Interstellar scattering near the galactic center

Abstract

A previously obtained dependence of the scattering angles of extragalactic radio sources on the background emissionmeasure toward the sources is used together with data on the background brightness temperature near the Galactic center at 43 and 10 GHz to calculate the scattering angles of compact extragalactic radio sources as a function of their angular separation from the compact source Sgr A*. It is shown that the distribution of the scattering angles of extragalactic sources as a function of the angular distance to Sgr A* has two scales. The extended component has an angular size at the half-intensity level of about 1° in longitude and about 0.5° in latitude. The calculated scattering angles for this component agree well with the observations. The narrow component is fairly symmetric and has a scale at the half-intensity level of about 5′. The scattering angle at the maximum of the narrow component is approximately a factor of 10 larger than the scattering angle for the broad component. The scattering angle begins to increase sharply at angular distances less than 6′ from Sgr A*. It is suggested that the narrow component is associated with the 7′ halo located in the Sgr A complex. A VLA survey of compact sources at 1400 MHz shows that compact extragalactic sources are not observed at angular distances of less than 14′ from Sgr A*, due to both enhanced scattering and a decrease in sensitivity due to the strong background emission with approach toward the Galactic center and the presence of strong compact sources in the sidelobes of the VLA beam, in particular, Sgr A*. The results obtained can be used to choose the optimal baselines and frequencies of interferometers in future observations of compact sources near the Galactic center.