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/ )

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

CO in VV 114 (Iono et al. 2004)

More examples: Optical selected galaxy mergers NGC520, Arp81 Xu et al (2000)

Arp 81 (ACS Optical View)

HI & CO in Arp81 Iono & Yun 2005

CO Contours overlaid on the optical images (false-color) Molecular gas density increases as merging advances

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 basis when compared to the IR-HCN correlation!

ALMA: Dense gas kinematics poor FIR resolution even with Spitzer ALMA: Sub-mm continuum + HCN lines (at high-z)

The power index N=1.

Kennicutt 1998

Normal disk spirals

IR circumnuclear starbursts

Wu et al 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