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The G lobal S tar F ormation L aw in Dense M olecular G as. Yu Gao Purple Mountain Observatory Chinese Academy of Sciences . Sept. 7 , 2011 @SED2011 ( IAUS 284). Outline of this talk.
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The Global Star Formation Law in Dense Molecular Gas Yu Gao Purple Mountain Observatory Chinese Academy of Sciences Sept. 7, 2011@SED2011 (IAUS 284)
Outline of this talk • What are the star formation (SF) recipes? SFR-gas (HI, CO) scaling laws: Schmidt law, SF laws in other forms • Why do we need a SFR-DenseGas law • A linear FIR(SFR)-HCN (dense gas tracer)relation for all star-forming systems: SF law in DenseGas • Major Issues and debates • Conclusion+Solutions?!
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, Georges Lemaître& H0analogy)
Kennicutt 1998 n=1.4
IR circumnuclear starbursts Normal disk spirals
SFR vs. M(H2): No Unique Slope:1, 1.4, 1.7? H2-dominated LIRGs/ULIRGs HI ~ H2 HI-dominated LSB galaxies Extragalactic SF=CO until 90’s Gao & Solomon 2004b ApJ
SF thresholds may simply reflect the change of the dominant cold gas phase in galaxies from HI ->H2 & from H2->denseH2 Bigiel’s talk @SFR50 Schruba+2011 ~linear in H2!
CO detected in a few positions in NGC 4244 Deep CO obs. (~20 hrs on 12m) in UGC 7321 (HST/WFPC2 R+I image) First CO detections in a few edge-on LSB spirals using the 12m (Matthews & Gao 2001) DenseCoresin nuclear regions of LSB spirals?
SFR(FIR) ~ M(H2) correlation of LSBs roughly follows that of normal spiral disks (nuclear regions, linear relation: L_FIR/M(H2)=31) Matthews, Gao, Uson & Combes 2005
GMCsenbed in diffuse atomic gas(HI), the gas reservoir for molecular clouds, and the supply for future star formation. PDRs
High DensityTracersMerging/interactions trigger gasinfall to nuclearregionsNucleiof Galaxies shouldpossessdensergasas GMCs have to survive to tidal forces (must bedenser) Criticaldensity: 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) highercriticaldensity to beexcited (>105cm-3) * & High dipolemoment molecules HCN, HNC, HCO+, CS (mu ~ 30x > CO), etc.. * X factor ? CO-to-H2, HCN-to-DenseH2 conversions
Dense gas is the essential fuel for high mass star/SSC formation in Galaxies HCN Surveys in 53 Galaxies: Gao & Solomon 2004a ApJS Far-IR, HCN, CO Correlations: Gao & Solomon 2004b ApJ
SFR Dense Molecular Gas
Baan, Henkel, Loenen + 2008 HCN,CS,HNC etc. in SF gals. • 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 Best case studies: Arp 220 & NGC 6240 (Greve + 2009)
Gao, Carilli, Solomon & Vanden Bout 2007 ApJ, 660, L93 13 HCN @high-z
Fit to Galaxies Wu, Evans, Gao et al. 2005 ApJL Fit to GMCs Wu+2010 Fit to both GMCs & Gals..
SSCs in nearby galaxies could fill in the gap in FIR-HCN corr. Gap?
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 mKat dV~20 km/s.
Correlations between 8um-HCN & 24um-HCN. The solid lines: fixed slope of 1.
Correlation between 70um-HCN Correlation between 160um-HCN
M31GMCs (Rosolowsky, Pineda & Gao 2011 MN) Chen & Gao in prep.
∑Mdense vs. ∑SFR Liu & Gao 2011 arXiv:1106.2813 Dense H2 show the best correlation with SFR (linear Liu & Gao 2011).
Bi-modal SF laws in high-z gals (Daddi+2010; Genzel+2010) also exist in local gals
Juneau+2009; Narayanan+2008; Krumholz & Thompson 2007; Mao+2010; Henkel+ Poster SFR – CO & SFR -- HCN indexes
FIR – CS(5-4): linear! (SMT 10m, still limited data; Wang, Zhang & Shi 2011 MN) FIR – CS(3-2): linear! (IRAM 30m)
Poster by Z. Zhang fresh results from the APEX telescope
Concluding Remarks • SF: quiescent (few Dense Cores=DCs), normal, active/burst modes (starbursts: active formation ofDCs) • DCsin Dense Molecular Gas Complexes High Mass Stars/Clusters (SF in different environments: SMGs/hi-zQSOs; 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 H2DCs/StarsClusters