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

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.

20 pages, 21 figures

References in corpus (9)

• The structure of galactic disks: Studying late-type spiral galaxies using SDSS
• Observations of Thick Disks in the Hubble Space Telescope Ultra Deep Field
• The effect of radial migration on galactic disks
• Constraints on Radial Migration in Spiral Galaxies - II. Angular momentum distribution and preferential migration
• Shrinking Galaxy Disks with Fountain-Driven Accretion from the Halo
• Exponential Disks from Stellar Scattering: III. Stochastic Models
• The Extended Disk Galaxy Exploration Science Survey: Description and Surface Brightness Profile Properties
• Near-exponential surface densities as hydrostatic, nonequilibrium profiles in galaxy discs
• Power-law Sersic profiles in hydrostatic stellar galaxy discs

Cited by in corpus (2)

• Efficient radial migration by giant molecular clouds in the first several hundred Myr after the stellar birth
• Exponential galaxy discs as the quasi-stationary distribution in a Markov chain model simulating stellar scattering