Engineering topics

Cowie, L. L.

Publications and source records attributed to Cowie, L. L..

At least 37 records · Page 2

NASA NTRS1983

The filaments of NGC 1275 - A collision between a galaxy and an accretion flow?

Morphological evidence is presented (in the form of 3-A-bandpass Fabry-Perot images made with a CCD camera) that the two bright emission-line systems seen toward the galaxy NGC 1275 arise from a high-velocity impact of a foreground galaxy upon the accretion flow of gas cooling in the center of the Perseus cluster. The uniquely high optical-line luminosity of NGC 1275, in comparison with other central galaxies in clusters observed to have cooling flows, may be explained by energy deposited during the collision. Using additional information from 21-cm and extinction measurements, a rough model of the interaction is developed. Problems remain with this model - such as the likelihood of a gas-rich system penetrating to the cluster center. The kinematic structure of the optically emitting gas shows additional complex structure near the nucleus of NGC 1275.

Kaaret, P.↗

NASA NTRS1983

Interstellar absorption lines in the spectrum of 3C 273

Absorption lines near the rest wavelengths of the Si IV doublet (wavelengths 1393.76, 1402.77) with equivalent widths 533 and 235 mA are reported in the spectrum of QSO 3C 273. The profiles show the nature of hot gas along a high-latitude (b = 64 deg) line of sight through our entire halo. The lines have full width at half-maxima of 100 km/s, compared to an instrumental resolution of 35 km/s, are centered at zero velocity, and show no evidence for components with equivalent widths greater than 50 mA at an absolute value of v greater than 75 km/s. The absence of high peculiar velocities is consistent with published spectra of halo gas over shorter pathlengths. The implications of this result are discussed, with particular reference to the possible connection between galactic halos and QSO absorption lines, for which C IV and Si IV usually have a broader velocity range.

York, D. G.↗

NASA NTRS1983

Two-dimensional spectrophotometry of the cores of X-ray luminous clusters

The results of a two-dimensional spectrophotometric survey of the core regions of 11 rich clusters of galaxies are presented. A number of these clusters have spectacular optical emission line systems in their cores. Both morphologically and kinematically, the emission line regions divide into extended, 20-100 kpc systems of long linear filaments associated with the cluster core and more compact, homogeneous elongated regions associated with the dominant central cluster galaxy. It is suggested that the present results can be expected, as hot X-ray emitting gas cools in the cluster center. Luminosities almost entirely agree with expected values. The morphology of the systems can be understood if the filaments form initially in the cooling flow and, in some cases, are subsequently accreted by the central galaxy.

Hu, E. M.↗

NASA NTRS1983

On the possibility of detecting very hot gas through absorption-line studies

The possibility of detecting hot (T greater than or approximately equal to 10 to the 6th K) diffuse interstellar and intergalactic gas through absorption-line studies is assessed. Optical studies of semiforbidden lines are shown to be just beyond feasibility. X-ray absorption-line studies are concluded to be within the capability of future soft X-ray spectrographs, however. In particular, it is shown that the Bragg crystal spectrometer aboard Einstein could possibly have been used to detect O VII and O VIII lines against the Crab if they are present at plausible levels. Improvement of instrument parameters by only a small factor could make this the most important method available for studying hot interstellar, galactic halo and possibly even intergalactic gas.

York, D. G.↗

NASA NTRS1982

Orion's cloak as a model for supershells of gas around OB associations

Orion OB1 was the association most heavily observed by the Copernicus satellite UV spectrometer, which detected very unusual, strong interstellar UV absorption lines. Negative velocity gas at -100 km/sec was also noted, together with the absence of a corresponding, very high positive velocity feature. These and other characteristics have led to the present inferrence of a radially expanding, thin, uniform and low column density shell of fast moving gas which surrounds the Ori OB1 and Lambda Ori regions. Inside this shell is a more slowly moving inhomogeneous region of higher density gas which produces the more sporadically distributed gas at velocities in the 30-100 km/sec range. Within this framework, it is suggested that the most recent supernova is seen in the highest velocity gas, while the composite effects of the history of supernova formation lie in the denser, slower material.

Cowie, L. L.↗

NASA NTRS1982

The structure and evolution of galacto-detonation waves - Some analytic results in sequential star formation models of spiral galaxies

Waves of star formation in a uniform, differentially rotating disk galaxy are treated analytically as a propagating detonation wave front. It is shown, that if single solitary waves could be excited, they would evolve asymptotically to one of two stable spiral forms, each of which rotates with a fixed pattern speed. Simple numerical solutions confirm these results. However, the pattern of waves that develop naturally from an initially localized disturbance is more complex and dies out within a few rotation periods. These results suggest a conclusive observational test for deciding whether sequential star formation is an important determinant of spiral structure in some class of galaxies.

Cowie, L. L.↗

NASA NTRS1982

Radio emission from supernova remnants in a cloudy interstellar medium

The van der Laan (1962) theory of SNR radio emission is modified in light of the inhomogeneity of the interstellar medium, and in order to allow for particle acceleration in shock fronts. It is proposed that most of the radio emission in 10-20 pc radius SNRs originates in cold interstellar clouds that have been crushed by the high pressure hot gas within the expanding remnant. Under these circumstances, simple reacceleration of ambient interstellar cosmic ray electrons can account for the surface brightness-diameter distribution of observed remnants, with the additional, relativistic particle energy compensating for the decreased filling factor of the radio-emitting regions. Warm interstellar gas, at about 8000 K, may also be compressed within very large SNRs (of radius of 30-100 pc) and account for both the giant radio loops, when these SNRs are seen individually, and the anomalously bright galactic nonthermal radio background, which may be the superposition of a number of such features.

Blandford, R. D.↗

NASA NTRS1982

The gaseous galactic halo as inferred from the line spectra of the galaxies Markarian 509 and Fairall 9

Narrow interstellar absorption lines of S II 1259.52, Si II 1260.42, and Fe II 1608.46 due to gas in the disk and the halo of the Galaxy have been detected in the spectrum of the Seyfert galaxy Mrk 509 with the International Ultraviolet Explorer. This gas is also seen at higher resolution in the Ca II and Na I absorption lines in two components at LSR velocities of +6 and +62 km/s. In addition, narrow Ly-alpha and C IV absorption near the Seyfert redshift seem to be present in the spectrum. Si II 1260.42 absorption from the galactic disk and from the Magellanic Stream or the halo of the SMC have been detected with the IUE in the spectrum of Fairall 9. The observations of these two objects when combined with existing results are shown to be consistent with a corotating galactic halo having a height of less than 10 kpc at the sun.

York, D. G.↗

NASA NTRS1982

Distribution of gas in the halo of the galaxy

Results are presented from a high-resolution, ultraviolet study of interstellar gas situated away from the plane of the Milky Way Galaxy, using the nuclei of Seyfert galaxies Mkn 509 and F9 as background probes. In these directions, low-velocity, galactic gas were detected as well as two extragalactic clouds, one probably associated with the Magellanic Stream and the other with Mkn 509. These data were combined with results from other lines of sight to show that the ultraviolet species extend about 10 kpc from the plane, assuming the high-latitude gas corotates with the galactic disk. Complimentary observations of the optical Ca II and Na I species suggests that these do not extend as far - perhaps 2 to 3 kpc from the plane. Further, the exceedingly complex velocity structure found only in Magellanic Cloud directions suggests that these sight-lines are not typical of high-latitude gas in general.

Blades, J. C.↗

NASA NTRS1981

Interstellar, intracluster and supercluster gas

Hot tenuous plasmas with temperatures in the range 1,000,000 to 3.10 to the 8th power K and densities of .000001 to .001 per cu cm are considered. The dominant radiation processes are thermal bremsstrahlung and collisional line excitation and all except the very hottest objects will have observable X-ray emissions lines. The most important unifying point is the angular scales involved. For the interstellar gas, structure ranging in size from about 10 ft to tens of degrees is expected.

Cowie, L. L.↗

NASA NTRS1981

A search for expanding supershells of gas around OB associations

The results of a search for giant, rapidly expanding shells of gas surrounding OB associations are recorded. Two out of thirteen nearby associations (Orion and Carina) show supershells of gas with radii of about 100 pc, expansion velocities of about 100 km/sec, and ages of about 400,000 years. Observations of the newly discovered Carina supershell are presented. It is suggested that these features are formed by supernova explosions in the associations. The birthrate of association supernovae than lies between 1.5 x 10 to the -14th/cu pc per/year and 10 to the -13th/cu pc per/year.

Cowie, L. L.↗

NASA NTRS1981

A high-resolution optical survey of interstellar absorption lines toward globular clusters and extragalactic objects. II - Further data

A status report is given on an ongoing survey of narrow-line, visual absorption components, which has been expanded to include lines of sight to the 3C 273 quasar, the nucleus of M87, and the NGC 1851, NGC 2808, and 47 Tuc globular cluster cores. The number density of components of the halo is calculated to lie between one-third and one per 100 kpc, though with substantial uncertainty. This estimate is held to be consistent with the galactic halo interpretation of the origin of high column density, quasar absorption-line systems.

Songaila, A.↗

NASA NTRS1981

On the origin and distribution of C IV and Si IV ions in the neighboring interstellar medium

IUE spectrometer observational data are reported on the C IV, Si IV, and N V absorption lines in 46 distant stars at FWHM resolution of 30 km/sec. A simple analytic theory is given for these contributions, including the effects of ionization by a soft X-ray background. While for some of the most distant field stars a photoionized contribution may be ruled out, C IV absorption lines are detected which are shallow and broad. It is suggested that what are being detected in these cases are contributions from the hot interstellar gas that is responsible for the O VI absorption lines. Most of the Si IV and C IV lines probably arise in the H II regions surrounding the observed star or its neighbors.

Cowie, L. L.↗

NASA NTRS1981

The distribution and morphology of X-ray-emitting gas in the core of the Perseus cluster

An unresolved source has been found coincident with the nucleus of NGC 1275 in a high-resolution X-ray image of the core of the Perseus cluster. Absorption in the optical features at high velocity with respect to NGC 1275, which are thought to be associated with a foreground galaxy, does not produce any detectable X-ray absorption. The emission tends to become asymmetric in the presence of the lower-velocity filaments, but no obvious, detailed correlation is found between X-ray enhancements and individual filaments. Deprojection of surface brightness to yield temperature and density profiles of the intracluster gas shows results consistent with a quasi-hydrostatic radiative accretion flow onto NGC 1275, and the pressure-driven mass inflow onto the central galaxy is then 200-400 solar masses/yr. The possibility of a problem in the relation of line-of-sight velocity dispersion to cluster gravitational mass is confirmed.

Fabian, A. C.↗

NASA NTRS1981

The Gemini-Monoceros X-ray enhancement - A giant X-ray ring

A 1.5 deg spatial resolution map of the 1/4 KeV diffuse X-ray background enhancement in the Gemini and Monoceros constellations is found to show a circular ring-shaped emission feature with a diameter of 20 deg. The feature and possible X-ray contributions from the Mon OB1 association and neighboring supernova remnants are discussed. From 300 pc, the region has a radius of 50 pc, with an emitting electron density of 0.01 per cu cm. A shell of expanding neutral hydrogen and nonthermal radio spur is observed outside the ring with the X-ray emitting pulsar PSR 0656 + 14 lying close to the center of the ring. Origins of the ring are discussed, ruling out formation by the association Mon OB1. The ring is considered to be a field supernova remnant formed by the progenitor of the central pulsar, and providing constraints on theories of remnant evolution. This conclusion is found to agree with estimated supernova rates, and the absence of additional examples of this stage of evolution is an observational selection effect.

Nousek, J. A.↗

NASA NTRS1981

Supernova remnant revolution in an inhomogeneous medium. I - Numerical models

The first numerical simulations of supernova remnant evolution in an inhomogeneous gas are presented. Evolution in the lowest density substrate (the intercloud) is assumed to be spherically symmetric with a large intercloud filling factor and many dense regions (clouds) within the remnant; however, mass momentum and energy transfer between cloud and intercloud are included and the position and morphology of individual clouds tracked. Evolution is considered in several different models of the interstellar medium, both those in which the intercloud gas is diffuse (0.001 to 0.01/cu cm) and those in which it is relatively dense (n approximately 0.3/cu cm) under a variety of assumptions about the efficiency of thermal evaporation from the clouds into the intercloud medium.

Cowie, L. L.↗

NASA NTRS1981

X-ray emission from M87 - A pressure confined cooling atmosphere surrounding a low mass galaxy

A conventional model for the mass of the galaxy M87 (500-billion solar masses) is used to show that a radiative cooling flow agrees well with spectroscopically derived mass flow rates (10 solar masses per year) and surface brightness profiles of X-ray emission around the galaxy. A temperature of 100-million K, and a density of a few x 10 to the -4th/cu cm are obtained without the use of a massive halo. The atmosphere contains 1-trillion solar masses of gas, with a temperature profile increasing outward and gas acting as a reservoir for the flow. The present models indicate that thermal conductivity in M87 is less than 0.002 of its classical value, and does not significantly affect properties of the surrounding gas.

Binney, J.↗

NASA NTRS1981

Galactic disk absorption lines in the spectrum of the quasar 3C 273

It is pointed out that the line of sight to 3C 273 is unique because of the possibility of obtaining both radio and optical absorption line measurements. This information may make it possible to understand in detail the structure of the interstellar medium along the line of sight. To take advantage of this possibility, high-resolution optical spectra were obtained of 3C 273. The observations were made using the echelle spectrograph at the Cassegrain focus of the 4 m telescope at the Cerro Tololo Inter-American Observatory in 1980 January. Both Na D and Ca K lines were obtained on the same plate. In the visual lines, the galactic disk absorption shows a main component at 20 km/sec with a trailing wing in the negative velocities.

Cowie, L. L.↗