A search for neutral carbon towards two z=4.05 submillimetre galaxies, GN20 and GN20.2

Using the IRAM Plateau de Bure Interferometer (PdBI) we have searched for the upper fine structure line of neutral carbon [CI((3)P(2) -> 3P1), nu(rest) = 809 GHz] and (12)CO(J = 7 -> 6) (nu(rest) = 806 GHz) towards the submillimetre galaxies (SMGs) GN20 (SMMJ123711.9+622212, z = 4.055) and GN20.2 (SMMJ123708.8+622202, z = 4.051). The far-infrared continuum is detected at 8 sigma significance in GN20, with a flux density of S(1.8mm) = 1.9 +/- 0.2mJy, while no continuum is detected in GN20.2. Both sources are statistically undetected in both CI((3)P(2) -> (3)P(1)) and (12)CO(J = 7 -> 6) lines; we derive line luminosity limits for both CI and CO of L' less than or similar to 2 x 10(10) K km s(-1) pc(2). Assuming carbon excitation temperatures of T(ex) = 30K (the galaxies' measured dust temperatures), we infer CI mass limits of M(CI) < 5.4 x 10(6) M(circle dot) (GN 20) and M(CI) < 6.8 x 10(6) M(circle dot) (GN 20.2). The derived C I abundance limits are < 1.8 x 10(-5) for GN20 and < 3.8 x 10(-5) for GN20.2, implying that the systems have Milky Way level carbon enrichment (X[CI]/X[H(2)]) or lower, similar to high-redshift carbon-detected systems (at 5 x 10(-5)) but about 50 times less than the carbon enrichment of local starburst galaxies. Observations of GN20 and GN20.2 in high-resolution MERLIN+VLA (Multi-Element Radio Linked Interferometer Network+Very Large Array) radio maps of GOODS-N (Great Observatories Origin Deep Survey-North) are used to further constrain the sizes and locations of active regions. We conclude that the physical gas properties of young rapidly evolving systems like GN20 and GN20.2 are likely significantly different than starburst/ULIRG (ultraluminous infrared galaxy) environments in the local Universe yet similar to z similar to 2 SMGs. Unless gravitationally amplified examples can be found, observations of galaxies like GN20 will require the order of magnitude increase in sensitivity of the Atacama Large Millimetre Array (ALMA) to constrain their C I and high-J CO content, despite the fact that they are the brightest systems at z similar to 4.