Smallest GMC Structures Resolved in CO Absorption by ALMA

Transcription

1 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

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

3 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 <10miliarcsec NRO45 Emission ALMA/QSO Absorption Velocity NRO45 ~15arcsec Velocity (b) Clumpy medium ALMA/QSO <10miliarcsec NRO45 Emission ALMA/QSO Absorption Velocity NRO45 ~15arcsec Velocity

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

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

6 Emission Profiles from NRO45 Tmb [K] CO(1-0), 13CO(1-0), C18O(1-0) Tmb [K] Churchwell et al J : J Velocity [km/s] Sample several molecular clouds along velocity

7 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

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

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

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

11 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)

12 Case A: τ CO(1-0) >>1 Multi-component Gaussian Fit Velocity Disp.~ Sound Speedà Droplets supported by thermal pressure, not by turbulent pressure τ 13CO(1-0) Native ~0.04km/s resolution Velocity [km/s] 8km/s τ0 v0 dv [km/s] [km/s] # # # # # # # # #

13 Excitation Temperature (T ex ) from τ 21 /τ 10 The uncertainty of beam filling factor is NOT a problem AU resolution justifies One-zone approx. & LTE assumption Absorption Coefficient LTE α ν J+1,J dν J+1,J = hν J+1,J 4π (n J B J,J+1 n J+1 B J+1,J )= c 2 8πν J+1,J 2 n J A g J+1,J 1 exp hν J+1,J J kt ex g J+1 τ 21 τ 10 = α 21 ds α 10 ds τ 21 = 2exp hν /k 10 τ 10 T ex 1+ exp hν /k 10 T ex A J+1,J ν J+1,J 3 J +1 g J+1 n J+1 n J = g J+1 g J g J = 2J +1 exp hν J+1,J kt ex T ex = hν 10 /k ln ( 2τ 21 /τ )/2

14 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 4K 2K T CMB ~2.7K 6K 4K Velocity [km/s]

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

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

17 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 àdroplets exist in CO absorption; ambient gas at very low level Absorption (ALMA) Emission (Nobeyama 45m) Exp(-τ) T mb [K] 12CO 13CO 18CO

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

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