Density of warm ionized gas near the Galactic Center: Low radio frequency observations

TL;DR

This study uses low-frequency radio observations to analyze the scattering and absorption of signals near the Galactic Center, revealing that actual scattering is less severe than models predicted and that the absorbing medium is patchy.

Contribution

First detailed low-frequency survey of the Galactic Center region showing discrepancies with existing electron distribution models and revealing the patchy nature of the ionized medium.

Findings

Scattering sizes decrease linearly with distance from the GC.

Observed scattering is over an order of magnitude less than NE2001 model predictions.

Most extragalactic sources are not scatter broadened at 1.4 GHz despite hyperstrong scattering predictions.

Abstract

We have observed the Galactic Center (GC) region at 0.154 and 0.255 GHz with the GMRT. A total of 62 compact likely extragalactic sources are detected. Their scattering sizes go down linearly with increasing angular distance from the GC up to about 1 deg. The apparent scattering sizes of sources are more than an order of magnitude down than predicted earlier by the NE2001 model of Galactic electron distribution within 359.5 deg < l < 0.5 deg and -0.5 deg <b <0.5 deg (Hyperstrong scattering region) of the Galaxy. High free-free optical depths are observed towards most of the extended nonthermal sources within 0.6 deg from the GC. Significant variation of optical depth indicate the absorbing medium is patchy at an angular scale of 10' and electron density is ~10 per cc that matches with the NE2001 model. This model predicts the extragalactic (EG) sources to be resolved out from 1.4 GHz interferometric surveys. However, 8 likely EG sources out of 10 expected in the region are present in 1.4 GHz catalog. Ionized interfaces of dense molecular clouds to the ambient medium are most likely responsible for strong scattering and low radio frequency absorption. However, dense GC clouds traced by CS $J=1-0$ emission are found to have a narrow distribution of ~0.2 deg across the Galactic plane. Angular distribution of most of the EG sources seen through the so called Hyperstrong scattering region are random in $b$, and typically ~7 out of 10 sources will not be seen through to the dense molecular clouds, and it explains why most of them are not scatter broadened at 1.4 GHz.