The peculiar galaxy IC 1182: An ongoing merger?

Abstract

High resolution broad and narrow band images and long slit spectroscopy of the peculiar galaxy IC 1182 are pre- sented. The analysis of the broad band images reveals a distorted morphology with a large, heavily obscured disk-like structure and several knots in the central region. Galactic material, some of it in the form of two slender tails, is detected well beyond the main body of the galaxy. The second, fainter tail and several knots are reported here for the first time. The galaxy has color indices of an early type object except U − B, which is significantly bluer than what is typical for this kind of galaxy. The narrow band images centered on different emission lines show that the galaxy is a very powerful emitter. Most of the knots detected in the central region and in the prominent tail emerging eastward from the galaxy are very luminous in Hα ,a nd have typical sizes about 1 kpc (FWHM). The emission in the main lines extends all over the galaxy, with plumes and arc-like structures seen in Hα at large distances from the center. The observed, uncorrected Hα flux corresponds to a total luminosity of 3.51 × 10 41 erg s −1 , about 3 times that of the starburst galaxy Arp 220. We have found that the internal extinction deduced from the observed Balmer decrement is high all along the slit, with EB−V ≈ 1, so the corrected SFR could amount to 90 Mper year. On this basis IC 1182 is found to be a very powerful starburst galaxy. Surprisingly, the source is not in the IRAS Point Source Catalogue. The emission knots detected in the central region of the galaxy have line ratios that place them close to the border of the region occupied by active nuclei in the diagnostic diagrams. Using the best determined diagnostic ratio, (O III)/Hβ vs. (N II)/Hα ,t hey can still be classified as extreme HII-like regions. We notice that the same kind of line ratios are also measured at different places in the galaxy, adding to the idea that the nuclear line ratios can be explained in terms of stellar photoionization. The metallicity we have measured for the ionized gas in the two brightest central knots is low, 0.1 Zand 0.06 Zrespectively, and their measured helium abundance is also lower than solar. In the main body of the galaxy, besides the reported knots, the distribution of the ionized gas resembles that of an inclined disk about 12 kpc in size. The spectroscopic data show however a complex rotation pattern. We interpret them as corresponding to two identifiable disk galaxies with observed rotation amplitudes of 200 km s −1 and 100 km s −1 respectively. The stellar absorption lines detected in the bigger system do not show any clear rotation pattern. The data presented here indicate that IC 1182 is a high luminosity starburst system. Its global properties and peculiarities can be understood as corresponding to two systems that can still be recognized, in the process of merging, with two tidal tails emerging from the central region of the galaxy. In the main tail there are several candidates forming tidal dwarf galaxies. The measured low metallicity of the ionized gas, together with the low amplitude of one of the systems, suggests that the process involves a late-type, gas-rich spiral galaxy that is supplying most of the gas to the system.