The effects of the viewing angle on star formation in the Milky Way
Sara Rezaei Kh.
Wednesday, Dec. 7th, 9:40CET
Star formation properties of the molecular clouds are primarily derived from various observational quantities such as shape and column density. These observations, however, are limited to the 2D projected view of the clouds in the plane of the sky. Interstellar medium appears highly filamentary; specifically in regions of higher column densities which typically correspond to higher star formation rates. Nevertheless, there have been observations of molecular clouds such as California which show relatively low star formation efficiency despite its filamentary shape in the plane of the sky and its column densities in the same range as that of Orion A molecular cloud with an order of magnitude higher star formation rate. This raises the question of whether or not shape and column density are reliable tools to evaluate the star formation efficiency of the molecular clouds. We look into this subject via mapping the 3D structure of the clouds in dust using the latest Gaia data, combined with 2MASS and WISE photometry. We discover that, despite the apparent filamentary structure in the plane of the sky, California is a flat ~120-pc-long sheet extending along the line of sight. We show that not only do 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 expand our analysis to a larger sample of the local star-forming regions, and show how the clouds' surface area, mass, and surface density change significantly with different viewing angles. Our results underline the importance of 3D information in interpreting star formation relations and challenge studies that rely solely on the column density thresholds to determine star formation properties in molecular clouds.