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At least 1,711 records · Page 95
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NASA NTRS1974

On the nature of the nonthermal radio emission from the galactic center

Some recent observations of the galactic center appear at first sight to have made the character of the radio source Sgr A unclear. The flatness of the usually quoted spectrum, the lack of any measured polarization, the presence of hydrogen recombination lines, and the radio-infrared luminosity ratio are all consistent with Sgr A being a rather hot H II region. It is shown here, however, that when flux measurements between 80 MHz and 91000 MHz are all included and the multiple nature of the source is accounted for, the spectral index exceeds 0.4 - much too steep to be bremsstrahlung. There is, therefore, no question that Sgr A is a nonthermal source. It is further argued on the basis of a comparison of appearance, energetics and the recent results which no longer put Sgr A exactly at the center of the galaxy that Sgr A could be a supernova remnant rather than a miniature of active extragalactic radio sources.

Jones, T. W.↗
NASA NTRS2022

Thermal Imaging of Dust Hiding the Black Hole in NGC 1068

In the widely accepted ‘unified model’1 solution of the classification puzzle of active galactic nuclei, the orientation of a dusty accretion torus around the central black hole dominates their appearance. In ‘type-1’ systems, the bright nucleus is visible at the centre of a face-on torus. In ‘type-2’ systems the thick, nearly edge-on torus hides the central engine. Later studies suggested evolutionary effects2 and added dusty clumps and polar winds3 but left the basic picture intact. However, recent high-resolution images4 of the archetypal type-2 galaxy NGC 10685,6, suggested a more radical revision. The images displayed a ring-like emission feature that was proposed to be hot dust surrounding the black hole at the radius where the radiation from the central engine evaporates the dust. That ring is too thin and too far tilted from edge-on to hide the central engine, and ad hoc foreground extinction is needed to explain the type-2 classification. These images quickly generated reinterpretations of the dichotomy between types 1 and 27,8. Here we present new multi-band mid-infrared images of NGC 1068 that detail the dust temperature distribution and reaffirm the original model. Combined with radio data ( J.F.G. and C.M.V.I., manuscript in preparation), our maps locate the central engine that is below the previously reported ring and obscured by a thick, nearly edge-on disk, as predicted by the unified model. We also identify emission from polar flows and absorbing dust that is mineralogically distinct from that towards the Milky Way centre.

Violeta Gamez Rosas↗
NASA NTRS1988

Extinction, profile asymmetry, and tidal effects in Seyfert and starburst galaxies

Possible correlations between luminosity, extinction, and morphological properties of 246 Seyfert and starburst galaxies are investigated. It is shown that the asymmetry of the forbidden O III 5007 A profile is correlated with the signature of dust in both Seyfert types, and therefore establishes a direct connection between them. A positive correlation is found between soft X-ray luminosities and optical reddening for Seyfert 2's and a negative correlation for Seyfert 1's. These and other correlations are discussed in the context of current models of active nuclei. No statistically significant differences are found between Seyferts with and without companions, except that interacting Seyfert 2's have excess infrared and radio-continuum emission.

Dahari, Oved↗
NASA NTRS1995

ROSAT PSPC and HRI observations of the composite starburst/Seyfert 2 galaxy NGC 1672

The nearby barred spiral galaxy NGC 1672 has been observed with the Position Sensitive Proportional Counter (PSPC) and High Resolution Imager (HRI) instruments on board the ROSAT X-ray satellite. NGC 1672 is thought to have an obscured Seyfert nucleus, and it has strong starburst activity as well. Three bright X-ray sources with luminosities 1-2 x 10(exp 40) erg/s are clearly identified with NGC 1672. The strongest lies at the nucleus, and the other two lie at the ends of NGC 1672's prominent bar, locations that are also bright in H alpha and near-infrared images. The nuclear source is resolved by the HRI on about the scale of the recently identified nuclear ring, and one of the sources at the ends of the bar is also probably resolved. The X-ray spectrum of the nuclear source is quite soft, having a Raymond-Smith plasma temperature of approximately equals 0.7 keV and little evidence for intrinsic absorption. The ROSAT band X-ray flux of the nuclear source appears to be dominated not by X-ray binary emission but rather by diffuse gas emission. The absorption and emission properties of the sources, as well as their spatial extents, lead us to models of superbubbles driven by supernovae. However, the large density and emission measure of the nuclear X-ray source stretch the limits that can be comfortably accommodated by these models. We do not detect direct emission from the putative Seyfert nucleus, although an alternative model for the nuclear source is thermal emission from gas that is photoionized by a hidden Seyfert nucleus. The spectra of the other two X-ray sources are harder than that of the nuclear source, and have similar difficulties with regard to superbubble models.

Brandt, W. N.↗
NASA NTRS1991

Cosmological investigations from SIRTF

An overview is presented of the Space Infrared Telescope Facility (SIRTF), a one-meter-class cryogenically-cooled observatory for IR astronomy from space that will be the IR component of NASA's family of Great Observatories. SIRTF will operate at an altitude of 100,000 km and achieve its five year lifetime without the complication of on-orbit cryogenic replenishment and have twice the on-target efficiency that could be realized in LEO. The observations will permit the development of a direct empirical picture of the time evolution of cosmic phenomena, such as evolution of the chemical and stellar content of galaxies and the growth and decay of the quasar population.

Werner, M.↗
NASA NTRS2016

The Carnegie Chicago Hubble Program: The Mid-Infrared Colours of Cepheids and the Effect of Metallicity on the CO Band-Head at 4.6 Micron

We compare mid-infrared (IR) 3.6 and 4.5 micron Warm Spitzer observations for Cepheids in the Milky Way and the Large and Small Magellanic Clouds. Using models, we explore in detail the effect of the CO rotation-vibration band-head at 4.6 micron on the mid-IR photometry. We confirm the temperature sensitivity of the CO band-head at 4.6 micron and find no evidence for an effect at 3.6 micron. We compare the ([3.6]-[4.5]) period-colour relations in the MW, LMC and SMC. The slopes of the period-colour relations for the three galaxies are in good agreement, but there is a trend in zero-point with metallicity, with the lowest metallicity Cepheids having redder mid-IR colours. Finally, we present a colour-[Fe/H] relation based on published spectroscopic metallicities. This empirical relation, calibrated to the metallicity system of Genovali et al., demonstrates that the ([3.6]-[4.5]) colour provides a reliable metallicity indicator for Cepheids, with a precision comparable to current spectroscopic determinations.

Scowcroft, Victoria↗
NASA NTRS1980

X-ray observations of the Seyfert 1 galaxies AKN 120 and MCG 8-11-11

A new X-ray source, H0523-00, with the optically variable Seyfert 1 galaxy AKN 120 is identified. The source has a 2-10 keV X-ray flux of 2 x 10 to the -11th ergs/sq cm s which corresponds to a 2-10 keV X-ray luminosity of 10 to the 44th ergs/s. X-ray observations over a 1.5 year time span combined with contemporaneous optical photometry show a decrease in the optical with no corresponding decrease in the X-ray. In contrast, similar observations of MCG 8-11-11 show a contemporaneous decrease in optical and X-ray fluxes. It is noted that the infrared and X-ray spectral slopes for these two objects are similar, with the optical being steeper by roughly one unit.

Mushotzky, R. F.↗
NASA NTRS1984

The IRAS minisurvey

Before the main Infrared Astronomical Satellite (IRAS) all-sky survey was started, a preliminary survey of 900 sq deg was carried out. Some results from this 'minisurvey' are given here. The completeness of the minisurvey at galactic latitudes from 20 to 40 deg drops sharply at flux densities below 0.4, 0.4, 0.5, and 2.5 Jy at 12, 25, 60, and 100 microns, respectively. The corresponding surface densities of point sources brighter than these flux levels are 1.1, 0.4, 0.65, and 1.25/sq deg, respectively. Outside the galactic plane, the majority of the sources at 12 and 25 microns are stars, while galaxies make up a significant proportion of 60 micron sources. The 100 micron band is dominated by emission from interstellar dust over much of the minisurvey area.

Rowan-Robinson, M.↗
NASA NTRS1980

X-ray observations of the Seyfert 1 galaxies AKN120 and MCG8-11-11

A new X-ray source, H0523-00, with the optically variable Seyfert 1 galaxy AKN 120 is identified. The source has a 2-10 keV X-ray flux of 2 x 10 to the -11th ergs/sq cm s which corresponds to a 2-10 keV X-ray luminosity of 10 to the 44th ergs/s. X-ray observations over a 1.5 year time span combined with contemporaneous optical photometry show a decrease in the optical with no corresponding decrease in the X-ray. In contrast, similar observations of MCG 8-11-11 show a contemporaneous decrease in optical and X-ray fluxes. It is noted that the infrared and X-ray spectral slopes for these two objects are similar, with the optical being steeper by roughly one unit.

Mushotzky, R.↗
NASA NTRS2010

James Webb Space Telescope (JWST) and Star Formation

The 6.5-m aperture James Webb Space Telescope (JWST) will be a powerful tool for studying and advancing numerous areas of astrophysics. Its Fine Guidance Sensor, Near-Infrared Camera, Near-Infrared Spectrograph, and Mid-Infrared Instrument will be capable of making very sensitive, high angular resolution imaging and spectroscopic observations spanning 0.7 - 28 ?m wavelength. These capabilities are very well suited for probing the conditions of star formation in the distant and local Universe. Indeed, JWST has been designed to detect first light objects as well as to study the fine details of jets, disks, chemistry, envelopes, and the central cores of nearby protostars. We will be able to use its cameras, coronagraphs, and spectrographs (including multi-object and integral field capabilities) to study many aspects of star forming regions throughout the galaxy, the Local Group, and more distant regions. I will describe the basic JWST scientific capabilities and illustrate a few ways how they can be applied to star formation issues and conditions with a focus on Galactic regions.

Greene, Thomas P.↗
NASA NTRS2011

The James Webb Space Telescope

The James Webb Space Telescope is the scientific successor to the Hubble and Spitzer Space Telescopes, and is currently the largest scientific project under construction in the United States. It will be a large (6.6m) cold (50K) telescope launched in about 5 years into orbit around the second Earth-Sun Lagrange point. It is a partnership of NASA with the European and Canadian Space Agencies. Science with the James Webb Space Telescope falls into four themes. The End of the Dark Ages: First Light and Reionization theme seeks to identify the first luminous sources to form and to determine the ionization history of the universe. The Assembly of Galaxies theme seeks to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and black holes within them evolved from the epoch of reionization to the present. The Birth of Stars and Protoplanetary Systems theme seeks to unravel the birth and early evolution of stars, from infall onto dust-enshrouded protostars, to the genesis of planetary systems. The Planetary Systems and the Origins of Life theme seeks to determine the physical and chemical properties of planetary systems around nearby stars and of our own, and investigate the potential for life in those systems. Webb will have four instruments: The Near-Infrared Camera, the Near-Infrared multi-object Spectrograph, and the Tunable Filter Imager will cover the wavelength range 0.6 to 5 microns, while the Mid-Infrared Instrument will do both imaging and spectroscopy from 5 to 28.5 microns. I will conclude the talk with a description of recent technical progress in the construction of the observatory.

Gardner, Jonathan P.↗
NASA NTRS2011

The James Webb Space Telescope

The James Webb Space Telescope is the scientific successor to the Hubble and Spitzer Space Telescopes, and is currently the largest scientific project under construction in the United States. It will be a large (6.6m) cold (50K) telescope launched into orbit around the second Earth-Sun Lagrange point. It is a partnership of NASA with the European and Canadian Space Agencies. Science with the James Webb Space Telescope falls into four themes. The End of the Dark Ages: First Light and Reionization theme seeks to identify the first luminous sources to form and to determine the ionization history of the universe. The Assembly of Galaxies theme seeks to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and black holes within them evolved from the epoch of reionization to the present. The Birth of Stars and Protoplanetary Systems theme seeks to unravel the birth and early evolution of stars, from infall onto dust-enshrouded protostars, to the genesis of planetary systems. The Planetary Systems and the Origins of Life theme seeks to determine the physical and chemical properties of planetary systems around nearby stars and of our own, and investigate the potential for life in those systems. Webb will have four instruments: The Near-Infrared Camera, the Near-Infrared multi-object Spectrograph, and the Tunable Filter Imager will cover the wavelength range 0.6 to 5 microns, while the Mid-Infrared Instrument will do both imaging and spectroscopy from 5 to 28.5 microns. I will conclude the talk with a description of recent technical progress in the construction of the observatory.

Gardner, Jonathan P.↗
NASA NTRS1988

Engineering planetary lasers for interstellar communication

Transmitting large amounts of data efficiently among neighboring stars will vitally support any eventual contact with extrasolar intelligence, whether alien or human. Laser carriers are particularly suitable for high-quality, targeted links. Space laser transmitter systems designed by this work, based on both demonstrated and imminent advanced space technology, could achieve reliable data transfer rates as high as 1 kb/s to matched receivers as far away as 25 pc, a distance including over 700 approximately solar-type stars. The centerpiece of this demonstration study is a fleet of automated spacecraft incorporating adaptive neural-net optical processing active structures, nuclear electric power plants, annular momentum control devices, and ion propulsion. Together the craft sustain, condition, modulate, and direct to stellar targets an infrared laser beam extracted from the natural mesospheric, solar-pumped, stimulated CO2 emission recently discovered at Venus. For a culture already supported by mature interplanetary industry, the cost of building planetary or high-power space laser systems for interstellar communication would be marginal, making such projects relevant for the next human century. Links using high-power lasers might support data transfer rates as high as optical frequencies could ever allow. A nanotechnological society such as we might become would inevitably use 10 to the 20th power b/yr transmission to promote its own evolutionary expansion out of the galaxy.

Sherwood, Brent↗
NASA NTRS2011

The James Webb Space Telescope

The James Webb Space Telescope is the scientific successor to the Hubble and Spitzer Space Telescopes, and is currently the largest scientific project under construction in the United States. It will be a large (6.6m) cold (50K) telescope launched in about 5 years into orbit around the second Earth-Sun Lagrange point. It is a partnership of NASA with the European and Canadian Space Agencies. Science with the James Webb Space Telescope falls into four themes. The End of the Dark Ages: First Light and Reionization theme seeks to identify the first luminous sources to form and to determine the ionization history of the universe. The Assembly of Galaxies theme seeks to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and black holes within them evolved from the epoch of reionization to the present. The Birth of Stars and Proto planetary Systems theme seeks to unravel the birth and early evolution of stars, from infall onto dust-enshrouded protostars, to the genesis of planetary systems. The Planetary Systems and the Origins of Life theme seeks to determine the physical and chemical properties of planetary systems around nearby stars and of our own, and investigate the potential for life in those systems. Webb will have four instruments: The Near-Infrared Camera, the Near-Infrared multi-object Spectrograph, and the Tunable Filter Imager will cover the wavelength range 0.6 to 5 microns, while the Mid-Infrared Instrument will do both imaging and spectroscopy from 5 to 28.5 microns. I will conclude the talk with a description of recent technical progress in the construction of the observatory.

Gardner, Jonathan P.↗
NASA NTRS2011

The James Webb Space Telescope

The James Webb Space Telescope is the scientific successor to the Hubble and Spitzer Space Telescopes, and is currently the largest scientific project under construction in the United States. It will be a large (6.6m) cold (50K) telescope launched in about 5 years into orbit around the second Earth-Sun Lagrange point. It is a partnership of NASA with the European and Canadian Space Agencies. Science with the James Webb Space Telescope falls into four themes. The End of the Dark Ages: First Light and Reionization theme seeks to identify the first luminous sources to form and to determine the ionization history of the universe. The Assembly of Galaxies theme seeks to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and black holes within them evolved from the epoch of reionization to the present. The Birth of Stars and Proto planetary Systems theme seeks to unravel the birth and early evolution of stars, from infall onto dust-enshrouded protostars, to the genesis of planetary systems. The Planetary Systems and the Origins of Life theme seeks to determine the physical and chemical properties of planetary systems around nearby stars and of our own, and investigate the potential for life in those systems. Webb will have four instruments: The Near-Infrared Camera, the Near-Infrared multi-object Spectrograph, and the Tunable Filter Imager will cover the wavelength range 0.6 to 5 microns, while the Mid-Infrared Instrument will do both imaging and spectroscopy from 5 to 28.5 microns. I will conclude the talk with a description of recent technical progress in the construction of the observatory.

Gardner, Jonathan P.↗
NASA NTRS2021

A Jovian analogue orbiting a white dwarf star

Studies have shown that the remnants of destroyed planets and debris-disk planetesimals can survive the volatile evolution of their host stars into white dwarfs, but few intact planetary bodies around white dwarfs have been detected. Simulations predict that planets in Jupiter-like orbits around stars of ≲8 Mꙩ (solar mass) avoid being destroyed by the strong tidal forces of their stellar host, but as yet, there has been no observational confirmation of such a survivor. Here we report the non-detection of a main-sequence lens star in the microlensing event MOA-2010-BLG-477Lb using near-infrared observations from the Keck Observatory. We determine that this system contains a 0.53 ± 0.11 Mꙩ white-dwarf host orbited by a 1.4 ± 0.3 Jupiter-mass planet with a separation on the plane of the sky of 2.8 ± 0.5 astronomical units, which implies a semi-major axis larger than this. This system is evidence that planets around white dwarfs can survive the giant and asymptotic giant phases of their host’s evolution, and supports the prediction that more than half of white dwarfs have Jovian planetary companions. Located at approximately 2.0 kiloparsecs towards the center of our Galaxy, it is likely to represent an analogue to the end stages of the Sun and Jupiter in our own Solar System.

J. W. Blackman↗
NASA NTRS1994

Large-scale characteristics of interstellar dust from COBE DIRBE observations

Observations from the COBE Diffuse Infrared Background Experiment of the 140 and 240 micrometer emissions from the Galatic plane region (absolute value of b less than 10 deg) are combined with radio surveys that trace the molecular (H2), neutral atomic (H I), and extended low-density (n(sub e) approximately 10 to 100/cm(exp 3)) ionized (H II) gas phases of the interstellar medium to derive physical conditions such as the dust temperature, dust-to-gas mass ratio, and far-infrared emissivity (1) averaged over these gas phases along each line of sight and (2) within each of these three gas phases. This analysis shows large-scale longitudinal and latitudinal gradients in the dust temperature and a decrease in dust temperature with increasing Galactocentric distance. The derived dust temperatures are significantly different from those derived in similar analyses using the Infrared Astronomical Satellite (IRAS) 60 and 100 micrometer data, suggesting that small (5 A approximately less than radius approximately less than 200 A) transiently heated dust particles contribute significantly o the Galactic 60 micrometer emission. It is found that 60% to 75% of the far-infrared luminosity arises from cold (approximately 17 to 22 K) dust associated with diffuse H I clouds, 15% to 30% from cold (approximately 19 K) dust associated with molecular gas, and less than 10% from warm (approximately 29 K) dust in extended low-density H II regions, consistent with the results of the IRAS analyses of the Galactic 60 and 100 micrometer emission. Within 2 deg of longitude of the Galactic center, the derived gas-to-dust mass ratio along the line of sight, G(sub d), reverses its general trend of decreasing G(sub d) toward the inner Galaxy and increases by a factor of approximately 2 to 3 toward the Galactic center. One possible explanation for this result is that the ratio of H2 column density to (12)CO intensity is lower in the Galactic center region than in the Galactic disk.

Sodroski, T. J.↗
NASA NTRS1992

Ly-alpha emission from cooling flows and measures of the dust content of rich clusters of galaxies

Ly-alpha and UV continuum fluxes from measurements across the emission-line regions of 10 'cooling flow' clusters of galaxies are tabulated. Comparison of Ly-alpha emission fluxes obtained with the IUE satellite with optical Balmer emission fluxes is used to demonstrate that the lines of sight to the centers of these clusters show an average excess reddening E(B-V) about 0.19 above foreground extinction. While some fraction of this may derive from localized extinction in the central (cooling flow) region of the cluster, a substantial portion of this effect should arise from an extended cluster component. The case where this extinction originates in the hot intracluster gas of the cluster halo is considered. Estimates for the corresponding 100-micron infrared emission, assuming grain properties similar to those of interstellar grains, place the expected emission at the about 1/2 Jy level for nearby clusters. Measurements from IRAS pointed mode observations of clusters are consistent with the expected values. These results are used to show that ISO and SIRTF may detect clusters to cosmological distances.

Hu, Esther M.↗