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At least 667 records · Page 37
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NASA NTRS1993

A composite large-scale CO survey at high galactic latitudes in the second quadrant

Surveys undertaken in the 2nd quadrant of the Galaxy with the CfA 1.2 m telescope have been combined to produce a map covering about 620 sq deg in the 2.6 mm CO(J = 1 - 0) line at high galactic latitudes. There is CO emission from molecular 'cirrus' clouds in about 13 percent of the region surveyed. The CO clouds are grouped together into three major cloud complexes with 29 individual members. All clouds are associated with infrared emission at 100 micron, although there is no one-to-one correlation between the corresponding intensities. CO emission is detected in all bright and dark Lynds' nebulae cataloged in that region; however not all CO clouds are visible on optical photographs as reflection or absorption features. The clouds are probably local. At an adopted distance of 240 pc cloud sizes range from O.1 to 30 pc and cloud masses from 1 to 1600 solar masses. The molecular cirrus clouds contribute between 0.4 and 0.8 M solar mass/sq pc to the surface density of molecular gas in the galactic plane. Only 26 percent of the 'infrared-excess clouds' in the area surveyed actually show CO and about 2/3 of the clouds detected in CO do not show an infrared excess.

Heithausen, A.↗
NASA NTRS1994

A high resolution atlas of the galactic plane at 12 microns and 25 microns

High resolution images of the 12 micron and 25 micron IRAS survey data from each HCON crossing the Galactic Plane are being created for those regions that the original IRAS processing labeled as confused. This encompasses the area within 100 deg longitude of the Galactic Center and within 3 deg to 10 deg of the Plane. The procedures used to create the images preserve the spatial resolution inherent in the IRAS instrument. The images are separated into diffuse and point source components and candidate sources are extracted from the point source image after non-linear spatial sharpening. Fluxes are estimated by convolving the candidate sources with the point response function and cross-correlating with the original point source image. A source is considered real if it is seen on at least two HCON's with a rather generous flux match but a stringent position criterion. A number of fields spanning a range of source densities from low to high have been examined. Initial analysis indicates that the imaging and extraction works quite well up to a source density of about 100 sources per square degree or down to roughly 0.8 Janskys.

Price, Stephan D.↗
NASA NTRS1994

Galactic arm models of gamma-ray burst sources

Using the BATSE observations, evidence was presented recently that at least some of the gamma-ray burst sources could be in our Galactic spiral arms. Other analyses have disagreed with this conclusion, leading to an active debate as to whether this can be true. Taking a different approach, a large range of possible spiral arm spatial distributions for the gamma-ray burst sources is investigated here, and it is shown that none of these are able to simultaneously fit the current BATSE (V/V(sub max)), (cos theta), and (sin(exp 2) b) observations if all the sources are in the spiral arms. If claims continue to be made that the burst sources are in the Galactic spiral arms, it will be necessary to produce an acceptable spatial distribution for the sources.

Smith, I. A.↗
NASA NTRS1994

Observations of enhanced sub-iron (Sc-Cr) to Iron abundance ratios in the low energy galactic cosmic rays in Spacelab-3 and their implications

The Anuradha cosmic ray experiment in Spacelab-3, flown in the orbit at 350 km with an inclination of 57 deg for about six days, was used to measure the low energy galactic cosmic ray (GCR) heavy ions using a specially designed CR-39 detector module incorporating the arrival time information of the particles. The abundances of sub-iron (Sc-Cr) and iron particles in the low energy interval of 30-300 MeV/N were determined from the measurements made in four different depths of the Cr-39 detector module of 150 layers. From these studies we obtained sub-iron (Sc-Cr) to iron abundance ratios of 0.8 to 1.2 in 30-300 MeV/N energy range. It is found that these ratios are enhanced by a factor of two as compared to interplanetary ratios of about 0.5. It is shown that the enhancement of the ratio inside the earth's magnetosphere is probably due to the degree of ionization of low energy Sc to Cr and Fe ions in the galactic cosmic rays and to the rigidity filtering effects of the geomagnetic field. Further studies are needed to understand fully the phenomena and their implications.

Biswas, S.↗
NASA NTRS1994

Dynamics and X-ray emission of a galactic superwind interacting with disk and halo gas

There is a general agreement that the conspicuous extranuclear X-ray, optical-line, and radio-contiuum emission of starbursts is associated with powerful galactic superwinds blowing from their centers. However, despite the significant advances in observational studies of superwinds, there is no consensus on the nature of the emitting material and even on the emission mechanisms themselves. This is to a great extent a consequence of a poor understanding of dynamical processes in the starburst superwind regions. To address this issue, we have conducted two-dimensional hydrodynamical simulations of galactic superwinds. While previous similar studies have used a single (disk) component to represent the ISM of the starburst galaxy, we analyze the interaction of the wind with a two-component disk-halo ambient interstellar medium and argue that this two-component representation is crucial for adequate modeling of starbursts. The emphasis of this study is on the geometry and structure of the wind region and the X-ray emission arising in the wind material and the shocked gas in the disk and the halo of the galaxy. The simulation results have shown that a clear-cut bipolar wind can easily develop under a range of very different conditions. On the other hand, a complex 'filamentary' structure associated with the entrained dense disk material is found to arise within the hot bubble blown out by the wind. The flow pattern within the bubble is dominated equally by the central biconic outflow and a system of whirling motions r elated to the origin and development of the 'filaments'. The filament parameters make them a good candidate for optical-emission-line filamentary gas observed in starburst halos. We find that the history of mass and energy deposition in the starburst region of the galaxy is crucial for wind dynamics. A 'mild' early wind, which arises as a result of the cumulative effect of stellar winds from massive stars, produces a bipolar vertical cavity in the disk and halo gas without strongly affecting the gaseous disk, thus creating conditions for virtually free vertical escape of the hot gas at the later, much more violent supernova-dominated phases of the starburst. We calculate the luminosity, mass, and effective temperature of the X-ray emitting gas in the 'soft' (0.1 to 0.7 keV, 0.7 to 2.2 keV, and 0.1 to 2.2 keV) and 'hard' (1.6 to 8.3 keV) energy bands and estimate the contribution of different gaseous components to the X-ray flux in these bands. Analysis of these parameters enables us to make conclusions regarding the nature of the X-ray-emitting material. We have inferred that the bulk of the soft thermal X-ray emission from starbursts arises in the wind-shocked material of the disk and halo gas rather than in the wind material itself. This enables us to predict that the integrated soft X-ray spectra of starbursts need not show an overabundance of heavy elements which are believed to be produced copiously in the centers of starbursts. Unlike soft X-ray emission, the hard component of thermal X-ray emission is found to originate in the wind material ejected from the starburst region. However, the derived ratio of hard-to-soft X-ray luminosities is too small compared to that observed in starbursts. We conclude therefore that the observed hard X-ray emission of starbursts is probably not associated with the thermal emission of hot wind or ambient shocked gas. Typical temperatures of the bulk of the soft X-ray-emitting material in our very different models have been found to agree well with the ones estimated on the basis of the ROSAT data for the soft component of X-ray emission of nearby starbursts. We predict that temperatures of the extranuclear soft X-ray-emitting gas in starburst galaxies with heavy element abundances near solar should be close to T(sub Xs = 2 to 5 x 10(exp 6)K.

Suchkov, Anatoly A.↗
NASA NTRS1994

Galactic stars applied to tests of the criterion for convection and semiconvection in an inhomogeneous star

The question of the effective criterion for convection and semiconvection to use in the inhomogeneous layers of models of massive stars is addressed here in terms of a choice between the Schwarzschild (temperature-gradient) criterion and the Ledoux (density-gradient) criterion. These two extremes give rise to and suppress, respectively, a fully convective zone (FCZ) in the layers immediately above the hydrogen-burning shell, if the star's mass is neither too low nor too high. Specifically, the existence of a large FCZ delays or prevents the star during core helium burning from becoming a red supergiant. The applicable range of initial stellar masses for a solar metallicity is approximately 13 to 30 solar mass. Nine different tests for the effective criterion for convection and semiconvection in Galactic supergiants, as well as a test using SN 1987A of the Large Magellanic Cloud, are performed, or reexamined, by using both old and new theoretical data, while three of the Galactic tests are repeated for association supergiants separately from cluster supergiants. Although eight of the 13 tests yield inconclusive results, five do support the Ledoux criterion, and three do so strongly.

Stothers, Richard B.↗
NASA NTRS1994

Anisotropic scattering of OH/IR stars toward the Galactic center

We have made angular broadening measurements in the satellite line of the OH molecule at 1612 MHz of eight OH/IR stars located near the Galactic center with the Very Large Array. Early observations with the Very Long Baseline Array had revealed a region of pronounced scattering approximately centered on Sgr A*. These new observations show that the compact line emission from these stars is scattered by an amount that is 2-3 times larger than had been previously seen. Additionally, we measured noncircular scattering disks for these objects, evidence that the electron density variations are distributed anisotropically. The present data suggest that the enhanced turbulence is located near the Galactic center. However, we cannot completely rule out that the scattering is caused by an unrelated region along the line of sight.

Frail, Dale A.↗
NASA NTRS1994

The galactic globular cluster system

We explore correlations between various properties of Galactic globular clusters, using a database on 143 objects. Our goal is identify correlations and trends which can be used to test and constrain theoretical models of cluster formation and evolution. We use a set of 13 cluster parameters, 9 of which are independently measured. Several arguments suggest that the number of clusters still missing in the obscured regions of the Galaxy is of the order of 10, and thus the selection effects are probably not severe for our sample. Known clusters follow a power-law density distribution with a slope approximately -3.5 to -4, and an apparent core with a core radius approximately 1 kpc. Clusters show a large dynamical range in many of their properties, more so for the core parameters (which are presumably more affected by dynamical evolution) than for the half-light parameters. There are no good correlations with luminosity, although more luminous clusters tend to be more concentrated. When data are binned in luminosity, several trends emerge: more luminous clusters tend to have smaller and denser cores. We interpret this as a differential survival effect, with more massive clusters surviving longer and reaching more evolved dynamical states. Cluster core parameters and concentrations also correlate with the position in the Galaxy, with clusters closer to the Galactic center or plane being more concentrated and having smaller and denser cores. These trends are more pronounced for the fainter (less massive) clusters. This is in agreement with a picture where tidal shocks form disk or bulge passages accelerate dynamical evolution of clusters. Cluster metallicities do not correlate with any other parameter, including luminosity and velocity dispersion; the only detectable trend is with the position in the Galaxy, probably reflecting Zinn's disk-halo dichotomy. This suggests that globular clusters were not self-enriched systems. Velocity dispersions show excellent correlations with luminosity and surface brightness. Their origin is not well understood, but they may well reflect initial conditions of cluster formation, and perhaps even be used to probe the initial density perturbation spectrum on a approximately 10(exp 6) solar mass scale. Core radii and concentrations play a role of a 'second parameter' in these correlations. While a global manifold of cluster properties has a high statistical dimensionality (D greater than 4), a subset of structural, photometric, and dynamical parameters forms a statistically three-dimensional family, as expected from objects following King models; we propose to call this set of quantities the King Manifold. Some of the observed correlations may be usable as distance indicator relations for globular clusters.

Djorgovski, S.↗
NASA NTRS1994

Gas flow in a two component galactic disk

Numerical simulations of two-component (stars + gas) self-gravitating galactic disks show that the interstellar gas can significantly affect the dynamical evolution of the disk even if its mass fraction (relative to the total galaxy mass) is as low as several percent. Aided by efficient energy dissipation, the gas becomes gravitationally unstable on local scale and forms massive clumps. Gravitational scattering of stars by these clumps leads to suppression of bar instability usually seen in heavy stellar disks. In this case, gas inflow towards the galactic center is driven by dynamical friction which gas clumps suffer instead of bar forcing.

Noguchi, M.↗
NASA NTRS1994

The source of the relativistic particles in the galactic center arc

Three fields in the molecular cloud M0.20-0.033, located near the midpoint of the filamentary 'Galactic center arc,' have been imaged in the CS J = 2-1 line with the OVRO millimeter interferometer. Several molecular clumps have been found to coincide with the endpoints of a number of the arc's nonthermal filaments (NTFs), suggesting that the relativistic particles present along the filaments originate in these molecular cloudlets. The most striking correlation involves a clump that is elongated along the direction of a pair of NTFs, and that approximately coincides with both their endpoints. These observations thus provide clues to the particle acceleration mechanism operative in the Galactic center arc: magnetic energy is likely liberated as the ambient field, tangled by the clumps' motions, reconnects in the advancing clumps' leading, externally ionized, surface layers.

Serabyn, E.↗
NASA NTRS1994

Short period Miras and Galactic Structure

Red Giants on the Asymptotic Giant Branch typically are more luminous than M(sub K) = -7 mag. Therefore, a new Two Micron Sky Survey (2MASS) which will go as faint as m(sub K) = 10 mag will be able to observe most of these stars in the Milky Way. Such a complete census will enable us to develop a much better understanding of Galactic Structure. It will be important to separate the luminous red giants into their different subclasses because these subclasses trace different Galactic Populations. For example, Miras with periods less than 300 days can be used to study the 'thick disk', while Miras with periods greater than 300 days belong to the 'thin disk'.

Jura, M↗
NASA NTRS1994

Probing the galactic disk and halo. 1: The NGC 3783 sight line

We report a study of Galactic disk and halo absorption toward the Seyfert galaxy NGC 3783 which has Galactic coordinates l = 287.46 and b = +22.95. The data were obtained with the Goddard High Resolution Spectrograph operating at medium resolution with the Large Science Aperture, which produces a line spread function having a sharp core (full width at half maximum (FWHM) approximately 20 km/s) and broad wings extending to +/- 70 km/s. Ion species detected in absorption near zero LSR velocity include C IV and N V for high ions, and C I, Mg II, Si II, and S II for low ions. Absorption from a high-velocity cloud (HVC) at a velocity of +240 km/s along the sight line is also detected in the ion species of S II, Si II, and probably C I. This is the first reported case where S II and C I absorption has been detected in a HVC. The S II lines are especially useful since metal abundance estimates based on S are largely unaffected by dust grains. The study is aided by the availability of 21 cm emission data.

Lu, Limin↗
NASA NTRS1994

Crystallography of rare galactic honeycomb structure near supernova 1987a

Near supernova 1987a, the rare honeycomb structure of 20-30 galactic bubbles measures 30 x 90 light years. Its remarkable regularity in bubble size suggests a single-event origin which may correlate with the nearby supernova. To test the honeycomb's regularity in shape and size, the formalism of statistical crystallography is developed here for bubble sideness. The standard size-shape relations (Lewis's law, Desch's law, and Aboav-Weaire's law) govern area, perimeter and nearest neighbor shapes. Taken together, they predict a highly non-equilibrium structure for the galactic honeycomb which evolves as a bimodal shape distribution without dominant bubble perimeter energy.

Noever, David A.↗
NASA NTRS1994

The ultraviolet emission properties of five low-redshift active galactic nuclei at high signal-to-noise ratio and spectral resolution

We analyze the ultraviolet (UV) emission line and continuum properties of five low-redshift active galactic nuclei (four luminous quasars: PKS 0405-123, H1821 + 643, PG 0953 + 414, and 3C 273, and one bright Seyfert 1 galaxy: Mrk 205). The HST spectra have higher signal-to-noise ratios (typically approximately 60 per resolution element) and spectral resolution (R = 1300) than all previously published UV spectra used to study the emission characteristics of active galactic nuclei. We include in the analysis ground-based optical spectra covering H beta and the narrow (O III) lambda lambda 4959, 5007 doublet. New results are obtained and presented.

Laor, Ari↗
NASA NTRS1994

X-ray color analysis of the spectra of active galactic nuclei

The identification and detection of X-ray absorption and emission features depends on the resolution and the signal-to-noise ratio (S/N) of the observation, the understanding of the instrument response, and the Galactic line-of-sight absorption. Since many of the active galactic nucleus (AGN) data sets are limited in their S/N and full modeling of the physical conditions is rather complicated, we suggest a new analysis method based on 'X-ray colors.' The two sets of X-ray colors, defined for low (ROSAT Position Sensitive Proportional Counter (PSPC)) and medium (Broad Band X-Ray Telescope (BBXRT)) and ASCA Solid-State Imaging Spectrometers (SIS) resolution experiments, are used to separate regions of different physical conditions in a two-dimensional color-color plane. They are similar but superior to previous methods using the X-ray 'hardness ratio' in being able to reveal more of the physical properties of the source. We illustrate the use of such diagrams by studying a number of AGNs suspected of showing absorption features. A sample of 14 AGNs observed by the ROSAT PSPC is presented which includes several objects with suspected 'warm absorbers' along the line-of-sight to the nucleus, several others exhibiting intrinsic continuum variations, and a number of control objects thought to be featureless. Our new observations show, for the first time, the color variation as a function of time for three of the Seyfert 1 sources: NGC 4051, Mrk 335, and Mrk 766. The variations suggest that in two sources we are witnessing real changes in continuum shape, while one (NGC 4051) is consistent with having a warm absorber. Four of the objects observed by BBXRT are reanalyzed using our X-ray colors. Out of these, we discuss in detail the case of NGC 4151 and show that the color-color analysis agrees very well with previous, detailed spectral fitting methods. In particular, we confirm that the observed BBXRT observation of this source is not consistent with the warm absorber explanation and must be interpreted as a combination of a variable hard component and another component. We explain the limitations in measuring low-energy emission features in ASCA SIS observations and present color-color diagrams which can be used as a powerful diagnostic tool in the analysis of data collected by this experiment.

Netzer, Hagai↗
NASA NTRS1995

Elliptical accretion disks in active galactic nuclei

We present a calculation of the profiles of emission lines originating in a relativistic, eccentric disk, and show examples of the resulting model profiles. Our calculations are motivated by the fact that in about one-quarter of the double-peaked emission lines observed in radio-loud active galactic nuclei (and in the mildly active nucleus of NGC 1097), the red peak is stronger than the blue peak, which is contrary to the prediction of relativistic, circular disk models. Using the eccentric disk model we fit some of the observed profiles that cannot be fitted with a circular disk model. We propose two possible scenarios for the formation of an eccentric disk in an active galactic nucleus: (a) tidal perturbation of the disk around a supermassive black hole by a smaller binary companion, and (b) formation of an elliptical disk from the debris resulting from the tidal disruption of a star by the central black hole. In the former case we show that the eccentricity can be long-lived because of the presence of the binary companion. In the latter case, although the inner parts of the disk may circularize quickly, we estimate that the outer parts will maintain their eccentricity for times much longer than the local viscous time. We suggest that it may be possible to detect profile variability on much shorter timescales than those ranging from a decade to several centuries by comparing the evolution of the line profile with detailed model predictions. We argue that line-profile variability may also be the most promising discriminant among competing models for the origin of asymmetric, double-peaked emission lines.

Eracleous, Michael↗
NASA NTRS1994

Dynamics of massive black holes as a possible candidate of Galactic dark matter

If the dark halo of the Galaxy is comprised of massive black holes (MBHs), then those within approximately 1 kpc will spiral to the center, where they will interact with one another, forming binaries which contract, owing to further dynamical friction, and then possibly merge to become more massive objects by emission of gravitational radiation. If successive mergers would invariably lead, as has been proposed by various authors, to the formation of a very massive nucleus of 10(exp 8) solar mass, then the idea of MBHs as a dark matter candidate could be excluded on observational grounds, since the observed limit (or value) for a Galactic central black hole is approximately 10(exp 6.5) solar mass. But, if successive mergers are delayed or prevented by other processes, such as the gravitational slingshot or rocket effect of gravitational radiation, then a large mass accumulation will not occur. In order to resolve this issue, we perform detailed N-body simulations using a modfied Aarseth code to explore the dynamical behavior of the MBHs, and we find that for a 'best estimate' model of the Galaxy a runaway does not occur. The code treates the MBHs as subject to the primary gravitational forces of one another and to the smooth stellar distribution, as well as the secondary perturbations in their orbits due to another and to the smooth stellar distribution, as well as the secondary perturbations in their orbits due to dynamical friction and gravitational radiation. Instead of a runaway, three-body interactions between hard binaries and single MBHs eject massive objects before accumulation of more than a few units, so that typically the center will contain zero, one, or two MBHs. We study how the situation depends in detail on the mass per MBH, the rotation of the halo, the mass distribution within the Galaxy, and other parameters. A runaway will most sensitively depend on the ratio of initial (spheroid/halo) central mass densities and secondarily on the typical values for the mass per MBH, with the rough dividing line, using Galactic parameters, being M(sub BH) less than or = 10(exp 6.5) solar mass. Using parameters from Lacey & Ostriker (1985) and our most accurate model for Galaxy, no runaway occurs.

Xu, Guohong↗
NASA NTRS1994

Highly ionized gas in the Galactic halo

We reexamine the values of electron density n(sub e) and gas pressure P/k in the interstellar medium (ISM) of the Galactic halo, as inferred from C IV emission and absorption lines and using current C IV atomic data. In a homogeneous model with 4.7 less than or equal to log T less than or equal to 5.3, the data are consistent with 0.01 less than or equal to n(sub e) less than or equal to 0.02/cu cm and 2200 less than or equal to P/k less than or equal to 3700/cu cm K, a factor of 2-3 higher than advocated by Martin & Bowyer (1990) and comparable to the thermal pressure in the disk. If some of the C IV absorption arises from nonemitting, photoionized gas, then the inferred density and pressure will increase accordingly. The volume filling factor for homogeneous models ranges from 0.5% to 5%. Because of the constraints arising from filling factor and radiated power, most of the C IV must arise from gas near the peak of the cooling curve, at log t less than or equal to 5.6. We relate both emission-line and absorption-line observations to recent models in which turbulent mixing layers and isobarically cooling supernova remnants (SNRs) provide significant amounts of halo gas at approximately 10(exp 5.3) K and process 20-40 solar mass/yr with a power of approximately 10(exp 41) ergs/sec. Since the observed C IV and N V absorption scale heights have been reported to differ, at 4.9 kpc and 1.6 kpc, respectively, we examine inhomogeneous models with different exponential scale heights of T, P, and SN energy input. The ISM may change its character with distance above the Galactic plane, as superbubbles and mixing layers dominate over isolated SNRs as the source of the C IV. For appropiate scale heights, the midplane pressure is twice the homogeneous values quoted above. The O IV lambda 1034 diffuse emission line, which can be used as a temperature diagnostic of the hot gas, is predicted to be comparable in strength to that of C IV lambda 1549 (approximately 6000 photons/cu cm/s/sr. The ions C IV, N V, and O VI are predicted to show a decreasing trend in vertical extent, with the O IV scale height perhaps as low as approximately 1 kpc.

Shull, J. Michael↗