天の川銀河の分子ガスの密度 頻度
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天の川銀河の分子ガスの密度頻度
半田利弘 ( 鹿児島大学 )
天の川銀河研究会2012/9/6@ 鹿児島大学
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星間ガスと物質循環
▶ 星間物質■ 星間ガス
電離ガス、中性原子ガス、分子ガス■ 星間塵
▶ 星形成の母胎■ 宇宙での物質循環■ 「希薄な星間ガス」から「星」へ
▶ 天の川銀河内での様子を調べる■ 分布■ 物理的性質(温度、密度)
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Gas density: 2 concepts▶ ISM has a fine structure.
■ sub-cloud scale structure▶ “gas density” with a limited
resolution1. thermo-dynamical density n → excitation2. averaged gas density <r> → mass in a volume
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Gas density structure▶ Geometrical approach
■ High resolution mapping▶ Statistical approach
■ Gas density histogram■ “Probability Density Function”
steady stateuniform condition
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Previous works▶ Column density
■ star forming regions■ a whole galaxy: LMC in HI
▶ Volume density■ HI & HII in MWG
Wada et al. 2000
Berkhuijsen & Fletcher 2008
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AMANOGAWA-2SB survey▶ 12CO (2-1) & 13CO (2-1) survey▶ with AMANOGAWA telescope
■ Dish: 60 cm, Beamsize: 9 arcmin■ RX: 2SB = waveguide sideband-separating SIS
simultaneous observations in both linesTsys=120 K @ zenith
■ Spectrometer: AOS
Nakajima et al. (2007)
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Survey specifications▶ The Galactic plane
■ Grid spacing: 7.5’■ Velocity resolution: 1.3 km s-1
■ Noise level: ~0.05 K
■ grid and velocity resolution = Colombia surveyDame et al. 2001
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Integrated intensity maps▶ Distribution on the sky
12CO(2-1)
13CO(2-1)
180 150 120 90 60 30
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l-v diagrams▶ Longitude-velocity diagrams12CO(2-1)
180 150 120 90 60 30Galactic Longitude [deg]
+100km/s
13CO(2-1)
+100km/s
180 150 120 90 60 30
Galactic Longitude [deg]
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samples▶ In this talk, data for 5o<l<90o, |b|<5o
■ to reduce bias by the local clouds
12CO(2-1)
180 150 120 90 60 30
Galactic Longitude [deg]
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CO intensity correlations▶ 12CO(2-1) vs 12CO(1-0)
■ Ratio<1.0 → subthermally excited▶ 12CO(2-1) vs 13CO(2-1)
■ Optical depth effect
12CO(2-1)
12CO(1-0) 12CO(2-1)
13CO(2-1)R12/1-0=0.64±0.058
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Gas density histogram▶ Statistics of averaged gas density
■ Relative volume in Msun pc-3 bin
▶ Conversion from observational data■ Line intensity → molecular gas mass■ Line velocity → distance & geometrical depth
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Conversion: volume▶ Distance estimation of each voxel
■ The kinetic distance v → d■ Cross section area in the beam W d= A
▶ Depth of each voxel■ Differential of the kinetic distance Dv → Dd
▶ Volume of each voxel■ V= W d Dd
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Conversion: mass density▶ Molecular gas mass
■ XCO=1.8x1020 cm-2/(K km s-1) Dame et al. 2001■ Typical intensity ratio T12, T13 → T1-0
Intensity correlation / simple excitation■ N(H2)=XCO ∫T dv → M(H2)
▶ Volume of each voxel■ V= W d Dd
▶ Molecular gas density in Msun pc-3
■ r =M/V
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XCO for 3 CO lines▶ for 12CO(2-1)
■ Observed standard ratio R12/1-0=0.64■ X12= X1-0 /R12/1-0=2.9x1020 cm-2 /(K km s-1)
▶ for 13CO(2-1) ■ assumptions
LTE with 10 Koptically thin 13CO(2-1)abundance 12CO/13CO=60, 12CO/H2=4.3x10-5
■ X13= 1.1x1021 cm-2 /(K km s-1)
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Kinetic model of MWG▶ The pure circular rotating disk▶ with IAU standard kinematics
■ Q0=220km s-1, R0=8.5kpc
▶ Geometrical thickness of Gal. disk■ assume: gas is confined in a ±100pc uniform disk■ not include the far side volume beyond z>100pc
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Gas density histogram▶ Gas density – volume in MWG
■ fairly well fit by log-normal■ slight depression at high density end
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Simple empirical relations ▶ Only simple radiation transfer eq.
■ TMB,13=η13 Tc,13 (1-exp(-τ13)); TMB,12=η12 Tc,12
▶ Linear relations■ (η13 Tc,13)/(η12 Tc,12)=α; η13 Tc,13=β τ13 ■ α, β : 2 constants
■ Tc,13 → typical τ13
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Optical depth correction▶ Gas density – volume in MWG
■ t-corrected : well fit by log-normal
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Model dependence▶ Galactic constants (recent VLBI obs.)
■ W0=Q0/R0=30 km s-1 kpc-1 Nagayama et al. 2010■ → Q0=210km s-1, R0=7kpc
▶ Radial variation of XCO■ X1-0=1.4x1020 exp(r/11) Arimoto et al. 1996
▶ Thickness of the galactic disk■ without any consideration (infinite thick disk)
▶ Reject local gas near the Sun■ only Vfar<100 Vnear
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GDH with different models▶ Still log-normal like
variable XCO
infinitly thick disk only near subcentral
Galactic constants
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Why log-normal?Vazquez-Semadeni 1994
▶ 密度:直前の密度を増幅・減衰する過程■ ランダムな増幅度決定←乱流 ?■ 増幅度は直前の密度の値によらない■ 多数の変化
▶ この場合の現在の密度は…■ ρ = ρ0 f1 f2 f3 … fn
■ よって、 logρ = log ρ0 +log f1 +log f2 … +log fn
▶中心極限定理から log ρは正規分布
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Nearby galaxies▶ Sample: Nobeyama CO atlas
■ Nobeyama 45m telescope■ 12CO(1-0)
▶ Gas “Column” Density Histogram
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Nobeyama CO atlas▶ 12CO (1-0) survey▶ with Nobeyama 45m telescope
■ beamsize: 15 arcsec■ RX: BEARS (25 beam SIS)■ Spectrometer: AOS
Kuno et al. 2007
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Sample galaxies▶ 40 spiral galaxies Kuno et al. 2007
■ morphology: Sa-Sc■ distance: d<25Mpc■ inclination: i<70deg (face-on)■ IRAS 100um flux >10Jy■ no/less interacting
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method▶ ICO(1-0) → N(H2)
■ using XCO=1.8x1020 cm-2/(K km s-1) Dame et al. 2001
▶ Inclination correction■ assume a disk with constant thickness
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Results■ lognormal type: ~24/40
■ Non-lognormal type: ~16/40
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What controll GDH shape?▶ correlation coefficient
■ compare with some parameters▶ observational effect?
■ N(pixel), linear resolution, noise level, inclination■ No correlation → not due to obs. effects
▶ other obs. property of galaxy?■ morphology(SA/SB), molecular mass ■ No correlation → to study more!
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summary▶ H2 density histogram over MWG
■ observational counter part of PDF▶ Some galaxies shows log-normal,
although about 40% do not.
logr=-2.0[Msun pc-3], s=0.80[dex]