Observational constraints on disk heating as a function of Hubble type

Current understanding of the secular evolution of galactic disks suggests that this process is dominated by two or more heating'' mechanisms, which increase the random motions of stars in the disk. In particular, the gravitational influence of giant molecular clouds and irregularities in the spiral potential have been proposed to explain the observed velocity dispersions in the solar neighborhood. Each of these mechanisms acts on different components of the stellar velocity, which affects the ratio sigma(z)/sigma(R) of the vertical and radial components differently. Since the relative strengths of giant molecular clouds and spiral irregularities vary with Hubble type, a study of sigma(z)/sigma(R) as function of Hubble type has the potential to provide strong constraints on disk heating mechanisms. We present major- and minor-axis stellar kinematics for four spiral galaxies of Hubble type from Sa to Sbc and use the data to infer the ratios sigma(z)/sigma(R) in the galaxy disks. We combine the results with those for two galaxies studied previously with the same technique, with Milky Way data, and with estimates obtained using photometric techniques. The results show that sigma(z)/sigma(R) is generally in the range 0.5-0.8. There is a marginally significant trend of decreasing sigma(z)/sigma(R) with advancing Hubble type, consistent with the predictions of disk heating theories. However, the errors on individual measurements are large, and the absence of any trend is consistent with the data at the 1 sigma level. As a by-product of our study, we find that three of the four galaxies in our sample have a central drop in their stellar line-of-sight velocity dispersion, a phenomenon that is increasingly observed in spiral galaxies.