Letter to the Editor

Star formation in outer rings of S0 galaxies

V. UGC 4599: An S0 with gas probably accreted from a filament

¹ Sternberg Astronomical Institute of the Lomonosov Moscow State University, University av. 13, 119234 Moscow, Russia
e-mail: olga@sai.msu.su
² Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz, 369167, Russia
e-mail: moisav@gmail.com; e-mail: doparin@mail.ru
³ Instituto de Astrofísica de Canarias, C/Via Lactea s/n, 38205 La Laguna Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁵ Observatorio Gemini Sur, NOIRLAB, c/o Recinto AURA, Colina El Pino s/n, La Serena, Chile

Received: 28 September 2022
Accepted: 12 December 2022

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.