Bars and boxy/peanut bulges in thin and thick discs

Abstract

We explore trends in the morphology and line-of-sight (los) velocity of stellar populations in the inner regions of disc galaxies using N-body simulations with a thin (kinematically cold) and a thick (kinematically hot) disc which form a bar and a boxy/peanut (b/p) bulge. The bar in the thin disc component is ∼50% stronger than the thick disc bar and is more elongated, with an axis ratio almost half that of the thick disc bar. The thin disc b/p bulge has a pronounced X-shape, while the thick disc b/p is weaker with a rather boxy shape. This leads to the signature of the b/p bulge in the thick disc being weaker and further away from the plane than in the thin disc. Regarding the kinematics, we find that the los velocity of thick disc stars in the outer parts of the b/p bulge can be higher than that of thin disc stars, by up to 40% and 20% for side-on and Milky Way-like orientations of the bar, respectively. This is due to the different orbits followed by thin and thick disc stars in the bar-b/p region, which are affected by two factors. First, thin disc stars are trapped more efficiently in the bar-b/p instability and thus lose more angular momentum than their thick disc counterparts and second, thick disc stars have large radial excursions and therefore stars from large radii with high angular momenta can be found in the bar region. We also find that the difference between the los velocities of the thin and thick disc in the b/p bulge (∆v los ) correlates with the initial difference between the radial velocity dispersions of the two discs (∆σ). We therefore conclude that stars in the bar-b/p bulge will have considerably different morphologies and kinematics depending on the kinematic properties of the disc population they originate from.

Stellar populations across galaxy bars in the MUSE TIMER project

“…These results indicate that intermediate age stellar populations are concentrated on more elongated orbits closer to the bar major axis than older stellar populations. They are consistent with the findings from idealised thin (kinematically cold/young) plus thick (kinematically hot/old) disc N-body galaxy simulations in Fragkoudi et al (2017). In their figure 2, they show that the colder component forms a strong and thin bar, while the hotter component forms a weaker and rounder bar (see also Wozniak 2007;Athanassoula et al 2017;Debattista et al 2017;Fragkoudi et al 2018).…”

“…We see that the youngest population has an elongated bar shape, much more so than the oldest population which is rounder. This difference in the shape of the bar according to the age and kinematics of the underlying population was shown using idealised simulations in Fragkoudi et al (2017) and Athanassoula et al (2017), and was termed kinematic fractionation by Debattista et al (2017). Therefore we see that the younger populations are more clustered along the bar major axis than the oldest populations, due to kinematic fractionation, giving rise to the V-shape we see in the observations.…”

“…Finally, recent N-body simulations have shown that stars could be trapped into bar orbits with different elongations based on the initial kinematics of the stars or the gas out of which they form (Athanassoula et al 2017;Debattista et al 2017;Fragkoudi et al 2017). This could lead to different populations dominating at different locations in the bar.…”

Quantifying the (X/peanut)-shaped structure of the Milky Way – new constraints on the bar geometry

“…Multiple studies, based on stellar populations and numerical simulations, have shown evidence that the Milky Way's central 'bulge' is not (primarily) the remnant of past merger events, i.e., a 'classical' bulge, but rather it was built predominantly from disc stars through the buckling and secular evolution of the Galactic bar, the latter itself originating from the disc (Shen et al 2010, Ness et al 2012Di Matteo et al 2014;Di Matteo 2016;Abbott et al 2017; see also Fragkoudi et al 2017). This result is consistent with the X/P morphology and indicates that the X/P 'bulge' and bar are aligned, since one has formed from, and is still the thick central part of, the other (see also Martinez-Valpuesta & Gerhard 2011, Romero-Gómez et al 2011and Wegg, Gerhard & Portail 2015.…”

The structural evolution of galaxies with both thin and thick discs

“…Debattista et al (2016) recently studied bar formation in galaxies that contain disc populations with differing random motions. They showed that radially cooler populations form stronger bars, the edge-on profiles of which are vertically thinner and peanut-shaped, whereas the hotter populations form a weaker bar with a vertically thicker edge-on box shape (see also Fragkoudi et al 2017).…”