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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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2644930

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)


2644930
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
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


Gas density structure
Gas density structure

  • Geometrical approach

    • High resolution mapping

  • Statistical approach

    • Gas density histogram

    • “Probability Density Function”

      • steady state

      • uniform condition


Previous works
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


Amanogawa 2sb survey
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)


Survey specifications
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


Integrated intensity maps
Integrated intensity maps

  • Distribution on the sky

12CO(2-1)

180 150 120 90 60 30

13CO(2-1)


L v diagrams
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]


Samples
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]


Co intensity correlations
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)


Gas density histogram
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


Conversion volume
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


Conversion mass density
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


X co for 3 co lines
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)


Kinetic model of mwg
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


Gas density histogram1
Gas density histogram

  • Gas density – volume in MWG

    • fairly well fit by log-normal

    • slight depression at high density end


Simple empirical relations
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


Optical depth correction
Optical depth correction

  • Gas density – volume in MWG

    • t-corrected : well fit by log-normal


Model dependence
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


Gdh with different models
GDH with different models

  • Still log-normal like

variable XCO

Galactic constants

infinitly thick disk

only near subcentral


Why log normal
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


  • Nearby galaxies
    Nearby galaxies

    • Sample: Nobeyama CO atlas

      • Nobeyama 45m telescope

      • 12CO(1-0)

    • Gas “Column” Density Histogram


    Nobeyama co atlas
    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


    Sample galaxies
    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


    Method
    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


    Results
    Results

    • lognormal type: ~24/40

    • Non-lognormal type: ~16/40


    What controll gdh shape
    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!


    Summary
    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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