Unraveling the Star Formation Scaling Laws in Galaxies (review + 2 new results)

1. Unraveling the Star Formation Scaling Laws in Galaxies(review + 2 new results) rants Robert Kennicutt Institute of Astronomy University of Cambridge M51: FUV,Ha,24mm

2. Basic Observations: circa 1998 • Galaxies exhibit an immense diversity in star formation properties, varying by >107 in absolute SFR, SFR/mass and SFR/area. • Over this range the SFR/area is correlated with gas surface density, following a truncated Schmidt power law with index N = 1.4 +-0.1 • the correlation of with dense gas (e.g., HCN) is roughly linear • The Schmidt law shows a turnover below a threshold surface density that varies between galaxies. • in gas-rich, actively star-forming galaxies this transition is seen as a radial transition in the SFR/area • some gas-poor disks reside in the threshold regime at all radii

3. IR-luminous starbursts circumnuclear starbursts BCGs, ELGs

4. starbursts normal galaxies Kennicutt 1998, ApJ, 498, 541 Gao, Solomon 2004, ApJ, 606, 271

7. NGC 1291 Ha + R: SINGG survey Meurer et al. 2006 Blue: Carnegie Atlas Sandage & Bedke 1994

8. Questions: Schmidt Law • Is the correlation really this good (or this bad)? • Why all the discrepant results yesterday (and today)? • Do all galaxies follow the same Schmidt law? • Is the scatter driven by a second parameter? • Is the Schmidt law the result of a more fundamental underlying SF scaling law? • over what range of physical scales is the law valid? • Is the SFR correlated more strongly with the total (atomic + molecular) surface density or with the molecular surface density alone? • What is the physical origin of the relation?

10. “Schmidt law”: SFR vs gas density power law “Silk law”:SFR vs gas density/dynamical time

11. “pressure law” Blitz & Rosolowsky 2006, ApJ, 650, 933

12. Questions: Schmidt Law • Is the correlation really this good (or this bad)? • Why all the discrepant results? • Do all galaxies follow the same Schmidt law? • Is the scatter driven by a second parameter? • Is the Schmidt law the result of a more fundamental underlying SF scaling law? • over what range of physical scales is the law valid? • Is the SFR correlated more strongly with the total (atomic + molecular) surface density or with the molecular surface density alone? • What is the physical origin of the relation?

13. Questions: Thresholds • Do the observed Ha edges of galaxies trace proportional changes in the SFR/area? • Does the SFR in the sub-threshold regime follow a (modified) Schmidt law? Or is it triggered entirely by local compression events? • What is the physical nature of the threshold?

14. Key Caveats, Considerations (aka Rant #1) • The “star formation law” and “star formation rate” mean different things on different linear scales • disk-averaged scale (1-20 kpc, >100 Myr): useful empirical recipes, but physical significance difficult to infer • radial averages (~1 kpc, 20-300 Myr): breaks some parametric degeneracies, but still smooths nonlinear phenomena over 10-100’s of cloud scales • cloud scale or below (50-500 pc, 3-10 Myr): probes “star formation efficiency”, but with large observational scatter. “SFRs” really are measures of cluster luminosities. SF law on larger scales may have very different form.

15. Rant #2: Beware of the local coincidence between ISM phase vs gravitational instabilities H2 HI grav bound diffuse Log NH Log P/k (pressure)

16. Rant #3 • It is important to match the SF and gas tracers to the application of interest • color-magnitude diagrams: nirvana: range of ages, stellar masses • Ha, Pa, 24mm knots: massive stars, last 3-10 Myr • FUV continuum: massive stars, last 0-200 Myr • diffuse dust continuum emission, 20-200mm + PAH emission: massive and intermediate mass stars, last 0-2 Gyr • CO, HI clumps: probably bound clouds, <10 Myr • diffuse CO, HI: anybody’s guess, probably ~0.1-1 Gyr

18. Draine et al. 2007, astro-ph/0703213

19. GALEX FUV + NUV (1500/2500 A) Ha + R IRAC 8.0 mm MIPS 24 mm

20. M81 Ha + R

21. Calzetti et al. 2007, ApJ, 666, 870

22. M 81 24µm 70µm 160µm

24. R. Kennicutt (IoA, Cambridge), L. Armus (SSC), A. Bollato (UMd), B. Brandl (Leiden), D. Calzetti (UMass), D. Dale (UWyo), B. Draine (Princeton), C. Engelbracht (UofA, USA), K. Gordon (UoA, USA), B. Groves (Leiden), L. Hunt (Oss Arcetri, Italy), J. Koda (Caltech), O. Krause, A. Leroy, H.W. Rix (MPIA), H. Roussel (IAP), M. Sauvage (CEA), E. Schinnerer (MPIA), J.D. Smith (Toledo), L. Vigroux (IAP), F. Walter (MPIA), M. Wolfire (UMd) + TBD PHOENIGS: from the SINGS `ashes’Project for Herschel On an Extragalactic Normal Infrared Galaxies Survey • Broad Science Objectives: • Trace and characterize the flow of energy through the ISM in galaxies; • Link heaters-emitters: use Herschel spatial resolution to enable definitive modeling of radiative transfer of dust and gas cooling in galaxies; • Probe the nature/origin of extended cold dust envelopes; link warm-cold dust emission; • Improve dust and spectral diagnostics of star formation and ISM properties. • Approach: • An objectively selected sample of nearby galaxies (SINGS-inspired), optimized to cover a broad and representative range of properties, and broad range of local physical environments; • Exploit angular resolution for resolving infrared components and dust heating populations. • Leverage existing and new ancillary data: from UV to radio • Data and high-level data products would be delivered quickly to the broad community.

25. Observational Wishlist • Spatially resolved measurements, vs disk-averaged or azimuthally-averaged data • Extinction corrections (Ha, UV), and corrections for unextincted star formation (infrared, radio) • Accurate molecular mass measurements (how reliable is CO?) • IMF

26. NGC 628 (M74) C. Tremonti

27. SINGS Galaxies

29. The Global Schmidt Law Revisited Work in progress! • analyze galaxies with spatially-mapped star formation (Ha, Pa, FIR), HI, and CO • enlarged, diversified samples • normal galaxy sample 3x larger • larger ranges in gas and SFR densities • large subsamples of circumnuclear starbursts, low-metallicity galaxies incorporated • densities averaged within active SF regions • explicit corrections for [NII], extinction • point-by-point analysis of SINGS + BIMA SONG galaxies

30. Kennicutt (1998) sample expanded sample constant extinction

31. expanded sample constant extinction Ha + 24mm extinction corrections

32. total FIR Pa + Ha Ha + 24mm Ha + Hb HI + CO (const X)

33. HI + H2 HI H2

35. metal-poor dwarf galaxies

37. The Spatially Resolved Star Formation Law in M51 Kennicutt et al. 2007, ApJ, in press (astro-ph/0708.0922) FUV,Ha,24mm - Use spatially-resolved measures of CO, HI, and SFR to characterize SFR vs gas surface density relation on a point-by-point basis - Use combinations of Ha + Pa and Ha + 24 mm emission to correct for extinction in SFR measurements - Probe scales from 300 - 1850 pc (IR/HII regions to unbiased sampling of the disk) Calzetti et al. 2005, ApJ, 633, 871

38. M51: BIMA SONG Survey Helfer et al. 2003

39. NGC 6946– THINGS VLA HI Survey F. Walter et al.

40. Local Schmidt Law in M51 Kennicutt et al. 2007

45. Tentative Conclusions • The disk-averaged Schmidt law in galaxies is rooted in a local relationship that persists to scales of <500 pc • In M51 the SF density is tightly coupled to the local H2 surface density, and not with HI density • A kinematic star formation law does not seem to extend as well to local scales • The disk-averaged SF law is confirmed with more/better observations. Some metal-poor galaxies lie systematically above the mean relation.

46. Tentative Conclusions • The combination of Ha and 24 mm imaging provides a reliable method for obtaining extinction-corrected ionizing fluxes of HII regions. The combination of Ha and FIR luminosities can provide reliable extinction-corrected SFRs of galaxies as a whole. • As SFR estimators become more reliable, empirical characterization of the SF law will become increasingly limited by the accuracy and depth of cold gas tracers, especially for molecular gas.