Formation of S0 galaxies through mergers

Abstract

Context. Lenticular (S0) galaxies are known to derive from spiral galaxies. The fact that S0s nearly obey the Tully-Fisher relation (TFR) at z ∼ 0 (as spirals have done in the last ∼9 Gyr) is considered an argument against their major-merger origin because equal mergers of two disc galaxies produce remnants that are outliers of the TFR. Aims. We explore whether a scenario that combines an origin by mergers at z ∼ 1.8 − 1.5 with a subsequent passive evolution of the resulting S0 remnants since z ∼ 0.8-1 is compatible with observational data of S0s in the TFR both at z ∼ 0.8 and z ∼ 0. Methods. We studied a set of major and minor merger experiments from the GalMer database that generate massive S0 remnants that are dynamically relaxed and have realistic properties. We analysed the location of these remnants in the photometric and stellar TFRs assuming that they correspond to z ∼ 0.8 galaxies. We then estimated their evolution in these planes over the last ∼ 7 Gyr considering that they have evolved passively in isolation. The results were compared with data of real S0s and spirals at different redshifts. We also tested how the use of V circ or V rot,max affects the results. Results. Just after ∼ 1-2 Gyr of coalescence, major mergers generate S0 remnants that are outliers of the local photometric and stellar TFRs (as already stated in previous studies), in good agreement with observations at z ∼ 0.8. After ∼ 4 -7 Gyr of passive evolution in isolation, the S0 remnants move towards the local TFR, although the initial scatter among them persists. This scatter is sensitive to the indicator used for the rotation velocity: V circ values yield a lower scatter than when V rot,max values are considered instead. In the planes involving V rot,max , a clear segregation of the S0 remnants in terms of the spin-orbit coupling of the model is observed, in which the remnants of retrograde encounters overlap with local S0s hosting counter-rotating discs. The location of the S0 remnants at z ∼ 0 agrees well with the observed distribution of local S0 galaxies in the σ 0 -M K , V circ -σ 0 , and V rot,max -σ 0 planes. Conclusions. Massive S0 galaxies may have been formed through major mergers that occurred at high redshift and have later evolved towards the local TFR through passive evolution in relative isolation, a mechanism that would also contribute to the scatter observed in this relation.

Combining stellar populations with orbit-superposition dynamical modelling: the formation history of the lenticular galaxy NGC 3115

“…Given such a significant contribution to the stellar mass budget, persistent minor mergers could also explain how NGC 3115 transformed into an S0 galaxy, by building up the 'thick' disk and diluting any spiral arms that may have been present in the progenitor object. While previous works have claimed that either environmental perturbations (Bekki & Couch 2011), internal disk instabilities (Saha & Cortesi 2018), or major mergers (Querejeta et al 2015;Tapia et al 2017;Diaz et al 2018;Fraser-McKelvie et al 2018) are the likely formation paths for S0 morphologies, our model is inconsistent with these mechanisms. NGC 3115 is a field S0, making environmental perturbations unlikely, and both internal disk instabilities and major mergers would likely destroy the old disk structure that we find in our model.…”

Disc galaxies formed from major mergers in Illustris

“…This paper was a proof of concept that major mergers can produce disc galaxies in the idealized scenario of two isolated galaxies, and allowed to study and follow in detail and in a controlled environment the formation of the disc. Similar work then followed, such as Tapia et al (2017), Eliche-Moral et al (2018) and Sauvaget et al (2018), who confirmed these results. However, such simulations are run in isolation, therefore they neglect the effect of the environment, and make assumptions on the parameters of the merger such as the orbit, or on the structures of the two merging galaxies.…”

Galaxy interactions in IllustrisTNG-100, I: The power and limitations of visual identification

“…Figure 1 illustrates the typical merger sequence, from initial approach (far left) to final coalescence (far right). These encounters lead to significant changes in stellar and gas morphology (e.g., Mihos 1995;Mihos et al 1995;Malin & Hadley 1997;Côté et al 1998;Knierman et al 2003;Lotz et al 2008;Wen & Zheng 2016;Tapia et al 2017), including the production of non-axisymmetric torques which enable gaseous inflows (e.g., Duc et al 2004;Blumenthal & Barnes 2018), which may feed the central black hole, producing heightened activity of the nucleus (e.g., Cutri & McAlary 1985;Dahari 1985;Heckman et al 1986a,b;Ellison et al 2011;Hewlett et al 2017;Trakhtenbrot et al 2017). Interacting and merging galaxies have been shown to host heightened rates of star formation (e.g., Joseph & Wright 1985;Kennicutt et al 1987;Whitmore & Schweizer 1995;Vigroux et al 1996;Mirabel et al 1998;Bridge et al 2007;Scudder et al 2012;Moreno et al 2015;Rich et al 2015;Moreno et al 2019).…”