The relationship between nuclear rings and the overall galaxy morphology

Context. Nuclear rings are composed of gaseous material and/or stars and have a typical size ranging from 100 s pc to ~1 kpc. Their study is crucial to unravelling the processes involved in the evolution of the central regions of galaxies. Nuclear rings are long-lived structures, which allows the molecular gas inside to become sufficiently dense to initiate star formation. This makes them contributors to young circumnuclear populations and thus a crucial element in the study of secular evolution. However, the morphology of nuclear rings, and their potential correlations with that of the galactic hosts, remains an open subject. Aims. We examined 52 star-forming nuclear rings from the Atlas of Images of Nuclear Rings and correlated the overall galaxy morphology, in particular non-axisymmetric features, with the morphology of the nuclear ring. Methods. We divided the sample into different classes according to two visual classifications, one based on the overall morphology of the galaxy and the other based on the morphology of the nuclear ring. We defined three classes of nuclear rings: two-armed rings dominated by two dust lanes, twoarms+ rings crowded with secondary dust lanes in addition to the two main ones, and many-armed rings with multiple armlets of a similar prevalence. We employed unsharp-masked Hubble Space Telescope images to study the structure of nuclear rings. Results. We find that two-armed rings are more common in early-type grand design galaxies with strong bars. Twoarms+ rings are related to later-type and more weakly barred galaxies, both grand design and multi-armed. Lastly, many-armed rings are typically associated with later-type flocculent, multi-armed galaxies with the weakest bars. In addition, we examined the regions inside the nuclear rings and observe nuclear spirals in 28 galaxies (~90% of those galaxies for which the interior of the nuclear ring is resolved). Conclusions. We conclude that the global morphology of the host galaxy – more precisely, the presence and properties of a bar – plays a fundamental role in determining the morphology of the nuclear ring and the nuclear region. Specifically, the strength of non-axisymmetries strongly influences the morphological characteristics of a galaxy, from its innermost to its outermost regions. We suggest that two-armed rings are associated with a 180° structure forced by a strong bar, many-armed rings are associated with a very weak or absent 180° structure, and twoarms+ rings are found in intermediate cases.