The mysterious $6565\,\AA$ absorption feature of the Galactic halo

Abstract

We consider various possible scenarios to explain the recent observation of a claimed broad H$\alpha$ absorption in our Galactic halo with peak optical depth $\tau \simeq 0.01$ and equivalent width $W \simeq 0.17 \, \rm \AA$. We show that the absorbed feature cannot arise from the circumgalactic and ISM H$\alpha$ absorption. As the observed absorption feature is quite broad $\Delta\lambda \simeq 30 \, \rm \AA$, we also consider CNO lines that lie close to H$\alpha$ as possible alternatives to explain the feature. We show such lines could also not account for the observed feature. Instead, we suggest that it can arise from diffuse interstellar bands (DIBs) carriers or polyaromatic hydrocarbons (PAHs) absorption. While we identify several such lines close to the H$\alpha$ transition, we are unable to determine the molecule responsible for the observed feature, partly because of selection effects that prevents us from identifying DIBs/PAHs features close to H$\alpha$ using local observations. Deep integration on a few extragalactic sources with high spectral resolution might allow us to distinguish between different possible explanations.