However, unlike Galactic sources such as W3(OH), the emission is unpolarized and the 1667 MHz line is stronger than the 1665 MHz line.
but how is this possible? Does not the 1665 MHz line and 1667 MHz line correspond to a particular transition and dipole moment? Won't emitted photons naturally have a polarization?
Furthermore, they state:
The characteristics of the λ18 cm OH mega-maser emission differ from those of Galactic maser sources, such as the main line intensity ratio (T1667MHz/T1665 MHz > 1), large linewidth (>100 km s^−1), and unpolarized emission.
But why is unpolarized emission expected from extragalactic masers (arising near AGN) and not galactic ones (arising in circumstellar and interstellar environments)?