THE RADIAL SCALE LENGTH OF THE MILKY-WAY

The radial scale length of the exponential component of the disc of th e Milky Way has been determined in the near infrared. We have used the TMGS (Two Micron Galactic Survey) database which contains positions a nd K-magnitudes of about 700.000 stars distributed in serveral regions along the galactic plane. From those we have selected areas more than 5 degrees off the plane to minimize the effect of extinction and the contributions of the young disc; and with longitudes ranging from 30 d egrees to 70 degrees, to avoid contaminations from the central bulge, the bar and the molecular ring, in the inner end, and from the local a rm, the warp and the truncation of the disc, in the outer end. We obse rved stars with magnitude 9 less than or equal to m(K) less than or eq ual to 10. The use of the NIR K-band also reduces the effect of extinc tion. In the observation region, m(K) = 9.5 mag stars are K2-K5 III st ars, with an absolute magnitude that is nearly constant, which also gr eatly simplifies the problem. We have obtained the value 2.1+/-0.3 kpc for the radial scale length, which isa typical value when compared wi th external galaxies of similar types.