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天の川銀河の分子ガスの密度 頻度

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 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）. Gas density: 2 concepts. ISM has a fine structure. sub-cloud scale structure

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天の川銀河の分子ガスの密度 頻度

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  1. 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)

  2. 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）

  3. Gas density: 2 concepts • ISM has a fine structure. • sub-cloud scale structure • “gas density” with a limited resolution • thermo-dynamical density n→ excitation • averaged gas density <r> → mass in a volume

  4. Gas density structure • Geometrical approach • High resolution mapping • Statistical approach • Gas density histogram • “Probability Density Function” • steady state • uniform condition

  5. 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

  6. 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 lines • Tsys=120 K @ zenith • Spectrometer: AOS Nakajima et al. (2007)

  7. 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 survey Dame et al. 2001

  8. Integrated intensity maps • Distribution on the sky 12CO(2-1) 180 150 120 90 60 30 13CO(2-1)

  9. l-v diagrams • Longitude-velocity diagrams 12CO(2-1) +100km/s +100km/s 13CO(2-1) 180 150 120 90 60 30 Galactic Longitude [deg] 180 150 120 90 60 30 Galactic Longitude [deg]

  10. 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]

  11. CO intensity correlations • 12CO(2-1) vs12CO(1-0) • Ratio<1.0 →subthermally excited • 12CO(2-1) vs13CO(2-1) • Optical depth effect 12CO(2-1) 13CO(2-1) R12/1-0=0.64±0.058 12CO(1-0) 12CO(2-1)

  12. 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

  13. 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 distanceDv→Dd • Volume of each voxel • V= W dDd

  14. 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 ∫Tdv → M(H2) • Volume of each voxel • V= WdDd • Molecular gas density in Msun pc-3 • r =M/V

  15. XCO for 3 CO lines • for 12CO(2-1) • Observed standard ratio R12/1-0=0.64 • X12=X1-0 /R12/1-0=2.9x1020cm-2 /(K km s-1) • for 13CO(2-1) • assumptions • LTE with 10 K • optically thin 13CO(2-1) • abundance 12CO/13CO=60, 12CO/H2=4.3x10-5 • X13=1.1x1021 cm-2 /(K km s-1)

  16. 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

  17. Gas density histogram • Gas density – volume in MWG • fairly well fit by log-normal • slight depression at high density end

  18. Simple empirical relations • Only simple radiation transfer eq. • TMB,13=η13Tc,13 (1-exp(-τ13)); TMB,12=η12Tc,12 • Linear relations • (η13Tc,13)/(η12Tc,12)=α; η13Tc,13=βτ13 • α, β : 2 constants • Tc,13→ typical τ13

  19. Optical depth correction • Gas density – volume in MWG • t-corrected : well fit by log-normal

  20. Model dependence • Galactic constants (recent VLBI obs.) • W0=Q0/R0=30 km s-1 kpc-1Nagayama et al. 2010 • → Q0=210km s-1, R0=7kpc • Radial variation of XCO • X1-0=1.4x1020exp(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

  21. GDH with different models • Still log-normal like variable XCO Galactic constants infinitly thick disk only near subcentral

  22. 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 =ρ0f1f2f3 … fn • よって、logρ= log ρ0 +logf1 +logf2 … +logfn • 中心極限定理からlog ρは正規分布

  23. Nearby galaxies • Sample: Nobeyama CO atlas • Nobeyama 45m telescope • 12CO(1-0) • Gas “Column” Density Histogram

  24. Nobeyama CO atlas Kuno et al. 2007 • 12CO (1-0) survey • with Nobeyama 45m telescope • beamsize: 15 arcsec • RX: BEARS (25 beam SIS) • Spectrometer: AOS

  25. 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

  26. 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

  27. Results • lognormal type: ~24/40 • Non-lognormal type: ~16/40

  28. 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!

  29. 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]

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