Ionized Gas and Merger-triggered Star Formation in the Galaxy NGC 1222

Abstract

We obtained narrowband images in the emission lines Hα, [N II]λ6583, and [O III]λ5007 at the 2.5 m telescope of the Caucasus Mountain Observatory (CMO) of the Sternberg Astronomical Institute of the Moscow State University with photometer MaNGaL, which showed that the ionized gas in the galaxy NGC 1222 resides in a restricted area of the central part of the host galaxy near to the eastern companion and in the companion to the south-southwest of it. There are star-forming regions there that ionize the gas. Also, we obtained long-slit spectra at the 6 m Big Telescope Alt-azimuthal of the Special Astrophysical Observatory of the Russian Academy of Sciences in two position angles crossing most of the star-forming regions. We derived line-of-sight velocity curves for different components of the ionized gas and revealed kinematic features in the positions of impact of companions, which confirm their accretion origin. We applied several methods for estimating the oxygen abundance in the gas and found differences in the metallicity of the gas in the companion to the south-southwest, in the star-forming regions in the central part of NGC 1222, and presumably in the surrounding gas. Since current star formation is observed only in a restricted central area of NGC 1222 near the eastern companion, as well as in the south-southwestern companion and along its fall trajectory, we conclude that local star formation in the extensive low-metallicity gas disk of neutral hydrogen was triggered by a merger about 6 Myr ago.