Evolution of Extreme Starbursts & The Star Formation Law

Yu Gao

Purple Mountain Observatory, CAS

The Schmidt Law

• 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)

• Gao & Solomon (2004): n=1 in terms of dense H2 only.

Extreme Starbursts (ES)

• Compact radio sources in ULIGs (e.g. Condon et al. 1992)

• Heavily obscured compact nuclear sources from near/mid-IR (e.g. Scoville et al. 2000)

• Downes & Solomon (1998): ES=10^9Msun H2, 10^11Lsun in ~100pc regions

• ES are norm in most ULIGs & many LIGs

Extreme Starbursts: Questions

• How did the molecular gas get there?• Does this happen quite often in gas-

rich mergers? • In the context of galaxy evolution: a

disk-disk merger sequence?• Mergers in high-z Universe?• “Overlap” starbursts? Some nearby

examples of the ongoing mergers• Connections: Bulge—Starburst—AGN?

Bulge—Starburst?—Massive BH

Gebhardt et al. 2000

Ferrarese & Merritt 2000

HI Atomic Gas, PDR

Barnes 2002

INTRODUCTION II: Galaxy Evolution (Merging)

• Observations: Statistically Gas is Being Depleted when the Merging Advances (Gao & Solomon 1999)

• CO Imaging of Merger Sequence (Gao, Lo, Gruendl & Hwang 1999)

• Early Stage Galaxy Mergers (e.g., N6670, Wang, Lo, Gao & Gruendl 2001;

Taffy, Gao, Zhu, & Seaquist 2003)

Gao & Solomon 1999, total H2 content decreases; confirmed by Georgakakis, Forbes & Norris 2000

More examples: simulations with & without BH (Di Matteo, Springel, & Hernquist 2005)

Early Merger N6670 (Wang et al. 2001)

Gao, Zhu & Seaquist 2003, AJ, 126, 2171 (astro-ph/0307490)

Gao, Lo, Lee^2 2001, SFE (20cm/CO) contour map

CO in VV 114 (Iono et al. 2004)

More examples:

Optical selectedgalaxy mergersNGC520, Arp81

Xu et al (2000)

Arp 81 (ACS Optical View)

HI & CO in Arp81

Iono & Yun 2005

CO Contoursoverlaid on the optical images (false-color)

Moleculargas densityincreasesas mergingadvances

CO in a late stage merger NGC 6240 (Tacconi et al 1999)

Summary: the Overview of ES

• Dense gas is the ultimate material to make stars in star-forming regions and galaxies

• ES are extremely concentrated regions of huge amount of dense gas

• Simulations & observations reveal how gas settles into inner disks and nuclear regions (& becomes much denser) so that starbursts can be initiated ES

• Dense gas (HCN) is the key to star formation

Dense gas is essential, the fuel for high mass star formation

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

Normalized IR—HCN correlation=SFE—dense gas fraction correlation

IR-CO correlation may not have much physical basiswhen compared to the IR-HCN correlation!

ALMA: Dense gas kinematicspoor FIR resolution even with Spitzer

ALMA: Sub-mm continuum + HCN lines (at high-z)

The power index N=1.

Kennicutt1998

Normal disk spirals

IR circumnuclear starbursts

Wu et al. 2005 In prep.

New Star Formation Law

• Dense Molecular Gas High Mass Stars

• SFR ~ M(DENSE) ~ density of dense gas

e.g. gas density >~100,000 cc

• HI H_2 DENSE H_2 Stars

Schmidt law : HI Stars

Kennicutt : HI+H_2 Stars

Gao & Solomon: Dense H_2 Stars