Evolution induced by dry minor mergers onto fast-rotator S0 galaxies

Abstract

Context. Numerical studies have shown that the properties of the S0 galaxies with kinematics intermediate between fast and slow rotators are difficult to explain by a scenario of major mergers. Aims. We investigate whether the smoother perturbation induced by minor mergers can reproduce these systems. Methods. We analysed collisionless N-body simulations of intermediate and minor dry mergers onto S0s to determine the structural and kinematic evolution induced by the encounters. The original primary galaxies represent gas-poor fast-rotator S0b and S0c galaxies with high intrinsic ellipticities. The original bulges are intrinsically spherical and have low rotation. Different mass ratios, parent bulges, density ratios, and orbits were studied. Results. Minor mergers induce a lower decrease of the global rotational support (as provided by λ e ) than encounters of lower mass ratios, which results in S0s with properties intermediate between fast and slow rotators. The resulting remnants are intrinsically more triaxial, less flattened, and span the whole range of apparent ellipticities up to e ∼ 0.8. They do not show lower apparent ellipticities in random projections than initially; on the contrary, the formation of oval distortions and the disc thickening increase the percentage of projections at 0.4 < e < 0.7. In the experiments with S0b progenitor galaxies, minor mergers tend to spin up the bulge and to slightly decrease its intrinsic ellipticity, whereas in the cases of primary S0c galaxies they keep the rotational support of the bulge nearly constant and significantly decrease its intrinsic ellipticity. The remnant bulges remain nearly spherical (B/A ∼ C/A > 0.9), but exhibit a wide range of triaxialities (0.20 < T < 1.00). In the plane of global anisotropy of velocities (δ) vs. intrinsic ellipticity ( e,intr ), some of our models extend the linear trend found in previous major merger simulations towards higher e,intr values, while others clearly depart from it (depending on the progenitor S0). This is consistent with the wide dispersion exhibited by real S0s in this diagram compared with ellipticals, which follow the linear trend drawn by major merger simulations. Conclusions. The smoother changes induced by minor mergers can explain the existence of S0s with intermediate kinematic properties between fast and slow rotators that are difficult to explain with major mergers. The different trends exhibited by ellipticals and S0 galaxies in the δ -e diagram may be pointing to the different role played by major mergers in the build-up of each morphological type.

Formation of S0 galaxies through mergers

“…However, here we would like to point out that, whatever that contribution has been, any S0s deriving from the major merger between two spirals would also contribute to the trend observed by Dressler of transformation of spirals into S0s. It is beyond the scope of the present paper to quantify the relevance of major mergers in terms of creating S0s (recent estimates indicate that they may have been essential in the buildup of ∼50% of present massive S0s at most, see Tapia et al 2014), but it is a mechanism that surely needs to be taken into account, and may especially explain the origin of the S0s that reside in groups and less dense environments. S0s are at least as common in groups as in clusters, and galaxy interactions are the dominant evolution mechanism in this regime (Wilman et al 2009;Mazzei et al 2014a,b).…”

“…Mergers that differ only slightly in their initial conditions are capable of producing quite different sort of remnant systems, and all of them with bulge-disc structural coupling consistent with observations. If we also account for the observational and computational evidence that points to a merger origin for a significant fraction of S0s (and in particular, for the most massive ones, see Eliche-Moral et al 2010a;Bernardi et al 2011a,b;Méndez-Abreu et al 2012;Barway et al 2013;Wilman et al 2013;Tapia et al 2014), it seems unjustified to exclude major mergers from the current scenarios of S0 formation and evolution.…”

“…In fact, intermediate mergers (with mass ratios from 4:1 to 7:1) are not present in the database, but many studies suggest that intermediate encounters and multiple minor mergers may have been as relevant for the evolution of S0 galaxies as major merger events (see Maller et al 2006;Bournaud et al 2007;Wilman et al 2013;Tapia et al 2014, and references therein).…”

Formation of S0 galaxies through mergers

“…Gunn & Gott 1972;Larson et al 1980;Moore et al 1996;Quilis et al 2000;Peng et al 2015) are often considered the main drivers of this transformation. However, numerical studies have shown that mergers, in particular major ones (mass ratios from 1:1 to 3:1), can also give rise to remnant galaxies that can be classified as S0s in terms of their morphology, structure, and scaling relations (Bekki 2001;Bekki & Couch 2011;Eliche-Moral et al 2011, 2013Borlaff et al 2014;Tapia et al 2014;Querejeta et al 2015a,b). Furthermore, Athanassoula et al (2016) have recently shown that major mergers can build up remnants of even later types (see also Springel & Hernquist 2005).…”

Formation of S0 galaxies through mergers

“…In fact, many authors find observational evidence of a major-merger origin of many S0s at z < 1 (see Peirani et al 2009;Yang et al 2009;Hammer et al 2009aHammer et al ,b, 2012Tapia et al 2014). So, the question of whether major mergers can produce Type III S0s remains unsettled.…”