The shape of galaxy disks : how the scale height increases with galactocentric distance ⋆

Letter to the Editor The shape of galaxy disks: how the scale height increases with galactocentric distance Abstract. We present the results of a detailed study of vertical surface brightness profiles of edge-on disk galaxies. Although the exponential disk scale height is constant to first order approximation, we show that for the large majority of galaxies in our sample, the scale height increases with distance along the major axis. The effect is strongest for early-type galaxies, where the increase of the scale height can be as much as a factor of 1.5 per scale-length, but is almost 0 for the latest-type galaxies. The effect can be understood if early-type disk galaxies have thick disks with both scale lengths and scale heights larger than those of the dominant disk component. Its origin appears to be linked to the processes that have formed the thick disk. The behaviour of the thickness of galactic disks is of major importance for understanding the evolution of disk galaxies. Since the exponential vertical scale height can be related directly to the vertical velocity dispersion in (isothermal) disks (e.g., Bahcall & Casertano, 1984), we may be able to constrain models that describe the dynam-ical heating mechanisms of galactic disks by studying this parameter. Van der Kruit & Searle (1981a,b; 1982) found, for their sample of edge-on spirals, that the vertical scale parameter , z 0 = 2 h z , is in good approximation independent of position along the major axis. Later studies of NGC have confirmed this result. From two-dimensional modeling of a sample of 10 edge-on spiral and lenticular galaxies, Shaw & Gilmore (1990) found that the radial variation of scale heights is typically within ± 3% of the derived mean for the main disk component, with no obvious dependence on colour or model type adopted. 1.1. Why should the scale height be constant? Van der Kruit & Searle (1982), following a suggestion by Fall, pointed out that the scale height would be constant during the secular evolution of a galactic disk if: – the disk is continuously heated by, e.g., the random acceleration of the disk stars by giant molecular clouds or spiral structure (Spitzer & Schwarzschild, 1951). – at all times the star formation rate is proportional to the surface number density of the giant molecular clouds. In this case the radial distribution of both the vertical velocity dispersion and the surface mass …