2015A&A...580A.112B

To study the atomic, molecular, and ionized emission of giant molecular clouds (GMCs) in the Milky Way, we initiated a large program with the Karl G. Jansky Very Large Array (VLA): ``THOR: The HI, OH, Recombination line survey of the Milky Way''. We map the 21cm HI line, 4 OH lines, up to 19 Hα recombination lines and the continuum from 1 to 2GHz of a significant fraction of the Milky Way (l=15°-67°, |b|≤1°) at an angular resolution of ∼20". Starting in 2012, as a pilot study we mapped 4 square degrees of the GMC associated with the W43 star formation complex. The rest of the THOR survey area was observed during 2013 and 2014. In this paper, we 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. This approach might yield reasonable results for regions of low-mass star formation, however, it is not sufficient to describe GMCs. We analyzed strong continuum sources to measure the optical depth along the line of sight, and thus correct the HI 21cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the HI mass of this region more accurately and our analysis reveals a lower limit for the HI mass of M=6.6_–1.8x10⁶M_☉ (v_LSR=60-120km/s), which is a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The HI column densities are as high as N_HI∼150M_☉/pc2≃1.9x10²²cm^–2, which is an order of magnitude higher than for low-mass star formation regions. This result challenges theoretical models that predict a threshold for the HI column density of ∼10M_☉/pc², at which the formation of molecular hydrogen should set in. By assuming an elliptical layered structure for W43, we estimate the particle density profile. For the atomic gas particle density, we find a linear decrease toward the center of W43 with values decreasing from n_HI=20cm^–3 near the cloud edge to almost 0/cm3 at its center. On the other hand, the molecular hydrogen, traced via dust observations with the Herschel Space Observatory, shows an exponential increase toward the center with densities increasing to n_H2>200cm^–3, averaged over a region of ∼10pc. While atomic and molecular hydrogen are well mixed at the cloud edge, the center of the cloud is strongly dominated by H₂ emission. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results, which challenge current theoretical models, are an important characterization of the atomic to molecular hydrogen transition in an extreme environment.