Gas and dust in the Cloverleaf quasar at redshift 2.5

We observed the upper fine structure line of neutral carbon, CI $(\rm ^3P_2\!\to^3\!P_1)$, ($\nu_{\rm rest} = 809$ GHz), the ${^{12}{\rm CO}(J=3\!\to\!2)}$, line ($\nu_{\rm rest} = 345$ GHz) and the 1.2 mm continuum emission from H1413+117 (Cloverleaf quasar, $z=2.5$) using the IRAM interferometer. Together with the detection of the lower fine structure line (Barvainis et al. 1997), the Cloverleaf quasar is now only the second extragalactic system, besides M 82, where both carbon lines have convincingly been detected. Our analysis shows that the carbon lines are optically thin and have an excitation temperature of ${{T}_{\rm ex}}$,≈ 30 K. CO is subthermally excited and the observed line luminosity ratios are consistent with ${n({\rm H}_2)}$,≈ 10$^{3-4}\,{\rm cm}^{-3}$ at ${{T}_{\rm kin}}$,= 30–50 K. Using three independent methods (CI, dust, CO) we derive a total molecular gas mass (corrected for magnification) of $M({{\rm H}_2})$ ≈ 1.2 ± 0.3 $\times$ 10$^{10}\,{{M}_{\odot}}$. Our observations suggest that the molecular disk extends beyond the region seen in CO(7–6) to a zone of more moderately excited molecular gas that dominates the global emission in CI, and the low J CO lines.