Characterizing gas flows through observations of the disk-circumgalactic medium interface with the Habitable Worlds Observatory

How gas gets into, through, and out of galaxies is critical to understanding galactic ecosystems. The disk-circumgalactic medium (CGM) interface region is uniquely suited for studying processes that drive gas flows. Matter and energy that enter and leave a galaxy pass through this region; however, the precise pathways are yet to be explored. We discuss future observations that will facilitate the discovery of the gas flow pathways in galaxies and the telescope parameters necessary for making those observations. We advocate for high spectral resolution ultraviolet spectroscopic capabilities on the Habitable Worlds Observatory (HWO) that will enable observations at a wavelength range of 940 to 3500 Å (minimum range 970 to 3000 Å) and at a resolution of 100,000 (minimum of 50,000). We advocate for a multi-object spectrograph with thousands of sub-arcsec slitlets and a field of view 6′×6′. We also recommend that the spectrograph be sensitive enough to achieve a signal-to-noise ratio of 10 or higher within a few hours for a continuum source of 21 AB magnitude and estimate an optimal aperture size of 8 m. These capabilities would enable the characterization of gas in the disk-halo interface, leading to breakthroughs in our understanding of the gas flows and galactic ecosystems.