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

Abstract

Inversion of interstellar (IS) gas or dust absorbing columns measured along the path to stars distributed in distance and direction allows reconstructing the distribution of interstellar matter (ISM) in three dimensions. A low resolution IS dust map based on reddening measurements towards 23,000 nearby stars is used to illustrate the potential of the more detailed maps that are expected within the next several years. The map reveals the location of the main IS cloud complexes up to distances on the order of 600 to 1200 pc depending on directions. Owing to target selection biases towards weakly reddened, brighter stars, the map is especially revealing in terms of regions devoid of IS matter. It traces the Local Bubble and its neighboring cavities, including a conspicuous, giant, ≥1000 pc long cavity in the third quadrant located beyond the so-called βMa tunnel. This cavity is bordered by the main constituents of the Gould belt, the well-known and still unexplained rotating and expanding ring of clouds and young stars, inclined by ~ 20° to the galactic plane. Comparing the dust distribution with X-ray emission maps and IS gas observations shows that the giant cavity contains a large fraction of warm, fully ionized and dust-poor gas in addition to million K, X-ray bright gas. This set of structures must reflect the main events that occurred in the past; today however even the formation of the Gould belt is still a matter of controversy.It has been suggested recently that the Cretaceus-Tertiary (KT) mass extinction is potentially due to a gamma-ray burst (GRB) that occurred in the massive globular cluster (GC) 47 Tuc during its close encounter with the Sun ~70 Myrs ago. Such a hypothesis is based on computations of the cluster and Sun trajectories and the frequency of short GRBs in GC's. 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. On the other hand, a burst must have produced huge ionization and radiation pressure effects on the ISM. Therefore, identifying (or not) the corresponding imprints should provide additional clues to the extinction source and the ISM history. Interestingly, first-order estimates suggest that the Gould belt dynamics and age could match the expected consequences of the cluster crossing, and that the giant ionized, dust-free cavity could be related to gas ionization and dust evaporation by an intense flux of hard radiation such as produced by a GRB. Moreover, dust-gas decoupling during the crossing and after the burst could have produce a highly inhomogeneous dust to gas ratio, potentially explaining the high variability and pattern of the D/H ratio in the gas phase. Future Gaia data should confirm or dismiss this hypothesis.