The Stellar and Gas Kinematics of the LITTLE THINGS Dwarf Irregular Galaxy NGC 1569

In order to understand the formation and evolution of dIm galaxies, one needs to understand their three-dimensional structure. We present measurements of the stellar velocity dispersion in NGC 1569, a nearby post-starburst dIm galaxy. The stellar vertical velocity dispersion, $σ_ z$, coupled with the maximum rotational velocity derived from \ionH1 observations, $V_ max$, gives a measure of how kinematically hot the galaxy is, and, therefore, indicates its structure. We conclude that the stars in NGC 1569 are in a thick disk with a $V_ max / σ_ z$ = 2.4 $±$ 0.7. In addition to the structure, we analyze the ionized gas kinematics from \ionO3 observations along the morphological major axis. These data show evidence for outflow from the inner starburst region and a potential expanding shell near supermassive star cluster (SSC) A. When compared to the stellar kinematics, the velocity dispersion of the stars increase in the region of SSC A supporting the hypothesis of an expanding shell. The stellar kinematics closely follow the motion of the gas. Analysis of high resolution \ionH1 data clearly reveals the presence of an \ionH1 cloud that appears to be impacting the eastern edge of NGC 1569. Also, an ultra-dense \ionH1 cloud can be seen extending to the west of the impacting \ionH1 cloud. This dense cloud is likely the remains of a dense \ionH1 bridge that extended through what is now the central starburst area. The impacting \ionH1 cloud was the catalyst for the starburst, thus turning the dense gas into stars over a short timescale, $∼$ 1 Gyr. We performed a careful study of the spectral energy distribution using infrared, optical, and ultraviolet photometry producing a state-of-the-art mass model for the stellar disk. This mass modeling shows that stars dominate the gravitational potential in the inner 1 kpc.