Orbits and action changes during star-clump encounters responsible for the origin of exponential discs in dwarf galaxies

TL;DR

This study uses simulations to show that star-clump encounters in dwarf galaxies cause angular momentum changes and orbital alterations, leading to the formation of exponential stellar profiles over several billion years.

Contribution

It provides detailed insights into how star-clump interactions dynamically shape galaxy disc profiles, emphasizing the role of encounter geometry and clump distribution.

Findings

Star-clump encounters primarily drive angular momentum changes.

Stars tend to move inward or outward depending on initial position and encounter direction.

Exponential profiles form regardless of the initial conditions or encounter biases.

Abstract

Previous studies found that stellar scattering by massive clumps can lead to the formation of exponential profiles in galaxy discs, but details on how a star is moved around have not been fully explained. We use a GADGET-2 simulation where an exponential profile forms from an initially Gaussian disc in about 4 Gyr for a low-mass galaxy like a dwarf irregular. We find that nearly all large angular momentum changes of stars are caused by star-clump encounters with the closest approach less than 0.5 kpc. During star-clump encounters, stars may increase their random motions, resulting in an increase in the average radial and vertical actions of the whole stellar population. The angular momentum change and the radial action change of an individual star are influenced by the direction from which the star approaches a clump. A star initially at a higher galactic radius relative to the scattering clump usually gets pulled inwards and loses its angular momentum during the encounter, and one at a lower radius tends to shift outwards and gains angular momentum. The increase in the radial action is the largest if a star encounters a clump from the azimuthal direction, and is the smallest from a radial approach. The angular momentum change due to encounters has an inward bias when the clump profile has a steep radial decline, and a shallow decline can make the bias outwards. The stellar profile evolution towards an exponential seems to occur regardless of the direction of the bias.