Smallest GMC Structures Resolved in CO Absorption by ALMA Jin Koda - PowerPoint PPT Presentation

Smallest GMC Structures Resolved in CO Absorption by ALMA Jin Koda Stony Brook University Sabbatical Last year: National Astronomical Observatory of Japan & Joint ALMA Observatory Collaborators: Nick Scoville (Caltech), Tsuyoshi Sawada (NAOJ, JAO), Sachiko Onodera (Meisei U.), Tetsuo Hasegawa, Seiichi Sakamoto (NAOJ) Contact Scientist: Daniel Espada

Evolution of Gas in Galaxies Energy cascade Inner gas-rich part Spiral arm gradients Small à large à small ~1kpc; ~20-30km/s molecular clouds Cloud-cloud ~300pc; ~10km/s Cloud internal ~40pc; ~4km/s Clumps w/i cloud ~1pc; ~1km/s Outer gas-poor part Sound speed at ~10K <<1pc; ~0.2km/s “HI à H2 à HI” phase transition (old textbook picture) Cloud structures at this smallest scale? Observationally, NOT detected yet. Synthesis of literature works (see Koda, Scoville & Heyer 2016)

Cloud Structure: Continuous or Droplets? Why should we care? Because it may affect, for example: cooling (through density), star formation timescale (droplet collisions?) Molecular absorptions toward compact QSOs with ALMA give highest spatial & velocity res. (a) Continuous medium ALMA/QSO NRO45 Emission <10miliarcsec Velocity ALMA/QSO Absorption NRO45 ~15arcsec Velocity (b) Clumpy medium ALMA/QSO NRO45 Emission <10miliarcsec Velocity ALMA/QSO Absorption NRO45 ~15arcsec Velocity

Two QSOs directly behind MW Spatial resolution limited by the sizes of the QSOs <10milliarcsec J1924+1540 (l,b)~(50.63, -0.03 ) VLBA images Size < 10 milli-arcsec ~ 100AU at 10kpc J1851+0035 (l,b)~(33.50, +0.19 ) Churchwell et al. 2009

Observation Parameters Molecles Transition Resolution NRO45 Emission CO, 13CO, C18O J=1-0 15“ 0.34 km/s ALMA Absorption CO, 13CO, C18O J=1-0 & 2-1 <10milliarcsec ~0.04 km/s Nobeyama 45m telescope (NRO45): ~15 arcsec beam ~0.07 pc ~0.4 pc ~0.7 pc Distance 1kpc 5kpc 10kpc ~0.00005 pc ~0.0002 pc ~0.0005 pc ~10 AU ~50 AU ~100 AU ALMA+QSO: <~ 10 mili-arcsec Velocity Resolution Resolve sound speed of ~10K gas (~0.2km/s) Spatial Resolution Trace ~10-100 AU scale structures

Churchwell et al. 2009 Emission Profiles from NRO45 CO(1-0), 13CO(1-0), C18O(1-0) J1924+1540: Tmb [K] J1851+0035 Tmb [K] Velocity [km/s] Sample several molecular clouds along velocity

NRO45 Emission & ALMA Absorption Emission (NRO45) ~15arcsec T mb [K] Absorption (ALMA) <~10mas Exp(-τ) Note: NRO45 12CO(1-0) Emission * 0.2 for plots

Case A: τ 12CO(1-0) >>1 Heart of cloud? Case τ 12CO(1-0) 13CO Absorption 12CO&13CO Emission A >>1 Present Present (Nobeyama 45m) T mb [K] Emission 12CO Absorption Exp(-τ) (ALMA) 13CO 18CO

Case B: τ 12CO(1-0) ~1 Cloud edge? Case τ 12CO(1-0) 13CO Absorption 12CO&13CO Emission B ~1 Absent Present (Nobeyama 45m) T mb [K] Emission 12CO Absorption Exp(-τ) (ALMA) 13CO 18CO

Case C: τ 12CO(1-0) ~0 Cloud edge? Case τ 12CO(1-0) 13CO Absorption 12CO&13CO Emission C ~0 Absent Present (Nobeyama 45m) T mb [K] Emission 12CO Absorption Exp(-τ) (ALMA) 13CO 18CO

Case A: τ CO(1-0) >>1 CO Saturated, but Multiple Droplets in 13CO 13CO emission & τ 13CO profiles different à Spatial variations w/i NRO45 beam CO saturated ( τ 12CO(1-0) >>1) à molecular gas between droplets as well 1/5*CO(1-0) Emission 13CO(1-0) Emission τ 13CO(1-0) smoothed τ 13CO(1-0)

Case A: τ CO(1-0) >>1 Multi-component Gaussian Fit Velocity Disp.~ Sound Speed à Droplets supported by thermal pressure, not by turbulent pressure τ0 v0 dv [km/s] [km/s] τ 13CO(1-0) # 1 0.17 10.52 0.10 # 2 0.15 10.84 0.15 # 3 0.11 12.35 0.16 # 4 0.05 12.91 0.08 # 5 0.11 12.35 0.82 # 6 0.05 14.07 0.21 # 7 0.41 16.05 0.20 # 8 0.30 16.43 0.15 # 9 0.22 16.88 0.18 Velocity [km/s] 8km/s Native ~0.04km/s resolution

Excitation Temperature (T ex ) from τ 21 /τ 10 The uncertainty of beam filling factor is NOT a problem. 10-100 AU resolution justifies One-zone approx. & LTE assumption Absorption Coefficient LTE ⎡ ⎤ ⎛ ⎞ h ν J + 1, J h ν J + 1, J g J + 1 c 2 ⎢ ⎥ α ν J + 1, J d ν J + 1, J = ( n J B J , J + 1 − n J + 1 B J + 1, J ) = A J + 1, J 1 − exp − 2 n J ⎜ ⎟ 4 π 8 πν J + 1, J g J kT ex ⎢ ⎥ ⎝ ⎠ ⎣ ⎦ ∫ α 21 ds τ 21 ⎡ ⎤ ⎛ ⎞ ⎛ ⎞ τ 21 = 2exp − h ν 10 / k ⎟ 1 + exp − h ν 10 / k = ⎢ ⎥ ⎜ ⎜ ⎟ ∫ τ 10 τ 10 T ex T ex ⎝ ⎠ ⎝ ⎠ α 10 ds ⎢ ⎥ ⎣ ⎦ 3 J + 1 A J + 1, J ∝ ν J + 1, J − h ν 10 / k g J + 1 T ex = ( ) /2 g J = 2 J + 1 ⎡ ⎤ 2 τ 21 / τ 10 + 1 − 1 ln ⎡ ⎤ h ν J + 1, J n J + 1 g J + 1 ⎢ ⎥ ⎣ ⎦ = exp − ⎢ ⎥ n J g J kT ex ⎢ ⎥ ⎣ ⎦

Case A: τ CO(1-0) >>1 Case B: τ CO(1-0) ~1 Excitation Temperature from Absorption Temperature ~4-6K – colder than typically assumed ~10K CO(1-0) emission & absorption Temperature [K] 10K 8K 6K 6K 4K 4K T CMB ~2.7K 2K Velocity [km/s]

Case A: τ CO(1-0) >>1 Excitation Temperature (from Emission) Ambient CO gas filling volume between droplets T CMB ~ 3 K Optically-thick CMB )(1 − e −τ ) e -τ à 0 T mb = f beam ( T ex − T T mb = f area ( T ex − T CMB ) CO(1-0) T mb [K] Absorption T ex ~ 4-6 K Exp(-τ) Left figure T mb ~ 2-5 K T mb [K] CO(1-0) f beam ≈ 1 Exp(-τ) CO emission filling Optically-thick along velocity entire space of NRO45 beam

Case A: τ CO(1-0) >>1 Sound-speed droplets seen in 13CO absorption T ex ~4-6 K Extended component between droplets see in 12CO T ex ~4-6 K

Case B: τ CO(1-0) ~1 Case C: τ CO(1-0) ~0 Case B: τ 12CO(1-0) ~1, Case C: τ 12CO(1-0) ~0 Similar analysis à Droplets exist in CO absorption; ambient gas at very low level (Nobeyama 45m) T mb [K] Emission 12CO Absorption Exp(-τ) (ALMA) 13CO 18CO

Case B ( τ 12CO(2-1) ~1 ) & Case C ( ~0 ) Droplets seen in 12CO Only little inter-droplet gas near the edge

Synthesis as Summary Smallest Structures in Molecular Clouds Explored with ALMA Sound-speed droplets Extended gas seen in 13CO absorption between 13CO clumps T ex ~4-6K T ex ~4-6 K Case A Case B Case C Droplets seen in Only little inter-droplet 12CO absorption gas near the edge