THE STELLAR AND GAS KINEMATICS OF THE LITTLE THINGS DWARF IRREGULAR GALAXY NGC 1569

Abstract

In order to understand the formation and evolution of dIm galaxies, one needs\nto understand their three-dimensional structure. We present measurements of the\nstellar velocity dispersion in NGC 1569, a nearby post-starburst dIm galaxy.\nThe stellar vertical velocity dispersion, $\\sigma_{\\rm z}$, coupled with the\nmaximum rotational velocity derived from \\ion{H}{1} observations, $V_{\\rm\nmax}$, gives a measure of how kinematically hot the galaxy is, and, therefore,\nindicates its structure. We conclude that the stars in NGC 1569 are in a thick\ndisk with a $V_{\\rm max} / \\sigma_{\\rm z}$ = 2.4 $\\pm$ 0.7. In addition to the\nstructure, we analyze the ionized gas kinematics from \\ion{O}{3} observations\nalong the morphological major axis. These data show evidence for outflow from\nthe inner starburst region and a potential expanding shell near supermassive\nstar cluster (SSC) A. When compared to the stellar kinematics, the velocity\ndispersion of the stars increase in the region of SSC A supporting the\nhypothesis of an expanding shell. The stellar kinematics closely follow the\nmotion of the gas. Analysis of high resolution \\ion{H}{1} data clearly reveals\nthe presence of an \\ion{H}{1} cloud that appears to be impacting the eastern\nedge of NGC 1569. Also, an ultra-dense \\ion{H}{1} cloud can be seen extending\nto the west of the impacting \\ion{H}{1} cloud. This dense cloud is likely the\nremains of a dense \\ion{H}{1} bridge that extended through what is now the\ncentral starburst area. The impacting \\ion{H}{1} cloud was the catalyst for the\nstarburst, thus turning the dense gas into stars over a short timescale, $\\sim$\n1 Gyr. We performed a careful study of the spectral energy distribution using\ninfrared, optical, and ultraviolet photometry producing a state-of-the-art mass\nmodel for the stellar disk. This mass modeling shows that stars dominate the\ngravitational potential in the inner 1 kpc.\n