THOR - The HI, OH, Recombination Line Survey of the Milky Way - The pilot study: HI observations of the giant molecular cloud W43

Abstract

To study the atomic, molecular and ionized emission of Giant Molecular Clouds (GMCs), 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 H_alpha recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way (l=15-67deg, |b|<1deg) at ~20" resolution. In this paper, we focus on the HI emission from the W43 star-formation complex. Classically, the HI 21cm line is treated as optically thin with column densities calculated under this assumption. This might give 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, 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 W43 more accurately and our analysis reveals a lower limit of M=6.6x10^6 M_sun, 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)~150 M_sun/pc^2 ~ 1.9x10^22 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 HI column density of ~10 M_sun/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 profiles. While at the cloud edge atomic and molecular hydrogen are well mixed, the center of the cloud is strongly dominated by molecular hydrogen. 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 and challenges current theoretical models.

References in corpus (14)

• Theory of Star Formation
• Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• The VLA Galactic Plane Survey
• The WISE Catalog of Galactic HII Regions
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• CO-dark gas and molecular filaments in Milky Way type galaxies
• The Parallax of W43: a Massive Star Forming Complex near the Galactic Bar
• Magnetically Dominated Strands of Cold Hydrogen in the Riegel-Crutcher Cloud
• Optically thick HI dominant in the local interstellar medium; an alternative interpretation to "dark gas"
• The formation of the W43 complex: constraining its atomic-to-molecular transition and searching for colliding clouds
• The W43-MM1 mini-starburst ridge, a test for star formation efficiency models
• Carbon in different phases ([CII], [CI], and CO) in infrared dark clouds: Cloud formation signatures and carbon gas fractions
• Excitation temperature of the warm neutral medium as a new probe of the Lyman-α radiation field

Cited by in corpus (49)

• HI4PI: A full-sky HI survey based on EBHIS and GASS
• Cloud-Scale Molecular Gas Properties in 15 Nearby Galaxies
• The SILCC (SImulating the LifeCycle of molecular Clouds) project - II. Dynamical evolution of the supernova-driven ISM and the launching of outflows
• SILCC-Zoom: The dynamical and chemical evolution of molecular clouds
• CHIMPS: the ${}^{13}$CO/C${}^{18}$O (J=3-2) Heterodyne Inner Milky Way Plane Survey
• Physical Properties of Molecular Clouds at 2 parsec Resolution in the Low-Metallicity Dwarf Galaxy NGC 6822 and the Milky Way
• Analytic HI-to-H2 Photodissociation Transition Profiles
• Dust-Gas Scaling Relations and OH Abundance in the Galactic ISM
• Offset between stellar spiral arms and gas arms of the Milky Way
• The HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and HI overview
• The HI-to-H2 Transition in a Turbulent Medium
• Where is OH and Does It Trace the Dark Molecular Gas (DMG)?
• The Lognormal Probability Distribution Function of the Perseus Molecular Cloud: A Comparison of HI and Dust
• GLOSTAR -- Radio Source Catalog I: $28^{\circ }<\text{l}<36^{\circ }$ and $|b|<1^{\circ }$
• Continuum sources from the THOR survey between 1 and 2 GHz
• H2O Southern Galactic Plane Survey (HOPS): Paper III - Properties of Dense Molecular Gas across the Inner Milky Way
• The Multi-phase Turbulence Density Power Spectra in the Perseus Molecular Cloud
• Molecular gas and star formation in an absorption-selected galaxy: Hitting the bull's eye at z = 2.46
• The history of dynamics and stellar feedback revealed by the HI filamentary structure in the disk of the Milky Way
• Radio continuum emission in the northern Galactic plane: Sources and spectral indices from the THOR survey
• OH maser emission in the THOR survey of the northern Milky Way
• HI-to-H2 Transition Layers in the Star-Forming Region W43
• The Galactic dynamics revealed by the filamentary structure in atomic hydrogen emission
• Cloud formation in the atomic and molecular phase: HI self absorption (HISA) towards a Giant Molecular Filament
• Exploring the properties of warm and cold atomic hydrogen in the Taurus and Gemini regions
• Inferring the composition of super-Jupiter mass companions of pulsars with radio line spectroscopy
• The diffuse gamma-ray emission toward the Galactic mini starburst W43
• FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN). VI. Dense gas and mini-starbursts in the W43 giant molecular cloud complex
• On the accuracy of HI observations in molecular clouds -- More cold HI than thought?
• OH absorption in the first quadrant of the Milky Way as seen by THOR
• A lack of constraints on the cold opaque HI mass: HI spectra in M31 and M33 prefer multi-component models over a single cold opaque component
• The Interstellar Medium and Star Formation of Galactic Disks. I. ISM and GMC properties with Diffuse FUV and Cosmic Ray Backgrounds
• A Survey for Hydroxyl in the THOR Pilot Region around W43
• Atomic and molecular gas properties during cloud formation
• Dynamical effects of the radiative stellar feedback on the H I-to-H2 transition
• Large-scale Map of Millimeter-wavelength Hydrogen Radio Recombination Lines around a Young Massive Star Cluster
• The Carina Nebula and Gum 31 molecular complex: II. The distribution of the atomic gas revealed in unprecedented detail
• FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45-m Telescope (FUGIN) IV: Galactic Shock Wave and Molecular Bow Shock in the 4-kpc Arm of the Galaxy
• Mapping low frequency carbon radio recombination lines towards Cassiopeia A at 340, 148, 54 and 43 MHz
• A global view on star formation: The GLOSTAR Galactic plane survey. IX. Radio Source Catalog III: 2<l<28, 36<l<40, 56<l<60 and |b|<1, VLA B-configuration
• The "Maggie" filament: Physical properties of a giant atomic cloud
• Properties of atomic hydrogen gas in the Galactic plane from THOR 21-cm absorption spectra: a comparison with the high latitude gas
• The Carina Nebula and Gum 31 molecular complex III: The distribution of the 1-3 GHz radio continuum across the whole nebula
• Molecular Bow Shock in the 3-kpc Norma Arm
• Low-Frequency Radio Recombination Lines Away From the Inner Galactic Plane
• Cold atomic gas identified by HI self-absorption. Cold atomic clouds toward giant molecular filaments
• Exploring the pattern of the Galactic HI foreground of GRBs with the ATCA
• Hydrogen in diffuse molecular clouds in the Milky Way: Atomic column densities and molecular fraction along prominent lines of sight
• The Neutral Atomic Hydrogen in the solar neighborhood (NeAtHood) project I. Ghost in the shell: Neutral atomic hydrogen in the extended Orion nebula