The global star formation laws in galaxies recent updates
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The Global Star Formation Laws in Galaxies: Recent Updates. Yu Gao Purple Mountain Observatory Chinese Academy of Sciences . May 15, 2012 @5 th sino-french. Outline of this talk.

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The Global Star Formation Laws in Galaxies: Recent Updates

Yu Gao

Purple Mountain Observatory

Chinese Academy of Sciences

May 15, 2012 @5th sino-french

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 indense gas

  • Major issues and debates (CS surveys)

  • Conclusions

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 H0analogy)

Hubble law




IR circumnuclear starbursts

Normal disk spirals

SFR vs. M(H2): No Unique Slope:1, 1.4, 1.7?



HI ~ H2


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


~linear in H2!

GMCsenbed in diffuse atomic gas(HI), the gas reservoir for

molecular clouds,

and the supply for future star formation.


Stars are forming in giant molecular clouds (GMCs)

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 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 formation in galaxies

HCN Surveys in 53 Galaxies: Gao & Solomon 2004a ApJS

Far-IR, HCN, CO Correlations: Gao & Solomon 2004b ApJ


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


Fit to both

GMCs & Gals..

CARMA+45m CO (Koda + 09)


  • 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.

HCN contours

are overlaid on

the CO image


  • Correlations between 8um-HCN & 24um-HCN. The solid lines: fixed slope of 1.


M33GMCs (Rosolowsky, Pineda & Gao 2011)

Chen & Gao in prep.

∑Mdense vs. ∑SFR

Liu & Gao arXiv:1106.2813

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

K-S SF Law in high-redshift galaxies

Daddi et al. 2010; Genzel+2010

Two major SF modes:1. a long-lasting mode for disks (local spirals and BzKs)

2. a rapid starburst for LIRGs ULIRGs & SMGs/QSOs

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

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

Juneau+2009; Narayanan+2008; Krumholz & Thompson 2007; Iono+2008; Mao+2010

SFR – CO & SFR -- HCN indexes

  • Dense gas over a range of 10^4-8 /cm^3

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 H2DCs/StarsClusters

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  Stars

Kennicutt : HI + H2 Stars

Gao & Solomon: Dense H2 Stars

From Cores to High-z: Dense GasMassive SF

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