Star formation in outer rings of S0 galaxies

Abstract

Aims. Though S0 galaxies are usually thought to be “red and dead”, they often present weak star formation organized in ring structures in their outer disks. We try to clarify the nature of this phenomenon and its difference from star formation in spiral galaxies. The nearby luminous S0 galaxy NGC 1211 is studied here. Methods. By applying Fabry-Perot scanning interferometry at the Russian 6m telescope, we measured the gas rotation in the outer ring of the galaxy. We studied the pattern and properties of the brightest star formation regions with the tunable filter Mapper of Narrow Galaxy Lines (MaNGaL) at the 2.5 m telescope of the Caucasian Mountain Observatory of the SAI MSU. Results. The gas excitation is strongly inhomogeneous along the ring: in the eastern part, HII-type excitation dominates, while the western part of the ring is partly excited by shocks, as revealed by the emission-line diagnostic diagram. The gas metallicity of the emission clumps in the outer ring also has a broad distribution. The star formation rate is currently falling and is in general too weak for the galaxy gas content. We conclude that the gas in the ring has been accreted recently, about 300 Myr ago, probably from an inclined direction, and that the accretion event has provoked a brief, ineffective starburst.