Probing the Properties of Neutral Clouds in the Vicinity of the Local Bubble: Linking three-dimensional Dust with Neutral Hydrogen Absorption
The Local Bubble (LB) contains significant internal structure, with many interstellar clouds lying within or on its walls. By observing time variability of atomic neutral hydrogen (HI) absorption profiles in the direction of pulsars, it was suggested that some of the HI structures exhibit variations in optical depth representative of the tiny scale atomic structure (TSAS). However, the exact location of these structures has not been known and understanding their origin in the framework of LB formation and evolution has been impossible. We use three-dimensional dust maps to extract density profiles of the hydrogen volume density along the line-of-sight (LOS) to four selected pulsars. We attempt to correlate individual density components with the Gaussian components of the HI absorption spectra, revealing the TSAS towards B1929+10 likely lies outside of the LB. Comparing column density images integrated over the entire LOS and over sub-volumes of the 3D dust maps show a mixture of diffuse and clumpy structure. Using estimates of the mean HI volume density and maximum total neutral hydrogen density along the LOS, we place upper and lower limits on ionisation fraction and thermal pressure, respectively, to characterise the properties of the local ISM towards each pulsar. Our method of using HI absorption data in conjunction with three-dimensional dust data holds potential for broader applications, especially to validate the structures and properties of dust clouds in the LB and beyond.