Possibility of Determining Intergalactic Gas Density by Radio Observations of Flares of Remote Sources

Abstract

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−(e²N/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_gL_g∼(10²⁰ to 10²¹) cm⁻². Therefore at N_mg∼10⁻⁵ cm⁻³ the product N_mgL>N_gL_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.