Evolution along the sequence of S0 Hubble types induced by dry minor mergers II. Bulge-disk coupling in the photometric relations through merger-induced internal secular evolution

Abstract

Context. Galaxy mergers are widely discussed as one possible evolution mechanism for lenticular galaxies (S0s), because even minor mergers induce structural changes that are difficult to reconcile with the strong bulge-disk coupling observed in the photometric scaling relations of these galaxies. Aims. We check if the evolution induced onto S0s by dry intermediate and minor mergers can reproduce the S0 photometric scaling relations. Methods. We analyse the bulge-disk decompositions of the collisionless N-body simulations of intermediate and minor mergers onto S0s presented previously to determine the evolution induced by the mergers in several relevant photometric planes. Results. The mergers induce an evolution in the photometric planes that is compatible with the data of S0s, even in the relations that indicate a strong bulge-disk coupling. Mergers drive the formation of the observed photometric relation in some cases and induce a slight dispersion compatible with data in others. Therefore, this evolutionary mechanism tends to preserve the observational photometric relations. In the photometric planes where the morphological types segregate, the mergers always induce evolution towards the region populated by S0s. No clear trends with the mass ratio of the encounter, the central satellite density, or the spin-orbit coupling are found for the range of values studied. Long-pericentre orbits generate more concentrated disks and expanded bulges than initially, while short-pericentre orbits do the opposite. The structural coupling of the bulge and the disk is preserved or reinforced in the models because mergers trigger internal secular processes in the primary disk that induce significant bulge growth. This happens even though the encounters do not induce bars in the disks. Conclusions. Intermediate and minor mergers can be considered to be plausible mechanisms for the evolution of S0s if one includes their photometric scaling relations, because they can preserve and even strengthen any pre-existing structural bulge-disk coupling by triggering significant internal secular evolution, even without bars or dissipational effects. Satellite accretions thus seem to unavoidably entail internal secular evolution, meaning that it may be quite complex to isolate the effects of the internal secular evolution driven by mergers from the one due to purely intrinsic disk instabilities in individual early-type disks at the present.