Star Formation in Outer Rings of S0 galaxies. V. UGC 4599 -- an S0 with gas probably accreted from a filament

TL;DR

This study investigates the star formation in the outer ring of S0 galaxy UGC 4599, revealing subsolar metallicity and suggesting the ring formed from gas accreted via a cosmic filament, differing from typical S0 rings.

Contribution

It provides detailed spectroscopic and kinematic analysis of UGC 4599's outer ring, highlighting its unique low metallicity and proposing a gas accretion origin from a filament as a novel insight.

Findings

Outer ring gas is ionized by young stars.

Gas metallicity is subsolar, [O/H] = -0.4 dex.

Outer disk likely formed from cosmological filament accretion.

Abstract

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 moderate-luminosity nearby S0 galaxy, UGC 4599, is studied here. By applying long-slit spectroscopy at the Russian 6m 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 ionized by young stars, we have determined the gas oxygen abundance by using conventional strong-line calibration methods. We have 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 are studied with the tunable filter MaNGaL at the 2.5m telescope of the Caucasian Mountain Observatory of the Sternberg Astronomical Institute (CMO SAI MSU). The gas metallicity in the ring is certainly subsolar, [O/H]$=-0.4 \pm 0.1$~dex, that is different from the majority of the outer starforming rings in S0s studied by us which have typically nearly solar metallicity. The total stellar component of the galaxy which is old in the center is less massive than its extended gaseous disk. We conclude that probably the ring and the outer disk of UGC~4599 are a result of gas accretion from a cosmological filament.