Abstract
Using the IRAM 30 m telescope we detected the lower fine structure line of neutral carbon (C I$(\rm ^3\!P_1\to{}^3\!P_0)$, $\nu_{\rm rest} = 492$ GHz) towards three high–redshift sources: IRAS FSC 10214 ($z=2.3$), SMM J14011+0252 ($z=2.5$) and H1413+117 (Cloverleaf quasar, $z=2.5$). SMM J14011+0252 is the first high–redshift, non–AGN source in which C I has been detected. The C I$(\rm ^3\!P_1\to{}^3\!P_0)$ line from FSC 10214 is almost an order of magnitude weaker than previously claimed, while our detection in the Cloverleaf is in good agreement with earlier observations. The C I$(\rm ^3\!P_1\to{}^3\!P_0)$ linewidths are similar to the CO widths, indicating that both lines trace similar regions of molecular gas on galactic scales. Derived C I$(\rm ^3\!P_1\to{}^3\!P_0)$ masses for all three objects are of order few $\times10^7$ $M_{\odot}$ and the implied C I$(\rm ^3\!P_1\to{}^3\!P_0)$/12CO($J=3\to2$) line luminosity ratio is about 0.2. This number is similar to values found in local galaxies. We derive a C I abundance of $5\times10^{-5}$ which implies significant metal enrichment of the cold molecular gas at redshifts 2.5 (age of the universe 2.7 Gyr). We conclude that the physical properties of systems at large lookback times are similar to today's starburst/AGN environments.