Showing papers on "Spiral galaxy published in 1977"

Clusters of galaxies

Abstract: Observational properties of clusters of galaxies and some theoretical interpretations are reviewed. The major subjects of the review include catalogs of clusters, the static properties of rich clusters, cluster dynamics, X-ray emission from clusters of galaxies, and cluster radio emission. Observed static properties discussed are: richness, classification schemes, galactic content, cD galaxies, density profiles, sizes, and the optical luminosity function. Several characteristic time scales of clusters of galaxies are examined along with mass segregation, the dynamical evolution of clusters, cluster masses, mass-to-light ratios, the missing-mass problem, and the mean galactic and cluster mass densities in the universe. The main characteristics of clusters that contain X-ray emission are summarized, emphasizing the relation of the X-ray emission to optical properties. The most important properties of cluster radio emission are outlined, the relation of this emission to the other cluster properties studied is indicated, and evidence suggesting the influence of an intracluster gas on the radio galaxies and their relativistic electrons is noted.

Lectures on density wave theory

Abstract: Some elements of stellar dynamics.- The density distribution perpendicular to the galactic plane.- The stability of solutions.- The dynamics of a rotating disk of non-interacting particles.- The theory of spiral structure of galaxies.- Gas dynamical effects on density waves.- Observable consequences of the density wave theory.- Evolution and origin of density waves.

Cosmic ray propagation in a closed galaxy

Abstract: A simple model of cosmic ray propagation is proposed from which the major experimental results can be derived: The model reproduces the observed nuclear abundances and accounts for the observed changes of nuclear composition with energy, the high degree of isotropy of cosmic ray flux at all energies, and the high degree of its constancy throughout the history of the Solar System. It is consistent with the observed size distribution of extensive airshowers, the intensity and energy distribution of the electron component, and the diffuse emission of γ-rays and radio waves. The model is characterized by the two basic assumptions: (1) that cosmic rays have been injected at an unchanging rate by sources located in the galactic spiral arms and (2) that a large-scale magnetic field retains all particles in our galaxy, where they interact with interstellar gas, so that all complex nuclei are finally fragmented and their energy dissipated in meson production and electro-magnetic interactions.

X-ray clusters of galaxies - Correlation of X-ray luminosity with galactic content

Abstract: All 14 X-ray clusters of galaxies detected by the Ariel 5 satellite at high galactic latitudes with redshifts of less than 0.05 are analyzed optically for galaxy content. A strong correlation is found between cluster X-ray luminosity and galaxy type: the fraction of spiral galaxies in a cluster decreases rapidly with increasing X-ray luminosity. The observed correlation is consistent with a combined model of thermal bremsstrahlung emission from a hot intracluster gas and stripping of the spiral galaxies caused by the ram pressure of the same intracluster gas.

Spectral types in the Orion OB1 association.

Abstract: Spectral types are derived for 152 stars in the Northwest, Belt, and Outer Sword regions. The classifications show that the following are present: (1) stars above the ZAMS in the O9B3 and the B7-A3 regions; most of these have greater luminosities than class V or are double-lined binaries or peculiar stars; (2) eleven Ap stars of which three are Si rich, two are He rich, five are He poor, and one is Mg poor; (3) four moderate Be stars; (4) only two stars with broad hydrogen lines like the ones that are so prevalent in the Orion Nebula Cluster; (5)14 "sn" stars that have both sharp and broad He I lines, perhaps due to tenuous shells; (6) one classical shell star; (7) several Sirius-type Am stars, although most of the Am stars in the region are foreground stars. Key words: associations-peculiar stars-spectral classification

A galactic vacuum cleaner

Abstract: A REMARKABLE feature of the distributions of CO, H2, HI, HII and dust in the Galaxy is that their most dense regions form a broad ring-like structure surrounding the galactic centre, along with a dense nuclear disk located at the centre1–4. Between the ring and the nuclear disk there exists a region of very low density. The radius of the nuclear disk is approximately 600 pc, and the ring extends from an inner radius of 2–4 kpc out to a radius greater than 15 kpc for HI, but to only 8–10 kpc for the interstellar components expected to be generated by spiral shocks (namely CO, H2 and HII). We point out here that this structure is a possible consequence of a postulated stellar bar at the centre of the galaxy.

The relationship between the galactic matter distribution, cosmic-ray dynamics, and gamma-ray production

Abstract: Theoretical considerations and analysis of the results of ..gamma..-ray astronomy suggest that the galactic cosmic rays are dynamically coupled to the interstellar matter through the magnetic fields; hence the the cosmic-ray density should be enhanced where the matter density is greatest on the scale of galactic arms. This concept has been explored in a galactic model using recent 21 cm radio observations of neutral hydrogen and 2.6 mm observations of carbon monoxide, which is considered to be a tracer of molecular hydrogen. The model assumes that (1) cosmic rays are galactic and not universal; (2) on the scale of galactic arms, the cosmic-ray column (surface) density is proportional to the total interstellar-gas column density; (3) the cosmic-ray scale height is significantly larger than the scale height of the matter; and (4) ours is a spiral galaxy characterized by an arm-to-interarm density ratio of about 3:1. The second assumption implies that the ..gamma..-ray emission is proportional to the square of the mass on the scale of galactic arms. The theoretical predictions for ..gamma..-rays produced by matter--cosmic-ray interactions in this model show a good correlation with the galactic ..gamma..-ray observations, not only in terms of the absolute intensities along the galactic planemore » but also in terms of the positions of specific galactic features near longitudes 315/sup 0/, 330/sup 0/--335/sup 0/, 340/sup 0/--345/sup 0/, 0/sup 0/, 25/sup 0/,and 35/sup 0/.« less

The spiral structure in the inner parts of the Galaxy

Abstract: The spiral structure of the inner parts of the Galaxy is studied using 21 cm line data and stellar data. To study the neutral hydrogen distribution in the galactic layer a parameter η=τ(dV/dr) proportional to the mean densities is calculated using a first approximation for the velocity gradients due to differential rotation. The obtained distribution η(R, Z) shows spiral features completely consistent with the early star distribution and with the Hii regions. The corrugation effect of the galactic layer is observed in all the studied zones in neutral hydrogen and in the distribution of the OB stars in the Carina zone. The pattern obtained indicates four spiral arms for the inner parts of the Galaxy, three of which are identified also in the stellar data (arms -I, -II, and -III) and the more distant -IV in Hii regions. The local arm according to the stellar data of Kilkennyet al. forms a feature completely similar to the arms -I and -II and there are no indications that this arm is a special material branch between two main spiral arms as has been supposed in order to conciliate the neutral hydrogen pattern with the stellar distribution. The pitch angles for the spiral arms are approximately 13°–17°. The observed wave form distribution of the hydrogen cloud layer is completely consistent with the theoretical predictions of Nelson (1976) but there are no indications of such an effect in the intercloud hydrogen. The corrugated cloud layer has a width of 100 pc, a wave amplitude of 70 pc, and a wavelength which grows with the galactic center distance (approx. 2 kpc in the zones next to the galactic nucleus and 2.6–3.0 kpc in the zones next to the Sun). To each wavelength correspond two spiral arms. The spiral features in our Galaxy show characteristics quite similar to the features in the Andromeda nebula, not only in the component materials (neutral hydrogen, Hii regions and possibly also dust and stars) but also in their kinematics.

Dynamics of early-type galaxies. III. The rotation curve of the S0 galaxy NGC 4762

Abstract: The rotation curve of the flat S0 galaxy NGC 4762, derived from measurements of the absorption lines of Ca II H and K out to a distance of 11' (5 kpc) from the nucleus is presented This curve shows a moderate central gradient and a steady increae up to a value of 165 km s/sup -1/, with no definite indication that the turnover velocity has been reached In a simple model, it is estimated that the mass up to the last observed point is 35 x 10/sup 10/ Msub solar The corresponding galaxies appear to possess similar amounts of angular momentum 2 figures, 2 tables

A dynamical interpretation of the Hubble sequence of galaxies

Abstract: Brosche (1970) has proposed a theory in which the energy loss due to collisions among gas clouds contained in a galaxy constitutes the driving mechanism for its evolution, through virial equilibrium states which, from an initial spherical shape, makes it to contract towards an elongated form; moreover, the value of the total angular momentum, assumed as given by uniform rotation, is assumed to determine the galaxy type on the Hubble sequence and to strongly influence the contraction time from the initial spherical to the final flat configuration.

The galactic magnetic field.

Abstract: Estimates for the scale, geometry and strength of the magnetic field in the galactic system can be derived from observations of polarization properties of radio emission from the Galaxy, extragalactic radio sources and pulsars, and polarization of starlight. Within distances of about 500 parsecs (1 parsec = 3.26 lightyears) from the solar system the magnetic field is directed towards galactic longitude l≈45°, while at distances extending over a few kiloparsec its average direction is towards l≈90°. Seen on a large scale the magnetic field in the Galaxy may be directed parallel to the galactic plane and along the spiral arms. The field may consist of a regular component and a random component with small scale variations of about 50 parsec in size. The strength of both components is of the same order of magnitude, about 2×10–6 Gauss (this is about 6×10–6 times the magnetic field strength of the earth).

Evidence from optical polarisation for a galactic-scale uniform magnetic field in M104

Abstract: LINEAR polarisation of light has been observed in several galaxies1–4, but the only previous detailed polarisation pattern has been mapped in M822,3,5,6 from which information about the luminous and physical structure of the galaxy has been deduced7. Clearly, detailed knowledge of the polarisation pattern in a nebula is a powerful tool for further understanding of the nebula. In this letter we present a map of the linear optical polarisation of the Sombrero Galaxy (M104) giving the first clear evidence for the existence of a magnetic field which maintains the same projected direction over an estimated volume of some 103 kpc3.

The NGC 1275 enigma

Abstract: The following arguments suggest that NGC 1275 does not consist of a giant elliptical (E) galaxy that is colliding with (or is superimposed on) a late-type spiral (L): 1The total diameter of the region containing young associations is 33 (100/H) kpc. This size is characteristic of ScI galaxies. Neither the morphology nor the integrated luminosity of the L component of NGC 1275 supports such a classification. 2The chaotic appearance of the L component of NGC 1275 is unlikely to be due to tidal damage. This is so because: (a) the E and L components are still approaching each other, (b) their relative velocity is ≈ 3000 km s−1, (c) no stripped galaxy core (which would survive a catastrophic tidal encounter) is seen near NGC 1275. 3The core of the Perseus cluster contains only one (anemic) spiral. The a priori probability that NGC 1275 represents a chance superimposition (or collision) of a spiral and an elliptical galaxy is therefore low. 4The assumption that the L component of NGC 1275 is superimposed on, but not interacting with, the E component does not account for (a) the presence of an active SEYFERT nucleus, (b) the peculiar filamentary HII shell, discovered by LYNDS, (c) the presence of recently-formed stars, (d) the X-ray emission and the radio emission of NGC1275.

Lack of dust in quasar absorption line systems

Abstract: THE origin of the absorption line systems in quasars is still uncertain1. Most such systems apparently have column densities of atomic hydrogen of the order of 1019cm−2, but, at least two quasars, 1331 + 170 (ref. 2) and PHL957 (ref. 3), have such strong Lyman a absorption lines that atomic hydrogen column densities of the order of 1021 cm−2 are indicated. It should be possible to observe the dust produced 2,200 A extinction bump as it is redshifted into the visible4, and to determine whether the absorption line systems are produced in spiral galaxies where the dust content is similar to that in the interstellar medium. McKee and Petrosian4 argue that the emission line regions of quasars generally lack dust, and that toward PHL957 the 2,200 A feature is absent. From recent published data, I argue that dust similar to that found in the interstellar medium is not found toward the quasars 1331 + 170 and PHL 957. This could explain why H2 is not found toward PHL 957, and it also indicates that the absorption line systems in quasars are not produced in spiral galaxies similar to ours.

The missing intercloud medium and spiral galaxies

Abstract: IT is the purpose of this letter first to point out that several independent lines of evidence seem to indicate that the ‘intercloud medium’ does not exist in its usually quoted state with density ≳ 0.1 cm−3 and temperature T ∼ 104 K. Rather, a hot, tenuous medium (n ≲ 10−2 cm−3, T ∼ 106 K) seems more consistent with observations in the neighbourhood of the Sun. That such might be the situation has been previously suggested (see refs 1 and 2). A mechanism for producing such a medium has been proposed by Cox and Smith3. Second, this letter points out several implications of such a “missing intercloud medium” on the large-scale structure of spiral galaxies.