Bars and spirals in tidal interactions with an ensemble of galaxy mass models

MNRAS 474, 5645–5671 (2018) doi:10.1093/mnras/stx3129 Advance Access publication 2017 December 5 Bars and spirals in tidal interactions with an ensemble of galaxy mass models Alex R. Pettitt1‹ and J. W. Wadsley2 1 Department 2 Department of Physics, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan of Physics and Astronomy, McMaster University, Hamilton, L8S 4M1, Canada Accepted 2017 November 30. Received 2017 November 28; in original form 2017 October 29 We present simulations of the gaseous and stellar material in several different galaxy mass models under the influence of different tidal fly-bys to assess the changes in their bar and spiral morphology. Five different mass models are chosen to represent the variety of rotation curves seen in nature. We find a multitude of different spiral and bar structures can be created, with their properties dependent on the strength of the interaction. We calculate pattern speeds, spiral wind-up rates, bar lengths, and angular momentum exchange to quantify the changes in disc morphology in each scenario. The wind-up rates of the tidal spirals follow the 2:1 resonance very closely for the flat and dark matter-dominated rotation curves, whereas the more baryon-dominated curves tend to wind-up faster, influenced by their inner bars. Clear spurs are seen in most of the tidal spirals, most noticeable in the flat rotation curve models. Bars formed both in isolation and interactions agree well with those seen in real galaxies, with a mixture of ‘fast’ and ‘slow’ rotators. We find no strong correlation between bar length or pattern speed and the interaction strength. Bar formation is, however, accelerated/induced in four out of five of our models. We close by briefly comparing the morphology of our models to real galaxies, easily finding analogues for nearly all simulations presenter here, showing passages of small companions can easily reproduce an ensemble of observed morphologies. Key words: methods: numerical – ISM: structure – galaxies: kinematics and dynamics – galaxies: spiral – galaxies: structure. 1 I N T RO D U C T I O N Disc galaxies are known to display a wide variety of structures in both their stellar and gaseous components (Hubble 1936; de Vaucouleurs 1959; Lintott et al. 2008; Baillard et al. 2011). The most prominent of these features are the striking inner bars and spiral arms, with some galaxies, such as our own Milky Way, believed to harbour both. Quite how these structures are generated, and what maintains them, has been the subject of many decades of observations and theoretical studies. Interactions between galaxies are believed to be commonplace, be they between similar sized galactic discs, dwarfs, or dark matter subhaloes (Soifer et al. 1984). They can induce changes in the star formation properties of the galaxies (Larson & Tinsley 1978; Keel et al. 1985; Kennicutt et al. 1987) and many examples exist of galaxies that appear in mid-interaction, be they early stages of mergers (Elmegreen et al. 2000; Hopkins et al. 2013; Renaud, Bournaud & Duc 2015) or fly-by encounters (Yun 1999; Struck et al. 2005; Querejeta et al. 2016). Cosmological simulations  E-mail: interactions – galaxies: suggest that interactions are an essential part of a galaxy’s history (e.g. Lotz et al. 2010; Bournaud et al. 2014; Kannan et al. 2015), and thus play an important part in the evolution of spiral and bar features. The classical picture of spiral arms is that they exist as density wave-like feature, with gas and stars flowing through the spiral pattern (Lin & Shu 1964; Kalnajs 1973). Gas and dust lanes are seen to trace these spiral arm features (Kennicutt et al. 2003; Walter et al. 2008; Rahman et al. 2011), with gas believed to experience strong shocks as it falls into the spiral potential well (Fujimoto 1968; Roberts 1969). While the details of this theory have changed somewhat over the years (e.g. Bertin et al. 1989; Bertin & Lin 1996), the keystone idea has remained the same until somewhat recently. Results of numerical simulations have fuelled the theory of dynamical spiral arms, where arms are recurrent transients that rotate with the material speed of the disc, winding up as they do so (Sellwood & Carlberg 1984; Elmegreen & Thomasson 1993), rather than at a near-fixed pattern speed like the classical density wave picture. These two different theories seem at odds in a number of respects, including the prevalence of star–gas arm offsets, arm lifetimes, and locations of shocks (Dobbs & Pringle 2010; Wada, Baba & Saitoh 2011; Grand, Kawata & Cropper 2012; Grand et al. 2015;  C 2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society ABSTRACT 5646 A. R. Pettitt and J. W. Wadsley MNRAS 474, 5645–5671 (2018) numerical resolution to properly capture the dynamics of the stellar (and gaseous) disc. The aim of this work is to study the response of a variety of disc galaxies to perturbing satellite passages, specifically focusing on bar and spiral features, via a suite of N-body and hydrodynamical simulations. For instance, how are bar lengths and pattern speeds changed in a tidal interaction, and how long-lived are tidal spiral arms for galaxies with varying levels of shear? This paper is organized as follows. Section 2 details the computational method and the generation of the initial conditions for both the galaxies and the interaction scenarios. Results are presented and discussed in Section 3, where we discuss the general morphology, pattern speeds, spiral arms, bars, and observational analogues. We then conclude in Section 4. 2 N U M E R I C A L S I M U L AT I O N S 2.1 Numerics Simulations were performed using the N-body, smoothed particle hydrodynamics code GASOLINE2 (Wadsley, Stadel & Quinn 2004; Wadsley, Keller & Quinn 2017) . Gravity is solved using a binary tree, and the system integrated using a kick-drift-kick leapfrog. We use 64 neighbours and the standard cubic spline kernel. Selfgravity is active for all components, using a fixed gravitational softening of 50pc. The gas is isothermal with a temperature of 10 000 K, in effect simulating the warm interstellar medium (ISM) and halting gravitational collapse of the gas. The effects of cooling, and resulting star formation/feedback processes in tidal spirals are not included here, though were the focus of Pettitt et al. (2017), which we refer the reader to for an in depth discussion of the multiphase ISM and star-forming properties of such discs. 2.2 Galaxy models We choose to setup galaxies using the GALIC initial conditions generator (Yurin & Springel 2014), where the galaxy is decomposed into a exponential disc,1 , and Hernquist profile bulge and halo. We use 1 million particles for the gas disc, 1 million for stellar disc, 1 million for the dark halo, 50 000 for the stellar bulge, and 10 000 for the companions. The angular momentum transfer between the halo and disc particles plays an importa (...truncated)