Star Formation: Near and Far

1. Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt

2. Far: Whole Galaxy Relations Solid circles are disk-averaged normal spirals Open circles are central regions of normal disks Squares are circumnuclear starbursts Slope is 1.4±0.15 Spirals Starbursts Kennicutt 1998, ARAA 36, 189

3. black: normal galaxies red: starbursts green: circumnuclear starbursts blue open: Low metals (<~1/3 solar), mostly dwarfs Blue line: slope of 1.4, not a fit RCK, in preparation

4. SFR/Mass Increases with SFR • SFR/Mass of molecular gas increases with SFR • Factor of ~ 100 • “Efficiency” increasing • But what does this really mean? Star formation efficiency Star formation Rate • Solomon & Vanden Bout (2005 ARAA)

5. The Dense Gas SF Relation • LFIR correlates better with L(HCN) • Smaller scatter • Higher rate • SFR rate linearly proportional to amount of dense gas • “Efficiency” for dense gas stays the same Star formation rate Amount of dense molecular gas • Gao & Solomon (2004) ApJ 606, 271

6. Whole Galaxy Prescriptions • Kennicutt(1998) • SFR(Msun yr–1kpc–2) = 2.5x10–4gas(Msun pc–2) • Gao and Solomon (2004) • SFR (Msun/yr) ~ 1.8 x 10–8M(dense) (Msun) • SFR(Msun yr–1kpc–2) = 1.8x10–20dense(Msun pc–2)

7. What Does SSFR Mean? • SSFR is grand average over: • Whole galaxy, with huge variations in • SFR, Sgas, metallicity, … • Time • ~5 Myr for Ha • ~ 30-100 Myr for UV, • 5-100 Myr for FIR (short for starbursts)

8. What Does Sgas Mean? • “Sgas” is not the mean surface density of any structure. • At best, the filling factor x mean cloud emission times X(CO) • Higher “Sgas” really means more clouds in beam

9. CO: Limited Dynamic Range CO can be off by large factors in some regions. It clearly fails for AV > 10 mag. Need AV >0.4 mag for CO, but issues below 3 mag (Pineda et al. 2010) Heiderman et al. 2010

10. Not so Bad on Average • 12CO underestimates AVat gas> 200 M pc–2 by 30% • Constant value of 13CO vsgas, underestimating gasby factors of 4-5 • Correcting for 12CO, would flatten the slope of the Kennicutt-Schmidt relation (but does not explain big offset)

11. Intermediate: Resolved Studies • Radial cuts or averages • Martin and Kennicutt (2001): threshold • Schruba et al. (2011): SF continues even when HI > H2 • Pixel by pixel: e.g., • Kennicutt et al. (2007) • Bigiel et al. (2008) • Blanc et al. (2009)

12. Sub-kpc scales Study of 18 nearby galaxies with sub-kpc resolution in HI, CO. SFR from UV+24 micron Threshold around 10 Msunpc–2 in total gas: transition from HI to H2 Bigiel et al. 2008

13. CO, SF continue into HI region Schruba et al. 2011 SFR ~ I(CO) even in HI dominated outer parts

14. Star Formation Prescriptionsfor sub-kpc scales • Kennicutt et al. (2007) M51 • SFR(Msun yr–1kpc–2) = 1.7x10–437mol(Msun pc–2) • Bigielet al. (2008) • SFR(Msun yr–1kpc–2) = 7.9x10–30mol(10 Msun pc–2) • SFR(Msun yr–1kpc–2) = 7.9x10–40mol(Msun pc–2) • Blanc et al. (2009) M51 • SFR(Msun yr–1kpc–2) = 5.1x10–20.82mol(Msun pc–2) • Includes 0.43 dex scatter in SFR and includes limits • Issues of tracer, diffuse emission, fitting method

15. Star Formation PrescriptionsTheory • Schmidt (1959) • SFR ~ n, n = 1 or 2 (or Sn, 2009) • Krumholz et al. (2009) • SFR = f(gas, f(H2), Z, clumping) • Nearly linear with mol below ~ 100 Msun pc–2 • Steepens above 100 Msun pc–2 • Other dynamical relations

16. The Predictions

17. Very Near: Clouds in Solar Neighborhood Spitzer Programs c2d + Gould Belt: 20 nearby molecular clouds (blue circles) Cluster Project: 35 young stellar clusters (red circles) 90% of known stellar groups and clusters within 1 kpc (complete to ~ 0.1 MSun)

18. Whole Clouds (2-16 pc) Almost all clouds within 300 pc Total SFR from YSO counting /area Total mass/area Heiderman et al. 2010

19. Clouds within 1 kpc Adds Orion, Mon R2, S140, Cep OB3, all forming more massive stars, and North America nebula, less active not complete to 1 kpc, but representative

20. It’s Worse than that… Gray is extinction, red dots are YSOs, contours of volume density (blue is 1.0 Msun pc–3; yellow is 25 Msun pc–3)

21. Really Near: Within Clouds Heiderman et al. 2010

22. Less Near: Add Clouds to 1 kpc N = 2.67 N = 1.87 Gutermuth et al. subm.

23. Cep OB3 Gutermuth et al. subm.

24. Still Less Near: Dense Clumps Survey of dense clumps across MW. (n ~ 105 to 106 cm–3) Birthsites of large clusters. Follow linear relation very similar to dense gas relation for starbursts, as long as LFIR > 104.5Lsun. L(IR) L(HCN J = 1-0) Wu et al. (2005)

25. Dense Clumps on Sgas-SSFR Using LFIR to get SFR, likely underestimates. Includes fit from Wu et al.

26. Combine with Nearby Clouds Fit with broken powerlaw with slopes of 4.6 below and 1.1 above a turnover Sgas = 129+-14 Msun pc–2. (see Lada et al. 2010) Gutermuth et al. favor continued rise with SSFR ~ Sgas2 throughout. All agree: well above all exgal relations except for dense gas relation.

27. Lessons from Nearby Clouds • SFR >10 times prediction of relations for galaxies • SFR determined on sub-pc scales << exgal resolution • On scales where SF actually happens… • Dependence on Smol may be very strong, at least up to Smol~ 100 Msun pc–2

28. Speculation • The underlying SF law is linear in Sgas above a noisy threshold ~ 100 Msunpc–2 • 10 times exgal relations around threshold. • Fraction of gas above threshold (fdense) increases with <S> as S0.5 for <S> >100 Msunpc–2 • When <S> ~ 100 Sth, fdense ~1 • KS prescription and Dense gas prescription agree • What about linear relations in resolved studies of non-starbursts? • fdense ~ constant below <S> ~ 100 Msunpc–2?

29. Issues for Resolved Studies • SFR have be restricted to local SF • Remove diffuse emission • Use tracer with short timescale • Clouds are not resolved, much less clumps • “Sgas” is still not that of any structure • Small number statistics cause larger spread • Massive stars can destroy clouds • SF tracers and gas may even anti-correlate

30. Observe the Solar Neighborhood from Outside Size and location of beam/pixel causes huge variations All centered on Sun 100 pc: No SF, no CO 300 pc: SF, CO, but no Ha, little 24 mm 500 pc: SF, Ha, CO

31. What would we see? 300 pc, count YSOs, 500 pc, count YSOs 300 pc, using Ha, remove diffuse emission 500 pc, using Ha, remove diffuse emission, assume standard L(Ha) to SFR Bigiel et al. 2011

32. The Larger Context of MW • Surveys in mm continuum finding 1000’s of dense clumps • Bolocam Galactic Plane Survey (>8000 sources) • http://irsa.ipac.caltech.edu/data/BOLOCAM_GPS/ • ATLASGAL survey from APEX • Future SCUBA2 survey • Herschel Galactic Plane Survey (HIGAL) • Infrared Dark Clouds (IRDC) • MSX, GLIMPSE, MIPSGAL • New models of Galaxy, VLBA distances, … • Provide link to extragalactic star formation

33. The Improved Milky Way Model Green and blue dots show VLBA measurements of distance, which align star-forming regions along spiral arms much better than previous distances.

34. Summary • Star formation highly concentrated to dense regions • Steep increase in SSFR to at least Sgas > 120 Msun pc–2 • 10-20 x more SF than predicted by any prescriptions • SFR ~ Mass of gas above a threshold density • Non-linear nature of KS relation: • A consequence of fdense ~ <Sgas>0.5? • Resolved studies of galaxies must watch for systematic issues

35. Backup Slides

36. A Popular Explanation for Non-linear Relation • Free-fall time depends on volume density • tff ~ r–0.5 • Common theoretical approach • Krumholz and Thompson • Narayanan et al. • SFR ~ Mass/tff • dr*/dt ~ r/r–0.5 ~ r1.5 • Local version of Kennicutt relation

37. Any evidence for this? ~ SFR Mean density from virial mass and radius of well-studied sample of dense clumps. <n> ~ M/r3 (Wu et al. 2010)

38. Nor in YSO Counting Yellow stars are from Class I and Flat SED SFRs in c2d+GB Clouds.

39. Milky Way Estimates • Volume filling factor of molecular gas (as traced by CO) is about 0.005 (Heyer, prelim estimate) • Volume filling factor of clumps (density of few x 103cm–3) < 10–5 (M. K. Dunham, prelim estimate)