The Milky Way Compared to External Galaxies

Abstract

Kapteyn’s work to map the Sidereal System was based on star counts. Now that we know that the Stellar System is comparable to many external spiral galaxies, this information can be used to further constrain models for the structure of our Galaxy. It is now possible to also use surface brightness distributions, both in the optical from the Pioneer spacecrafts and in the near-infrared with the COBE/DIRBE experiment. Star counts and surface photometry are discussed in a comparative sense. It turns out, that at least away from directions close to the Galactic center, surface brightness is dominated by the contribution from the disk, while faint star counts are dominated by the halo population. If low Galactic latitudes are ignored, surface brightness data can be used to constrain the parameters of the old stellar disk.

I repeat an earlier discussion in which older star counts are compared with current models for the distribution of stars in the Galactic disk. Herschel, in his famous “Star Gauges” of 1785, counted stars rather consistently to a visual magnitude of about 15. This is consistent with Herschel’s own values for the “gage or space penetrating powers” of his telescopes and his “extent of telescopic vision”. Counts early this century by Kapteyn and van Rhijn seem to have significantly incorrect magnitude scales compared to the prediction of models, in the sense of being too bright by half a magnitude or more at photograpic magnitude 18.5. Yet, the counts are consistent across the sky and with limits on current parameters for the Galactic disk.

A discussion is given of the value of the exponential scalelength of the Galactic disk. A value of 4.5 to 5.0 kpc is still preferred in spite of smaller values from surveys in the near-infrared. Finally, I show that as far as structural parameters are concerned, NGC 891, 5033 and 5375 closely resemble the Milky Way Galaxy.