CO (J = 2-1) line observations of the galactic center molecular cloud complex. II. Dynamical structure and physical conditions

Tomoharu Oka, Tetsuo Hasegawa, Masahiko Hayashi, Toshihiro Handa, Seiichi Sakamoto

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Abstract

A large-scale 12C16O (J = 2-1) survey of the inner few hundred parsecs of the Galaxy has been conducted using the University of Tokyo-Nobeyama Radio Observatory 60 cm survey telescope. We have taken about 700 12C16O (J = 2-1) spectra in the region -2°.5 ≤ l ≤ 2°.5 and |b| ≤ 1° with 0°.125 grid spacing, covering the entire region of the huge molecular cloud complex in the Galactic center. We refer to the CO (J = 1-0) data taken with the Columbia 1.2 m telescope and calculate the J = 2-1 to J = 1-0 intensity ratio. Velocity channel maps and longitude-velocity maps of CO (J = 2-1) line are presented, with corresponding maps of J = 2-1/J = 1-0 intensity ratio. Large-scale CO maps enable us to identify several giant molecular cloud complexes and many characteristic features of molecular gas. We identify 15 molecular cloud complexes larger than ∼30 pc in our CO (J = 2-1) data. Their virial masses are at least 1 order of magnitude larger than the masses estimated from the CO luminosity. This discrepancy can be removed if we notice that they may not be gravitationally bound but are in pressure equilibrium with the hot gas and/or magnetic field in this region. Using the expressions of virial mass and CO mass for a cloud in the pressure equilibrium case, we get the X-factor for the Galactic center molecular clouds as X = 0.24 × 1020 cm-2 (K km s-1)-1, which is 1 order of magnitude lower than that in the Galactic disk (X = 3.0 × 1020 cm-2 [K km s-1]-1). We estimate the total molecular mass in the Galactic center as M/(H2) = 2 × 107 M as a lower limit; the actual total gas mass within the central 400 pc of the Galaxy must be M/(H2) = (2-6) × 107 M. We diagnose the physical conditions of the molecular gas in the Galactic center using the intensity ratio between the J = 2-1 and J = 1-0 lines. Although the CO J = 2-1/J = 1-0 line intensity ratio is high (∼0.74) in the midplane, molecular gas at |b| ≥ 0°.25 exhibits low J = 2-1/J = 1-0 ratios (∼0.6). The overall J = 2-1/J = 1-0 luminosity ratio is R(2-1)/(1-0) = 0.64 ± 0.01 if we include all the emission within |b| ≤ 1°, -2°.5 ≤ / ≤ 2°.5, and

Original languageEnglish
Pages (from-to)730-761
Number of pages32
JournalAstrophysical Journal
Volume493
Issue number2 PART I
DOIs
Publication statusPublished - 1998
Externally publishedYes

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molecular clouds
molecular gases
gas
luminosity
telescopes
galaxies
Columbia (Orbiter)
high temperature gases
longitude
physical conditions
observatories
spacing
coverings
observatory
grids
radio
magnetic field
estimates
gases
magnetic fields

Keywords

  • Galaxy: center
  • ISM: clouds
  • ISM: molecules
  • Radio lines: ISM
  • Surveys

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

CO (J = 2-1) line observations of the galactic center molecular cloud complex. II. Dynamical structure and physical conditions. / Oka, Tomoharu; Hasegawa, Tetsuo; Hayashi, Masahiko; Handa, Toshihiro; Sakamoto, Seiichi.

In: Astrophysical Journal, Vol. 493, No. 2 PART I, 1998, p. 730-761.

Research output: Contribution to journalArticle

Oka, Tomoharu ; Hasegawa, Tetsuo ; Hayashi, Masahiko ; Handa, Toshihiro ; Sakamoto, Seiichi. / CO (J = 2-1) line observations of the galactic center molecular cloud complex. II. Dynamical structure and physical conditions. In: Astrophysical Journal. 1998 ; Vol. 493, No. 2 PART I. pp. 730-761.
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AU - Oka, Tomoharu

AU - Hasegawa, Tetsuo

AU - Hayashi, Masahiko

AU - Handa, Toshihiro

AU - Sakamoto, Seiichi

PY - 1998

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N2 - A large-scale 12C16O (J = 2-1) survey of the inner few hundred parsecs of the Galaxy has been conducted using the University of Tokyo-Nobeyama Radio Observatory 60 cm survey telescope. We have taken about 700 12C16O (J = 2-1) spectra in the region -2°.5 ≤ l ≤ 2°.5 and |b| ≤ 1° with 0°.125 grid spacing, covering the entire region of the huge molecular cloud complex in the Galactic center. We refer to the CO (J = 1-0) data taken with the Columbia 1.2 m telescope and calculate the J = 2-1 to J = 1-0 intensity ratio. Velocity channel maps and longitude-velocity maps of CO (J = 2-1) line are presented, with corresponding maps of J = 2-1/J = 1-0 intensity ratio. Large-scale CO maps enable us to identify several giant molecular cloud complexes and many characteristic features of molecular gas. We identify 15 molecular cloud complexes larger than ∼30 pc in our CO (J = 2-1) data. Their virial masses are at least 1 order of magnitude larger than the masses estimated from the CO luminosity. This discrepancy can be removed if we notice that they may not be gravitationally bound but are in pressure equilibrium with the hot gas and/or magnetic field in this region. Using the expressions of virial mass and CO mass for a cloud in the pressure equilibrium case, we get the X-factor for the Galactic center molecular clouds as X = 0.24 × 1020 cm-2 (K km s-1)-1, which is 1 order of magnitude lower than that in the Galactic disk (X = 3.0 × 1020 cm-2 [K km s-1]-1). We estimate the total molecular mass in the Galactic center as M/(H2) = 2 × 107 M⊙ as a lower limit; the actual total gas mass within the central 400 pc of the Galaxy must be M/(H2) = (2-6) × 107 M⊙. We diagnose the physical conditions of the molecular gas in the Galactic center using the intensity ratio between the J = 2-1 and J = 1-0 lines. Although the CO J = 2-1/J = 1-0 line intensity ratio is high (∼0.74) in the midplane, molecular gas at |b| ≥ 0°.25 exhibits low J = 2-1/J = 1-0 ratios (∼0.6). The overall J = 2-1/J = 1-0 luminosity ratio is R(2-1)/(1-0) = 0.64 ± 0.01 if we include all the emission within |b| ≤ 1°, -2°.5 ≤ / ≤ 2°.5, and

AB - A large-scale 12C16O (J = 2-1) survey of the inner few hundred parsecs of the Galaxy has been conducted using the University of Tokyo-Nobeyama Radio Observatory 60 cm survey telescope. We have taken about 700 12C16O (J = 2-1) spectra in the region -2°.5 ≤ l ≤ 2°.5 and |b| ≤ 1° with 0°.125 grid spacing, covering the entire region of the huge molecular cloud complex in the Galactic center. We refer to the CO (J = 1-0) data taken with the Columbia 1.2 m telescope and calculate the J = 2-1 to J = 1-0 intensity ratio. Velocity channel maps and longitude-velocity maps of CO (J = 2-1) line are presented, with corresponding maps of J = 2-1/J = 1-0 intensity ratio. Large-scale CO maps enable us to identify several giant molecular cloud complexes and many characteristic features of molecular gas. We identify 15 molecular cloud complexes larger than ∼30 pc in our CO (J = 2-1) data. Their virial masses are at least 1 order of magnitude larger than the masses estimated from the CO luminosity. This discrepancy can be removed if we notice that they may not be gravitationally bound but are in pressure equilibrium with the hot gas and/or magnetic field in this region. Using the expressions of virial mass and CO mass for a cloud in the pressure equilibrium case, we get the X-factor for the Galactic center molecular clouds as X = 0.24 × 1020 cm-2 (K km s-1)-1, which is 1 order of magnitude lower than that in the Galactic disk (X = 3.0 × 1020 cm-2 [K km s-1]-1). We estimate the total molecular mass in the Galactic center as M/(H2) = 2 × 107 M⊙ as a lower limit; the actual total gas mass within the central 400 pc of the Galaxy must be M/(H2) = (2-6) × 107 M⊙. We diagnose the physical conditions of the molecular gas in the Galactic center using the intensity ratio between the J = 2-1 and J = 1-0 lines. Although the CO J = 2-1/J = 1-0 line intensity ratio is high (∼0.74) in the midplane, molecular gas at |b| ≥ 0°.25 exhibits low J = 2-1/J = 1-0 ratios (∼0.6). The overall J = 2-1/J = 1-0 luminosity ratio is R(2-1)/(1-0) = 0.64 ± 0.01 if we include all the emission within |b| ≤ 1°, -2°.5 ≤ / ≤ 2°.5, and

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KW - Radio lines: ISM

KW - Surveys

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