Multi-wavelength probes of the Milky Way's Cold Interstellar Medium: Radio HI and Optical KI Absorption with GASKAP and GALAH

Data and Analysis Notebooks for MNRAS Publication This document includes the data and analysis notebooks associated with the study entitled "Multi-wavelength probes of the Milky Way’s Cold Interstellar Medium: Radio HI and Optical KI Absorption with GASKAP and GALAH" (accepted for publication in the Monthly Notices of the Royal Astronomical Society, MNRAS). Title: Multi-wavelength probes of the Milky Way’s Cold Interstellar Medium: Radio HI and Optical KI Absorption with GASKAP and GALAH Abstract:We present a comparative analysis of interstellar hydrogen (HI) and potassium (KI) absorption from the radio and optical surveys, GASKAP and GALAH, 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 HI absorption with interstellar KI absorption detected in GALAH spectra of nearby stars (within 12 arcmin angular distance or a spatial separation of ∼0.75 pc), we reveal a strong kinematic correlation between these two tracers of the cold neutral ISM. The velocity offsets between matched HI and KI absorption components are small, with a mean (median) offset of −1.3 (−1.2) km s−1 and a standard deviation of 2.3 km s−1. The high degree of kinematic consistency suggests a close spatial association between Ki and cold HI gas. Correlation analyses reveal a moderate positive relationship between HI and KI line-of-sight properties, such as KI column density with HI column density or HI brightness temperature. We observe a ∼63% overlap in the detection of both species towards 290 (out of 462) GASKAP HI absorption lines of sight, and estimate a median KI/HI abundance ratio of ∼2.3×10^(−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 interstellar medium through its diverse tracers. AuthorsHiep Nguyen $^{1★}$Sven Buder 1,2Juan D. Soler 3N. M. McClure-Griffiths $^{1}$J. R. Dawson $^{4,5}$James Dempsey $^{1,6}$Helga Dénes $^{7}$John M. Dickey $^{8}$Ian Kemp $^{9}$Denis Leahy $^{10}$Min-Young Lee $^{11}$Callum Lynn $^{1}$Yik Ki Ma $^{12}$Antoine Marchal $^{1}$Marc-Antoine Miville-Deschênes $^{13}$Eric G. M. Muller $^{1}$Claire E. Murray $^{14,15}$Gyueun Park $^{11}$Nickolas M. Pingel $^{16}$Hilay Shah $^{1}$Snežana Stanimirović $^{16}$Jacco Th. van Loon $^{17}$ $^{1}$ Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia$^{2}$ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia$^{3}$ Istituto di Astrofisica e Planetologia Spaziali (IAPS). INAF. Via Fosso del Cavaliere 100, 00133 Roma, Italy$^{4}$ Department of Physics and Astronomy and MQ Research Centre in Astronomy, Astrophysics, and Astrophotonics, Macquarie University, NSW 2109, Australia$^{5}$ Australia Telescope National Facility, CSIRO Space and Astronomy, PO Box 76, Epping NSW 1710, Australia$^{6}$ CSIRO Information Management and Technology, GPO Box 1700 Canberra, ACT 2601, Australia$^{7}$ School of Physical Sciences and Nanotechnology, Yachay Tech University, Hacienda San José S/N, 100119, Urcuquí, Ecuador$^{8}$ School of Natural Sciences, Private Bag 37, University of Tasmania, Hobart, TAS, 7001, Australia$^{9}$ International Centre for Radio Astronomy Research (ICRAR), Curtin University, Bentley, WA 6102, Australia$^{10}$ Department of Physics and Astronomy, University of Calgary, Calgary, AB, Canada T2N 1N4$^{11}$ Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Daejeon 34055, Republic of Korea$^{12}$ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany$^{13}$ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, F-91191 Gif-sur-Yvette, France$^{14}$ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA$^{15}$ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA$^{16}$ Department of Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI 53703, USA$^{17}$ Lennard-Jones Laboratories, Keele University, ST5 5BG, UK