The Two Tightest Correlations: FIR-RC and FIR-HCN

Yu GAO

Purple Mountain Observatory, NanjingChinese Academy of Sciences

Stars are forming in giant molecular clouds (GMCs)

Diffuse atomic gas（ HI） , the gas reservoir for the molecular clouds,and the supply of future star formation. PDRs

Zhang, Gao & Wang arXiv: 0911.4815

Break-down of corr.: one example

Zhang, Gao & Wang arXiv: 0911.481570um and 160um overlaid with CO(1-0)

Zhang, Gao & Wang arXiv: 0911.4815HI and RC (radio continuum) images overlaid with CO(1-0)

Dense gas is the essential fuel for high mass SF in Galaxies

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

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

SFR

Dense Molecular Gas

More CO data of ULIGs (Solomon et al. 1997)that Lco > ~ 10^10 K km/s pc^2

Total Molecular Gas Mass

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

^^^ ||~15hrs VLA, stacking 3 lens components

r.m.s.~19uJy/beam

More than 45hrs VLA

13 HCN @high-z

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

Wu, Evans, Gao et al. 2005 ApJL

Krumholz & Thompson 2007

Fit to GMCs

Fit to Galaxies

Fit to All

SSCs in nearby galaxies could fill

in the gap in FIR-HCN corr.

Gap？

Resovled Local (FIR-HCN) SF Law ? In Dense Gas (M51 & N6946)

70um maps & comparedto HCN measurements

Work in progress!

Still far from the size scale of SSCs(by more than twoorders of magnitude)

Baan, Henkel,Loenen et al. 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.

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

SFR vs. M_dense(H2): linear correlation

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

HI-dominatedLSB galaxies

HI ~ H2

H2-dom

inated

Gao & Solomon 2004b ApJ

Liu & Gao 2010

RC-HCN corr.

Liu & Gao 2010 ApJ submitted

Gao & Solomon 2004b Liu & Gao 2010

Summary

• Dense Cores in Dense Molecular Gas Complexes High Mass Stars & SSCs -> SFR

• (FIR-HCN Linear Correlation) SFR ~ M(DenseH2): the dense gas mass in galaxies & all star-forming systems?

• SF thresholds: change of the dominant cold gas phase in galaxies from HI ->H2 & from H2->denseH2

• SFR: Counting Dense Cores/SSCs in Galaxies?• HI (LSBs) H2 (Spiral disks) denseH2

(starbursts/ULIRGs) Stars/SSCs• Resolved local correlations are the key SFR

FIR-radio vs. FIR-HCN correlations

• FIR, radio continuum (RC), HCN & CO• FIR-RC > FIR/RC-HCN > FIR/RC-CO• Stars Death SN/SNRs RC• Stars Form UV/dust FIR• CO HCN SF FIR RC• FIR-RC & FIR-HCN corr. the strongest!• Comparison of different SFR indicators

Differ both spatially and temporally