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Astron. Astrophys. 355, 929-948 (2000)

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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:

  1. the infalling gas has a lower angular momentum than the circular motions in the disc, and mixing with the gas in the disc induces a net radial inflow with a velocity up to a few km sec-1;

  2. viscosity in the gas layer induces radial inflows in the inner parts of the disc and outflows in the outer parts, with velocities of [FORMULA]0.1 km sec-1;

  3. gravitational interactions between gas and spiral density waves lead to large-scale shocks, dissipation and therefore radial inflows of gas (or outflows in the outer parts) with typical velocities of [FORMULA]0.3 km sec-1 (e.g. Bertin & Lin 1996 and references therein); much larger velocities can be achieved in the inner few kpc in the presence of a barred potential.

In summary, radial flows are plausible with velocities of [FORMULA]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.

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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