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The evolution of the Milky Way disc
II. Constraints from star counts at the galactic poles
M. Haywood 1,
A.C. Robin 2, 3 and
M. Crézé 3
Received 5 April 1996 / Accepted 9 May 1996
Classical models of the stellar populations in our Galaxy based on exponential density laws turn out to be unsuitable to match comprehensively recent data from deep surveys at a broad variety of wavelengths. Actually, we show that such models may generate spurious interpretations of deep star-count data, resulting in biaised estimates of the density characteristics of each population. Instead, a fully synthetic approach must take into account the time dependence of observable star properties as well as the dynamical constraints relating age, kinematics and space distributions. We present here comparisons of colour magnitude star-count data with the predictions of models in which the age of disc stars works as an explicit parameter connecting the star's intrinsic properties with the dynamical properties of the generation it belongs to. Hence, the history of star formation, the initial mass function, and the heating rate of the stellar disc can be tested against their respective impact on star-count data.
Star counts at the galactic pole combined with the more local data investigated in Paper I strongly favour star formation scenarios with constant or increasing SFR, while those implying a maximum of star formation activity in the early disc followed by a substantial decrease are excluded. The slope of the IMF in the mass range 1 to 3 is found between 1.8 and 2.0 (associated with a constant SFR). Such an IMF does not produce efficient gas recycling, as a result the gas infall required to key the SFR nearly constant must be about 3.5 .pc-2.Gyr-1.
The vertical density distribution of disc stars decreases much faster than the conventionnal 300-350 pc exponential scale height and simple valued exponentials used in classical models are not an adequate description of the density laws. Instead, the dynamical consistent density laws adopted in the Besancon model do fit available magnitude and colour distributions of stars over a very wide range of magnitudes. It is also shown that the thick disc contribution to faint star counts has been heavily underestimated.
The whole dynamical equilibrium and fit to local and non local data
is obtained under a scenario involving a moderate contribution of
unseen matter to the local potential (local surface density of less
than 8 .pc-2) and rather limited
increase of the velocity dispersion of disc stars with age
( reached after more than 3 Gyrs).
Key words: Galaxy: evolution Galaxy: stellar content Galaxy: structure
Send offprint requests to: M. Haywood
Online publication: June 30, 1998