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Astron. Astrophys. 354, L21-L24 (2000)

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3. Asymmetry

The asymmetry method used here, described in detail by Conselice, Bershady & Jangren (2000), gives a simple, quantified measure of how a galaxy deviates from perfect axi-symmetry. Like Abraham et al. (1996), our algorithm consists of rotating a galaxy 180o about a center, subtracting the rotated image from the original, and dividing the sum of the absolute value of the pixels in the residual image by the sum of pixel values in the original image. The higher the intensity of the residuals, the larger the asymmetry. In our implementation, however, (a) we repeat the asymmetry computation using different center estimates until a minimum asymmetry is found, and (b) we make a correction for noise.

The rotational asymmetry of normal galaxies increases with the `lateness' of their morphological type (Conselice 1997), indicating that at least some component of their asymmetry is associated with the flocculent appearance of star-formation within galaxies. Asymmetry may also arise, however, from large-scale, dynamical perturbations. Other methods of asymmetry measurement (e.g. Zaritsky & Rix 1997; Kornreich, Haynes, & Lovelace 1998) are particularly sensitive to this dynamical component. In contrast, our rotational measurement is sensitive to both dynamical and flocculent components of asymmetry. As we show in the next section, interacting and irregular galaxies can be distinguished on the basis of the relative amplitudes of these two asymmetry components via the color-asymmetry diagram. Asymmetry is a powerful quantitative morphological parameter, and in conjunction with the color of a galaxy, it can be used to determine the physical nature of a galaxy.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000