FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope (FUGIN). VII. molecular fraction of HI clouds

Abstract

In this study, we analyze molecular gas formation in neutral atomic hydrogen (HI) clouds using the latest CO data obtained from the four-beam receiver system on a 45-m telescope (FOREST) unbiased Galactic plane imaging survey with the Nobeyama 45-m telescope (FUGIN) and HI data taken from the Very Large Array (VLA) Galactic plane survey (VGPS). We applied a dendrogram algorithm to the HI data cube to identify HI clouds, and we calculated the HI mass and molecular gas mass by summing the CO line intensity within each HI cloud. On the basis of the results, we created a catalog of 5,737 identified HI clouds with local standard of rest (LSR) velocity of $V_{\mathrm{LSR}}\le -20$ km s${}^{-1}$ in Galactic longitude and latitude ranges of $20^{\circ }\le l\le 50^{\circ }$ and $-1^{\circ }\le b\le 1^{\circ }$, respectively. We found that most of the HI clouds are distributed within a Galactocentric distance of 16 kpc, most of which are in the Cold Neutral Medium (CNM) phase. In addition, we determined that the high-mass end of the mass HI function is well fitted with the power-law function with an index of 2.3. Although two sequences of self-gravitating and diffuse clouds are expected to appear in the M${}_{\mathrm{tot}}$-M${}_{{\mathrm{H}}_2}$ diagram according to previous works based on a plane-parallel model, the observational data show only a single sequence with large scattering within these two sequences. This implies that most of the clouds are mixtures of these two types of clouds. Moreover, we suggest the following scenario of molecular gas formation: An HI-dominant cloud evolved with increasing H${}_2$ mass along a path of $M_{{\mathrm{H}}_2}\propto M_{\mathrm{tot}}^2$ by collecting diffuse gas before reaching and moving along the curves of the two sequences.

arXiv comment13 pages, 11 figures and 2 tables

References in corpus (24)

• The Milky Way in Molecular Clouds: A New Complete CO Survey
• Structural Analysis of Molecular Clouds: Dendrograms
• The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• The VLA Galactic Plane Survey
• An Origin of Supersonic Motions in Interstellar Clouds
• Mass models of the Milky Way
• Simulating the formation of molecular clouds. II. Rapid formation from turbulent initial conditions
• Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: II. The Molecular Gas Disk
• Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: I. The HI Disk
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) I: Project Overview and Initial Results
• The Atomic-to-Molecular Transition in Galaxies. III. A New Method for Determining the Molecular Content of Primordial and Dusty Clouds
• Formation of Turbulent and Magnetized Molecular Clouds via Accretion Flows of HI Clouds
• Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: III. The Total Neutral Gas Disk
• The Vertical Distribution and Kinematics of HI and Mass Models of the Galactic Disk
• Cold Milky Way Hi gas in filaments
• Structure and Stability of Phase Transition Layers in the Interstellar Medium
• Extensive [CI] Mapping toward the Orion-A Molecular Cloud
• Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: IV. 3D Molecular Fraction and Galactic-Scale HI-to-H${}_2$ Transition
• Environmental Effects on Gaseous Disks of the Virgo Spiral Galaxies
• Stacking Analysis of 12CO and 13CO Spectra of NGC3627: Existence of non-optically thick 12CO emission?
• Toward Understanding the Origin of Turbulence in Molecular Clouds: Small Scale Structures as Units of Dynamical Multi-Phase Interstellar Medium

Cited by in corpus (4)

• The "Maggie" filament: Physical properties of a giant atomic cloud
• Nobeyama 45 m Local Spur CO survey. I. Giant molecular filaments and cluster formation in the Vulpecula OB association
• Molecular Fraction in the Galactic Center: The Central Molecular and HI Zones
• Ammonia mapping observations of the Galactic infrared bubble N49: Three NH${}_3$ clumps along the molecular filament