The Stellar and Gas Kinematics of Several Irregular Galaxies

We present long-slit spectra of three irregular galaxies from which we determine the stellar kinematics in two of the galaxies (NGC 1156 and NGC 4449) and ionized gas kinematics in all three (including NGC 2366). We compare this to the optical morphology and H I kinematics of the galaxies. In the ionized gas, we see a linear velocity gradient in all three galaxies. In NGC 1156 we also detect a weak linear velocity gradient in the stars of (5 ± 1/ sin i) km s-1 kpc-1 to a radius of 1.6 kpc. The stars and gas are rotating about the same axis, but this is different from the major axis of the stellar bar, which dominates the optical light of the galaxy. In NGC 4449 we do not detect organized rotation of the stars and place an upper limit of km s-1 kpc-1 to a radius of 1.2 kpc. For NGC 4449, which has signs of a past interaction with another galaxy, we develop a model to fit the observed kinematics of the stars and gas. In this model the stellar component is in a rotating disk seen nearly face-on, while the gas is in a tilted disk with orbits whose planes precess in the gravitational potential. This model reproduces the apparent counterrotation of the inner gas of the galaxy. The peculiar orbits of the gas are presumed as a result of acquisition of gas in the past interaction.