Why is star formation correlated with molecular gas? Simon Glover

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1 Why is star formation correlated with molecular gas? Simon Glover

2 Schruba (2012)

3 Good evidence that molecular gas and star formation are correlated in local spirals (see also K. Sandstrom s talk) Obvious hypothesis: need molecular gas in order to form stars (because of cooling?) But why? H2 cooling ineffective at GMC temperatures, CO not much better than C+

4 In order to form stars, need low Jeans mass Jeans mass is low in cold, dense gas Cold dense gas clouds are also good place to form molecules Is star formation correlated with molecular gas simply because molecules and stars form in similar environments?

5 Two ways to test this: Observational: can stars form in molecule-poor environments? Numerical: what happens if we switch off molecule formation? For an observational test, we need: Sample of metal-poor star-forming systems (since these are likely to be molecule-poor) Some way of measuring H2 content in these systems

6 Schruba et al (2012)

7 Small-scale models

8 Obvious starting point: see what happens if we disable CO formation and/or H2 formation in molecular cloud simulations Basic model: SPH simulation of Msun cloud, mass resolution 0.5 Msun Run 5 different variants with different microphysics

9 Model A: detailed chemistry, cooling, but no shielding Model B: atomic cooling, no chemistry Model C: atomic, H2 cooling; only hydrogen chemistry Models D1,D2: full chemistry & cooling

10 Glover & Clark (2012a, MNRAS, 421, 9)

11 Glover & Clark (2012a)

12 Glover & Clark (2012a)

13 Glover & Clark (2012a)

14 Presence of molecular gas has only small influence on star formation rate H2 cooling never important in dense cloud conditions CO important if present; but C+ good substitute when CO absent

15 This study was somewhat artificial, since we just switched off bits of the chemistry. Are there real systems where we might expect star formation without (much) molecular gas? Yes! We just need to look at low metallicity...

16 Glover & Clark (2012b, MNRAS, 426, 377)

17 Glover & Clark (2012b)

18 Glover & Clark (2012b)

19 Krumholz (2012)

20 Star-forming gas eventually becomes molecular At low Z, this happens after runaway collapse has already begun Supports idea that molecular gas often traces star formation, but isn't necessary

21 Large-scale models

22 To model molecular gas in galactic disk, need: High resolution (~ 1 pc for self-consistent models) Molecular chemistry, thermal physics Dust shielding, H2 self-shielding Large-scale disk model (e.g. stellar potential) Stellar feedback (esp. supernovae) AREPO disk simulations (Smith et al, 2014) Include everything _except_ stellar feedback SILCC project (Walch et al, in prep.) Medium resolution, no large-scale disk model

23 Smith et al (2014)

24 Smith et al (2014)

25 Smith et al (2014)

26 Smith et al (2014)

27 Smith et al (2014)

28 Smith et al (in prep.)

29 Log Smith et al (in prep.)

30 Smith et al (in prep.)

31 Log Smith et al (in prep.)

32 Walch et al (in prep.)

33 Conclusions CO traces star formation well at metallicities near solar, but not at lower metallicity H2 and star formation correlate well over larger range of metallicities, but eventually this correlation also breaks down Molecular gas traces star formation because both trace cold, dense gas

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35 Extra slides

36 Glover & Clark (2014)

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40 At Z > 0.1 Zsun, H2 cooling unimportant, regardless of density or ISRF strength At lower metallicity, H2 cooling important when G0/n small H2 therefore plays important role in enabling formation of GMCs when Z, G0 both small