Star formation in outer rings of S0 galaxies. III. UGC 5936 -- an S0 with currently accreted satellite matter

TL;DR

This study investigates star formation in the outer ring of the S0 galaxy UGC 5936, revealing ongoing star formation fueled by accreted gas from a satellite, with detailed analysis of gas properties and metallicity.

Contribution

It provides new insights into the nature of star formation in S0 galaxy rings and the role of satellite accretion in fueling this process.

Findings

Star formation in the ring is ionized mainly by young stars.

The gas metallicity in the ring is solar.

Current accretion of cold gas from a satellite fuels star formation.

Abstract

Though S0 galaxies are usually thought to be `red and dead', they demonstrate often star formation organized in ring structures. We try to clarify the nature of this phenomenon and its difference from star formation in spiral galaxies. The luminous S0 galaxy with a large ring, UGC 5936, is studied here. By applying long-slit spectroscopy along the major axis of UGC 5936, we have measured gas and star kinematics, Lick indices 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 ratios diagnostic diagrams and having ensured that it is ionized mostly by young stars, we have determined the gas oxygen abundance by using popular strong-line methods. Also we have proved the spatial proximity of the south-eastern dwarf satellite to UGC 5936 and have measured its gas metallicity. The ionized gas of the ring is excited by young stars and has solar metallicity. Star formation in the ring is rather prolonged, and its intensity corresponds to the current HI content of UGC 5936 (to the Kennicutt-Schmidt relation). The whole morphology of the HI distribution implies current accretion of the cold gas from the satellite onto the outer disc of UGC 5936; due to the satellite location and rotation in the plane of the stellar disc of the host galaxy, the accretion is smooth and laminar providing the favorable condition for star formation ignition.