[eng] Astronomers have known of the existence of lenticular galaxies (S0) almost as long as they have known that there are other galaxies besides the Milky Way. While it is accepted that spiral galaxies emerge from the collapse of primordial vast clouds of gas and that when they merge they often give place to elliptical galaxies, the origin of S0 galaxies is still subject to debate. A clear bimodality in the properties of these galaxies, which are relatively abundant in low- and high-density environments, suggests that multiple formation mechanisms may be at play. In the field or in small groups, S0 galaxies can evolve from mergers. Conversely, hydrodynamic interactions are expected to transform spirals into S0s within large galaxy aggregations. This thesis is devoted to reviewing the main properties of galaxies classified as S0. Our goal is to gather abundant and robust information about relevant parameters of this poorly understood morphological type and their possible dependence on the environment to constrain their formation. For the first time, we explore the entire optical spectrum of these galaxies, seeking clues to infer their evolution. We start the exploration with single-fibre spectra from the SDSS of a sample of ∼70,000 nearby S0s and their global properties. A principal component analysis (PCA) is used to reduce the high complexity of the spectral data through its projections into a low-dimensional space, thereby facilitating a bias-free, machine-learning-based classification of the galaxies. The procedure reveals that the S0 population consists of two main classes with statistically inconsistent properties. While the bulk population is made by passive lenticulars with inactive spectra, the other is characterized by active galaxies that, despite their early-type morphology, show star formation rates that can be similar to those observed in late-type spirals. The main ionisation source of the active galaxies is star formation. However, in the Seyfert and LINER S0 systems detected in radio and X-ray, activity is driven, respectively, by nuclear accretion and post-AGB stars. Applied to spatially resolved spectra from MaNGA, the PCA can be used to study the radial configuration of activity in the galaxies. The extension of the PCA to these spectra leads us to identify star-forming rings in S0 galaxies typically associated with a positive activity gradient, and assemble the largest catalogue of these objects ever identified through this kind of data. Assessment of the rings indicates that they are relatively abundant (∼30%) in fully-formed S0s with a frequency that sharply increases with the mass of the hosts, but are uncorrelated with the environment. Rings are twice more frequent among the members of the passive class than in the active, and likely feed on residual gas from the disc. These results link the rings with the capture by the S0s of tiny dwarf satellites that closely orbit them. Finally, we examine the radial activity profiles of S0s as a function of their properties. The comparison reveals that the radial activity gradient of these galaxies is tightly related to their PCA and BPT classifications, and star formation status. The passive class often shows low-level, flat activity profiles, while their active counterparts generally have negative activity gradients, associated with high specific star formation rates. Altogether, our results support a scenario where minor mergers actively play a rejuvenation role in the recent evolution of S0 galaxies, while clusters operate in the opposite direction by quenching their activity. The framework we have developed provides a unified picture of activity in S0 galaxies in the optical domain. Combined with physical quantities and line diagnostics, the framework is a valuable tool for interpreting key trends in S0s that should be transferable to other morphologies.

We acknowledge financial support from the Spanish state agency MCIN/AEI/10.13039/501100011033 and by ’ERDF A way of making Europe’ funds through research grants PID2019–106027GB–C41 and PID2019–106027GB–C43. MCIN/AEI/10.13039/501100011033 has also provided additional support through the Centre of Excellence Severo Ochoa’s award for the Instituto de Astrofísica de Andalucía under contract SEV–2017–0709 and the Centre of Excellence María de Maeztu’s award for the Institut de Ciències del Cosmos at the Universitat de Barcelona under contract CEX2019–000918–M. J.L.T. acknowledges support by the PRE2020–091838 grant from MCIN/AEI/10.13039/501100011033 and by ’ESF Investing in your future’. H.D.S. acknowledges support by the PID2020-115098RJI00 grant from MCIN/AEI/10.13039/501100011033 and from the Spanish Ministry of Science and Innovation and the European Union - NextGenerationEU through the Recovery and Resilience Facility project ICTS-MRR-2021-03-CEFCA

We acknowledge financial support from the Spanish Agencia Estatal de Investigaci´on and European FEDER funds through the research project AYA2016-76682-C3. JLT was also partially supported by a collaboration scholarship from the Royal Academy of Sciences and Arts of Barcelona and the University of Barcelona. Additional funding for this work has been provided by the State Agency for Research of the Spanish Ministerio de Ciencia, Innovaci´on y Universidades through the Centre of Excellence Severo Ochoa’s award for the Instituto de Astrof´ısica de Andaluc´ıa under contract SEV-2017-0709.

With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (SEV-2017-0709).

Programa de doctorado en Física de la Universidad de Barcelona. Leída el 12 de abril del 2024.

Peer reviewed