Interstellar electron density

Abstract

We impose the requirement that the spatial distribution of pulsars deduced from their dispersion measures using a model of the galactic electron density (ne) should be consistent with cylindrical symmetry around the galactic centre (assumed to be 10 kpc from the Sun). Using a carefully selected sub-sample of the pulsars detected by the I1 Molonglo Survey (I1 MS), we test a number of simple models and conclude that (i) the effective mean (ne) for the whole galaxy is 0-03728:::; cm-3, (ii) the scale height of electrons is greater than 300 pc and probably about 1 kpc or more, and (iii) there is little evidence for variation of ne with galactic radius RGC for RGC ≥ 5 kpc. Further, we make a detailed analysis of the contribution to ne from HII regions. Combining the results of a number of relatively independent calculations, we propose a model for the galactic electron density of the form ne (z) = 0.030 + 00020 exp (- 1 z 1 170) cm4 where z(pc) is the height above the galactic plane and the second term describes the contribution from HII regions. We believe the statistical uncertainties in the parameters of this model are quite small.