A Search for Dense Molecular Gas in High Redshift Infrared-Luminous Galaxies
Abstract
We present a search for HCN emission from four high redshift far infrared (IR) luminous galaxies. Current data and models suggest that these high-z IR luminous galaxies represent a major starburst phase in the formation of spheroidal galaxies, although many of the sources also host luminous active galactic nuclei (AGN), such that a contribution to the dust heating by the AGN cannot be precluded. HCN emission is a star formation indicator, tracing dense molecular hydrogen gas within star-forming molecular clouds (n(H$_2$) = 10^5 cm^{-3}). HCN luminosity is linearly correlated with IR luminosity for low redshift galaxies, unlike CO emission which can also trace gas at much lower density. We report a marginal detection of HCN (1-0) emission from the z=2.5832 QSO J1409+5628, with a velocity integrated line luminosity of L_HCN'=6.7\pm2.2 x10^9 K km s^{-1} pc^2, while we obtain 3sigma upper limits to the HCN luminosity of the z=3.200 QSO J0751+2716 of L_HCN'=1.0x10^9 K km s^{-1} pc^2, L_HCN'=3.2x10^8 K km s^{-1} pc^2 for the z= 2.565 starburst galaxy J1401+0252, and L_HCN'=1.0x10^{10} K km s^{-1} pc^2 for the z = 6.42 QSO J1148+5251. We compare the HCN data on these sources, plus three other high-z IR luminous galaxies, to observations of lower redshift star-forming galaxies. The values of the HCN/far-IR luminosity ratios (or limits) for all the high-z sources are within the scatter of the relationship between HCN and far-IR emission for low-z star-forming galaxies (truncated).Keywords
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