Browsing by Author "Block, DL"

Now showing 1 - 5 of 5
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    A dust-penetrated classification scheme for bars as inferred from their gravitational force fields
    (IOP Publishing, 2000) Buta, R; Block, DL; University of Witwatersrand; University of Alabama Tuscaloosa
    The division of galaxies into "barred" (SB) and "normal" (S) spirals is a fundamental aspect of the Hubble galaxy classification system. This "tuning fork" view was revised by de Vaucouleurs, whose classification volume recognized apparent "bar strength" (SA, SAB, SB) as a continuous property of galaxies called the "family." However, the SA, SAB, and SB families are purely visual judgments that can have little bearing on the actual bar strength in a given galaxy. Until very recently, published bar judgments were based exclusively on blue light images, where internal extinction or star formation can either mask a bar completely or give the false impression of a bar in a nonbarred galaxy. Near-infrared camera arrays, which principally trace the old stellar population in both normal and barred galaxies, now facilitate a quantification of bar strength in terms of their gravitational potentials and force fields. In this paper, we show that the maximum value, Q(b), of the ratio of the tangential force to the mean axisymmetric radial force in a barred disk galaxy is a quantitative measure of the strength of a bar. Q(b) does not measure bar ellipticity or bar shape but rather depends on the actual forcing due to the bar embedded in its disk. We show that a wide range of true bar strengths characterizes the category "SB," while the de Vaucouleurs category "SAB" corresponds to a narrower range of bar strengths. We present Q(b) values for 36 galaxies, and we incorporate our bar classes into a dust-penetrated classification system for spiral galaxies.
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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.
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    A technique for separating the gravitational torques of bars and spirals in disk galaxies
    (University of Chicago Press, 2003-09) Buta, R; Block, DL; Knapen, JH; University of Alabama Tuscaloosa; University of Witwatersrand; University of Hertfordshire
    We describe a Fourier-based method of separating bars from spirals in near-infrared images. The method takes advantage of the fact that a bar is typically a feature with a relatively fixed position angle and uses the simple assumption that the relative Fourier amplitudes due to the bar decline with radius past a maximum in the same or a similar manner as they rose to that maximum. With such an assumption, the bar can be extrapolated into the spiral region and removed from an image, leaving just the spiral and the axisymmetric background disk light. We refer to such a bar-subtracted image as the "spiral plus disk'' image. The axisymmetric background ( Fourier index m = 0 image) can then be added back to the bar image to give the "bar plus disk ''image. The procedure allows us to estimate the maximum gravitational torque per unit mass per unit square of the circular speed for the bar and spiral forcing separately, parameters that quantitatively de. ne the bar strength Q(b) and the spiral strength Q(s) following the recent study of Buta & Block. For the first time, we are able to measure the torques generated by spiral arms alone, and we can now de. ne spiral torque classes, in the same manner as bar torque classes are delineated. We outline the complete procedure here using a 2.1 mum image of NGC 6951, a prototypical SAB( rs) bc spiral having an absolute blue magnitude of - 21 and a maximum rotation velocity of 230 km s(-1). Comparison between a rotation curve predicted from the m = 0 near-infrared light distribution and an observed rotation curve suggests that NGC 6951 is maximum disk in its bar and main spiral region, implying that our assumption of a constant mass-to-light ratio in our analysis is probably reliable. We justify our assumption on how to make the bar extrapolation using an analysis of NGC 4394, a barred spiral with only weak near-infrared spiral structure, and we justify the number of needed Fourier terms using NGC 1530, one of the most strongly barred galaxies ( bar class 7) known. We also evaluate the main uncertainties in the technique. Allowing for uncertainties in vertical scale height, bar extrapolation, sky subtraction, orientation parameters, and the asymmetry in the spiral arms themselves, we estimate Q(b) = 0.28 +/- 0.04 and Q(s) = 0.21 +/- 0.06 for NGC 6951.