Considerations with stacking absorption spectra: cold HI gas in cirrus region of the Milky Way

TL;DR

This study investigates how stacking multicomponent HI spectra affects measurements of optical depth and spin temperature, revealing that while optical depth estimates are lower limits, spin temperature remains reliably correlated with true values, and detects warm and cold neutral gas components.

Contribution

The paper demonstrates the effects of spectral stacking on HI measurements and provides new insights into the properties of cold and warm neutral gas in the Milky Way using GASKAP data.

Findings

Stacking produces lower limits for secondary component optical depths.

Spin temperature estimates are well correlated with true values despite stacking.

Detected warm neutral medium with a spin temperature of 1320 K.

Abstract

We use the Milky Way neutral hydrogen (HI) absorption and emission spectra from the Galactic Australian Square Kilometre Array Pathfinder (GASKAP) Phase II Pilot survey along with toy models to investigate the effects of stacking multicomponent spectra on measurements of peak optical depth and spin temperature. Shifting spectra by the peak in emission, 'primary' components shifted to 0 km s$^{-1}$ are correctly averaged. Additional components on individual sightlines are averaged with non-centred velocities, producing a broader and shallower 'secondary' component in the resulting stack. Peak optical depths and brightness temperatures of the secondary components from stacks are lower limits of their true average values due to the velocity offset of each component. The spin temperature however is well correlated with the truth since the velocity offset of components affects the emission and absorption spectra equally. Stacking 462 GASKAP absorption-emission spectral pairs, we detect a component with a spin temperature of 1320 $\pm$ 263 K, consistent with gas from the unstable neutral medium and higher than any previous GASKAP detection in this region. We also stack 2240 pilot survey spectra containing no Milky Way absorption, revealing a primary narrow and secondary broad component, with spin temperatures belonging to the cold neutral medium (CNM). Spatially binning and stacking the non-detections across the plane-of-sky by their distance from CNM absorption detections, the primary component's optical depth decreases with distance from known locations of cold gas. The spin temperature however remains stable in both components, over an approximate physical plane-of-sky distance of $\sim$ 100 pc.