2007ApJ...671L..13R

We report a sensitive search for the HCN(J = 2 ⟶ 1) emission line toward SDSS J114816.64+525150.3 (hereafter J1148+5251) at z = 6.42 with the Very Large Array (VLA). HCN emission is a star formation indicator, tracing dense molecular hydrogen gas [n(H₂)≥10⁴/cm3] within star-forming molecular clouds. No emission was detected in the deep interferometer maps of J1148+5251. We derive a limit for the HCN line luminosity of L'_HCN<3.3x10⁹ K.km/s.pc2, corresponding to a HCN/CO luminosity ratio of L'_HCN/L'_CO<0.13. This limit is consistent with a fraction of dense molecular gas in J1148+5251 within the range of nearby ultraluminous infrared galaxies (ULIRGs; median value: L'_HCN/L'_CO=0.17^+0.05_–0.08) and HCN-detected z>2 galaxies (0.17^+0.09_–0.08). The relationship between L'_HCN and L_FIRis considered to be a measure of the efficiency at which stars form out of dense gas. In the nearby universe, these quantities show a linear correlation, and thus, a practically constant average ratio. In J1148+5251, we find L_FIR/L'_HCN>6600. This is significantly higher than the average ratios for normal nearby spiral galaxies (L_FIR/L'_HCN=580⁺⁵¹⁰_–270) and ULIRGs (740⁺⁵⁰⁵_–50), but consistent with a rising trend as indicated by other z>2 galaxies (predominantly quasars; 1525⁺¹³⁰⁰_–475). It is unlikely that this rising trend can be accounted for by a contribution of active galactic nucleus (AGN) heating to L_FIRalone, and may hint at a higher median gas density and/or elevated star formation efficiency toward the more luminous high-redshift systems. There is marginal evidence that the L_FIR/L'_HCN ratio in J1148+5251 may even exceed the rising trend set by other z>2 galaxies; however, only future facilities with very large collecting areas such as the Square Kilometer Array (SKA) will offer the sensitivity required to further investigate this question.