Angular momentum transport and evolution of lopsided galaxies

TL;DR

This paper investigates how lopsided asymmetry in stellar and gas discs influences angular momentum transport and galaxy evolution, highlighting the role of dark matter halos in enabling outward angular momentum flow.

Contribution

It demonstrates that dark matter halos are essential for proper angular momentum flow in lopsided galaxies, extending Lindblad's density wave theory to outer galactic regions.

Findings

Lopsided modes can reverse angular momentum flow without dark matter halos.

Dark matter halos facilitate outward angular momentum transport in lopsided galaxies.

Lopsidedness may promote gas inflow from cosmic filaments, aiding galaxy evolution.

Abstract

The surface brightness distribution in the majority of stellar galactic discs falls off exponentially. Often what lies beyond such a stellar disc is the neutral hydrogen gas whose distribution also follows a nearly exponential profile at least for a number of nearby disc galaxies. Both the stars and gas are commonly known to host lopsided asymmetry especially in the outer parts of a galaxy. The role of such asymmetry in the dynamical evolution of a galaxy has not been explored so far. Following Lindblad's original idea of kinematic density waves, we show that the outer part of an exponential disc is ideally suitable for hosting lopsided asymmetry. Further, we compute the transport of angular momentum in the combined stars and gas disc embedded in a dark matter halo. We show that in a pure star and gas disc, there is a transition point where the free precession frequency of a lopsided mode, $\Omega -\kappa $, changes from retrograde to prograde and this in turn reverses the direction of angular momentum flow in the disc leading to an unphysical behaviour. We show that this problem is overcome in the presence of a dark matter halo, which sets the angular momentum flow outwards as required for disc evolution, provided the lopsidedness is leading in nature. This, plus the well-known angular momentum transport in the inner parts due to spiral arms, can facilitate an inflow of gas from outside perhaps through the cosmic filaments.