Evolution of galactic discs: multiple patterns, radial migration, and disc outskirts

Abstract

We investigate the evolution of galactic discs in N-body tree-SPH simulations. We find that discs, initially truncated at three scalelengths, can triple their radial extent, solely driven by secular evolution. At the same time, the initial radial metallicity gradients are flattened and even reversed in the outer discs. Both Type I (single exponential) and Type II (down-turning) observed disc surfacebrightness profiles can be explained by our findings. We show that profiles with breaks beyond the bar's outer Lindblad resonance, at present only explained as the effect of star-formation threshold, can occur even if no star formation is considered. We explain these results with the strong angular momentum outward transfer, resulting from torques and radial migration associated with multiple patterns, such as central bars and spiral waves of different multiplicity. We find that even for stars ending up on cold orbits, the changes in angular momentum exhibit complex structure as a function of radius, unlike the expected effect of transient spirals alone. We show that the bars in all of our simulations are the most effective drivers of radial migration through their corotation resonance, throughout the 3 Gyr of evolution studied. Focussing on one of our models, we find evidence for non-linear coupling among m = 1, 2, 3 and 4 density waves, where m is the pattern multiplicity. In this way the waves involved conspire to carry the energy and angular momentum extracted by the first mode from the inner parts of the disc much farther out than a single mode could. We suggest that the naturally occurring larger resonance widths at galactic radii beyond four scale-lengths may have profound consequences on the formation and location of breaks in disc density profiles, provided spirals are present at such large distances. We also consider the effect of gas inflow and show that when in-plane smooth gas accretion of ∼5 M /yr is included, the outer discs become more unstable, leading to a strong increase in the stellar velocity dispersion. This, in turn, causes the formation of a Type III (up-turning) profile in the old stellar population. We propose that observations of Type III surface brightness profiles, combined with an up-turn in the stellar velocity dispersions beyond the disc break, could be a signature of ongoing gas-accretion. The results of this study suggest that disc outskirts comprised of stars migrated from the inner disc would have relatively large radial velocity dispersions (>30 km s −1 at 6 scale-lengths for Milky Way-size systems), and significant thickness when seen edge-on.

The relation between chemical abundances and kinematics of the Galactic disc with RAVE

“…Possible processes that have been studied by means of N-body simulations include stellar diffusion driven by transient spiral arms (Sellwood & Binney 2002;Roškar et al 2008), by the interaction between spiral arms and the Galactic bar (Minchev & Famaey 2010;Minchev et al 2011;Brunetti et al 2011) or by depositing stars into the Galactic discs by mergers (Abadi et al 2003;Villalobos & Helmi 2008;Di Matteo et al 2012). Such a disc stirring can give rise to a number of phenomena in the chemo-kinematical properties of a galaxy, such as flattening in radial metallicity gradients (e.g., Schönrich & Binney 2009a;Minchev et al 2011;Pilkington et al 2012) or extended stellar density profiles (e.g., Sánchez-Blázquez et al 2009;Minchev et al 2012b), and can have a profound impact on the way we interpret the abundance-kinematic correlations in our own Galaxy as recently discussed by Minchev et al (2012d).…”

Constraining the Milky Way assembly history with Galactic Archaeology

“…While thought to always produce short-lived spirals, N-body simulations have been constructed to yield long-lived spiral density waves lasting for over five rotation periods, by introducing an inner Qbarrier to shield the 2:1 ILR (Donner & Thomasson 1994;Elmegreen & Thomasson 1993;Thomasson et al 1990;Zhang 1996). Minchev et al (2012b) studied the longevity of spiral structure in N-body Tree SPH simulations from the GalMer database (Di Matteo et al 2007). These simulations develop strong bar and spiral structure.…”

“…These simulations develop strong bar and spiral structure. The full description and simulation setup can be found in Minchev et al (2012b). Minchev et al (2012b)'s novel way of estimating the longevity of spirals in N-body discs consisted of following the time evolution of Fourier power spectrograms, which allowed to assessing the development of radial extent, amplitude, and patterns speed.…”

“…The full description and simulation setup can be found in Minchev et al (2012b). Minchev et al (2012b)'s novel way of estimating the longevity of spirals in N-body discs consisted of following the time evolution of Fourier power spectrograms, which allowed to assessing the development of radial extent, amplitude, and patterns speed. Unlike in most previous works, where power spectrograms are typically computed over time periods of 0.5-1 Gyr, in our analyses we decreased the time window to 300 Myr and computed spectrograms every 150 Myr from t = 150 to t = 900 Myr (using outputs every 10 Myr).…”

Boxy/Peanut/X-shaped Bulges: Steep Inner Rotation Curve Leads to Barlens Face-on Morphology

“…Recently, barlens morphology was also reported in simulations where disk galaxies were formed as a result of gas-rich mergers (Athanassoula et al 2016). However, in most corresponding high-resolution simulations, either with or without gas (e.g., Minchev et al 2012;Saha et al 2012;Di Matteo et al 2013), the formed bars have elongated shapes without round inner barlens components. In addition, a close inspection of the models of Athanassoula et al (2015) indicates that even there, many of the simulated images (see their Figure 2) contain a slight trace of X-shaped morphology in the face-on view.…”