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Session 103 - Galaxy Evolution.
Display session, Thursday, January 16
Metropolitan Ballroom,

[103.03] Morphological Peculiarity Indices for Distant and Local Galaxies

K. L. Wu (UCSC), S. M. Faber (UCO/Lick), T. R. Lauer (KPNO/NOAO)

We are developing several algorithms to quantify peculiarities in galaxy morphology. These algorithms, or peculiarity indices, will be applied to galaxy images and are sensitive to several different types of features. Two indices have been tested on a small local sample with a range of morphologies. The indices are the P/C Ratio (the ratio of the perimeter of a given light contour to the circumference of a circle with the same area), and (2) a concentration index (the ratio of the radii enclosing 80% and 20% of the total light). We find that both of these indices are able to distinguish between ``normal'' (i.e., Hubble Sequence type) galaxies and ``peculiar'' galaxies. This also holds true when the galaxies are ``redshifted'' out to z \sim 0.8.

In order to obtain contours that are neither dominated by noise nor featureless due to over-smoothing, moderate amounts of smoothing are required. Various degrees of smoothing were applied to determine the optimal smoothing length and to test the robustness of the indices against smoothing. A first-order correction for the ellipticity (inclination) of the galaxy was also developed.

We are currently extending our tests to several more local galaxies in order to determine the distribution of values for each index for the various Hubble types as well as for peculiars. We will also present similar results for other indices as they are developed. Ultimately, comparing the frequency and types of local morphological peculiarities to those seen at high z from deep HST images will help us attain a better understanding of the merging history of galaxies and their progenitors.

Program listing for Thursday