Ionized gas towards the Galactic centre - constraints from low-frequency recombination lines

Abstract

Observations of the H272 alpha recombination line towards the Galactic center show features near V(LSR) = 0, -50 and +36 km/s. The parameters of these features are combined with the available H166 alpha measurements to obtain the properties of the ionized gas present along the line of sight and also in the '3 kpc arm'. For the line-of-sight ionized gas, an electron density of about 7 cu cm and a pathlength through it about 10-60 pc.The emission measure and the electron temperature are in the range of 500-2900 pc/cm exp 6 and 2000-6000 K respectively. The ionized gas in the 3 kpc arm has an electron density of 30 cu cm and extends over 9 pc along the line of sight if an electron temperature of 10,000 K is assumed. Using the available upper limit to the intensity of the H351 alpha recombination line, it is shown that the distributed ionized gas responsible for the dispersion of pulsar signals should have a temperature of greater than 4500 K and a minimum filling factor of 20 percent.