4. Discussion and summary
We discuss the effect of interaction on the Tully-Fisher relation. The relationship between the HI-to-CO linewidth ratio and the HI-to-CO distance ratio is given as,
where k is the slope of the Tully-Fisher relation ( ). Table 2 gives the obtained values of the ratio. In order to estimate the error, we assume that the error in the linewidths is 15 km s-1, and the error in the slope of the Tully-Fisher relation is 0.50. We combine them with the dispersion of the sample. Then we find that the HI-to-CO distance ratio is 1.25 0.28 for the WIG, and 1.62 0.48 for the SIG. Distance derived from the HI linewidths is, thus, found to be 62% larger than that derived from the CO linewidths for SIG. When we observe distant galaxies, we may overlook tidal features, because they are faint. We suggest that the CO-line Tully-Fisher relation will be more reliable to measure the distances of galaxies for especially distant galaxies. The same should apply for galaxies in rich clusters, where galaxy-galaxy interaction is far more frequent and inevitable.
Table 2. HI-to-CO Linewidth ratio and distance ratio
Finally we mention that the effect of interaction should be taken into account not only in the Tully-Fisher relation but also in the dynamics and evolution of spiral galaxies. The present method of the WCO versus WHI comparison may give a clue to reveal dynamical properties of galaxies in the era and regions where galaxy-galaxy interaction would have a significant effect.
We summarize our results as follows:
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998