Author(s): , , , , , ,
Institution(s): 1. Max Planck Institute for Astronomy, 2. Max-Planck-Institut fuer Radioastronomie, 3. NRAO
To study the atomic, molecular and ionised emission of Giant Molecular Clouds (GMCs) in the Milky Way, we have initiated a Large Program with the VLA: 'THOR - The HI, OH, Recombination Line survey of the Milky Way'. We map the 21cm HI line, 4 OH lines, 19 Hα recombination lines and the continuum from 1-2GHz of a significant fraction of the Milky Way (l=15-67°, |b|<1°) at an angular resolution of ~20’’. In my talk, I will focus on the HI emission from the W43 GMC complex. Classically, the HI 21cm line is treated as optically thin with properties such as the column density calculated under this assumption. While this approach gives reasonable results for regions of low-mass star-formation, it is not sufficient to describe the atomic gas in close proximity to GMCs. In my talk, I will present a method using strong continuum sources to measure the optical depth, and thus correct the HI 21cm emission for optical depth effects and weak diffuse continuum emission. Our analysis puts a lower limit of M~6.6x106 Msun on the HI mass associated with the W43 GMC, which is a factor of 2.4 larger than the mass obtained using the optically thin assumption. The HI column densities reach NHI~150 Msun pc-2 ~ 1.9x1022 cm-2, which is an order of magnitude higher than seen in low mass star formation regions. This result challenges theoretical models that predict a threshold for the HI column density of ~10 Msun pc-2, at which the formation of molecular hydrogen should set in. Furthermore, we assume an elliptical layered structure for W43 to estimate the particle density profile. The HI particle density shows a linear decrease toward the centre of W43 and the molecular hydrogen, traced via dust observations with Herschel, shows an exponential increase toward the centre. While at the cloud edge atomic and molecular hydrogen are well mixed, the centre of the cloud is dominated by H2. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results are an important characterization of the atomic to molecular hydrogen transition in an extreme environment that challenges current theoretical models.