Short-term dynamical evolution of grand-design spirals in barred galaxies

MNRAS 454, 2954–2964 (2015) doi:10.1093/mnras/stv2220 Short-term dynamical evolution of grand-design spirals in barred galaxies Junichi Baba‹ Earth-Life Science Institute, Tokyo Institute of Technology, 2–12–1 Ookayama, Meguro, Tokyo 152–8551, Japan Accepted 2015 September 22. Received 2015 September 22; in original form 2015 July 30 ABSTRACT Key words: methods: numerical – galaxies: kinematics and dynamics – galaxies: spiral – galaxies: structure. 1 I N T RO D U C T I O N Spiral arms in disc galaxies can be classified into three categories: grand-design, multiple-arm, and flocculent spirals (Elmegreen & Elmegreen 1987). Grand-design spiral arms are large-scale coherent, symmetric, two-armed patterns (e.g. NGC 628), whose number fraction is more than 50 per cent in nearby spiral galaxies (Grosbøl, Patsis & Pompei 2004; Kendall, Kennicutt & Clarke 2011). Observations show that grand-design spirals are associated with bars or companions (Kormendy & Norman 1979; Seigar & James 1998; Kendall et al. 2011), suggesting that a bar or companion is essential in forming a grand-design spiral in a disc galaxy. In barred spiral galaxies, the grand-design spirals extend from the ends of the bars. Furthermore, although some studies found little or no correlation between bar strengths and spiral arm strengths (Durbala et al. 2009; Kendall et al. 2011), other studies have found correlations (Block et al. 2004; Salo, Laurikainen, Buta & Knapen 2010). These observations, therefore, suggest that bars drive grand-design spirals with the same pattern speed as the bars. This ‘bar-driven spiral hypothesis’ has been promoted by the ballistic closed-orbit theory, which is based on non-self-gravitating hydrodynamic simulations in fixed barred potentials (e.g. Sanders & Huntley 1976; Sanders 1977; Huntley, Sanders & Roberts 1978;  E-mail: Sormani, Binney & Magorrian 2015). This theory explains the physical mechanism responsible for the formation of the spirals in barred galaxies in terms of closed orbits of the gas (see also Wada 1994): the spiral arms in barred galaxies are regarded as kinematic density waves (i.e. crowding of gaseous closed orbits) driven by an external barred potential. However, spiral arms in observed barred galaxies are composed of not gas but stars; therefore, theories based on stellar dynamics are required for investigating the origin of spiral arms in barred galaxies. On the other hand, the ‘invariant manifold theory’ is another bardriven spiral theory (Romero-Gómez et al. 2006, 2007; Athanassoula et al. 2009b, 2010; Athanassoula, Romero-Gómez & Masdemont 2009a) and has been developed from stellar orbital theories in fixed barred potentials (Danby 1965). Essentially, the backbones of barred spirals are bunches of untrapped stars escaped from unstable Lagrangian points L1 and L2 close to the ends of the bar. This means that stars should move along the arms rather than across the arms (Athanassoula 2012). This behaviour is completely different from the quasi-stationary1 density wave theory, which predicts that 1 In general, the word stationary or steady means ‘not moving’. However, in this context, this word indicates that density waves do not propagate radially and that instead they propagate azimuthally with a single pattern speed. See Bertin & Lin (1996) for further discussion on the concept of quasi-stationary density waves.  C 2015 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society We investigate the short-term dynamical evolution of stellar grand-design spiral arms in barred spiral galaxiesusing a three-dimensional (3D) N-body/hydrodynamic simulation. Similar to previous numerical simulations of unbarred, multiple-arm spirals, we find that grand-design spiral arms in barred galaxies are not stationary, but rather dynamic. This means that the amplitudes, pitch angles, and rotational frequencies of the spiral arms are not constant, but change within a few hundred million years (i.e. the typical rotational period of a galaxy). We also find that the clear grand-design spirals in barred galaxies appear only when the spirals connect with the ends of the bar. Furthermore, we find that the short-term behaviour of spiral arms in the outer regions (R > 1.5–2 bar radius) can be explained by the swing amplification theory and that the effects of the bar are not negligible in the inner regions (R < 1.5–2 bar radius). These results suggest that although grand-design spiral arms in barred galaxies are affected by the stellar bar, the grand-design spiral arms essentially originate not as bar-driven stationary density waves, but rather as self-excited dynamic patterns. We imply that a rigidly rotating grand-design spiral could not be a reasonable dynamical model for investigating gas flows and cloud formation even in barred spiral galaxies. Dynamics of spirals in barred galaxies 2 The term ‘swing amplification’ is used to express the amplification process, which combines the shearing flow, epicyclic motions, and disc self-gravity, regardless of whether the process is a linear or non-linear phenomenon. Originally, the ‘swing amplification’ and its feedback cycle were based on the linear perturbation analyses of local shearing sheets (Goldreich & LyndenBell 1965; Julian & Toomre 1966; Toomre 1981), and previous N-body simulations of galactic discs partly supported this mechanism (Carlberg & Freedman 1985; Fujii et al. 2011). However, N-body simulations of galactic discs suggested the importance of non-linearity, such as mode–mode coupling and radial migration of stars (Baba et al. 2013; D’Onghia et al. 2013), in the dynamical evolution of spiral arms. Baba et al. 2013; D’Onghia et al. 2013). Grand, Kawata & Cropper (2012b) also found that spiral arms in barred galaxies are dynamic patterns whose rotation frequencies decrease with the radius in such a way that the rotation frequency is similar to the rotation of stars. In this study, to address the short-term (e.g. a few hundred million years) behaviour of grand-design spirals in barred spiral galaxies, we performed a three-dimensional (3D) N-body/hydrodynamic simulation of a Milky Way-like barred spiral galaxy. This paper is organized as follows. We describe the galaxy model and our numerical simulation methodologies in Section 2. In Section 3, we present our results concerning the short-term behaviour of granddesign spirals in the barred galaxy and effects of the bar on spiral dynamics. Finally, we summarize our results in Section 4. Longterm (e.g. ∼10 Gyr) behaviour of spirals in barred galaxies, as well as effects of a ‘live’ dark matter (DM) halo, will be presented in a forthcoming paper (Fujii et al., in preparation). 2 N U M E R I C A L S I M U L AT I O N S A N D A N A LY S I S We performed a 3D N-body/hydrodynamic simulation of a Milky Way-like galaxy with an N-body/smoothed particle hydrodynamics (SPH) simulation code ASURA-2 (Saitoh & Makino 2009, 2010). In this st (...truncated)