OBSERVATIONS OF DENSE MOLECULAR GAS IN A QUASAR HOST GALAXY AT z = 6.42: FURTHER EVIDENCE FOR A NONLINEAR DENSE GAS-STAR FORMATION RELATION AT EARLY COSMIC TIMES

摘要

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(H_2) ≥ 10~4 cm~(-3)] 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) ＜ 3.3 × 10~9 K km s~(-1) pc~2, 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_(-0.08)~(+0.05) and HCN-detected z ＞ 2 galaxies (0.17_(-0.08)~(+0.09)). The relationship between L′_(HCN) and L_(FIR) is 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_(-270)~(+510) and ULIRGs (740_(-50)~(+505)), but consistent with a rising trend as indicated by other z ＞ 2 galaxies (predominantly quasars; 1525_(-475)~(+1300)). It is unlikely that this rising trend can be accounted for by a contribution of active galactic nucleus (AGN) heating to L_(FIR) alone, 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.