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2004ApJ...606..271G - Astrophys. J., 606, 271-290 (2004/May-1)
The star formation rate and dense molecular gas in galaxies.
GAO Y. and SOLOMON P.M.
Abstract (from CDS):
The HCN-CO correlation is also much tighter than the IR-CO correlation. We suggest that the nonlinear correlation between LIRand LCOmay be a consequence of the stronger and perhaps more physical correlations between LIRand LHCNand between LHCNand LCO. Thus, the star formation rate indicated by LIR depends on the amount of dense molecular gas traced by HCN emission, not the total molecular gas traced by CO emission. One of the main arguments in favor of an active galactic nucleus (AGN) as the power source in ULIGs is the anomalously high ratio LIR/LCOor LIR/M(H2) or high star formation rate per M☉of gas, compared with that from normal spiral galaxies. This has been interpreted as indicating that a dust-enshrouded AGN is required to produce the very high luminosity. Viewed in terms of the dense gas mass the situation is completely different. The ratio LIR/LHCNor LIR/Mdense, a measure of the star formation rate per solar mass of dense gas, is essentially the same in all galaxies including ULIGs. The ratio LIR/Mdenseis virtually independent of galaxy luminosity and on average LIR/Mdense~90L☉/M☉, about the same as in GMC cores but much higher than in GMCs. We find that ULIGs simply have a large quantity of dense molecular gas and thus produce a prodigious starburst that heats the dust, produces the IR, and blocks all or most optical radiation. The HCN global luminosity may be used as an indicator of the star formation rate in high-redshift objects including hyperluminous galaxies.
The HCN/CO ratio is an indicator of the dense molecular gas fraction and gauges the globally averaged molecular gas density. We find that the HCN/CO ratio is a powerful starburst indicator. All galaxies in our sample with a high dense gas mass fraction indicated by LHCN/LCO>0.06 are LIGs or ULIGs. Normal spirals all have similar and low dense gas fractions LHCN/LCO=0.02-0.05. The global star formation efficiency depends on the fraction of the molecular gas in a dense phase.
Abstract Copyright: ∼
Journal keyword(s): Galaxies: ISM - Galaxies: Starburst - Infrared: Galaxies - ISM: Molecules - Radio Lines: Galaxies
Simbad objects: 75
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