Abstract
A number of simulators have argued that major mergers can sometimes preserve discs, but the possibility that they could explain the emergence of lenticular galaxies (S0s) has been generally neglected. In fact, observations of S0s reveal a strong structural coupling between their bulges and discs, which seems difficult to reconcile with the idea that they come from major mergers. However, in our recent papers we have used N-body simulations of binary mergers to show that, under favourable conditions, discs are first destroyed but soon regrow out of the leftover debris, matching observational photometric scaling relations. Additionally, we have shown how the merger scenario agrees with the recent discovery that S0s and most spirals are not compatible in an angular momentum–concentration plane. This important result from CALIFA constitutes a serious objection to the idea that spirals transform into S0s mainly by fading (e.g., via ram-pressure stripping, as that would not explain the observed simultaneous change in λ Re and concentration), but our simulations of major mergers do explain that mismatch. From such a 3D comparison we conclude that mergers must be a relevant process in the build-up of the current population of S0s.
Highlights
Ever since Hubble [3] placed them between ellipticals and spirals in his tuning-fork diagram, lenticular galaxies have been considered as a transition class and have not deserved too much attention until recently
The original structure is mostly destroyed in the intermediate stages (t ∼ 1 Gyr), leading to spectacular tidal tails, but, eventually (t ∼ 3 Gyr), the whole system ends up relaxing into a passive disc with a small but clear bulge: a lenticular galaxy
The observational scaling relation tells us that, for a given bulge size, only a certain range of disc sizes are allowed by nature in real lenticulars, and what we have found is that the S0s emerging from our major merger simulations have discs and bulges which are coupled in that way, in agreement with photometric observations
Summary
Ever since Hubble [3] placed them between ellipticals and spirals in his tuning-fork diagram, lenticular galaxies (or S0s) have been considered as a transition class and have not deserved too much attention until recently. Recent efforts have shown that S0s form a whole sequence, parallel to that of spirals, with bulge-to-total ratios and rotational support levels which span a large range of values, in a similar fashion to the change from early- to late-type spiral galaxies [5,6,7] In this context, the question arises as to whether this parallelism in properties is the consequence of an underlying genetic connection; in other words, are the different types of lenticulars (S0a, S0b, S0c), the end-products of a morphological transformation starting from the corresponding spiral galaxies (Sa, Sb, Sc)?. The presentation was mostly based on two recently published papers [1,2]
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