Star formation in outer rings of S0 galaxies

Abstract

Aims. Though S0 galaxies are usually thought to be ‘red and dead’, they often demonstrate weak star formation organised in ring structures and located in their outer disks. We try to clarify the nature of this phenomenon and its difference from star formation in spiral galaxies. The nearby moderate-luminosity S0 galaxy UGC 4599 is studied here. Methods. By applying long-slit spectroscopy at the Russian 6 m telescope, we have measured stellar kinematics for the main body of the galaxy and strong emission-line flux ratios in the ring. After inspecting the gas excitation in the ring using line ratio diagrams, and having shown that it is ionised by young stars, we determined the gas oxygen abundance by using conventional strong-line calibration methods. We inspected the gas kinematics in the ring with Fabry–Perot interferometer data obtained at the William Herschel Telescope. The pattern and properties of the brightest star formation regions were studied with the tunable filter MaNGaL at the 2.5 m telescope of the Caucasian Mountain Observatory of the SAI MSU. Results. The gas metallicity in the ring is certainly subsolar, [O/H] = −0.4 ± 0.1 dex, which is different from the majority of the outer star-forming rings in S0s we have studied in the past, which typically have nearly solar metallicity. The total stellar component of the galaxy, which is old in the centre, is less massive than its extended gaseous disk. We conclude that the ring and the outer disk of UGC 4599 are probably a result of gas accretion from a cosmological filament.