Deep Images of Mergers (DIM), a pilot project

We report the first results of a pilot project named “Deep Images of Mergers” (DIM) aiming at revealing optical shells or ripples around early-type galaxies. Shells are often thought to be rare and consist mostly of stars. Our goal is to locate these faint structures that are not easily seen in optical images and to search for any correlation with the HI gas which is often found far off the galaxy after interaction or merger. We used the Rogues gallery of HI maps of peculiar and interacting galaxies compiled by J. Hibbard et al. (2001) to select our pilot sample and present the results of three mergers, Centaurus A, Arp 230 and NGC 1052. The DIM project is currently using a combination of telescopes across Brazil: two Ritchey-Chretien 8” and 12”, a Celestron 11”, a reflector 450mm and an APM 140 mm refractor. In deep images taken with these relatively small telescopes we are able to see a large field of view and expand our search for faint shells at large distances. After combining all images to the same scale, the first results of the stacked images with dozens of hours of exposure reveal a large number of shells confirming the feasibility of the project and the importance of deep images when analyzing mergers. Our first results show that after 13 hours of exposure, Arp 230’s shells are detected as far as 27.5 kpc away from the core of the galaxy (assuming that Arp 230 is at 23.3 Mpc), confirming the shells detected with the Hubble Space Telescope. Shells in Centaurus A are seen 17 kpc away from the galaxy (assuming that Cen A is at 3.8 Mpc) after 35 hours of exposure. We are in the process of adding more data to Arp 230 and executing the observations of our next pilot target, NGC 1052. We will expand the sample to include as many mergers as possible and search for correlation with the extended HI gas.