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The Global Star Formation Laws in Galaxies: Recent Updates
Yu GaoPurple Mountain ObservatoryChinese Academy of Sciences
May 15, 2012 @5th sino-french
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Outline of this talk
• What are the star formation (SF) recipes? SFR-gas (HI, CO) scaling laws: Kennicutt - Schmidt law, SF laws in other forms
• Why do we need a SFR-dense gas law• A linear FIR(SFR)--HCN (dense gas
tracer) relation for all star-forming systems (GMCs-->high-z): SF law in dense gas
• Major issues and debates (CS surveys)• Conclusions
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Star formation laws
• Schmidt (1959): SFR~density(HI)^n, n=1-3, mostly 2-3 in ISM of our Galaxy.• Kennicutt (1989): Disk-average [SFR~ density(HI+H2)^n] n is not well constrained. ~1-3, wide spread.• Kennicutt (1998): n=1.4 ? Total gas (HI + H2) vs. Dense gas• Better SF law in dense gas? (Hubble law and H0 analogy)
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Hubble law
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Kennicutt1998
n=1.4
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Normal disk spirals
IR circumnuclear starbursts
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SFR vs. M(H2): No Unique Slope:1, 1.4, 1.7?
HI-dominatedLSB galaxies
HI ~ H2
H2-dom
inatedLIR
Gs/
ULIR
Gs
Gao & Solomon 2004b ApJExtragalactic SF=CO until 90’s
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Big
iel’s
talk
@S
FR50
SF thresholds maysimply reflect the change of the dominant cold gas phase in galaxiesfrom HI ->H2 & from H2->denseH2
Schruba+2011~linear in H2!
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GMCs enbed in diffuse atomic gas( HI) , the gas reservoir for molecular clouds,and the supply for future star formation. PDRs
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Stars are forming in giant molecular clouds (GMCs)
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High Density Tracers Merging/interactions trigger gas infall to nuclear regions
Nuclei of Galaxies should possess denser gasas GMCs have to survive to tidal forces (must be denser)
Critical density: the radiating molecule (eg, CO) suffers collisions at the rate: n(H2) sigma v = A (Einstein coefficient A ~ nu^3 mu^2)* High-J (>~3) levels of CO (nu ~ J)higher critical density to be excited (>105cm-3)* & High dipole moment molecules HCN, HNC, HCO+, CS (mu ~ 30x > CO), etc..* X factor ? CO-to-H2, HCN-to-DenseH2 conversions
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Dense gas is the essential fuel for high mass star formation in galaxies
HCN Surveys in 53 Galaxies: Gao & Solomon 2004a ApJS
Far-IR, HCN, CO Correlations: Gao & Solomon 2004b ApJ
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SFR
Dense Molecular Gas
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Baan, Henkel, Loenen + 2008
• Baan et al. (2008)• Kohno 2007, et al. (2003)• Imanishi (2006)• Aalto et al. 2007, 2002, 1995• Solomon et al. 1992• Nguyen et al. 1992• Henkel et al. 1990• Henkel, Baan, Mauersberger 1991
HCN,CS,HNC etc. in SF gals.
Best case studies: Arp 220 & NGC 6240 (Greve + 2009)
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Wu, Evans, Gao et al. 2005 ApJL
Fit to GMCs
Fit to Galaxies
Fit to both GMCs & Gals..
Wu+2010
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CARMA+45m CO (Koda + 09)
HST
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• Total useful on-source integration time >~110 hours.
• HCN spectra with S/N>3
(a channel width dV ~7 km/s).
• Typical rms ~1-2 mK at dV~20 km/s.
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HCN contoursare overlaid onthe CO image
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• Correlations between 8um-HCN & 24um-HCN. The solid lines: fixed slope of 1.
SFR
HCN
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Chen & Gao in prep.
M33GMCs (Rosolowsky, Pineda & Gao 2011)
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∑Mdense vs. ∑SFR
Dense H2 show the best correlation with SFR (linear Liu & Gao 2011).
Liu & Gao arXiv:1106.2813
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K-S SF Law in high-redshift galaxiesDaddi et al. 2010; Genzel+2010
CO->H2 conversion factor ?? αco (Msun (K km/s pc2)-1: 4.6 for local spirals 3.6 ± 0.8 for BzKs 0.8 for LIRGs/SMGs/QSOs
Two major SF modes:1. a long-lasting mode for disks (local spirals and BzKs) 2. a rapid starburst for LIRGs ULIRGs & SMGs/QSOs
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Bi-modal SF laws in high-z gals (Daddi+2010; Genzel+2010) also exist in local gals
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Juneau+2009; Narayanan+2008; Krumholz & Thompson 2007; Iono+2008; Mao+2010 SFR – CO & SFR -- HCN indexes
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• Dense gas over a range of 10^4-8 /cm^3
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Concluding Remarks
• SF: quiescent (few Dense Cores=DCs), normal, active/burst modes (starbursts: active formation of DCs)
• DCs in Dense Molecular Gas Complexes High Mass Stars/Clusters (SF in different environments: SMGs/hi-z QSOs; ULIRGs/Starbursts; Spirals; LSBs; DCs)
• SF thresholds: change of the dominant cold gas phase in galaxies from HI ->H2 & from H2->denseH2
• (FIR-HCN, CS Linear Correlations) SFR ~ M(DenseH2): the total mass of dense molecular gas in galaxies & all star-forming systems (spanning 10 orders of mag.)?
• SFR-DenseGas: Counting DCs(=SF units) in Galaxies?
Gao & Solomon: Dense H2 DCs/StarsClusters
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New Star Formation Law
• Dense Molecular Gas High Mass Stars• SFR ~ M(DENSE) ~ density of dense gas (e.g. gas density >~10^5 cc), linear• HI H2 DENSE H2 Stars
Schmidt law : HI StarsKennicutt : HI + H2 Stars
Gao & Solomon: Dense H2 Stars
From Cores to High-z: Dense GasMassive SF