Abstract
ABSTRACT We present and validate a novel semi-analytical approach to study the effect of dynamical friction (DF) on the orbits of massive perturbers in rotating stellar discs. We find that DF efficiently circularizes the orbit of co-rotating perturbers, while it constantly increases the eccentricity of counter-rotating ones until their angular momenta reverse, then once again promoting circularization. Such ‘drag toward circular co-rotation’ could shape the distribution of orientations of kinematically decoupled cores in disc galaxies, naturally leading to the observed larger fraction of co-rotating cores.
Highlights
We present and validate a novel semi-analytical approach to study the effect of dynamical friction (DF) on the orbits of massive perturbers in rotating stellar discs
The orbital evolution of massive perturbers (MPs, i.e. objects considerably heavier than the single bodies forming the galactic structure within which the perturber is moving) under the effect of dynamical friction (DF, see Binney & Tremaine 2008) has been studied in many different contexts and over a broad range of spacial scales: from the pairing and mergers of galaxies at kpc-scales, through the sinking and tidal disruption of globular clusters in the central kpc of galaxies (Binney & Tremaine 2008), down to the formation of massive black hole (MBH) binaries at pc-scales (MBHBs, see e.g. Dotti, Sesana & Decarli 2012, for a review)
Contrary to the cases of DF exerted by spherical structures with isotropic velocity fields (e.g. Chandrasekhar 1943), an analytical description of DF in composite systems with at least one rotating disc-like structure has not been developed yet
Summary
The interaction between an MP and a rotating system embedding it (either stellar or gaseous) has been found to reverse the direction of the MP angular momentum if it is initially anti-aligned with that of the background, or, if the two are aligned, to circularize the MP orbit Such behaviour has first been observed by Dotti, Colpi & Haardt (2006) in simulations of MBHs in circumnuclear gaseous discs, common in ultra-luminous infrared galaxies (Sanders & Mirabel 1996; Downes & Solomon 1998), and observed to form in gas-rich galaxy mergers (Barnes 2002; Mayer et al 2007; Capelo & Dotti 2017). Such proof is long due, because of the numerous and important consequences of the DTCC, among which: the slow-down of the MP orbital shrinking (see e.g. the discussion in Mayer 2013), the enhanced accretion on to orbiting MBHs once they circularize (Dotti et al 2009; Callegari et al 2011), the alignment of the spins of MBHs in circumnuclear discs, well before they bind in a binary (Dotti et al 2010), and the consequent low recoils expected at coalescence (but for a small tail of fast recoiling remnants, see Lousto et al 2012)
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