Soft merging in early-type galaxies

Statistical, photometric, and kinematical arguments suggest that encounters, accretion, and merging of galaxies are likely the key mechanisms for the formation of present-day ellipticals. While major merger events leave strong and clearly measurable signatures, those left by minor (or "soft") ones are usually rather tricky to observe. The authors present a new image-filtering technique capable to detect weak photometric signatures (down to 5% of the local value of the surface brightness) of structures which may be interpreted as signatures of past accretion phenomena. By applying this technique to a sample of 28 early-type galaxies observed with the ESO-NTT telescope, the authors have revealed faint structures such as nuclear disks, ripples, bars, distorted disks, warps, etc., in more than 90% of the galaxies. The revealed structures are also capable to explain some anomalies in the photometric properties which were known to exist by means of more traditional techniques.