Decoding the Schmidt Law: Gas Density & Star Formation | Real-world Application

1. … and its relationship to the the real world? What is this thing called . the Schmidt Law?. Barry F.Madore What is this thing called … the Schmidt Law?

2. “Shit Happens” Which one of these is a law? (1) “Mercy falls, like the gentle rain, from heaven” (2) “The rain in Spain falls mainly in the plain” (3) “Rain forms in clouds” (4) “Stars form in clouds” (5) “Schmidt happens” (6) “Star formation is like rabbit pie”

3. Heidelberg (2010) THE SCHMIDT LAW GAS DENSITY regulates the RATE of STAR FORMATION ? What gas? What density? Over what scale? ** What tracers? What rate(s)? ** (And, what about efficiency?) ** … and what about collisions, and bars, and flows, and density waves and …ack?

4. George Santayana “Those who ignore history are doomed to repeat it” -- George Santayana Recall that Schmidt (1959) originally stated that a power-law dependence was suggested because the exponential scale heights of the young stars were smaller than the scale height of the gas layer. In which direction is causality operating?

5. This is what started it all! SMC: Neutral Hydrogen and Individually Resolved OB Stars Sanduleak (1969) SMC: Neutral Hydrogen Column Density Sanduleak (1969)

6. Sanduleak (1969) Sanduleak (1969) substituted neutral hydrogen surface densities for gas volume densities, and used the area-average star density for the time-averaged star formation rate. Note: This is the first spatially resolved determination of the Schmidt Law in an extragalactic setting. SMC: Neutral Hydrogen and Individually Resolved OB Stars

7. M31 20um + Swift Andromeda (2010) M31: Andromeda WISE Early Release Data (12 and 22 um) SWIFT: High-Resolution UV Data

8. Bigiel(2090) Bigiel et al. (2009) Star Formation Rate versus Total Gas (HI + H2) Surface Density

9. Madore (2010) Cloud-Collapse and Stellar Evolutionary Timescales

10. Madore (2010) Madore (2010) Star Formation and Stellar Evolution Timescales Combined

11. Boissier (2007) Radially Averaged Schmidt Law

12. Kennicutt (1998) Kennicutt (1998) Globally-averaged Schmidt Law

13. Clumping Spongebob 1 Spongebob 2 • 10 1 37 gas density 10 10 1 1 <10> <10> <gas density> SFR = density**2 100 100 1 1369 sfr 100 100 1 1 <100> <342> <sfr> The Global Schmidt Law is dominated by “sub-grid physics” SBSP-1

14. Kennicutt (1998) Kennicutt (1998) Global Schmidt Law

15. Complications (2010) Additional (but non-trivial) Complications: Plane thickness variations within individual galaxies, and between galaxies. What controls theefficiencywith which gas gets converted into stars within any given cycle of the star formation process?

16. Bell the cat! What is this thing called the Schmidt Law? .. … and the real world? We need to finally “Bell the cat” and actually control the experiment. We need to remember that we are not reading the recipe. Causality plays a role, but we are observing input and end products simultaneously weighted by their various lifetimes. Galaxies are 3D objects but we observe them only in projection. Scales matter: Azimuthally-averaged and globally-averaged Schmidt Laws are not substitutes for the spatially resolved Schmidt Law. Finally, we need to understand the physics behind star formation efficiency. Understand the Magellanic Clouds. Barry F.Madore

17. What is this thing called the Schmidt Law? .. … and the real world? Bill the cat! We need to finally “Bell the cat” and actually control the experiment. We need to remember that we are not reading the recipe. Causality plays a role, but we are observing input and end products simultaneously weighted by their various lifetimes. Galaxies are 3D objects but we observe them only in projection. Scales matter: Azimuthally-averaged and globally-averaged Schmidt Laws are not substitutes for the spatially resolved Schmidt Law. Finally, we need to understand the physics behind star formation efficiency. Understand the Magellanic Clouds. Barry F.Madore

18. What is this thing called the Schmidt Law? … many things by many people M31 20um Sanduleak (1969) Schmidt-Boissier Law Schmidt-Sanduleak Law Schmidt-Kennicutt Law Schmidt-Calzetti Law SMC: Neutral Hydrogen and Individually Resolved OB Stars

19. What is this thing called the Schmidt Law? … many things by many people M31 20um Sanduleak (1969) Schmidt-Boissier Law Schmidt-Sanduleak Law Schmidt-Calzetti Law Schmidt-Kennicutt Law SMC: Neutral Hydrogen and Individually Resolved OB Stars

20. … and the real world? What is this thing called . the Schmidt Law?. “Kill Bill” - Uma Thurmon “And once again were are forced to think” - Martin Schwarschild * No animals were harmed in the making of this presentation Barry F.Madore What is this thing called … the Schmidt Law?

21. … and the real world? What is this thing called . the Schmidt Law?. We need to finally “Bell the cat” and control this experiment. We need to remember that we are not reading the recipe. Causality plays a role, but we are observing input and end products simultaneously weighted by their various lifetimes. Galaxies are 3D objects but we observe them only in projection. Scales matter: Azimuthally-averaged and globally-averaged Schmidt Laws are not substitutes for the spatially resolved Schmidt Law. Finally, we need to understand the physics behind star formation efficiency. Understand the Magellanic Clouds. Barry F.Madore What is this thing called … the Schmidt Law?