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Browsing by Author "Puerari, I"

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    Gravitational bar and spiral arm torques from K(s)-band observations and implications for the pattern speeds
    (IOP Publishing, 2004) Block, DL; Buta, R; Knapen, JH; Elmegreen, DM; Elmegreen, BG; Puerari, I; University of Witwatersrand; University of Alabama Tuscaloosa; University of Hertfordshire; Vassar College; International Business Machines (IBM); Instituto Nacional de Astrofisica, Optica y Electronica
    We have obtained deep near-infrared K(s)-band William Herschel Telescope observations of a sample of 15 nearby spiral galaxies having a range of Hubble types and apparent bar strengths. The near-infrared light distributions are converted into gravitational potentials, and the maximum relative gravitational torques due to the bars and the spirals are estimated. We find that spiral strength, Q(s), and bar strength, Q(b), correlate well with other measures of spiral arm and bar amplitudes and that spiral and bar strengths also correlate well with each other. We also find a correlation between the position angle of the end of the bar and the position angle of the inner spiral. These correlations suggest that the bars and spirals grow together with the same rates and pattern speeds. We also show that the strongest bars tend to have the most open spiral patterns. Because open spirals imply high disk-to-halo mass ratios, bars and spirals most likely grow together as a combined disk instability. They stop growing for different reasons, however, giving the observed variation in bar-spiral morphologies. Bar growth stops because of saturation when most of the inner disk is in the bar, and spiral growth stops because of increased stability as the gas leaves and the outer disk heats up.
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    The gravitational torque of bars in optically unbarred and barred galaxies
    (EDP Sciences, 2001) Block, DL; Puerari, I; Knapen, JH; Elmegreen, BG; Buta, R; Stedman, S; Elmegreen, DM; University of Witwatersrand; Instituto Nacional de Astrofisica, Optica y Electronica; Isaac Newton Group of Telescopes; University of Hertfordshire; International Business Machines (IBM); University of Alabama Tuscaloosa; Vassar College
    The relative bar torques for 45 galaxies observed at K-band with the 4.2 m William Herschel Telescope are determined by transforming the light distributions into potentials and deriving the maximum ratios of the tangential forces relative to the radial forces. The results are combined with the bar torques for 30 other galaxies determined from our previous K-band survey (Buta & Block 2001). Relative bar torques determine the degree of spiral arm forcing, gas accretion, and bar evolution. They differ from other measures of bar strength, such as the relative amplitude of the bar determined photometrically, because they include the bulge and other disk light that contributes to the radial component of the total force. If the bulge is strong and the radial forcing large, then even a prominent bar can have a relatively weak influence on the azimuthal motions in the disk. Here we find that the relative bar torque correlates only weakly with the optical bar type listed in the Revised Shapley-Ames and de Vaucouleurs systems. In fact, some classically barred galaxies have weaker relative bar torques than classically unbarred galaxies. The optical class is a poor measure of azimuthal disk forcing for two reasons: some infrared bars are not seen optically, and some bars with strong bulges have their azimuthal forces so strongly diluted by the average radial force that they exert only small torques on their disks. The Hubble classification scheme poorly recognizes the gravitational influence of bars. Applications of our bar torque method to the high-redshift universe are briefly discussed.
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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.