Possibility of Determining Intergalactic Gas Density by Radio Observations of Flares of Remote Sources
DETERMINATION of the mean gas concentration in intergalactic space (N mg) is an important problem which is still far from being solved. One may hope finally to determine the value of N mg in different indirect ways (see, for example, ref. 1). At the same time there is a possibility of finding N mg directly by measuring the radio signal retardation time τ(ν) = t (ν) - t (∞) for a frequency ν, where t (∞) is the time of the arrival of a signal of a rather high frequency. In a completely ionized gas the refractive index n = 1-(e 2 N/2πmν2), the group velocity u = cn and where L is the distance to the signal source assuming (1 - n)<<1 the accuracy of the expressions used is clear (for example, from ref. 2). For our Galaxy, at not very small galactic latitudes, ∫N gdl≡N g L g~(1020 to 1021) cm-2. Therefore at N mg~10-5 cm-3 the product N mg L>N g L g if L>1025 to 1026 cm. Thus, for extragalactic sources at more than 3 to 30 Mpc signal retardation in intergalactic space at N>~10-5 is greater than in the Galaxy and could be measured (I note, as an example, that at N~10-5, ν~3 × 107 Hz and L~1025, the time τ~100 s). To apply this method, which is widely used in the case of pulsars3, there must, however, exist rapidly varying and powerful extragalactic radio emission sources; these have not yet been found.