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Astron. Astrophys. 326, 915-918 (1997)

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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 ( [FORMULA] ). Table 2 gives the obtained values of the ratio. In order to estimate the error, we assume that the error in the linewidths is [FORMULA] 15 km s-1, and the error in the slope of the Tully-Fisher relation is [FORMULA] 0.50. We combine them with the dispersion of the sample. Then we find that the HI-to-CO distance ratio is 1.25 [FORMULA] 0.28 for the WIG, and 1.62 [FORMULA] 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:

  • (1) HI linewidths are larger than CO linewidths for interacting galaxies, and the linewidth ratio WHI / WCO 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.
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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998