THOR: The H i, OH, Recombination line survey of the Milky Way

Abstract

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 H , OH, Recombination line survey of the Milky Way". We map the 21 cm H  line, 4 OH lines, up to 19 Hα recombination lines and the continuum from 1 to 2 GHz 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 H  emission from the W43 GMC complex. Classically, the H  21 cm 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 H  21 cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the H  mass of this region more accurately and our analysis reveals a lower limit for the H  mass of M = 6.6 −1.8 × 10 6 M (v LSR = 60−120 km s −1 ), which is a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The H  column densities are as high as N H  ∼ 150 M pc −2 ≈ 1.9 × 10 22 cm −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 H  column density of ∼10 M pc −2 , 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 H  = 20 cm −3 near the cloud edge to almost 0 cm −3 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 H 2 > 200 cm −3 , averaged over a region of ∼10 pc. While atomic and molecular hydrogen are well mixed at the cloud edge, the center of the cloud is strongly dominated by H 2 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.

The Lognormal Probability Distribution Function of the Perseus Molecular Cloud: A Comparison of Hi and Dust

“…This is because hydrogen is converted from atomic to molecular (HI-to-H 2 transition) at N(HI) ∼ (9-11) × 10 20 cm −2 for Perseus . However, in general, the uniform HI column density distribution could also result from a large amount of optically thick HI gas (see Bihr et al 2015). The amount of the optically thick HI in Perseus was studied in Lee et al (2015).…”

“…In general, HI is an excellent tracer of low column density atomic gas in the far outskirts of molecular clouds. While it can be optically thick closer to giant molecular cloud (GMC) centers (see Bihr et al 2015), HI absorption measurements can be obtained to provide full column density information and to provide insight into the nature of the atomic to molecular transition in GMCs . In particular, we focus our study on the Perseus molecular cloud, whose atomic and molecular gas and dust content, as well as the HI-H 2 transition, has been well studied in a number of previous works (Lee et al 2012Stanimirović et al 2014;Lee et al 2015).…”

HI4PI: a full-sky H i survey based on EBHIS and GASS

“…H i absorption spectroscopy can in principle be used to overcome this shortcoming. For example, in the framework of the on-going THOR survey (The H i, OH, Recombination line survey of the Milky Way), Bihr et al (2015) calculate the optical depth toward the giant molecular cloud W 43 to estimate the true H i column density. Using Galactic continuum emitters for the H i absorption technique, however, has the intrinsic issue that only the fraction of H i gas in front of the continuum emitter can be accounted for.…”

Continuum sources from the THOR survey between 1 and 2 GHz

“…• ) during the 2012A semester (Project 12A-161, see also Bihr et al 2015). We used a hexagonal geometry for the mosaic for this 2…”

“…Starting in 2012 with a pilot study around the giant molecular cloud (GMC) associated with the W43 star formation complex (Bihr et al 2015;Walsh et al 2016), we observed a large fraction of the Galactic plane in the first quadrant of the Milky Way (l = 14−65…”