Spin-up of massive classical bulges during secular evolution

Abstract

Context. Classical bulges in spiral galaxies are known to rotate, but the origin of this observed rotational motion is not well understood. It has been shown recently that a low-mass classical bulge (ClB) in a barred galaxy can acquire rotation by absorbing a significant fraction of the angular momentum emitted by the bar. Aims. Our aim here is to investigate whether bars can also spin up more massive ClBs during the secular evolution of the bar, and to study the kinematics and dynamics of these ClBs. Methods. We use a set of self-consistent N-body simulations to study the interaction of ClBs with a bar that forms self-consistently in the disk. We use orbital spectral analysis to investigate the angular momentum gain by the classical bulge stars. Results. We show that the ClBs gain on average 2−6% of the disk's initial angular momentum within the bar region. Most of this angular momentum gain occurs via low-order resonances, particularly 5:2 resonant orbits. A density wake forms in the ClB which corotates and aligns with the bar at the end of the evolution. The spin-up process creates a characteristic linear rotation profile and mild tangential anisotropy in the ClB. The induced rotation is small in the centre, but is significant beyond ∼2 bulge half mass radii, where it leads to mass-weighted V/σ ∼ 0.2, and reaches a local V max /σ in ∼ 0.5 at around the scale of the bar. The resulting V/σ is tightly correlated with the ratio of the bulge size to the bar size. In all models, a box/peanut bulge forms suggesting that composite bulges may be common. Conclusions. Bar-bulge resonant interaction in barred galaxies can provide some spin-up of massive ClBs, but the process appears to be less efficient than for low-mass ClBs. Further angular momentum transfer due to nuclear bars or gas inflow would be required to explain the observed rotation if it is not primordial.

The Gaia-ESO Survey: Exploring the complex nature and origins of the Galactic bulge populations

“…In this way, the classical bulge will become photometrically and kinematically indistinguishable from the B/P bulge. In the case of a massive initial classical bulge, its central parts might be less affected (Saha et al 2016), providing a kinematic relic to be exploited by spectroscopic observations of the inner-bulge.…”

Bars and boxy/peanut bulges in thin and thick discs

“…This is, in fact, a characteristic signature of a kinematically hot disc component: while a dispersion dominated population, such as a classical bulge, can acquire some rotation, it cannot rotate faster than the surrounding disc component (see e.g. Fux 1997;Saha et al 2012Saha et al , 2016Saha & Gerhard 2013;Di Matteo et al 2014). …”

“…Owing to the ubiquitous nature of thick discs in observed (Burstein 1979;Tsikoudi 1979;Yoachim & Dalcanton 2006) and simulated galaxies (Abadi et al 2003;Bird et al 2013;Stinson et al 2013;Martig et al 2014), along with the various formation scenarios proposed for them (Quinn et al 1993;Brook et al 2004;Schönrich & Binney 2009;Qu et al 2011;Martig et al 2014;Minchev et al 2015;Haywood et al 2015), the question of the interplay between such a population and structures such as bars and b/p's naturally arises. In the Milky Way in particular, due to the small scalelength of the α-enhanced thick disc population (Bensby et al 2011;Bovy et al 2012), it follows that the chemically defined thick disc is centrally concentrated; this, along with recent results from chemical evolution models which indicate that the mass of the thick disc could be of the same order as that of the thin disc (Haywood et al 2013;Snaith et al 2015), point to the fact that the thick disc will have an important contribution in terms of mass in the central region of the Milky Way.…”

Sculpting Andromeda – made-to-measure models for M31’s bar and composite bulge: dynamics, stellar and dark matter mass

“…Moreover, on even longer time-scales, box/peanut bulges and bars can interact through resonances with the disc and thereby redistribute its material, generating for example surface brightness breaks, as well as ring-like substructures (Buta & Crocker 1991;Debattista et al 2006;Erwin et al 2008;Buta 2017). Bars also transfer their angular momentum to the spheroid components, such as classical bulges (Saha et al 2012(Saha et al , 2016, stellar haloes (Perez-Villegas et al 2017) and dark matter haloes (Athanassoula & Misiriotis 2002), changing their dynamical properties. Furthermore, Erwin & Debattista (2016) show also with observations that classical bulges can coexist with discy pseudobulges and box/peanut bulges building composite bulges, a scenario that has also been reproduced in galaxy formation simulations (Athanassoula et al 2016).…”