both broad line region clouds, which Holt et al. identify as the source of the extinction, and to models of the narrow line region gas. The weakness of both C III] 1909 and the broad component of Hα relative to C IV 1549, together with the absence of an absorption Balmer jump, indicate that the ionization parameter for the broad line region is a factor …
Membership Of The Rosette Nebula Cluster, Ngc 2244, Laurence A. Marschall, William F. Van Altena, Liang-Tai George Chiu
Membership Of The Rosette Nebula Cluster, Ngc 2244, Laurence A. Marschall, William F. Van Altena, Liang-Tai George Chiu
Physics and Astronomy Faculty Publications
Relative proper motions for 287 stars in the region of the young open cluster NGC 2244, located in the Rsette Nebula, have been determined from plates taken with the Yerkes 40-in. and the Allegheny 30-in. refractors. Probabilities of membership based on these proper motions are derived. Because probable members are chosen primarily from kinematic considerations, the sample should show a minimum photometric bias. Differential extinction across the cluster, however, is identified as a complication in the immediate interpretation of the sample in terms of color-magnitude diagrams or luminosity functions.
Physical Conditions In The Accretion Disk Of V603 Aquilae, Gary J. Ferland, D. L. Lambert, M. L. Mccall, G. A. Shields, M. Slovak
Physical Conditions In The Accretion Disk Of V603 Aquilae, Gary J. Ferland, D. L. Lambert, M. L. Mccall, G. A. Shields, M. Slovak
Physics and Astronomy Faculty Publications
Ultraviolet and optical spectra of the old nova V603 Aql are discussed. The UV-optical continuum is dominated by emission from the accretion disk. Emission lines from ions of H, He, C, N, and 0 are identified. These lines are probably formed in a circumstellar shell with radius comparable to the binary separation, density ~1010 cm-3, and a roughly solar chemical composition. This corona is probably heated by radiation emitted by the underlying accretion disk. Photoionization calculations of the structure and emission-line spectrum of the corona are presented, and the effects of this gas on the X-ray continuum …
A Cosmic Clock For The Classroom, Laurence A. Marschall
A Cosmic Clock For The Classroom, Laurence A. Marschall
Physics and Astronomy Faculty Publications
Teachers who watched the first episode of Carl Sagan's Cosmos show on the Public Broadcasting System may have been impressed by his use of the "Cosmic Calendar" to dramatically introduce the evolutionary time scale of the universe. In this calendar, which Sagan first represented in The Dragons of Eden, the 15 billion year history of the universe is compressed into a single year. Each month represents 1.25 billion years, each day 40 billion years, and each second 500 years. At this scale the entire recorded history of mankind flashes by during the final 10 seconds of the cosmic year. [ …
Interferometric Studies Of Interstellar Calcium Lines, Laurence A. Marschall, L. M. Hobbs
Interferometric Studies Of Interstellar Calcium Lines, Laurence A. Marschall, L. M. Hobbs
Physics and Astronomy Faculty Publications
Interferometric, photoelectric scans of the interstellar calcium K-lines in the spectra of 65 stars are presented. The scans were obtained with a PEPSIOS spectrometer having a passband with a full half-intensity width of 1.0 kms(-1) or 0.013A. The fivefold improvement in resolution over that used by Adams reveals numerous line components which correspond very well to those of the interstellar sodium lines, apart from frequent differences in relative intensities.