Formation of SO galaxies through mergers: antitruncated stellar discs resulting from major mergers

Abstract

Context. Lenticular galaxies (S0’s) are more likely to host antitruncated (Type-III) stellar discs than galaxies of later Hubble types. Major mergers are popularly considered too violent mechanisms to form these breaks. Aims. We have investigated whether major mergers can result into S0-like remnants with realistic antitruncated stellar discs or not. Methods. We have analysed 67 relaxed S0 and E/S0 remnants resulting from dissipative N-body simulations of major mergers from the GalMer database. We have simulated realistic R-band surface brightness profiles of the remnants to identify those with antitruncated stellar discs. Their inner and outer discs and the breaks have been quantitatively characterized to compare with real data. Results. Nearly 70% of our S0-like remnants are antitruncated, meaning that major mergers that result in S0’s have a high probability of producing Type-III stellar discs. Our remnants lie on top of the extrapolations of the observational trends (towards brighter magnitudes and higher break radii) in several photometric diagrams, due to the higher luminosities and sizes of the simulations compared to observational samples. In scale-free photometric diagrams, simulations and observations overlap and the remnants reproduce the observational trends, so the physical mechanism after antitruncations is highly scalable. We have found novel photometric scaling relations between the characteristic parameters of the antitruncations in real S0’s, which are also reproduced by our simulations. We show that the trends in all the photometric planes can be derived from three basic scaling relations that real and simulated Type-III S0’s fulfill: h_i ∝ R_brkIII, h_o ∝ R_brkIII, and μ_brkIII ∝ R_brkIII, where h_i and h_o are the scalelenghts of the inner and outer discs, and μ_brkIII and R_brkIII are the surface brightness and radius of the breaks. Bars and antitruncations in real S0’s are structurally unrelated phenomena according to the studied photometric planes. Conclusions. Mayor mergers provide a feasible mechanism to form realistic antitruncated S0 galaxies.