The Star Formation Rate and Dense Molecular Gas in Galaxies

• Published in: The Astrophysical journal 2004-05, Vol.606 (1), p.271-290
• Main Authors: Gao, Yu, Solomon, Philip M
• Format: Article
• Language: English
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/382999

HCN luminosity is a tracer of dense molecular gas, n(H sub(2)) > ~ 3 x 10 super(4) cm super(-3), associated with star-forming giant molecular cloud (GMC) cores. We present the results and analysis of our survey of HCN emission from 65 infrared galaxies, including nine ultraluminous infrared galaxies (ULIGs, L sub(IR) > ~ 10 super(12) L sub(o)), 22 luminous infrared galaxies (LIGs, 10 super(11) L sub(o) < L sub(IR) ~ 10 super(12) L sub(o)), and 34 normal spiral galaxies with lower IR luminosity (most are large spiral galaxies). We have measured the global HCN line luminosity, and the observations are reported in Paper I. This paper analyzes the relationships between the total far-IR luminosity (a tracer of the star formation rate), the global HCN line luminosity (a measure of the total dense molecular gas content), and the CO luminosity (a measure of the total molecular content). We find a tight linear correlation between the IR and HCN luminosities L sub(IR) and L sub(HCN) (in the log-log plot) with a correlation coefficient R = 0.94, and an almost constant average ratio L sub(IR)/L sub(HCN) = 900 L sub(o) (K km s super(-1) pc super(2)) super(-1). The IR-HCN linear correlation is valid over 3 orders of magnitude including ULIGs, the most luminous objects in the local universe. The direct consequence of the linear IR-HCN correlation is that the star formation law in terms of dense molecular gas content has a power-law index of 1.0. The global star formation rate is linearly proportional to the mass of dense molecular gas in normal spiral galaxies, LIGs, and ULIGs. This is strong evidence in favor of star formation as the power source in ultraluminous galaxies since the star formation in these galaxies appears to be normal and expected given their high mass of dense star-forming molecular gas. The HCN-CO correlation is also much tighter than the IR-CO correlation. We suggest that the nonlinear correlation between L sub(IR) and L sub(CO) may be a consequence of the stronger and perhaps more physical correlations between L sub(IR) and L sub(HCN) and between L sub(HCN) and L sub(CO). Thus, the star formation rate indicated by L sub(IR) 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 L sub(IR)/L sub(CO) or L sub(IR)/M(H sub(2)) or high star formation rate per M su