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Using the IRAM Plateau de Bure Interferometer (PdBI) we have searched for the upper fine structure line of neutral carbon [] and   12 CO( J = 7 → 6) (ν rest = 806 GHz)  towards the submillimetre galaxies (SMGs) GN 20 (SMM J123711.9+622212,   z = 4.055 ) and GN 20.2 (SMM J123708.8+622202,   z = 4.051 ). The far‐infrared continuum is detected at 8σ significance in GN 20, with a flux density of   S  1.8 mm = 1.9 ± 0.2 mJy , while no continuum is detected in GN 20.2. Both sources are statistically undetected in both  and   12 CO( J = 7 → 6)  lines; we derive line luminosity limits for both C  i  and CO of   L ′≲ 2 × 10 10  K km s −1  pc 2  . Assuming carbon excitation temperatures of   T  ex = 30 K  (the galaxies' measured dust temperatures), we infer C  i  mass limits of   M  C  i   &lt; 5.4 × 10 6  M ⊙   (GN 20) and   M  C  i   &lt; 6.8 × 10 6  M ⊙  (GN 20.2) . The derived C  i  abundance limits are  &lt;1.8 × 10 −5   for GN 20 and  &lt;3.8 × 10 −5   for GN 20.2, implying that the systems have Milky Way level carbon enrichment ( X [C  i ]/ X [H 2 ]) or lower, similar to high‐redshift carbon‐detected systems (at  5 × 10 −5  ) but about 50 times less than the carbon enrichment of local starburst galaxies. Observations of GN 20 and GN 20.2 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 GN 20 and GN 20.2 are likely significantly different than starburst/ULIRG (ultraluminous infrared galaxy) environments in the local Universe yet similar to   z ∼ 2  SMGs. Unless gravitationally amplified examples can be found, observations of galaxies like GN 20 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 ∼ 4 .

A search for neutral carbon towards two z = 4.05 submillimetre galaxies, GN 20 and GN 20.2