| Abstract | In this paper the surface brightness of the Galactic background in the Pioneer 10 background starlight experiment is analysed in terms of recent models for the distribution of stars in the Galaxy. Only areas of low Galactic extinction are used and the data are corrected for contributions of diffuse Galactic light. This restricts the analysis to those areas of sky with ∣b∣ > 20°. The procedure provides important, independent information on the photometric parameters of the old disk population. Use of a vertical scaleheight of the old disk dwarfs of 325 ± 25 pc leads to a radial scalelength of 5.5 ± 1.0 kpc. The colour index of the background starlight at high latitudes provides an indication that the scaleheight of the disk giants is similar to that of the old disk dwarfs. The local surface brightness of the Galactic disk is 23.8 ± O.1B-mag arcsec-2 = 20 ± 2Lsun,B pc-2, of which two-thirds is provided by the old disk population. The old disk has a central surface brightness of 22.5 ± 0.3B-mag arcsec-2 = 67 ± 20 Lsun,B pc-2 a colour index (B - V) = 0.95 ± 0.15 and a total luminosity of (1.1 ± 0.2) 1O10 Lsun,B. If the young population I has the same scalelength of 5.5 kpc these values for the total disk become respectively 22.1 ± 0.3B-mag arcsec-2 = 95 ± 30 Lsun,B pc-2, 0.84 ± 0.15 and (1.8 ± 0.3) 1010 Lsun,B. The effects of the longer scalelength on the mass models is discussed. A model is illustrated for which the disk has a central mass surface density of 375 ± 100 Msun pc-2, (M/L)old disk = 6.5 ± 2 Msun/Lsun,B and a total mass of(7 ± 2) × 10Msun. Within 25 kpc from the centre the corona contains about 2.5 times as much mass as the luminous material. Evidence is discussed that suggests that our Galaxy is of type Sb. Comparison is made of the scalelength and photometric dimension of the Galaxy with external galaxies for which detailed surface photometry is available. For a Hubble constant of 100 km s-1 Mpc-1 our Galaxy would be the largest spiral in the Local Supercluster; it is comparable in size to the largest spirals in the Virgo cluster if the distance to Virgo is 22 ± 6 Mpc or H = 60 ± 17 km s-1 Mpc-1. Essentially the same result is obtained when these parameters for M 31 are used, namely H = 65 ± 10km s-1. Comparison of the model with star counts obtained in the plan of Selected Areas early this century shows that - although the magnitude scales are significantly in error at faint magnitudes - the relative counts across the sky are in excellent agreement with the model. Comparison to Herschel's counts two centuries ago show that these go consistently as faint as V ̃ 15. |
