Showing papers on "Spiral galaxy published in 1974"

The local complex of O and B stars. I - Distribution of stars and interstellar dust

Abstract: The O-B5 stars, supergiants, young clusters, and associations within 1 kpc of the sun populate two flat systems inclined to each other by 19 to 22 deg. The historical background, statistical significance, composition, spatial arrangement of the contents, and interstellar extinction in the two belts are discussed. A more or less random distribution in space and in age characterizes the O-B5 stars of the 'galactic belt', which is aligned nearly along the Milky Way. The 'Gould belt' is inclined to the Milky Way (north in Sco-Oph and south in Orion), and exhibits a projected distribution of O-B5 stars in its mean plane that resembles a 'dragonfly', with five major features defining it. A crude 'diameter' of the system is 750 to 1000 pc, and the sun's position is eccentric, lying toward Ophiuchus. The nuclear age of the system, while not unique, may be characterized as 30 m.y. from the spectral type of the broad main-sequence turnup near B2-5. Most of the O-B2 stars and youngest stellar groups near the sun belong to the Gould belt, but both belts have approximately equal space densities of B3-B5 stars and similar average values of interstellar extinction.-

Aperture-synthesis observations of H I in the galaxy M82

Abstract: Maps of neutral hydrogen emission in the irregular galaxy M82 were obtained with a spatial resolution of 1' and a velocity resolution of 10.55 km s/sup -1/. A velocity map with spatial resolution of 1.5' was also constructed. These maps reveal a close association of the H I with optical features, including the filaments extending from the nucleus along the galaxy minor axis. A bar of H I at the east end of the major axis of M82 was found which may be due to interaction with M81. The H I distribution and velocities agree quite well with CO data near the nucleus of M82; however, both the systemic velocity of M82 and the velocity gradient determined from the radio data disagree with optically measured values. As much as 50% of the mass of M82 may be interstellar, and active star formation seems likely. 5 figures.

Statistics of Supernovae

Abstract: The frequency of SNe is strongly dependent on the intrinsic properties of their parent galaxies. The present investigation is an attempt to determine within a clearly defined sample volume the SN frequency as a function of two principal parameters: the type and the luminosity (or mass) of the parent galaxy — which seem to control essentially the intrinsic SN rate. In this respect it is an extension and improvement of an earlier paper (Tammann, 1970), where only Sb and Sc galaxies could be considered. Available distances have become in the mean time sufficiently accurate within the sample volume considered, that it seems possible to use individual intrinsic luminosities for the sample galaxies. The distance scale is based throughout on a value of the Hubble constant of 55 (Sandage and Tammann, 1974), whereas the earlier paper used H 0=75 km s−1 Mpc−1. The other major change in the present paper is that it allows for a newly found selection effect of the observable number of SNe in inclined spiral galaxies.

Physical Sciences: Detection of formaldehyde in external galaxies

Abstract: FURTHER to our observations of OH in the edge-on spiral galaxies NGC253 and NGC4945 (ref. 1), we now report the detection of H2CO absorption in both galaxies—the first evidence of this molecule in external galaxies.

The corrugation of the galactic layer

Abstract: The 21-cm line intensities in a (Z, R) distribution is studied at the locus of tangential points of the inner parts of the Galaxy using both Northern and Southern data. A corrugation effect is observed in the galactic neutral hydrogen layer with an average wave length of 2 kpc and a wave amplitude of 70 pc. The patterns obtained for the I and the IV quadrant indicate that the inner and the outer parts of the spiral arms are located, respectively, below and above the galactic plane. Also, with high angular resolution the corrugation pattern suggests the existence of ‘faults’ in a geological sense in the inter arm zones.

Distances to the sources of observed gamma-ray bursts

Abstract: WE suggest that the brief, intense outbursts of hard X rays, or soft γ rays of cosmic origin which have been termed γ-ray bursts1 are mainly, though not necessarily entirely, of Galactic origin. More specifically, the γ-ray bursts reported so far2 seem to originate in the local (Orion) spiral arm of our Galaxy, at distances of, typically, several hundred parsec. If we assume that the sources radiate into 4π sr, the absolute luminosities are ∼ 1039 erg per event. (It may be significant that this is of the order of the predicted Eddington limit radiation from accretion disks around collapsed stars.)

The systematic properties of clusters of galaxies

Abstract: Fifteen rich clusters of galaxies have been studied on photoelectrically calibrated photographic plates. These plates were measured using new automatic photometry techniques. Data in the literature have been supplemented by new observations to obtain velocity dispersions in nine of these clusters. Clusters can be divided by their galaxy content into three types: spiral rich, spiral poor, and those dominated by a cD galaxy. The luminosity functions of the latter two types are very similar, and differ significantly from that of galaxies in spiral rich clusters. The surface brightness profiles of the spiral poor and cD clusters agree in severalrespects with numerical models of collapsed clusters by Aarseth, but the spiral poor clusters are too irregular and have too little central concentration to be collapsed. All of the clusters have a local minimum in their profiles which cannot be explained. Spiral rich clusters tend to be less dense than average and cD clusters more dense, but, in the mean, all clusters follow a constant density law independent of mass. The cD clusters and others with a high ratio of elliptical to spiral galaxies have mass to light ratios of about 225, while the mass to light ratio of spiral rich clusters is less than half of this value. The collapse times calculated using these mass-luminosity ratios confirm that spiral rich clusters are still collapsing while the others are close to equilibrium. The degree of mass segregation in the collapsed clusters is close to that predicted by the Aarseth models and requires that most of the cluster mass be in the galaxies themselves rather than in a cluster-wide medium. The decrease of the number of spirals towards the center of the collapsed clusters supports the idea that SO galaxies are created by stripping the gas from spirals, and the spiral rich clusters can be expected to evolve into spiral poor clusters after collapse by this mechanism. The high proportion of elliptical galaxies in the cD clusters probably means that they are a distinct type of object, and not just a later evolutionary state of the spiral poor clusters.

The luminosity law for late-type main-sequence stars in the solar neighborhood

Abstract: Two thousand RI and 1700 UBV observations have been obtained for stars in northern (Vyssotsky and collaborators) and southern (Upgren and collaborators) spectrophometric surveys for K and M dwarfs. A comparison of the two surveys leads to the conclusion that the northern survey is nearly complete to VE = and for stars redder than (R- I) = + (K4-5) between the equator and the North Celestial Pole. These stars, plus a few supplementary objects mainly too bright for the survey, constitute northern sample I; and, except for a few objects without radial velocity or parallax determinations, the astrometric and photometric data are essentially complete. The southern survey also seems to be complete to VE = for stars with (R- I) between + (K2- 3) and + but there is an increasing deficiency of stars with increasing color. The astrometric data available for the northern sample I permit a distinction between young- and old

Optical interstellar line studies of a nearby cold cloud

Abstract: : Interstellar Ca II lines in the spectra of OB stars in a large region towards the galactic center are shown to arise in the cold hydrogen cloud seen in self-absorption in the 21-cm line. The distance to one of these stars is only 200 pc, which is thus an upper limit to the cloud distance. The cold hydrogen is almost certainly associated with the dust at about 150 pc distance which covers this entire area. OH molecules have been detected in this dust at the same velocity as the cold H I. The cold hydrogen is arranged in multiple velicity component sheets; density fluctuations within these sheets have a typical scale size of about 5 pc, which is the probable thickness of the sheets. The cloud extends for at least 100 pc along the galactic plane and at least 30 pc perpendicular to it. The possible origin of this complex of cold atomic hydrogen, dust, and molecules in a local spiral arm shock is discussed. (Modified author abstract)

The structure of spiral galaxies

Abstract: Chapter I (the Introduction) formulates the problem of the spiral structure. A survey of recent observational data on the structure of spiral galaxies is given in Chap. II. Data are submitted on the physical composition of the spiral arms, the distribution of the stellar and gaseous constituents of the galaxies, and the physical state of the interstellar medium. Chapter III deals with the wave theory of spiral structure. In the contemporary conception, the spiral arms are waves propagating in the galaxy at frequencies determined by some exciting mechanism (a generator), with their wave number determined by the dispersion properties of the galaxy. In this approach, the speed of rotation of the spiral pattern, which equals the frequency divided by the number of arms, does not depend on the distance to the center of the galaxy; this eliminates the problem of the twisting of the spiral arms by differential rotation. The conclusions of the wave theory are discussed, and a dispersion equation is given for the spiral structure of a galaxy. The problem of generation and maintenance of the sprial pattern is discussed. Numerical galaxy-modeling experiments are examined in Chap. IV. These experiments point to the formation of spiral structures of wave nature in disciform systems of gravitating particles. Critical remarks concerning the numerical experiments are offered. The Conclusion (Chap. V) discusses arguments for and against the wave theory. It is noted that this theory explains many of the observed facts and that many of its predictions have also been confirmed by observations.