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