Charting our Uncharted Milky Way

Abstract

Our position in the Milky Way, buried within its disk, makes it extraordinarily difficult to piece together the structure of our home Galaxy. We know the Milky Way is a barred spiral but many questions, including the precise number, location, and prominence of spiral features remain debated. Towards the goal of better understanding star formation and the structure of our Milky Way, we present four avenues of research developed to map the molecular gas and dust in the Galaxy. Specifically, using a combination of extraordinarily elongated gaseous filaments, numerical simulations of Milky Way analogs, 3D dust mapping of our solar neighborhood, and 4D spatial-kinematic views of individual star-forming regions, we are beginning to build new models of our Milky Way's interstellar medium both locally and towards the inner Galaxy. Towards the inner Galaxy, we systematically characterize the physical properties of the largest-scale filaments in the interstellar medium. We find that the diversity in their physical properties likely reflects different formation mechanisms and evolutionary histories, with the longest and densest filaments most likely to trace the Galaxy's gross spiral structure in position-position-velocity space. By producing synthetic observations of comparable filaments forming in an AREPO simulation of a Milky Way-like galaxy, we find that while large-scale filaments preferentially form in the mid-plane of the galaxy, additional physics (stellar feedback, magnetic fields) is needed to reproduce the range of observations. Within the solar neighborhood, we use 3D dust mapping techniques in combination with stellar distances from Gaia DR2 to produce the largest uniform catalog of accurate distances to local molecular clouds. Comparison with "gold-standard" maser distances obtained from VLBI observations indicate agreement to within 10%, with no systematic offsets out to 2.5 kpc. Using this new catalog, we present the discovery of a 2.7 kpc long coherent arrangement of stellar nurseries, which undulates about the Galactic plane with an amplitude of 160 pc and appears to be the Local Arm of our Galaxy nearby. Extensions of the 3D dust mapping technique applied to a single cloud in this structure demonstrate that 3D spatial views of dust can be knitted together with kinematic information from gas to create 4D views of the local interstellar medium. Ultimately, we plan to build on these and complementary techniques to produce an integrated 3D model of our Milky Way's stars, gas, and dust out to 6 kpc in the coming years.