A search for features in early-type galaxies

Abstract

We have conducted a search for dust lanes, incipient stellar disks, bars, shells, and other deviations from elliptical symmetry in a sample of 159 early-type galaxies. The data are from the CCD surface-photometry survey of Djorgovski (1985a), for which the selection effects are well understood. The image-processing technique used is division by a purely elliptical model image, constructed from the surfacephotometry profiles for a given object (best-fit surface brightness, ellipticity, and position angle as functions of semimajor axis). Our sample contains 116 elliptical galaxies, 33 SO galaxies, and ten intermediate types. Some galaxies that might better be classified as "dusty ellipticals" (in that they contain no obvious stellar disk) are included among the SOs. Forty-two of the ellipticals (36%) either definitely or very likely contain dust, either in patches or in well-defined lanes or rings. Five of the E/SO galaxies (50%) also show possible or definite dust, as do 15 (47%) of the SOs. Three of the elliptical galaxies definitely contain stellar disks, with several more possible candidates. Thus, approximately 50% of the elliptical galaxies show "features" of some kind. The detection of features is dependent on the signal-to-noise ratio of the data, so these numbers represent lower limits. The presence of dust in the galaxies classified as ellipticals does not seem to depend on any intrinsic characteristic of the galaxy. The dusty ellipticals do seem to prefer low-density environments. The "elliptical" galaxies with disks tend to have higher ellipticities and larger two-wave Fourier residuals, an effect also described by Carter ( 1987). These galaxies may be preferentially found in higher-density environments, and may be lower-luminosity systems, but these statements are based on very small-number statistics (ten galaxies out of a total of 116), and no strong conclusions may be drawn. However, our results clearly further blur the distinction between ellipticals and SOs.

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