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

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

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

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

Cold H2 Get around this by observing CO transitions.

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

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

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

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

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

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

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

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

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

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

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: 0.53-0.56 (Black & van Dishoeck 1987) Shock Models: 0.1-0.2 (Shull & Hollenback 1978)

Diagnostic Ratio

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

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

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

PDR Models

Observations

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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