Constraining the formation of inner bars. Photometry, kinematics and stellar populations in NGC 357

TL;DR

This study provides a detailed analysis of a double-barred galaxy, NGC 357, revealing that inner structures are likely formed through star redistribution rather than classical secular processes involving gas and star formation.

Contribution

First detailed photometric, kinematic, and stellar population analysis of a double-barred galaxy, challenging the traditional secular formation scenario.

Findings

Inner structures are nearly coeval and old.

Bulge and inner bar share similar metallicity and alpha-enhancement.

Outer bar is less metal-rich and more alpha-enhanced.

Abstract

Double-barred galaxies are common in the local Universe, with approximately one third of barred spirals hosting an smaller, inner bar. Nested bars have been proposed as a mechanism for transporting gas to the very central regions of the galaxy, trigger star formation and contribute to the growth of the bulge. To test this idea, we perform for the first time a detailed analysis of the photometry, kinematics and stellar populations of a double-barred galaxy: NGC 357. We find that this galaxy is either hosting a pseudobulge or a classical bulge together with an inner disc. We compare the relative mean luminosity-weighted age, metallicity and alpha-enhancement between the (pseudo)bulge, inner bar and outer bar, finding that the three structures are nearly coeval and old. Moreover, the bulge and inner bar present the same metallicity and overabundance, whereas the outer bar tends to be less metal-rich and more alpha-enhanced. These results point out that, rather than the classical secular scenario in which gas and star formation play a major role, the redistribution of the existing stars is driving the formation of the inner structures.