Transcript Document
Molecular Gas, Dense Molecular Gas and the Star Formation Rate in Galaxies (near and far) P. Solomon • Molecular Gas Mass as traced by CO emission and the star formation rate in spiral galaxies, LIRGS, ULIRGs and high z molecular galaxies (Early Molecular Galaxies, EMGs) • Dense molecular Gas as traced by HCN emission is a star formation rate indicator. The mass of dense molecular gas is the key to understanding the star formation rate • HCN observations at low and high z. • A new Star Formation Law HCN Observations: Solomon, Downes and Radford; ApJ 1992 Gao and Solomon; ApJ. 2005 Molecular Gas at High z: Solomon and Vanden Bout; ARAA, 2005 • CO(1-0) luminosity traces the mass of gas at H2 densities > 300 cm-3 associated with most of the mass in GMCs. • HCN(1-0) luminosity traces the mass of gas at H2 densities > 3 x 10 4 cm-3 associated with star forming GMC Cores. • Far IR luminosity traces the star formation rate due to absorption of OB stellar radiation by dust at T ~ 30 –70 K. For a given IMF this yields the total star formation rate. • SFR = 1.5 x 10-10 LFIR [Msun/yr] • LFIR/LCO is a measure of the star formation efficiency • LHCN/LCO is a measure of the dense gas mass fraction CO luminosity for spirals, ULIRGs and high z EMGs ARAA6 Star Formation Efficiency for local Spirals,ULIRGS & high z EMGs ARAA7 Star Formation Rate and molecular mass traced by CO in local Spirals, ULIRGS and high z luminous galaxies ARAA 8 • Slope = 1.7 Excluding ULIRGS, slope =1.0 STAR FORMATION LIFETIME ARAA9 Gas Mass of local ULIRGS and Early Molecular Galaxies at z >2 ARAA 10 Comparison of molecular gas in ULIRGS and high z EMGs Gas Mass [Mo] ULIRGS 0.5 - 2 x 1010 High z EMGs 0.5 - 11 x 1010 CO Diameters 0.8 - 2.4 kpc (FWHM) 0.8 - 7 kpc one may be larger EMGs are similar to ULIRGS with about 2 or 3 times as much molecular gas and slightly larger Can EMGs form massive ellipticals ? Probably not. They do not have enough gas and the gas is too concentrated in the center. LIR-LHCN for normal spirals, LIRGs and ULIRGs The slope is 1.0 Star Formation Efficiency and Dense Gas Fraction LIR --LHCN normalized by LCO. This removes all dependence on distance and total gas mass. The slope is 1.0 LIR--LHCN relation for Milky Way GMC cores, spirals, LIRGS and ULIRGS Fit to GMCs Fit to galaxies LIR/LHCN is independent of LIR (Star formation rate/dense gas mass) is the same for spirals and ULIRGs Star formation efficiency, LIR/LCO increases with SFR traced by LIR LHCN --LCO (LHCN/LCO) A Starburst Indicator All galaxies with a LHCN/LCO > 0.07 are Luminous IR Starbursts Star Formation Law The star formation rate is linearly proportional to the dense gas mass Mdense (Luminosity/Dense gas mass) LIR/Mdense = 90 Status of HCN Observations. at High-z • First detection with VLA in lensed Cloverleaf quasar (Solomon, Vanden Bout, Carilli, & Guelin 2003) • 5 detections [1@GBT, 3@VLA, 1@PdBI (HCN J=5-4)] + 4 more VLA upper limits (Carilli et al. 2005). Almost all are QSOs--starburst combinations • 4 new searches with VLA (Gao,etal) some sub-mm galaxies, >50hrs) difficult with current instruments LIR--LHCN with High z galaxies HCN/CO a starburst indicator at high z ? o o o o o • Summary • The star formation rate per Msun of dense gas is the same in Milky Way GMC cores, spiral galaxies, ULIRGS and (probably) high z IR starbursts (EMGs). • Star formation is very efficient for gas at densities n(H2) > 3x 104 cm-3 • The strongest starbursts in the universe are characterized by a high fraction of (dense molecular gas/total molecular gas)