1. Excitation of H2 in NGC 253 Marissa Rosenberg
Co-authors: Paul van der Werf and Frank Israel
Leiden Observatory, Netherlands

2. Overview Extragalactic molecular gas H2 excitation H2 in NGC 253
Comparison of H2 morphology
H2 diagnostic ratio
Comparison of shock and PDR models
Conclusions

3. Molecular Gas Excitation
Star formation happens in GMCs
H2 is most abundant
Many H2 vibrational transitions in NIR

4. Cold H2
But most H2 is cold and hard to directly observe!

5. Cold H2
Get around this by observing CO transitions.

6. Excitation of Hot H2 UV Excitation (Fluorescence) “Top Down”
Absorption of energetic photon
Cascade downwards emitting in NIR
Tgas = 10,000s K

7. Excitation of Hot H2 Shock Excitation “Bottom up”
Collisional excitation behind shock front
Cooling Lines emit in NIR
Tgas = 1000s K

8. Excitation of Hot H2 Degeneracy
If density is high, molecules are thermalized
Even if UV excitation is dominant, appears as if shock excited.

9. NGC 253 3.5 Mpc Barred Spiral Starburst ~0.5kpc Super Star Cluster
(Region 2)
Kinematic Center
(Region 3)
Pixel = 1.8 parsec

10. H2 in NGC 253
Say here,” So our main question is, “What is exciting the hot H2 gas in NGC 253”

11. Other Diagnostic Tracers
K Band Continuum
Continuum Emission -> dust
Traces underlying, older stellar population

12. Other Diagnostic Tracers
[FeII] 1.64 μm
Trapped in dust grains
Released into ISM by high v shocks (SNR)

13. Other Diagnostic Tracers
Brγ (HI 7->4)
λ < 912 Å, 13.6 eV
Traces most massive O stars
H2: λ < 1100 Å, 11.2 eV

14. Other Diagnostic Tracers
PAHs 3.3 μm
λ < 4200 Å, for NC>50
Traces slightly less massive O and B stars
ISAAC Continuum subtracted PAH 3.28 μm map

15. Mini Summary No morphological similarities between:
[FeII]  No SN excitation
Brγ  No excitation from Lyman photons
PAHs  Strong morphological correlation
Excitation by slightly less massive O and B stars
More quantitative measurement needed

16. Diagnostic Ratio 1-0 S(1) 2-1 S(1) 2-1 S(1)/1-0 S(1) v=1, Tex=6741 K
PDR Models: (Black & van Dishoeck 1987)
Shock Models: (Shull & Hollenback 1978)

17. Diagnostic Ratio

18. Diagnostic Ratio Important Constraint: Ratio < 0.2  55% of H2 flux
Exclude brightest Brγ flux regions
Maximum of 33% of gas is shock excited

19. Normalized Column Density
H2 Excitation Diagram
Normalized Column Density
Boltzmann Distribution with:
Upper Energy Level (103 K)

20. Shock Models Low Velocity Low Density Low Velocity High Density
Medium Velocity
Low Density
Medium Velocity
High Density
Shull & Hollenbach (1978)

21. PDR Models

22. Observations

23. Conclusion The hot H2 gas is predominately excited by fluorescence
Shocks may play a role in isolated regions
A maximum of 33% of the gas is shock excited
This result applies to molecular gas around the edge of molecular clouds, does not dictate excitation of gas inside the clouds.

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