Showing papers on "Spiral galaxy published in 1976"

Gas response to oval distortions in disk galaxies.

Abstract: Numerical hydrodynamical calculations have been carried out in order to investigate the response of a non-self-gravitating gaseous component to a rotating weak oval distortion in the axisymmetric gravitational field of a disk. The purpose of these time-dependent calculations was to determine the form of the steady-state gas density and flow distributions which would develop in the rotating frame of the oval distortion. This is possibly relevant to observed characteristics of gas distribution and kinematics in disk galaxies. The calculations reveal that the gas responds to the presence of the oval distortion by forming an open two-arm trailing spiral wave rotating with the angular speed of the oval distortion. The gas flow pattern is similar in form to predicted gas flow through small pitch-angle gravitational self-consistent spiral waves, although the deviations from uniform circular motion are large (approximately 75 km s/sup -1/ in regions of low gas density). These results may be understood from an investigation of periodic particle orbits in the rotating frame of the bar. Such an investigation implies that the presence of all three resonance regions (inner Lindblad, corotation, and outer Lindblad) within the gaseous disk is a necessary condition for such a spiral response. (AIP)

Shock-driven implosion of interstellar gas clouds and star formation.

Abstract: Two-dimensional hydrodynamical calculations are presented to demonstrate a mechanism of star formation which, within the context of the spiral density wave theory, can explain the narrowness of the spiral arms of galaxies delineated by the classic spiral tracers: the bright, young stars and their associated H II regions. The implosion of a standard interstellar cloud has been followed numerically after it encounters a shock in the intercloud medium. Parameters have been chosen to represent a cloud flowing into a spiral arm which is delineated by a shock in the interstellar gas. Although this work is motivated by spiral wave theory, the results should also be indicative of the evolution of a cloud struck, for example, by a supernova shock.

Photometric studies of spiral structure. I. The disks and arms of six Sb I and Sc I galaxies

Abstract: This paper presents detailed surface photometry of the galaxies NGC 3031, 4254, 4321, 5194, 5364, and 5457. Profiles showing the variation of surface brightness with azimuthal angle in each galaxy are given and used to separate the light into disk and arm components. The disk level at each radius is defined by the two main minima of the azimuthal light distribution, and the light above this level is assigned to the arms.The disks are exponential with an extrapolated center surface brightness of (B/sub 0/)/sub D/=21.67 +- 0.35 mag arcsec/sup -2/, in excellent agreement with Freeman. The disk colors are very uniform, with radial gradients mostly below the detection limit ( +- 0.010 mag kpc/sup -1/ in B-V), and lie in the narrow range of colors between old galactic clusters (B-Vapprox. =+0.7) and giant elliptical (B-Vapprox. =+0.9).The arms are significantly bluer than the disks and have a highly composite spectrum, as judged from the color indices. The ratio of arm intensity to disk intensity increases with radius, which explains the well-known outward bluing of spiral galaxies. There are indications that the ratio of total arm intensity to total disk intensity, which is a coarse measure of the strength of star formation,more » correlates with van der Kruit's compression strength of the gas. (AIP)« less

The peculiar velocity field in the local supercluster

Abstract: A general method of estimating the peculiar velocity field expected, under the gravitational instability picture, around the outer parts of a mass concentration like a cluster of galaxies is derived and applied to a preliminary analysis of the Sandage-Tammmann data on the distances and redshifts of nearby spiral galaxies. It is shown that there is evidence of a small peculiar velocity field of the sort expected. The field, if real, is consistent with a cosmologically flat universe, ..cap omega..=1; equally well, with a low-density cosmological model, ..cap omega..approx. =0.1. It is concluded that the data on the local peculiar velocity field do not yet offer a strong constraint on the density parameter ..cap omega... (AIP)

Statistics of extragalactic supernovae

Abstract: It is shown that supernovae of Type II are concentrated in spiral arms whereas those of Type I show no preference for spiral-arm regions. Rediscussion of available supernova statistics suggests that Tammann may have overestimated the dependence of supernova frequency on galaxy inclination. A study of the distribution of supernovae in elliptical galaxies indicates that the supernova rate per unit luminosity may be highest among (metal-poor) stars in the halos of E galaxies. All galaxies in which supernovae are known to have occurred have been classified on the DDO system. (AIP)

Aperture synthesis of neutral hydrogen in the galaxy M33

Abstract: Observations of neutral hydrogen in the galaxy M33 with resolutions of 2' and 10 km s/sup -1/ are described. Detailed modeling has revealed weak evidence for spiral density waves with an arm-interarm contrast of 3 to 2 and with a corotation radius in the range 7' to 12'. It has also disclosed substantial evidence for large-scale warping of the outer plane of this galaxy, similar to that found earlier in M83. (AIP)

Propagating star formation and irregular structure in spiral galaxies.

Abstract: A simple model is proposed which describes the irregular optical appearance often seen in late-type spiral galaxies. If high-mass stars produce spherical shock waves which induce star formation, new high-mass stars will be born which, in turn, produce new shock waves. When this process operates in a differentially rotating disk, our numerical model shows that large-scale spiral-shaped regions of star formation are built up. The structure is seen to be most sensitive to a parameter which governs how often a region of the interstellar medium can undergo star formation. For a proper choice of this parameter, large-scale features disappear before differential rotation winds them up. New spiral features continuously form, so some spiral structure is seen indefinitely. The structure is not the classical two-armed symmetric spiral pattern which the density-wave theory attempts to explain, but it is asymmetric and disorderly.The mechanism of propagating star formation used in our model is consistent with observations which connect young OB associations with expanding shells of gas. We discuss the possible interaction of this mechanism with density waves. (AIP)

On the formation of spiral and elliptical galaxies.

Abstract: The key factor distinguishing an elliptical from a spiral galaxy is proposed to be the amount of gas left over at the point of maximum collapse of the protogalaxy. The gas left depends on the ratio of the star formation time scale tau/sub s/ to the collapse time of the protogalaxy tau/sub c/. Observed properties of ellipticals imply that they formed out of relatively larger density perturbations at recombination, giving them a relatively small tau/sub s//tau/sub c/ (if a rho/sup 2/ star formation law is adopted); and thus they are expected to essentially complete their star formation by the time of maximum collapse. Observations of spiral galaxies and our Galaxy imply that they formed out of less dense density perturbations giving them a larger tau/sub s//tau/sub c/. In the latter, star formation is not nearly completed by the time of maximum collapse. A detailed numerical model including both stars and gas was used to follow the collapse of a spiral galaxy. The gas and stars collapse together until they hit the plane, whereupon the gas forms a disk and the stars relax to form a spheroidal halo. The competing effects of dissipation and turbulent viscosity are analyzed using a modelmore » of cloud formation in the early protogalaxy. The gas disk does not hit the plane at its equilibrium position, and both radial and nonaxisymmetric oscillations of the disk are expected before equilibrium is reached. ''Zeroth'' order Maclaurin spheroid models for equilibrium spiral and elliptical galaxies are presented, which relate many useful galactic parameters to the original size and angular momentum. Monte Carlo calculations suggest that the tidal interaction picture of Peebles is capable of explaining the angular momentum of our Galaxy and the observed distribution of ellipticities in elliptical galaxies. This also explains why ellipticals are found preferentially in large clusters: because protogalaxies arising from larger initial density fluctuations cluster more easily. (AIP)« less

Composition gradients across spiral galaxies. II. The stellar mass limit.

Abstract: The equivalent width of the H$beta$ emission from H ii regions in spiral galaxies increases with distance from the nucleus. This W (H$beta$) gradient is interpreted in terms of a radial gradient in the temperature of the hottest exciting stars. (T/subu/). From Searle's observations of M101, an increase $delta$ log T/subu/=0.02--0.13 from the intermediate to outermost spiral arms of M101 is inferred.There is also a radial decrease in the metal abundance (Z) across M101, and the T/subu/ gradient is consistent with the prediction of Kahn's recent theory that the upper mass limit for star formation should be smaller in regions of high Z. It is noted also that, even in the absence of changes in the upper mass limit, a T/subu/ gradient is expected because metal-rich stars of given mass have smaller effective temperatures. Several observational and theoretical improvements are needed before firm conclusions can be drawn, but it is clear that the presence of a T/subu/ gradient may lead to several important systematic changes in the interpretation of gradients in the properties of H ii regions across galaxies. AT/subu/ gradient reduces the Z gradient that is inferred from emission-line ratios, and it may help to explain why O iimore » is strong in the innermost regions where O iii is weak. A T/subu/ gradient may also partly camouflage a helium abundance gradient.« less

Consequences of a lifetime greater than 10 to the 7th power years for galactic cosmic rays

Abstract: The implications of recent determinations of the cosmic-ray lifetime are discussed. It is concluded that the observations are consistent with a 'dynamical halo' model in which cosmic rays are confined in an outward-moving galactic halo by self-generated hydromagnetic waves. Alternative models which do not incorporate a halo, but which have the cosmic rays propagate in regions of reduced density in the galactic disk, are also briefly discussed.

Color and metallicity gradients in E and S0 galaxies

Abstract: Photometric maps of the galaxies NGC 3115 (S0), NGC 3377 (E5), NGC 3379 (E0), and NGC 4762 (S0) are presented for four filter bandpasses: U, B, V, and K. In each of the galaxies, the color indices (U-B), (B-V), and (V-K) tend to decrease outward from the nucleus. The observed color gradients are greatest along the minor axes of the flattened systems. Color changes in these and other galaxies of like morphological type appear to be dominated by a single parameter which most logically seems to be the metal-to-hydrogen ratio Z. Because the (V-K) color index is dominated by light from K and M stars of luminosity class III, it is readily amenable to calibration in terms of Z. Grasdalen's (1975a) recent observations of (V-K) indices for globular cluster giants form the basis for this calibration. The metallicity gradients derived from the data are compared with those predicted from theoretical models recently reported by Larson (1975a); the comparison outside the nuclear regions is remarkably good. The (V-K) index places rather severe restrictions on the contribution of M dwarf stars to the integrated colors. If such stars are the major low-luminosity constituent of the galactic mass, it is difficult to justifymore » M/L ratios much in excess of 50. (AIP)« less

Spectral types in the Orion nebula cluster

Abstract: MK spectral types of 26 of the brightest cluster members show (1) nine stars with broad hydrogen-line cores, (2) two Am stars, of which one is in the region of the pre-main-sequence stars, (3) two stars with weak K lines similar to ones found in NGC 6475 and M 39, (4) three shell stars, of which one has hydrogen emission, (5) two composite spectra, and (6) no evolved (luminosity class IV or brighter) stars or Ap stars. Key words: open clusters - spectral classification

Dynamics of CO molecular clouds in the galaxy

Abstract: The CO velocities were compared with those of stars whose orbits are perturbed by the gravitational potential in the spiral arms. This model allowed agreement with the CO velocities for stars with ''ages'' < or =30 x 10/sup 6/ years. Some evidence was found that the ''age'' required for agreement with the CO velocities increases with galactic longitude through the region observed. (AIP)

The stellar content of the nuclear regions of Sc galaxies

Abstract: The stellar content of the nuclear regions of seven nearby Sc galaxies has been determined by means of population syntheses utilizing narrow-band spectrophotometric observations covering the wavelength range 3300A-10400A. A linear-programming fitting technique is used in conjunction with simple models of star formation to provide physically consistent population models incorporating multiple generations of stars. Evidence is found for substantial intrinsic reddening in the nuclear regions studied. Upper main-sequence stars are significant contributors in most cases, and the lower main sequence contributes insignificantly in all cases. All available indicators are consistent with M-giant-star dominance at the longest wavelengths. Absolute measurements of the emission-line spectra of the nuclear regions are presented and discussed. The 0-B stars arising naturally in the population models are found to be just sufficient to provide the observed nuclear ionization in all cases but NGC 5194, which may be collisionally ionized. The observed stellar populations are largely consistent with simple evolutionary models of star formation using a "local" initial mass function and a variety of time dependences ranging from exponential decays with time constants ~3-6 billion years to constant rates. A possibly significant correlation between stellar content and dynamical properties among four of the program galaxies is pointed out.

The distribution of γ-ray emissivity in the Galaxy

Abstract: The longitude distribution of gamma -ray intensity measured with the SAS-2 satellite has been unfolded to give the Calactic gamma -ray emissivity. It is found that the observations are consistent with either a cylindrically symmetric emissivity distribution, or a distribution uniform along spirals with tilt angles similar to those for spiral arms. The gamma -ray distribution in the Galaxy seems to be more in accord with models based on the molecular hydrogen distribution than those based on spiral structure.

The Radio Continuum Morphology of Spiral Galaxies

Abstract: Once Jansky realized that the extraterrestial microwave radiation which he had discovered was not coming from the Sun but from a direction near to that of the Galactic center (Jansky 1933) and after he established that the emission was associated with the Milky Way (Jansky 1935), he naturally concluded that either stars in the disk or interstellar matter in the Galaxy was responsible for the emission ofthis radiation. Reber ( 1940) confirmed this and was the first to try to detect similar emission from the nearest external spiral galaxy M31 in Andromeda. He gives a positive result at 160 MHz of about 3800 Jy, 1 corresponding to a main-beam bright­ ness temperature of � lOooK assuming that the true beam width of his telescope was about 12°. In a later paper (Reber 1944) he gives an inconclusive detection of about 960 Jy corresponding to � 25°K. We know now that the latter value of flux density is an order of magnitude too high to be a detection; the brightness temperature indicates that what Reber observed was probably part of the complicated structure in the galactic background [see for example the all-sky map of Landecker & Wielebinski ( 1970)]. However, with the Galaxy dominating the radio sky it is under­ standable that the nearest spiral galaxies were expected to be detectable. Ryle, Smith & Elsmore (1950) were therefore not surprised that four ofthe weakest sources in their first Cambridge Survey appeared to be associated with the brightest spiral galaxies. Their quoted flux densities are now known also to be in error; in their case it is most likely that their observations were adversely affected by confusion with neighboring sources. Detection of spiral galaxies turned out to be possible only with telescopes that were not confused at their detection limit and that had a resolving power comparable with the size of the galaxies. It was with such an instrument that Hanbury Brown & Hazard ( 1951) made the first incontrovertible observation of the

H$alpha$ emission from the disks of spiral galaxies

Abstract: Observations of the amount of H$alpha$ emission from the disks of 53 galaxies have been made using large entrance apertures so that the maximum possible fraction of the total area of the galaxy is included. These observations reveal a relationship between B--V color and H$alpha$ emission; for a given morphological type the bluer galaxies have more emission at H$alpha$. The effect of the strong emission lines on the colors is not sufficient to produce this trend. Furthermore, the H$alpha$ emission comes from discrete Hii regions ionized by starlight which are limited in the crudest approximation by the number of hot stars rather than the availability of gas. From the range in H$alpha$ emission, the ratio of hot stars to cooler stars corresponding to the observed range in B--V color is obtained. This is compared with a theoretical model by Searle, Sargent, and Bagnuolo which predicts B--V color as a function of the stellar luminosity function in the galaxy. The two separate methods of determining the change in ratio of hot-to-cool stars agree well. Further observational support is thus given to the models of Searle, Sargent, and Bagnuolo. (AIP)

A model for the local spiral structure of the Galaxy.

Abstract: Spatial distributions of optical spiral tracers and their motions in the region of the galactic plane from 1 = 280 to 340 deg are studied. When this region is related to the entire pattern of local spiral structure a major spiral arm, Sagittarius-Carina, is revealed from 1 = 280 to 30 deg. A model of the optical spiral pattern is proposed which adequately describes the available spatial and kinematic information for the spiral tracers.

The variable radio nucleus of M81

Abstract: Variations in the radio emission of the nucleus of the normal spiral galaxy M81 are reported. Observations at 2695 MHz between 1967 and 1975 show that the flux density decreased from 120 mJy to 64 mJy. (AIP)

Spatial distribution of neutral hydrogen in Virgo cluster galaxies of early type

Abstract: Early-type galaxies in the Virgo cluster were studied with the 3'.2, 1420 MHz beam at the Arecibo Observatory. For two SO and three Sa galaxies, neutral hydrogen was found in an extended disk with a ''hole in the middle.'' Flat rotation curves in the outer regions are indicated, as for later-type spirals. For two additional SO galaxies, neutral hydrogen emission, with a narrow Gaussian velocity profile, was found only from an unresolved region. For one further case, the large SO galaxy NGC 4762, emission with a broad velocity profile was found from an unresolved region. (AIP)

Ejection from the spiral galaxy NGC 1097

Abstract: Luminous features pointing at the spiral galaxy NGC 1097 were discovered with the U.K. 48 inch (1.2 m) Schmidt telescope. A series of deep, high-resolution photographs have now been taken with the 4 meter CTIO reflector in Chile. These plates show (1) three luminous jets, two of which are aligned in exactly opposite directions, and all of which emanate exactly from the center of NGC 1097; (2) that where the jets pass through the spiral arms of NGC 1097, they disrupt the gaseous component of those arms.The present photographic observations enable an age of 10/sup 7/ years and an ejection velocity of 5,000 km s/sup -1/, or greater, to be computed for the jets. The material of the jet consists of some compact bodies, possibly some plasma from the parent galaxy, and possibly some plasma and/or material given off from the ejecta in its outward passage through the galaxy. It is calculated that special circumstances make these particular jets visible and imply that many spiral galaxies may be similarly ejecting. (AIP)

Further evidence concerning the local density of red dwarfs.

Abstract: Recent evidence has suggested the existence of a large number of red dwarfs, highly concentrated to the galactic plane, in the solar neighborhood. These results are reexamined using photoelectrically determined B--V and (R--I)/subw/ colors for a sample of stars near the North Galactic Pole. Preliminary analysis of the data indicates that the density of the reddest stars is not nearly as great as was previously reported. (AIP)