Molecular Gas, Dense Molecular Gas and the Star Formation Rate in Galaxies (near and far) P. Solomon...
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![Page 1: 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.](https://reader036.fdocuments.us/reader036/viewer/2022062421/56649c955503460f94952ffb/html5/thumbnails/1.jpg)
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
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• 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
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CO luminosity for spirals, ULIRGs and high z EMGs ARAA6
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Star Formation Efficiency for local Spirals,ULIRGS & high z EMGs ARAA7
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• 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
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STAR FORMATION LIFETIME ARAA9
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Gas Mass of local ULIRGS and Early Molecular Galaxies at z >2 ARAA 10
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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.
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LIR-LHCN for normal spirals, LIRGs and ULIRGs
The slope is 1.0
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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
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LIR--LHCN relation for Milky Way GMC cores, spirals, LIRGS and ULIRGS
Fit to GMCs
Fit to galaxies
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LIR/LHCN is independent of LIR
(Star formation rate/dense gas mass) is the same for spirals and ULIRGs
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Star formation efficiency, LIR/LCO increases with SFR traced by LIR
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LHCN --LCO
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(LHCN/LCO) A Starburst Indicator
All galaxies with a LHCN/LCO > 0.07 are Luminous IR Starbursts
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Star Formation Law
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The star formation rate is linearly proportional to the dense gas mass Mdense
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(Luminosity/Dense gas mass) LIR/Mdense = 90
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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
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LIR--LHCN
with High z galaxies
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HCN/CO a starburst
indicator at high z ?
o oo
o
o
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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)