Disentangling the formation path of lenticular galaxies

Abstract

[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.

[cat] L’objectiu d’aquesta tesi és investigar la formació i evolució de les galàxies lenticulars (S0s) mitjançant la informació continguda en els seus espectres òptics. Realitzem una exploració dels espectres òptics de gairebé 70.000 S0s de l’SDSS utilitzant l’anàlisi de components principals (PCA) per classificar-les. Identifiquem dues classes principals: aproximadament un 70% de les lenticulars són galàxies passives; en canvi, un 25% de les lenticulars mostren espectres actius amb línies d’emissió associats a ritmes de formació estel·lar elevats, similars als observats en espirals tardanes, i es troben segregades en entorns de baixa densitat. La font d'ionització principal en aquestes S0 actives és la formació d'estrelles, mentre que en sistemes S0 de tipus Seyfert i LINER, l'activitat ve dominada per l’acreció nuclear i estrelles post-AGB. L'extensió de la PCA a espectres resolts espacialment de MaNGA permet l’estudi de la configuració radial de l’activitat a les galàxies S0. Trobem que els anells de formació estel·lar sovint observats en aquestes galàxies estan associats amb un gradient d’activitat positiu i es podrien atribuir a la captura per les lenticulars de satèl·lits nans propers. Quan examinem els perfils radials d’activitat de les lenticulars, trobem que aquests estan relacionats amb les classificacions PCA i BPT, així com amb el seu estat global de formació estel·lar. Les lenticulars passives mostren perfils plans de baix nivell d’activitat, mentre que els perfils de les actives tenen gradients negatius, fruit d’una formació estel·lar intensa concentrada a la regió central d’aquestes galàxies. En conjunt, els resultats presentats en aquesta tesi suggereixen que les fusions menors podrien rejovenir les galàxies S0, contrarestant l’efecte d’extinció de l’activitat produït pels cúmuls. La metodologia desenvolupada proporciona una imatge unificada de l'activitat a les galàxies S0, amb aplicacions potencials a altres morfologies galàctiques.