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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

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
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 f ormation laws
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)

slide5

Kennicutt

1998

n=1.4

slide6

IR circumnuclear starbursts

Normal disk spirals

slide7

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

slide8

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!

slide9

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

molecular clouds,

and the supply for future star formation.

PDRs

slide11

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

slide12

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

slide13

SFR

Dense Molecular Gas

slide14

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)

slide16

Fit to Galaxies

Wu, Evans, Gao

et al. 2005 ApJL

Fit to GMCs

Wu+2010

Fit to both

GMCs & Gals..

slide18
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.
slide19

HCN contours

are overlaid on

the CO image

slide20

SFR

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

HCN

slide22

∑Mdense vs. ∑SFR

Liu & Gao arXiv:1106.2813

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

slide23

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

slide25

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

SFR – CO & SFR -- HCN indexes

concluding remarks
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
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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