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)