The origin of star-forming rings in S0 galaxies

TL;DR

This study uses integral field spectroscopy to analyze star-forming rings in S0 galaxies, revealing their prevalence, properties, and possible origins related to galaxy evolution and resonances.

Contribution

It provides the first detailed analysis of the origin and characteristics of star-forming rings in S0 galaxies using MaNGA data, linking them to galaxy quenching and resonances.

Findings

Star-forming rings are common in S0 galaxies.

Ring presence correlates with inside-out quenching.

Frequency of rings increases with galaxy mass.

Abstract

Spatially resolved integral field spectroscopic maps in a sample of $532$ S0 galaxies from the MaNGA survey have unveiled the existence of inner rings ($\langle R\rangle\sim 1\,R_\mathrm{e}$) betraying ongoing star formation in a number of these objects. Activity gradients averaged over bins of galactocentric radius up to $\sim 1.5\,R_\mathrm{e}$ have been measured in the subspace defined by the first two principal components of the optical spectra of these galaxies. We find that the sign of the gradients is closely related to the presence of such rings in the spectral maps, which are especially conspicuous in the equivalent width of the H$\alpha$ emission line, EW(H$\alpha$), with a fractional abundance $\unicode{x2014}21\unicode{x2013}34\%\unicode{x2014}$ notably larger than that inferred from optical images. While the numbers of S0s with positive, negative, and flat activity gradients are comparable, star-forming rings are largely found in objects for which quenching proceeds from the inside out, in good agreement with predictions from cosmological simulations studying S0 buildup. Assessment of these ringed structures indicates that their frequency increases with the mass of their hosts, that they have shorter lifetimes in galaxies with ongoing star formation, that they may feed on gas from the disks, and that the local environment does not play a relevant role in their formation. We conclude that the presence of inner rings in the EW(H$\alpha$) is a common phenomenon in fully formed S0s, possibly associated with annular disk resonances driven by weakly disruptive mergers preferentially involving a relatively massive primary galaxy and a tiny satellite strongly bound to the former.