Fig. 1 shows a plot of the CO linewidths (Wi CO) versus the HI linewidths (Wi HI) for 17 interacting galaxies and 43 isolated galaxies. The CO and HI linewidths are corrected for the inclination. The CO and HI linewidths approximately coincide with each other for most of isolated galaxies (open circles). On the other hand, in most cases of interacting galaxies (filled symbols), the HI linewidths are clearly broader than those of CO.
Among the interacting galaxies, the differences between the CO and HI line widths in the SIG (filled squares) are larger than in the WIG (filled circles). Table 1 shows the number of galaxies, mean value and standard deviation of the ratio for each class. Here we excluded three galaxies which have small linewidth less than 200 km s-1, namely NGC 598, NGC 3893 and NGC 2976, from the statistics, because errors for small linewidth would amplify the linewidth ratio. The sample of mergers is small so that we cannot discuss their statistics.
Fig. 2 shows histograms of WHI / WCO for isolated galaxies, weakly and strongly interacting galaxies. The interacting galaxies have obviously larger values of the ratio than the isolated galaxies. The ratio WHI / WCO is proportional to strength of the interaction. This trend is caused by the tidal force of the galaxy - galaxy interaction. The CO gas is tightly confined to the luminous stellar disk, while the HI gas extends even beyond the optical disk. Therefore, the HI gas is likely to be disturbed by the tidal force, and the HI linewidth is broadened.
In order to compare distances derived from the CO and HI linewidths, we measured the distances using the Tully-Fisher relation in B-band. We assume that the same Tully-Fisher relation is adopted for the CO and HI linewidths, because there is no significant difference between the CO and HI linewidths for the isolated galaxies and the same relationship is better to compare between them. The B-band Tully-Fisher relation which we adopted is given by Pierce & Tully (1992),
where MB is the B-band absolute magnitude. In order to examine which of the linewidths is reliable for interacting galaxies, we plotted recession velocity versus distances derived from the Tully-Fisher relation with the CO and HI linewidths, and measured the Hubble constants. We compared the Hubble constants with those for the isolated galaxies. Fig. 3 shows the velocity-distance diagrams, namely the Hubble diagrams for interacting galaxies and isolated galaxies. The solid line and the dotted line are regression lines of the CO and HI data, respectively. The Hubble constants derived from the CO and HI linewidths for the isolated galaxies are and 60.9 5.7 km s-1 Mpc-1, respectively. The CO and HI data give a consistent value of H0 for the isolated galaxies. Here, the errors are due to the dispersion within the sample. On the other hand, the Hubble constants derived from the CO and HI linewidths for the interacting galaxies are 65.2 11.5 and 54.0 9.3 km s-1 Mpc-1, respectively. This indicates that the HI linewidths are broadened with the significance level of 89 %, and cause larger differences in H0 estimates for the interacting galaxies.
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998