Star-clump scattering and the formation of exponential density profiles in galactic disks

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Wu, Jian
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Struck, Curtis
Kerton, Charles
Kawaler, Steve
Evans, James
Wu, Xiaoqing
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Physics and Astronomy
The exponential shape of stellar surface density profiles in galactic disks have been known for decades. However, the mechanism for producing this shape has remained unclear. Clumps or clouds with a mass more than 106 solar masses are found in galaxies at both low and high redshifts. N -body simulations using the public GADGET-2 code show that scattering by these heavy masses in a disk can evolve the surface density profile of stars to a near-exponential shape with a central cusp, under a variety of initial stellar distributions. High-mass clumps in a disk with low gravitational stability drive faster evolution. In addition to the changes in the density profile, the stellar disk thickens, and velocity dispersions of stars gradually increase with time. Investigations of individual stellar orbits indicate that the effect of clumps on stellar profile changes take places through close star-clump encounters. When a star meets a clump that orbits the disk center at a slightly greater radius, the gravity of the clump is more likely to move the star outwards than inwards in the radial direction, leading to an increase in the stellar angular momentum. If the clump is at a smaller galactic radius than the star before the encounter, the star has a higher tendency to move inwards, losing angular momentum. Star-clump encounters, on average, also increase both the radial action and the vertical action of stars, which accounts for rises in velocity dispersions and disk thickening. The origin of an exponential profile can be understood with the aid of a Markov chain model where stars are scattered into adjacent radial bins. Simulations with this model again show that a near-exponential profile with a central cusp is the quasi-stationary distribution that any initial state evolves toward. Although the rate of profile evolution and the exponential scale length of the final profile depend on values of the outwards scattering probability p and the inwards scattering probability q, the general exponential shape of the quasi-stationary state holds regardless of the sign of p − q, i.e. the direction of radial scattering bias.
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