Contribution of turbulence to Tully-Fisher relation

Abstract

We have proposed a model for the flat rotaion curves of spiral galaxies in an earlier paper of ours (Prabhu & Krishan 1994). We could resolve the galactic velocity field for the outer regions into a turbulent and a gravity component. Since the Tully-Fisher relationship highlights a tight correlation between the galactic velocity and its luminosity, we think it is worthwhile to study the individual correlation between the luminosity of a galaxy and its turbulent and gravity components of velocity. Towards this end we have modelled the velocity fields of 76 galaxies and the individual correlations were studied in the U, B, V, I and I lowerscript 23.5 bands. This sample is severely limited by the fact that the overlap between the set of galaxies which have been photometrically observed in all the related bands and the set consisting of galaxies whose rotation curves are available is very small. Neverthless, the study revealed an interesting feature viz. the turbulent component of the velocity correlates better than the gravity component, for the U, B and V bands, whereas the gravity component correlates better with the luminosity in the I bands. This is expected since the long wavelength luninosity of a galaxy is very sensitive to its mass. The significant correlation between the short wavelength luninosity and the turbulent component of velocity indicates that our velocity model is doing well as it reproduces the expected correlations. Thus from the statistical significance of our results, we conclude that turbulence - apart from contributing to the spectra; line widths in the form of random motions - also gives rise to ordered motions, which manifest through rotation curves.