Browsing by Author "Zhang, Xiaolei"

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    PATTERN COROTATION RADII FROM POTENTIAL-DENSITY PHASE-SHIFTS FOR 153 OSUBGS SAMPLE GALAXIES
    (IOP Publishing, 2009-05-19) Buta, Ronald J.; Zhang, Xiaolei; University of Alabama Tuscaloosa; George Mason University
    The potential-density phase-shift method is an effective new tool for investigating the structure and evolution of galaxies. In this paper, we apply the method to 153 galaxies in the Ohio State University Bright Galaxy Survey (OSUBGS) to study the general relationship between pattern corotation radii and the morphology of spiral galaxies. The analysis is based on near-infrared H-band images that have been deprojected and decomposed assuming a spherical bulge. We find that multiple pattern speeds are common in disk galaxies. By selecting those corotation radii close to or slightly larger than the bar radius as being the bar corotation (CR) radius, we find that the average and standard deviation of the ratio R = r(CR)/r(bar), is 1.20 +/- 0.52 for 101 galaxies having well-defined bars. There is an indication that this ratio depends weakly on galaxy type in the sense that the average ranges from 1.03 +/- 0.37 for 65 galaxies of type Sbc and earlier, to 1.50 +/- 0.63 for 36 galaxies of type Sc and later. Our bar corotation radii are on average smaller than those estimated from single-pattern-speed numerical simulations, most likely because these simulations tend to find the pattern speed which generates a density response in the gas that best matches the morphology of the outer spiral structure. Although we find CR radii in most of the sample galaxies that satisfy conventional ideas about the extent of bars, we also consider the alternative interpretation that in many cases the bar CR is actually inside the bar and that the bar ends close to its outer Lindblad resonance instead of its CR. These "superfast" bars are the most controversial finding from our study. We see evidence in the phase-shift distributions for ongoing decoupling of patterns, which hints at the formation pathways of nested patterns, and which in turn further hints at the longevity of the density wave patterns in galaxies. We also examine how uncertainties in the orientation parameters of galaxies and in the shapes of bulges affect our results.
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    The Potential-Density Phase-Shift Method for Determining the Corotation Radii in Spiral and Barred Galaxies
    (American Astronomical Society, 2007-04-20) Zhang, Xiaolei; Buta, Ronald J.; University of Alabama Tuscaloosa
    We have developed a new method for determining the corotation radii of density waves in disk galaxies, which makes use of the calculated radial distribution of an azimuthal phase shift between the potential and density wave patterns. The approach originated from improved theoretical understanding of the relation between the morphology and kinematics of galaxies and of the dynamical interaction between density waves and the basic-state disk stars, which results in the secular evolution of disk galaxies. In this paper we present the rationales behind the method and the first application of it to several representative barred and grand-design spiral galaxies, using near-infrared images to trace the mass distributions, as well as to calculate the potential distributions used in the phase-shift calculations. We compare our results with those from other existing methods for locating the corotations and show that the new method both confirms the previously established trends of bar-length dependence on galaxy morphological types and provides new insights into the possible extent of bars in disk galaxies. The method also facilitates the estimation of mass accretion/excretion rates due to bar and spiral density waves, providing an alternative way of quantifying the importance of these features in disk galaxies. A preliminary analysis of a larger sample shows that the phase-shift method is likely to be a generally applicable, accurate, and essentially model-independent method for determining the pattern speeds and corotation radii of single or nested density wave patterns in galaxies. Other implications of the results of this work include that most of the nearby bright disk galaxies appear to possess quasi-stationary spiral modes; that these density wave modes, as well as the associated basic states of the galactic disks, slowly transform over the time span of a Hubble time due to a collective dissipation process directly related to the presence of the phase shift between the potential and density patterns; and that self-consistent N-particle systems contain physics not revealed by the passive orbit analysis approaches.