Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “SPIRAL GALAXY”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Accretion Disk Spectra of the Ultra-luminous X-ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources

Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and Galactic superluminal jet sources share the common spectral characteristic that they have unusually high disk temperatures which cannot be explained in the framework of the standard optically thick accretion disk in the Schwarzschild metric. On the other hand, the standard accretion disk around the Kerr black hole might explain the observed high disk temperature, as the inner radius of the Kerr disk gets smaller and the disk temperature can be consequently higher. However, we point out that the observable Kerr disk spectra becomes significantly harder than Schwarzschild disk spectra only when the disk is highly inclined. This is because the emission from the innermost part of the accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen for a face-on disk. The Galactic superluminal jet sources are known to be highly inclined systems, thus their energy spectra may be explained with the standard Kerr disk with known black hole masses. For ULXs, on the other hand, the standard Kerr disk model seems implausible, since it is highly unlikely that their accretion disks are preferentially inclined, and, if edge-on Kerr disk model is applied, the black hole mass becomes unreasonably large (greater than or approximately equal to 300 Solar Mass). Instead, the slim disk (advection dominated optically thick disk) model is likely to explain the observed super- Eddington luminosities, hard energy spectra, and spectral variations of ULXs. We suggest that ULXs are accreting black holes with a few tens of solar mass, which is not unexpected from the standard stellar evolution scenario, and their X-ray emission is from the slim disk shining at super-Eddington luminosities.

White, Nicholas E.↗
NASA NTRS1993

Global-, local-, and intermediate-scale structures in prototype spiral galaxies

The relationship between galactic spiral structure and the matter in the underlying disk constitutes one of the central problems in galactic dynamics. In Bertin et al. (1989), disk matter characterized by a low-dispersive speed is shown to be capable of playing a key role in the generation of large-scale spiral structure. In Roberts et al. (1992), this self-gravitating, low-dispersion disk matter is shown to be capable of playing an essential role in the formation of structure on local and intermediate scales. Both in computed cases where large-scale spiral structure is present and in those where it is not, the same dominant physical processes and fundamental dynamical mechanisms are active on local scales. The new perception, in which large-scale and small-scale phenomena operate somewhat independently as evidenced in the computational studies, permits a range of flocculent, multiarmed, and grand design spiral types to be simulated. In particular, grand design galaxies with ragged appearances exhibiting spurs, arm branchings, and interarm bridges in addition to the major spiral arms, similar to those often observed, can be generated.

Roberts, William W., Jr.↗
NASA NTRS1993

Mapping the stellar backbones of spiral galaxies

We present optical and IR surface photometry of M51 (NGC 5194) at B, V, R, I, J, K, and CO. These data are used to establish whether K-band images of spiral galaxies provide reliable maps of stellar surface mass density features such as massive spiral arms or bars. The main distorting agents in the mapping at shorter wavelengths are dust extinction and luminous young stars. From modeling the color changes across the main dust lanes in M51, we find the optical depths to be about 0.5 in the K band. For these optical depths the K-band flux is attenuated by only less than 10 percent even in the dust lanes. From monitoring the gravity-sensitive CO index across the spiral arms we find that young red supergiants do not significantly distort the K-band image. On this basis, we conclude that K-band images do trace the massive disk star population and allow to map azimuthal variation in the surface mass density of the stellar disk. In M51 we find the surface mass density contrast to range from 1.8 to 3, as expected from N-body simulations of the galaxy's tidal encounter with NGC 5195. The spiral arm amplitude in M51 clearly shows smooth, strong radial variations, with a maximum at about 130 arcsec and minima at 45 and 170 arcsec.

Rix, Hans-Walter↗
NASA NTRS1991

High resolution infrared astronomy satellite observations of a selected spiral galaxy

The H I, infrared, CO, H alpha and H beta band observations of M51, the prototypical grand-design spiral galaxy, are used to study the consequences of star formation for the distribution of H I and dust. Using the new Very Large Array (VLA) map of 21 cm emission, the Owens Valley Radio Observatory CO mosaic map, and an H alpha imate, new tests were performed with the idea of Tilanus and Allen that the H I is largely a photodissociation product in star-forming regions. It is confirmed that the H I spiral arms are generally coincident with the H II region arms, and offset downstream from the CO arms. The radial distributions of total gas, H alpha and H I surface density have a simple explanation in the dissociation picture. The distributions also demonstrate how the surface density of H I might be related to the star formation efficiency in molecule-rich galaxies. The large width of the H I regions along the arms compared to that of the giant H II regions can be understood in terms of a simple calculation of the expected size of an H I region associated with a typical giant H II region. The longer lifetime of the stars producing dissociating radiation vs. those producing ionizing radiation and the relatively long molecular formation timescale will also contribute to the greater width of the H I arms if stars are continuously forming on the arms. The lack of detailed coincidence of the H I and H II regions along the inner arms has a variety of possible explanations. Two simple tests were performed to probe the origins of the IRAS emission in M51. First, it was found that the infrared excess (IFE) of M51 is 24, suggesting that a substantial fraction of the infrared emission arises from dust heated by photons which do not originate in massive star-formaing regions. Second, radial cuts through the IRAS bands show that at 12, 25, and 60 microns, the arm-interarm contrast of the IRAS emission is substantially less than that of the H alpha emission, providing further evidence for the explanation and for the existence of a broadly distributed dust component. Deconvolved IRAS maps have improved resolution but do not change this finding.

Kulkarni, S. R.↗
NASA NTRS1989

Far-infrared emission and star formation in spiral galaxies

The correlations between the emission in the far-IR, H-alpha, and blue in a sample of normal spiral galaxies are investigated. It is found that the luminosities in these three bands are all tightly correlated, although both the strength of the correlations and their functional dependencies are a function of the galaxies' morphological types. The best-fit power laws to these correlations are different for the comparison of different quantities and deviate significantly from linearity in some cases, implying the presence of additional emission mechanisms not related to the general increase of luminosity with galactic mass. Clear evidence is found of two independent effects in the incidence of warm far-IR emission in late-type spirals. One is a luminosity effect shown by the presence of excess far-IR relative to H-alpha or optical emission in the more luminous galaxies. The other is a dependence on widespread star-formation activity.

Trinchieri, G.↗
NASA NTRS1993

Can cluster environment modify the dynamical evolution of spiral galaxies?

Over the past decade many effects of the cluster environment on member galaxies have been established. These effects are manifest in the amount and distribution of gas in cluster spirals, the luminosity and light distributions within galaxies, and the segregation of morphological types. All these effects could indicate a specific dynamical evolution for galaxies in clusters. Nevertheless, a more direct evidence, such as a different mass distribution for spiral galaxies in clusters and in the field, is not yet clearly established. Indeed, Rubin, Whitmore, and Ford (1988) and Whitmore, Forbes, and Rubin (1988) (referred to as RWF) presented evidence that inner cluster spirals have falling rotation curves, unlike those of outer cluster spirals or the great majority of field spirals. If falling rotation curves exist in centers of clusters, as argued by RWF, it would suggest that dark matter halos were absent from cluster spirals, either because the halos had become stripped by interactions with other galaxies or with an intracluster medium, or because the halos had never formed in the first place. Even if they didn't disagree with RWF, other researchers pointed out that the behaviour of the slope of the rotation curves of spiral galaxies (in Virgo) is not so clear. Amram, using a different sample of spiral galaxies in clusters, found only 10% of declining rotation curves (2 declining vs 17 flat or rising) in opposition to RWF who find about 40% of declining rotation curves in their sample (6 declining vs 10 flat or rising), we will hereafter briefly discuss the Amram data paper and compare it to the results of RWF. We have measured the rotation curves for a sample of 21 spiral galaxies in 5 nearby clusters. These rotation curves have been constructed from detailed two-dimensional maps of each galaxy's velocity field as traced by emission from the Ha line. This complete mapping, combined with the sensitivity of our CFHT 3.60 m. + Perot-Fabry + CCD observations, allows the construction of high-quality rotation curves. Details concerning the acquisition and reduction procedures of the data are given in Amram. We present and discuss our preliminary analysis and compare them with RWF's results.

Amram, P.↗
NASA NTRS1994

Azimuthally averaged radial S(sub 100 microns)/S(sub 60 microns) dust color temperatures in spiral galaxies

The IRAS S(sub 100 micron)/S(sub 60 micron) dust color temperature profiles are presented for two nearby spiral galaxies M 101 and M 81. The radial dust temperature profiles provided an important constraint on the origin of the far-infrared luminosity. The observed dust temperature is compared with that expected for diffuse interstellar dust heated by the general interstellar radiation field within each galaxy. The implications for the contribution of cirrus to the far-infrared luminosity of M 101 and M 81 are discussed.

Devereux, Nick A.↗
NASA NTRS1986

Optical rotation velocities and images of the spiral galaxy NGC 3198

H-alpha rotation velocities obtained from an echelle spectrogram to a radius of 2 arcmin for the spiral galaxy NGC 3198 agree well with the 21 cm rotation velocities of van Albada et al. (1985). The rotation curve is typical of Sc type galaxies in spite of NGC 3198's extended H I disk. From optical R and H-alpha-emission images, used to search for star-forming regions in the outer parts of this galaxy, several H-alpha knots are identified as far out as the Holmberg radius. With increasing radial distance out to 2 arcmin, the R-band surface brightness falls, the H-alpha is approximately constant, and the H I surface brightness rises.

Hunter, D. A.↗
NASA NTRS1990

Radio continuum and far-infrared emission of spiral galaxies: Implications of correlations

Researchers present a study extending the correlation seen between radio continuum and far-infrared emissions from spiral galaxies to a lower frequency of 408 MHz and also as a function of radio spectral index. The tight correlation seen between the two luminosities is then used to constrain several parameters governing the emissions such as the changes in star formation rate and mass function, frequency of supernovae that are parents of the interstellar electrons and factors governing synchrotron radio emission.

Rengarajan, T. N.↗
NASA NTRS1994

Diagnostics of dust content in spiral galaxies: Numerical simulations of radiative transfer

In order to find the best observable diagnostics for the amount of internal extinction within spiral galaxies, we have constructed realistic models for disk galaxies with immersed dust layers. The radiative transfer including both scattering and absorption has been computed for a range of model galaxies in various orientations. Standard galaxy surface photometry techniques were then applied to the numerical data to illustrate how different observables such as total magnitude, color and luminosity distribution behave under given conditions of dust distribution. This work reveals a set of superior diagnostics for the dust in the disk. These include not only the integrated parameters, but also the apparent disk structural parameters, the amplitude of the asymmetry between the near and far sides of the galaxy as divided by the apparent major axis and their dependence on the orientation of the galaxy with respect to the observer. Combining the above diagnostics with our impressions of real galaxies, we arrive at the qualitative conclusion that galaxy disks are generally optically thin. Quantitative conclusions will appear in subsequent work.

Byun, Y. I.↗
NASA NTRS1987

VLA continuum observations of barred spiral galaxies

Observations of NGC 613, NGC 1300, NGC 4314 and NGC 5383 using the VLA at frequencies of 1464.9 and 4885.1 MHz are reported. These objects are a subset of galaxies from which radio emission were searched. The selection criteria were: (1) they are barred spiral galaxies preferentially with different Hubble type; (2) they have a peculiar or hot spot nucleus; (3) they have been observed at far infrared wavelengths by IRAS; and (4) they are observable from the northern hemisphere. Their radio and far infrared properties are summarized and their composite spectra are shown.

Garcia-Barreto, J. Antonio↗
NASA NTRS1988

Dark and visible matter in spiral galaxies

Rotation-curve profiles are used to determine the dark-to-luminous mass ratio within the disk size for 43 spiral galaxies. It is noted that faint galaxies are halo-dominated and that bright galaxies are disk-dominated in the disk regions. The luminosity sequence is shown to be a dark-to-luminous sequence. By removing the dark-matter contribution from the velocity at the disk edge, the dispersion affecting the luminosity-kinematics relation is found to decrease in comparison with the conventional Tully-Fisher correlation.

Persic, M.↗
NASA NTRS1989

Far infrared structure of spiral galaxies from the IRAS CPC images

Significant extended far infrared (50 micron and 100 micron) structure was found for five face-on spiral galaxies (NGC2403, M51, M83, NGC6946, and IC342) from fourteen galaxies searched in the Infrared Astronomy Satellite (IRAS) chopped photometric channel (CPC) catalogue. Images were initially processed to remove instrumental and background artifacts, the isophotal centroids of each image determined, and multiple images of each galaxy (for each wavelength) superimposed and averaged to improve signal-to-noise. Calibration of these images was performed using IRAS survey array data. Infrared isophotes were then superimposed on optical (blue) images so that direct structural comparisons could be made.

Wainscoat, Richard J.↗
NASA NTRS1994

X-ray emission from the edge-on spiral galaxy NGC 4631

This grant supported research of the X-ray emission from the disk and halo of the edge-on spiral galaxy NGC 4631, using data from the ROSAT satellite. The data were obtained on the basis of a proposal submitted by the PI, which was highly ranked in the peer review. It is a pleasure to say that the goals of the project, imaging and spectroscopy of hot gas in the disk and halo of a vigorously star forming galaxy, have been achieved. The results of the project have been submitted for publication, and are in press. A list of the publications is included.

Walterbos, R. A. M.↗
NASA NTRS1993

H I 21 centimeter observations and I-band CCD surface photometry of spiral galaxies behind the Virgo Cluster and toward its antipode

Sample selection, radio and optical data acquisition and reduction, and observation results are presented for spiral galaxies behind the Virgo Cluster and toward its antipode. I-band CCD photometry was obtained for all the bright galaxies and part of the sample of faint galaxies in the two local volumes was studied. The statistical properties of the galaxy samples are discussed.

Lu, N. Y.↗
NASA NTRS1990

The interstellar halo of spiral galaxies: NGC 891

Researchers have detected the Warm Ionized Medium (WIM) phase in the galaxy NGC 891. They found that the radial distribution of the WIM follows the molecular or young star distribution - an expected dependence. The amount of the WIM in this galaxy exceeds that in our Galaxy. The major surprize is the large thickness of the WIM phase - about 9 kpc instead 3 kpc as in our Galaxy. Clearly, this is the most significant result of the observations. The presence of low ionization gas at high z as well as at large galactocentric radii (where young stars are rare) is an important clue to the origin of the halo and observations such as the one reported here provide important data on this crucial question. In particular, the ionization of gas at high absolute z implies that either the UV photons manage to escape from the disk of the galaxy or that the extragalactic UV background plays an important role. The bulk of the WIM in spiral galaxies is a result of star-formation activity and thus these results can be understood by invoking a high star formation rate in NGC 891. Only the concerted action of supernovae can get the gas to the large z-heights as is observed in this galaxy. Support for this view comes from our detection of many worms i.e., bits and pieces of supershells in the form of kilo-parsec long vertical filaments. Researchers also saw a 600-pc size supershell located nearly one kpc above the plane of the galaxy.

Kulkarni, Shrinivas R.↗
NASA NTRS1995

Gaseous Vortices in Barred Spiral Galaxies

During the course of examining many two-dimensional, as well as a smaller sample of three-dimensional, models of gas flows in barred spiral galaxies, we have been impressed by the ubiquitous presence fo vortex pairs, oriented roughly perpendicular to their bars, with one vortex on each side. The vortices are obvious only when viewed in the bar frame, and the centers of their velocity fields usually are near Lagrangian points L(sub 4,5). In all models that we have studied, the vortices form on essentially the same time scale as that for the development of gaseous spiral arms, typically two bar rotations. Usually the corotation radius, r(sub c), lies slightly beyond the end of the bar. Depending upon the mass distributions of the various components, gas spirals either into, or out of, the vortices: In the former case, the vortices become regions of high density, whereas the opposite is true if the gas spirals out of a vortex. The models described in this paper have low-density vortices, as do most of the models we have studied. Moreover, usually the vortex centers lie approximately within +/- 15 deg of L(sub 4,5). In the stellar dynamic limit, when pressure and viscous forces are absent, short-period orbits exist, centered on L(sub 4,5). These orbits need not cross and therefore their morphology is that of gas streamlines, that is, vortices. We believe that the gas vortices in our models are hydrodynamic analogues of closed, short-period, libration orbits centered on L(sub 4,5).

England, Martin N.↗
DOE OSTI2023

The profiles of bars in spiral galaxies

ABSTRACT We present an analysis of major-axis surface-brightness profiles of bars in a volume-limited sample of 182 barred spiral galaxies, using Spitzer 3.6 $\mu$m images. Unlike most previous studies, we use the entire bar profile, and we classify profiles into four categories. These are ‘Peak+Shoulders’ (P+Sh) – updating the classic ‘flat bar’ profile – and three subtypes of the classic ‘exponential’ profile: (true) Exponential, ‘Two-Slope’ (shallow inner slope + steeper outer slope), and ‘Flat-Top’ (constant inner region, steep outer slope). P+Sh profiles are preferentially found in galaxies with high stellar masses, early Hubble types, red colours, and low gas fractions; the most significant factor is stellar mass, and previous correlations with Hubble type can be explained by the tendency of higher-mass galaxies to have earlier Hubble types. The most common type of non-P+Sh profile is Exponential, followed by Flat-Top profiles; all non-P+Sh profiles appear to have similar distributions of stellar mass, Hubble type, colour, and gas fraction. We also morphologically classify the bars of an inclined subsample into those with and without boxy/peanut-shaped (B/P) bulges; as previously reported, the presence of a B/P bulge is very strong function of stellar mass. Essentially all bars with B/P bulges have P+Sh profiles; we associate the profile shoulders with the outer, vertically thin part of the bar. We find a small number of P+Sh profiles in bars without clear B/P bulges, which may indicate that P+Sh formation precedes the formation of B/P bulges.

Astronomy & Astrophysics↗