The Global Star Formation Law in Dense Molecular Gas

Yu GaoPurple Mountain ObservatoryChinese 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 & H0 analogy)

Kennicutt1998

n=1.4

Normal disk spirals

IR circumnuclear starbursts

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

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!

DenseCores in nuclear regions of LSB spirals?

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)

CO detected in a few positions in NGC 4244

Matthews, Gao, Uson & Combes 2005

SFR(FIR) ~ M(H2)correlation of LSBsroughly follows that of normal spiral disks(nuclear regions,linear relation: L_FIR/M(H2)=31)

GMCs enbed in diffuse atomic gas（ HI） , the gas reservoir for molecular clouds,and the supply for future star formation. PDRs

Stars are forming in giant molecular clouds (GMCs)

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

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

• 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)

13 HCN @high-z

Gao, Carilli, Solomon & Vanden Bout 2007 ApJ, 660, L93

Wu, Evans, Gao et al. 2005 ApJL

Fit to GMCs

Fit to Galaxies

Fit to both GMCs & Gals..

Wu+2010

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 mK at 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

Chen & Gao in prep.

M31GMCs (Rosolowsky, Pineda & Gao 2011 MN)

∑Mdense vs. ∑SFR

Dense H2 show the best correlation with SFR (linear Liu & Gao 2011).

Liu & Gao 2011 arXiv:1106.2813

Poster by Liu+

Bi-modal SF laws in high-z gals (Daddi+2010; Genzel+2010) also exist in local gals

Poster by Liu+

Juneau+2009; Narayanan+2008; Krumholz & Thompson 2007; Mao+2010; Henkel+ Poster SFR – CO & SFR -- HCN indexes

FIR – CS(2-1): linear! (IRAM 30m)

FIR – CS(3-2): linear! (IRAM 30m)

FIR – CS(5-4): linear! (SMT 10m, still limited data; Wang, Zhang & Shi 2011 MN)

Poster by Z. Zhangfresh results from the APEX telescope

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