*Astron. Astrophys. 326, 915-918 (1997)*
## 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
*W*_{CO}
versus
*W*_{HI} 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:
- (1) HI linewidths are larger than CO linewidths for
interacting galaxies, and the linewidth ratio
*W*_{HI} / *W*_{CO}
is proportional to the strength of interaction.
- (2) Distances derived from the HI Tully-Fisher relation are 25%
larger for the weakly interacting galaxies and 62% larger for the
strongly interacting galaxies than that derived from the CO
Tully-Fisher relation.
- (3) Therefore, the CO Tully-Fisher relation would give more
reliable distances for interacting galaxies. This implies that the CO
Tully-Fisher relation will give better distance measurement for
galaxies inside rich clusters, where the tidal interaction is
inevitable.
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998
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