Browsing by Author "Combes, F"

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    Dynamics of ringed barred spiral galaxies. I. Surface photometry and kinematics of NGC 1433 and NGC 6300
    (University of Chicago Press, 2001) Buta, R; Ryder, SD; Madsen, GJ; Wesson, K; Crocker, DA; Combes, F; University of Alabama Tuscaloosa; University of Wisconsin System; University of Wisconsin Madison; Duke University; UDICE-French Research Universities; Universite PSL; Observatoire de Paris
    This paper presents new near-infrared images and surface photometry and H alpha Fabry-Perot radial velocities for NGC 1433 and NGC 6300, two large and previously well-studied nearby ringed barred spiral galaxies. Stellar absorption-line radial velocities and new optical surface photometry are also presented for NGC 1433. The data are intended to set the stage for dynamical modeling of the two galaxies, with the principal goals being to derive mass distributions, bar mass-to-light ratios, bar pattern speeds, and resonance locations, parameters which have been derived for very few ringed disk galaxies. The presence of strong rings and pseudorings in the two galaxies allows the possibility to link specific orbital resonances with the observed rings. The new data allow us to derive a fairly complete composite rotation curve of NGC 1433, leading us to make some interesting preliminary judgments about the structure of the galaxy. The new data also verify the previous finding that the radial velocity of the Seyfert nucleus in NGC 6300 differs from the actual systemic velocity of the galaxy by nearly 100 km s(-1). We demonstrate in this paper that the offset is not an artifact of significant extinction in the inner regions of the galaxy.
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    Gravitational torques in spiral galaxies: Gas accretion as a driving mechanism of galactic evolution
    (EDP Sciences, 2002) Block, DL; Bournaud, F; Combes, F; Puerari, I; Buta, R; University of Witwatersrand; UDICE-French Research Universities; Universite PSL; Observatoire de Paris; Ecole Normale Superieure (ENS); Instituto Nacional de Astrofisica, Optica y Electronica; University of Alabama Tuscaloosa
    The distribution of gravitational torques and bar strengths in the local Universe is derived from a detailed study of 163 galaxies observed in the near-infrared. The results are compared with numerical models for spiral galaxy evolution. It is found that the observed distribution of torques can be accounted for only with external accretion of gas onto spiral disks. Accretion is responsible for bar renewal - after the dissolution of primordial bars - as well as the maintenance of spiral structures. Models of isolated, non-accreting galaxies are ruled out. Moderate accretion rates do not explain the observational results: it is shown that galactic disks should double their mass in less than the Hubble time. The best fit is obtained if spiral galaxies are open systems, still forming today by continuous gas accretion, doubling their mass every 10 billion years.
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    NGC-7217 - A SPHEROID-DOMINATED, EARLY-TYPE RESONANCE RING SPIRAL GALAXY
    (University of Chicago Press, 1995-09) Buta, R; Vandriel, W; Braine, J; Combes, F; Wakamatsu, K; Sofue, Y; Tomita, A; University of Tokyo; UDICE-French Research Universities; Universite PSL; Observatoire de Paris; Max Planck Society; Ecole Normale Superieure (ENS); Gifu University; Kyoto University; University of Alabama Tuscaloosa
    NGC 7217 is a well-known northern spiral galaxy which is characterized by flocculent spiral structure and a series of three optical ringlike zones: a nuclear ring 21 '' in diameter, a weak inner ring 63 '' in diameter, and a striking outer ring 2.'6 in diameter. The rings all have nearly the same shape and position angle in projection. The appearance of the galaxy suggests that it may be more axisymmetric than the typical spiral galaxy, since there is little evidence for the presence of a bar, oval, or stellar density wave. This makes the origin of the ring features uncertain. In an effort to understand this kind of ringed galaxy, which is by no means typical, we have obtained multicolor CCD BVRI images, accurate surface photometry, mappings of the CO and HI gas distributions, and rotational velocities from H alpha and HI spectral line data. Our deep surface photometry has revealed an important feature of NGC 7217 that was missed in previous studies: The region occupied by the rings of the galaxy is surrounded-by an extensive, nearly circular luminous halo. This halo cannot be merely an extension of the disk component because it is much rounder than the inner regions. Instead, we believe the light represents either the outer regions of the bulge or a separate stellar halo component. We are able to successfully model the luminosity profile in terms of an r(1/4) ''spheroid'' and an exponential disk with a spheroid-to-total disk (including rings) luminosity ratio of 2.3-2.4. This makes NGC 7217 one of the most spheroid-dominated spirals known, and the finding has important implications for the recent discovery by Merrifield and Kuijken of a significant population of counter-rotating stars in the galaxy. Although the spiral structure of NGC 7217 is flocculent in blue light, there is a definite two-armed stellar spiral in the region of the outer ring. This ring includes about 4.4% of the total blue luminosity and is the locus of most of the recent star formation in the galaxy. The ring is also where we find the HI gas to be concentrated. The galaxy is very gas poor (M(HI) = L(B)(0) = 0.024 M.IL.,(B)) for its morphological type. The HI rotational velocities agree well with published and our new H alpha-values. Fourier analysis reveals a very weak possible oval distortion in the stellar mass distribution. Using the I-band light distribution to define the potential, we carried out simulations of gas streaming with no self-gravity. A model with a bulge-to-disk mass ratio of 2.4 reproduces the observed optical ring morphology very well. This suggests to us that in spite of the extreme weakness of the observed nonaxisymmetry of this galaxy, this nonaxisymmetry is still sufficient to torque the gas into the usual resonance rings identified in other, more obviously barred galaxies. An additional noteworthy feature that we have identified in a B-I color index map is a symmetric, nuclear dust ring 17 '' in angular diameter. Other dust lanes are seen mainly on the near side of the galaxy.