Three-dimensional Shape Explains Star Formation Mystery of California and Orion A

The new Gaia data release (EDR3) with improved astrometry has opened a new era in studying our Milky Way in fine detail. We use Gaia EDR3 astrometry together with 2MASS and WISE photometry to study two of the most massive molecular clouds in the solar vicinity: Orion A and California. Despite having remarkable similarities in the plane of the sky in terms of shape, size, and extinction, California has an order of magnitude lower star formation efficiency. We use our state-of-the-art dust mapping technique to derive the detailed three-dimensional (3D) structure of the two clouds, taking into account both distance and extinction uncertainties, and a full 3D spatial correlation between neighboring points. We discover that, despite the apparent filamentary structure in the plane of the sky, California is a flat 120 pc-long sheet extending from 410 to 530 pc. We show that not only Orion A and California differ substantially in their 3D shapes, but also Orion A has considerably higher density substructures in 3D than California. This result presents a compelling reason why the two clouds have different star formation activities. We also demonstrate how the viewing angle of California can substantially change the cloud\u27s position in the Kennicutt-Schmidt relation. This underlines the importance of 3D information in interpreting star formation relations and challenges studies that rely solely on the column density thresholds to determine star formation activities in molecular clouds. Finally, we provide accurate distance estimates to multiple lines of sight toward various parts of the two clouds

3D map of the dust distribution in the Milky Way

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

The effect of viewing angle on the Kennicutt-Schmidt relation of the local molecular clouds

The Gaia data give us an unprecedented view to the three-dimensional (3D) structure of molecular clouds in the solar neighbourhood. We study how the projected areas and masses of clouds, and consequently the Kennicutt-Schmidt (KS) relation, depend on the viewing angle. We derive the probability distributions of the projected areas and masses for nine clouds within 400 pc of the Sun using 3D dust distribution data from the literature. We find that the viewing angle can have a dramatic effect on the observed areas and masses of individual clouds. The joint probability distributions of the areas and masses are strongly correlated, relatively flat, and can show multiple peaks. The typical ranges and 50% quartiles of the distributions are roughly 100-200% and 20-80% of the median value, respectively, making viewing angle effects important for all individual clouds. The threshold value used to define the cloud areas is also important; our analysis suggests that the clouds become more anisotropic for smaller thresholds (larger scales). On average, the areas and masses of the plane-of-the-sky and face-on projections agree, albeit with a large scatter. This suggests that sample averages of areas and masses are relatively free of viewing angle effects, which is important to facilitate comparisons of extragalactic and Galactic data. Ultimately, our results demonstrate that a cloud\u27s location in the KS relation is affected by the viewing angle in a non-trivial manner. However, the KS relation of our sample as a whole is not strongly affected by these effects, because the covariance of the areas and masses causes the observed mean column density to remain relatively constant