molecular gas, dense molecular gas and the star formation rate in galaxies (near and far) p. solomon...

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. 2005Molecular 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

• Slope = 1.7

Star Formation Rate and molecular mass traced by CO in local Spirals, ULIRGS and high z luminous galaxies ARAA 8

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] CO Diameters

ULIRGS 0.5 - 2 x 1010 0.8 - 2.4 kpc (FWHM)

High z EMGs 0.5 - 11 x 1010 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 oo

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• 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)