Michigan 160: Internal Kinematics and the Cosmic Distance Scale
Abstract
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_*_/L_B_ = 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1990
- DOI:
- 10.1086/169383
- Bibcode:
- 1990ApJ...364...23S
- Keywords:
-
- Cosmology;
- Distance;
- Irregular Galaxies;
- Radio Astronomy;
- Astronomical Models;
- Astronomical Spectroscopy;
- Brightness Distribution;
- Dark Matter;
- Galactic Rotation;
- Velocity Distribution;
- Astrophysics;
- GALAXIES: DISTANCES;
- GALAXIES: INDIVIDUAL NAME: MICHIGAN 160;
- GALAXIES: INTERNAL MOTIONS;
- GALAXIES: STRUCTURE