3D maps of the local interstellar medium: searching for the imprints of past events

Abstract

Inversion of interstellar gas or dust columns measured along the path to stars distributed in distance and direction allows reconstructing the distribution of interstellar matter (ISM) in 3D. A low resolution IS dust map based on the reddening of 23,000 stars illustrates the potential of future maps. It reveals the location of the main IS clouds within $\sim$1kpc and, owing to biases towards weakly reddened targets, regions devoid of IS matter. It traces the Local Bubble and neighboring cavities, including a giant, $\geq$1000 pc long cavity located beyond the so-called $\beta$CMa tunnel, bordered by the main constituents of the Gould belt (GB), the rotating and expanding ring of clouds and young stars, inclined by $\sim$ 20$^{\circ}$ to the galactic plane. From comparison with diffuse X-ray background and absorption data it appears that the giant cavity is filled with warm, ionized and dust-poor gas in addition to million K gas. This set of structures must reflect the main events that occurred in the past. It has been suggested that the Cretaceus-Tertiary mass extinction may be due to a gamma-ray burst (GRB) in the massive globular cluster (GC) 47 Tuc during its close encounter with the Sun $\sim$70 Myrs ago. Given the mass, speed and size of 47 Tuc, wherever it crossed the Galactic plane it must have produced at the crossing site significant dynamical effects on the disk stars and IS clouds, and triggered star formation. Interestingly, first-order estimates suggest that the GB dynamics and age could match the consequences of the cluster crossing. Additionally, the giant ionized, dust-free cavity could be related to an intense flux of hard radiation, and dust-gas decoupling after the burst could explain the high variability and pattern of the D/H ratio in the nearby gaseous ISM. Future Gaia data should confirm or dismiss this hypothesis.