MICHIGAN-160 - INTERNAL KINEMATICS AND THE COSMIC DISTANCE SCALE

The internal kinematics of neutral hydrogen in the irregular emission-line galaxy Michigan 160 are discussed. The line-of-sight gas velocities beyond the radius of maximum rotational velocity are well fitted by coplanar circular orbits. In the inner regions, there is a small barlike or warp asymmetry which appears to arise from the overall ringlike mass distribution and is associated with the recent onset of star formation. The stellar mass-to-light ratio, as deduced from the kinematics and CCD photometry, is M*/LB = 0.9 ± 0.3, in agreement with the results from the population evolutionary synthesis models of Arimoto and Yoshii. By virtue of the differing distance dependences of the observed (gaseous + stellar) mass and virial mass, an accurate absolute distance may be derived for Michigan 160. This method gives a distance of 41.8 ± 4.0 Mpc, corresponding to a Hubble constant of 70 ± 7 km s-1 Mpc-1. The rotation curve of Michigan 160 shows no evidence for a dark matter component. However, in the event that this is present in a manner which mimics one of the observed components, these results should be strictly interpreted as a 95% confidence level upper limit to the distance of 48.4 Mpc and a 95% confidence level lower limit on the Hubble constant of 59 km s-1 Mpc-1. A number of potential problems, such as the residual stellar gravity, gas self-pressure, finite disk thickness, overall disk equilibrium, and large-scale non-Hubble velocities, are addressed and, where possible, corrected for. The evolutionary status of Michigan 160 itself is discussed. Most plausibly, it is the precursor to an LMC-type galaxy in which star formation has been infrequent and sporadic, with much of its neutral gas reservoir yet to be processed.