2025MNRAS.538.2508K

We present a gas phase decomposition of the deep 21 cm H I spectrum of the target PKS1934-638 observed by the Australia Telescope Compact Array. Our measurements achieve $\sigma _{\tau } = 5.9 \times 10^{-4}$ making this one of the deepest 21 cm H I absorption spectra to date. Using the GaussPy package, we conducted a two phase Gaussian decomposition on the absorption spectrum measured on target and the associated GASS emission spectra at varying angular offsets. The spectral pairs were decomposed independently and permuted through all possible line-of-sight configurations to determine a weighted solution set of Gaussian components. Of the components derived, we identify two candidates in absorption that are likely unstable or warm, with spin temperatures $T_s$ = $1300\pm 100\, \rm {K}, 680\pm 70\, \rm {K}$. Cumulatively, these features represent a mass fraction of $\mathscr{f}_{\rm {UNM}} = 0.59\pm 0.22$ which is higher than has typically been reported. We also detect a component in emission only, with a possible detection of absorption, at $T_{k,{\rm max}} = 7610\pm 65\, \rm {K}$. The UNM is typically difficult to detect in absorption due to it's low optical depth, our results would make the detections among the few examples currently known. We believe our high mass fraction is attributable to the high optical depth sensitivity of our observations allowing marginal warm gas detections in absorption to come into prominence, and possibly also an atypical line of sight.