Multiwavelength probes of the Milky Way’s cold interstellar medium: radio H i and optical K i absorption with GASKAP and GALAH

We present a comparative analysis of interstellar hydrogen (H i) and potassium (K i) absorption from the radio and optical surveys, GASKAP (Galactic Australian Square Kilometre Array Pathfinder) and GALAH (Galactic Archaeology with HERMES), to study the physical and kinematic properties of the cold interstellar medium (ISM) in the Milky Way foreground towards the Magellanic Clouds. By comparing GASKAP H i absorption with interstellar K i absorption detected in GALAH spectra of nearby stars (within 12 arcmin angular distance or a spatial separation of $\sim$0.75 pc), we reveal a strong kinematic correlation between these two tracers of the cold neutral ISM. The velocity offsets between matched H i and K i absorption components are small, with a mean (median) offset of –1.3 (–1.2) $\mathrm{km\, s^{-1}}$ and a standard deviation of 2.3 $\mathrm{km\, s^{-1}}$. The high degree of kinematic consistency suggests a close spatial association between K i and cold H i gas. Correlation analyses reveal a moderate positive relationship between H i and K i line-of-sight properties, such as K i column density with H i column density or H i brightness temperature. We observe a $\sim$63 per cent overlap in the detection of both species towards 290 (out of 462) GASKAP H i absorption lines of sight, and estimate a median K i/H i abundance ratio of $\sim 2.3\times 10^{-10}$, in excellent agreement with previous findings. Our work opens up an exciting avenue of Galactic research that uses large-scale surveys in the radio and optical wavelengths to probe the neutral ISM through its diverse tracers.