2. Radial flows: previous literature
The possibility that radial flows play a role in establishing the radial metallicity gradients in galactic discs was first suggested by Tinsley & Larson (1978). Following Lacey & Fall (1985), we mention that radial gas flows in a disc can be driven by three main mechanisms:
In summary, radial flows are plausible with velocities of 0.1-1 km sec-1, and they are expected to be inflows over most of the disc. Observational upper limits permit radial inflows in the Galactic Disc with velocities up to 5 km sec-1 at the present time. For further details, see Lacey & Fall (1985) and references therein.
The first of the above mentioned mechanisms was modelled in detail by Mayor & Vigroux (1981), and later by Pitts & Tayler (1989, 1996), Chamcham & Tayler (1994). The effects of a generic inflow velocity profile on chemical evolution models has been explored by Lacey & Fall (1985), Tosi(1988), Götz & Köppen (1992), Köppen (1994), Edmunds & Greenhow (1995). A different approach is that of viscous disc models which, rather than imposing arbitrary radial velocity patterns, describe the evolution of the gas distribution in the disc self-consistently, following the model suggested by Lin & Pringle (1987). Viscous chemical models have been developed by Clarke (1989), Yoshii & Sommer-Larsen (1989) and Sommer-Larsen & Yoshii (1990), Thon & Meusinger (1998). All these studies show how radial inflows can steepen the metallicity gradients with respect to static models, especially if an outer cut-off of SF is assumed.
© European Southern Observatory (ESO) 2000
Online publication: March 21, 2000