Theoretical SEDs in Starbursts: SFRs in both the UV and IR

1. Theoretical SEDs in Starbursts:SFRs in both the UV and IR • Brent Groves • Max Planck Institute for Astrophysics

2. Made By Many • Michael Dopita, Ralph Sutherland, Jörg Fischera (RSAA,ANU) • Cristina Popescu, Richard Tuffs (MPIK) • Lisa Kewley (IfA, Hawaii) • Michiel Reuland (Leiden) • & Claus Leitherer (STSCI)

3. Why Starbursts? • Starburst Galaxies form many stars very quickly and hence: • Can give insight into the Initial Mass Function (IMF) • Are very bright and can be seen at the earliest epochs helping us understand galaxy formation • Are dominated by the youngest and most massive stars • and many other reasons...

4. Why Models? • Understanding the Physics behind Starbursts • Provide Pan Spectral Diagnostics to determine general parameters of Star-forming Galaxies • Provide Self-consistent, Theoretical Star Formation Rate Measures at several Wavelengths

5. Star Formation Rates • Can use any part of spectrum (eg. Hα, UV, IR) • Best is a full Spectral Energy Distribution (SED) • But must account for dust • ∴Need to calculate dust extinction and emission

6. Heating Dust • As the grains absorb the incident photons they heat up and emit thermal radiation • For large grains, the absorption and emission reach an equilibrium state so that the grain has a steady temperature • For small grains however things become stochastic...

7. Hot & Cold • Smallest grains • have small cross-section • hence low photon heating rate • However, small grains also • have low specific heat • one photon causes large increase in Temperature

8. Quick & Dirty Dust IR • Solve for Grain Temperature Probability Distribution • Convolve with Blackbody and integrate over dust sizes and types to get IR emission • Include the emission from PAH

9. From Star to Finish (SED) • Use stellar synthesis code (STARBURST99) to generate stellar spectrum of different aged bursts • Use radiative transfer code (MAPPINGS) to determine HII spectrum and hot dust extinction and emission • Use MAPPINGS to determine PDR spectrum of warm dust & PAH extinction and emission • Pass final spectrum through (diffuse, cold) dusty screen

10. Making Stars • The Stellar Emission • Instantaneous bursts of 104M☉ sampled at intervals of 1 Myr up to 10 Myr • Continuous at 1 M☉ yr-1 up to 108yrs for >10 Myr population

11. Blown Away • The HII Region • All Stellar light passes through HII region • Evaluated for 3 different Pressures: P/k=104, 106 and 107 cm-3 • Includes dynamical evolution of Stellar wind bubble

12. Hot & Bothered

13. Clearing the Mess • The Photodissociation Region • Molecular cloud covers fraction of HII region • Absorbs Far-UV and gives warm dust and PAH emission • Explore clearing timescale (~covering fraction) of PDR clouds • f(t)=exp(-t/τclear) • τclear=1, 2, 4, 8, 16 and 32 Myr

14. Adding PDRs

15. Dusty Screen • The dusty screen • Provides attenuation by diffuse dust • Does not include cold dust emission

16. Under Pressure

17. Seeing through...

18. A Fitting Example...

19. IRAS Diagnostics

20. Spitzer Diagnostics

21. The models then give the following relationships between SFR and standard indicators Rating Star Formation

22. Rating Star Formation Star Formation Rates for τclear=1Myr and for τclear=32 Myr and P/k=107 cm-3

23. SEDing The End • While limited, these self-consistent theoretical starburst SEDs: • Demonstrate that two parameters control the form of the SED • Pressure in the diffuse ISM • Molecular cloud clearing timescale • Can reproduce observed starburst SEDs over 3 decades of frequency

24. Pan-SED End • These models also • Explain the observed spread in observations on IRAS Colour-Colour Diagrams • Predict possible diagnostics in Spitzer Diagnostic Diagrams • Are a step forward in obtaining fully theoretical SFR indicators and explaining the links between different wavelength regimes