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Abstract

In this thesis, I present a new non-parametric model for inferring the three-dimensional (3D) distribution of dust density in the Milky Way. Our approach uses the extinction measured towards stars at different locations in the Galaxy at known distances. Each extinction measurement is proportional to the integrated dust density along its line of sight (l.o.s). Making simple assumptions about the spatial correlation of the dust density, we infer the most probable 3D distribution of dust across the entire observed region, including along sight lines which were not observed. This is possible because our model employs a Gaussian process to connect all l.o.s. The result is a smooth, 3D map of the dust density, which is the local property of the interstellar medium (ISM) rather than an integrated quantity. Owing to our smoothness constraint and its isotropy, the method provides one of the first maps without “fingers of God” artefact. I then present the first continuous map of the dust distribution in the Galactic disk out to 7 kpc within 100 pc of the Galactic midplane, using red giant stars from SDSS APOGEE DR14. The resulting map traces some features of the local Galactic spiral arms, even though the model contains no prior suggestion of spiral arms, nor any underlying model for the Galactic structure. This is the first time that such evident arm structures have been captured by a dust density map in the Milky Way. Our resulting map also traces some of the known giant molecular clouds in the Galaxy and puts some constraints on their distances, some of which were hitherto relatively uncertain. I also demonstrate a map of the 3D distribution of dust in the Orion complex. Orion is the closest site of high-mass star formation, making it an excellent laboratory for studying the ISM and star formation. We use data from the Gaia-TGAS catalogue combined with photometry from 2MASS and WISE to get the distances and extinctions of individual stars in the vicinity of the Orion complex. We find that the distance and depth of the cloud are compatible with other recent works, which show that the method can be applied to local molecular clouds to map their 3D dust distribution. We also use data from the recent second Gaia data release (GDR2) to update the map that shows complex dust clouds in the Orion region. I finally show a 3D map of hydrogen density in the local ISM. The hydrogen equivalent column densities were obtained from the Exploring the X-ray Transient and variable Sky project (EXTRAS), which provides equivalent NH values from X-ray spectral fits of observations within the XMM-Newton Data Release. A cross-correlation between the EXTRAS catalogue and the first Gaia Data Release was performed in order to obtain accurate parallax and distance measurements. The resulting map shows small-scale density structures which can not be modelled using analytic density profiles.