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

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▶ “gas density” with a limited

resolution1. thermo-dynamical density n → excitation2. averaged gas density <r> → mass in a volume

Gas density structure▶ Geometrical approach

■ High resolution mapping▶ Statistical approach

■ Gas density histogram■ “Probability Density Function”

steady stateuniform condition

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▶ 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)

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

Integrated intensity maps▶ Distribution on the sky

12CO(2-1)

13CO(2-1)

180 150 120 90 60 30

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]

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▶ 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

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▶ 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

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

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)

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 histogram▶ Gas density – volume in MWG

■ fairly well fit by log-normal■ slight depression at high density end

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

Optical depth correction▶ Gas density – volume in MWG

■ 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■ → 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

GDH with different models▶ Still log-normal like

variable XCO

infinitly thick disk only near subcentral

Galactic constants

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

Nearby galaxies▶ Sample: Nobeyama CO atlas

■ Nobeyama 45m telescope■ 12CO(1-0)

▶ Gas “Column” Density Histogram

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

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▶ 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■ lognormal type: ~24/40

■ Non-lognormal type: ~16/40

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▶ 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]