Showing papers on "Spiral galaxy published in 1986"

Dark matter in spiral galaxies. I. Galaxies with optical rotation curves

Abstract: Data obtained with a CCD camera are presented for 37 Sb and Sc galaxies with existing optical rotation curves. The CCD images are used to derive luminosity profiles using standard reduction methods, and multicolor images are obtained for several galaxies in order to investigate the effects of color gradients. A new method for decomposing the profiles into bulge and disk components is presented. It is found that optical rotation curves do not always place strong constraints on the amount of dark matter in galaxies. Three methods of constraining the halo or bulge/disk parameters are presented. The stellar M/L ratio is shown to correlate with galaxy mass, morphological type, and inclination. 31 references.

Bi-symmetric instabilities of the Kuz'min/Toomre disc

Abstract: Etude numerique de l'influence d'un mouvement aleatoire sur les modes insrables de formation des barres d'un type de modeles de disques stellaires. On montre que le taux de croissance des barres est une fonction simple du mouvement aleatoire et de la masse du halo

Morphological segregation in the Pisces-Perseus supercluster

Abstract: Evidence for continuous morphological segregation among galaxies along the entire Hubble sequence is sought for the wide range of galactic densities represented by the Pisces-Perseus supercluster. Observational data makes it possible to outline the gross structure of the supercluster. To the first order, enhancements in the projected surface density distribution of galaxies reflect volume density enhancements in the supercluster. The two-point autocorrelation function is computed for various subsets of the galaxy sample, and it is shown that early-type galaxies in the Pisces-Perseus region tend to cluster on smaller angular scales than later types. In addition, the population fractions of elliptical and S0 galaxies are found to grow with increasing density throughout the supercluster, while the spiral fraction correspondingly decreases. 35 references.

Do density waves trigger star formation

Abstract: A comparison of B - V colors, blue and far-IR surface brightnesses, metallicities, and star formation rates calculated from H-alpha and UV fluxes reveals no difference in the average star formation rates for galaxies with and without grand design spiral structure. This implies that strong density waves do not trigger a significant excess of star formation compared to that in similar galaxies without a wave. Density waves organize the gas and young stars into spiral arms because of the flow pattern, and they may influence the formation and destruction of the largest cloud complexes, but they appear to contribute less than 50 percent to the overall star formation rate. 46 references.

Unstable modes from galaxy simulations

Abstract: On propose des techniques numeriques qui permettent de calculer des frequences propres et des modes propres a partir de simulations sur ordinateur des disques stellaires

Enhanced star formation ― The importance of bars in spiral galaxies

Abstract: It was found that among an IR luminous subset of nearby spiral galaxies, nearly all of the systems with IRAS colors and luminosities indicative of enhanced star formation are barred. Radio continuum and IR spectroscopic results support the hypothesis that this emission originates within the central 2 kpc; possibly in a circumnuclear ring. It was also found that outer rings are over represented among these barred systems and suggest possible reasons for this phenomena.

The Pattern of H i Deficiency in the Virgo Cluster

Abstract: A sample of 160 galaxies in the Virgo region, including 16 new 21-cm profiles in the Virgo 5-degree core obtained with the 305-m Arecibo telescope, are examined to investigate the severe depletion of interstellar H I within spiral galaxies in the Virgo cluster core. A similar and non-Gaussian distribution is found for the distribution of H I deficiencies of both faint galaxies and brighter spirals, and populations of galaxies with normal abundances of interstellar H I, and those of gas poor objects exhibiting a late-type morphology, are both noted. One-sixth of the sample within the Virgo 5-degree core have lost more than 90 percent by mass of their original neutral hydrogen, and three quarters of the galaxies found within 2.5 degrees of M87 are H I poor by more than a factor of three. The most deficient galaxies are also found to be the ones with the smallest ratios of H I to optical disk size, and H I poor galaxies are redder than normal, indicating that star formation has been quenched. 57 references.

Studies of IRAS sources at high galactic latitudes – II. Results from a redshift survey at b > 60°: distribution in depth, luminosity function, and physical nature of IRAS galaxies

Abstract: On a identifie 530 sources IRAS d'un echantillon complet jusqu'aux flux limite S(60 μm)=0,5 Jy dans une zone de 844 degres carres du Pole galactique Nord. En dehors de 34 etoiles brillantes, 99% des sources coincident avec des galaxies, essentiellement de magnitudes 14 a 17. On a effectue des observations spectroscopiques pour 183 sources IRAS

Percolation and Galaxies

Abstract: A theory is presented in which much of the structure of spiral galaxies arises from a percolation phase transition that underlies the phenomenon of propagating star formation According to this view, the appearance of spiral arms is a consequence of the differential rotation of the galaxy and the characteristic divergence of correlation lengths for continuous phase transitions Other structural properties of spiral galaxies, such as the distribution of the gaseous components and the luminosity, arise directly from a feedback mechanism that pins the star formation rate close to the critical point of the phase transition The approach taken in this article differs from traditional dynamical views The argument is presented that, at least for some galaxies, morphological and other features are already fixed by general properties of phase transitions, irrespective of detailed dynamic or other considerations

The statistical distribution of the neutral-hydrogen content of S0 galaxies

Abstract: The distribution of relative global H I content M/sub HtsI//L/sub B/ has been derived for galaxies of types S0 and S0/a using a data set derived from recent H I observations in the literature. The relative H I content of these galaxies is found to show transitional properties between elliptical and spiral galaxies. The distribution of M/sub HtsI//L/sub B/ for S0/a's resembles that for spirals, and these galaxies may represent ''fossil'' spirals, i.e., galaxies whose gas has been severely depleted by star formation. The distribution for S0's, however, resembles that for ellipticals. The form of this distribution strongly suggests an external origin for most of the H I in S0 galaxies.

Molecules in Galaxies. III. The Virgo Cluster

Abstract: Using the 7 m Bell Laboratories antenna, observations of 47 spiral galaxies in the Virgo Cluster were made in the (C-12)O J = 1-0 line; CO emission from 25 of the galaxies was detected at greater than the 3 sigma level. Most of the galaxies were observed at the central position, as well as at four adjacent positions one beam-width away. A method is presented for estimating the total flux of the galaxy using this spatially undersampled map. The total CO emission is found to be correlated with radio continuum emission and 60 + 100 micron band far-infrared, reflecting direct relationships among molecular gas content, CO emission, and massive star formation in these galaxies. However, the total CO flux is only poorly correlated with visual blue luminosity. 49 references.

Molecular clouds in galaxies with different Z - Fragmentation of diffuse clouds driven by opacity

Abstract: Molecular clouds are formed from diffuse interstellar clouds when the external ultraviolet radiation field is prevented from penetrating into the cloud. The opacity is provided mainly by dust grains and the required column density to the cloud center is larger than about 5 x 10 to the 20th (solar Z/Z)/sq cm. This high-opacity criterion could have a significant impact on the radial trends observed in spiral galaxies, and on the distinctions between spiral and dwarf irregular galaxies.

Large-scale dissociation of molecular gas in galaxies by newly formed stars

Abstract: Molecular hydrogen is thought to form from atomic hydrogen on the surface of dust grains in the interstellar medium, and to be dissociated primarily by ultraviolet radiation from hot stars1. This process has been modelled on the 1 pc length scale for the case of dense interstellar clouds2 and near newly formed ionizing stars3. Observational evidence that molecular hydrogen is indeed dissociated in shells around young OB stars in the Galaxy has been presented recently4–6. We suggest here that the same dissociation process occurs on the kpc scale in active star-forming regions of some galaxies, and that this dissociation may strongly affect the observed morphology of atomic hydrogen in spiral arms.

The molecular content of interacting and isolated galaxies - the effect of environment on the efficiency of star formation

Abstract: Molecular gas observations of merging/interacting and isolated galaxies are presented in order to study the relationship between environment and the efficiency of star formation. The two galaxy samples differ primarily in their IR properties and are quite similar in their molecular gas contents. The ratios of IR luminosity to H2 mass have a mean value of 78 and 12 solar luminosity/solar mass for interacting and isolated galaxies, respectively. The highest star formation efficiencies (SFEs) appear to occur in the merging and interacting pairs. The SFE in merging/interacting galaxies is greater than that found in the spiral arms of M51 and may be roughly proportional to the rate of cloud-cloud collisions in the interacting systems.

IRAS Observations of a Small Sample of Blue Irregular Galaxies

Abstract: IRAS satellite IR observations are presently used to investigate the properties of the dust in a small sample of irregular galaxies chosen to cover a wide range in levels of star formation activity. The irregulars are found to have comparatively normal IR properties; the only exception is NGC 1569, an irregular galaxy with intense global star formation which seems to have a larger dust fraction at temperatures whose energies peak at 25-60 microns, by comparison with other irregulars. DDO 47, the system with lowest star formation activity, has the lowest far-IR color temperature and highest ratio of IR to H-alpha flux; this is suggested to be due to the decreased importance of radiation from young stars in heating the dust.

Large-scale anisotropy in the Hubble flow

Abstract: The apparent Local Group motion inferred from infrared observations of an all-sky sample of spiral galaxies is interpreted in terms of a large-scale streaming motion for these galaxies on a scale of 50 h -1 Mpc. If this streaming is induced by large-scale density fluctuations, the fluctuations required are compatible with the observed distribution of luminous matter, but only in a closed universe. In the context of current speculation regarding non-baryonic particle species which may dominate the mass of the universe, this result seems to be inconsistent with cold dark matter models.

Bubbles and braided jets in galaxies with compact radio nuclei

Abstract: Narrow-band H-alpha CCD images showing ionized gas in organized kiloparsec-scale structures in three galaxies with low-level active nuclei are presented. The edge-on spiral NGC 3079 contains an apparent loop structure which corresponds to previously reported nonthermal radio emission along the minor axis. The optical emission probably results from interaction between the ejected plasma and the ISM in the disk and halo. The S0 galaxy NGC 3998 exhibits an S-shaped structure centered on the nucleus, with no other evidence for spiral structure. In the spiral galaxy NGC 4258, the presence of continuum-free emission-line arms which coincide with the nonthermal radio arms is confirmed. The morphology of the arms suggests the presence of two double-sided jets which braid or wrap around one another and which bifurcate on both sides. It is concluded that the optical and radio emission in NGC 3079 and in NGC 4258 are related, and it is suggested that both are powered by a plasma which flows from the active nucleus and dissipates kinetic energy in the surrounding ISM.

A Schmidt Law for the Molecular Gas Profiles of Disk Galaxies

Abstract: An explanation for the observed difference in radial profiles of the atomic and molecular gas components of disk galaxies is proposed which involves GMC formation in local gravitational potential perturbations such as spiral arms. It is assumed that GMC form through binary cloud-cloud collisions within the potential of spiral density perturbations, with progenitors whose radial profile is that of the atomic gas, the rate of collisions being determined by how often a spiral arms is encountered. This model provides a good fit to the overall shape of the molecular gas distribution for disk galaxies where there is believed to be an underlying stationary density wave, with corotation resonance in the outer regions of the disk, as well as for those disk galaxies where the spiral arms are more probably transient. 41 references.

Distributions of (C-13)O emission in the disks of late-type spiral galaxies

Abstract: In studies of molecular clouds in external galaxies, the distribution of (C-12)O emissivity as a function of radius in a galaxy has been employed to infer the surface density of molecular hydrogen with galactocentric radius. In principle, observations of more optically thin molecular species, such as (C-13)O, can be used as an alternative tracer of the mass of molecular hydrogen in a galaxy. Since the (C-13)O lines have a much lower opacity than the saturated, nearly thermalized (C-12)O lines, their intensity variations as a function of galactocentric radius are more readily interpreted in terms of gas density variations. The first strip maps of the (C-13)O distributions in six spiral galaxies are presented, and the (C-13)O and (C-12)O radial distributions are compared. Attention is given to the reliability of H2 masses and H2 distributions derived from (C-12)O data.

The Centaurus cluster of galaxies – II. The bimodal velocity structure

Abstract: Les vitesses moyennes heliocentriques et les dispersions autour de la ligne de visee des deux composantes principales de l'amas, a moins de 3° du centre, sont 3041 et 586 kms −1 pour Cen 30 et 4570 et 262 kms −1 pour Cen 45

Star-gas density waves in spiral galaxies

Abstract: The steady state dynamics of spiral galaxies is analyzed as a two-component system consisting of stars and gas within the framework of the WKB density wave theory. The gravitational influence of the gas is included for the first time in a steady state calculation. The full set of equations for a star-gas galaxy is presented, and the equations are analyzed for small-amplitude forcing. Wave properties near the solar circle are examined, and it is found that the large-scale gas shock disappears for gas content above 8 percent. Instead, gas density profiles change to highly symmetric shapes as a result of the action of the gas self-gravity. The stellar wave is damped by the torque exerted by the gas.

The Morphological Types and Orbits of H i Deficient Spirals in Clusters of Galaxies

Abstract: On examine la distribution des types morphologiques et la cinematique des galaxies spirales deficientes en hydrogene neutre dans l'echantillon de Giovanelli et Haynes (1985)

Chemical composition gradient in NGC 2403 and the stellar mass limit

Abstract: Photoelectric spectrophotometry in the 3400 A-7400 A range is presented for five H II regions in the spiral galalxy NGC 2403. Steep gradients in the electron temperature, N/H, and O/H were found. By adopting the Y versus O/H relation the observations also imply gradients in neutral He/H(+). The maximum temperature of the most massive stars in giant H II regions across the disk of NGC 2403 is 35,000 K; this result implies that the mass of the most massive stars is very similar in each H II regioon and that it is independent of the gradient in heavy element abundances. By comparing the derived chemical abundances with models of galactic chemical evolution it is found that infall into the galactic disk is not important for NGC 2403.

IRAS observations of the nuclear bulge of M31

Abstract: IRAS observations of the nuclear bulge of M31 are reported. The 12-micron and 25-micron emission is attributed to circumstellar dust emission from late-type stars, while the 60 and 100-micron emission is attributed to interstellar dust emission. The total input rate of circumstellar gas and dust into the interstellar medium is estimated to be 0.015 solar masses/yr. The mass of dust in the interstellar medium estimated from the far-infared flux is about 1500 solar masses. The color temperature of the far-infrared-emitting dust is 45 K. The time required to accumulate the observed mass in interstellar dust is about 10 million yr. Either supernova-generated winds or star formation can deplete this gas without violating the observations.