Ammonia in the interstellar medium of a starbursting disc at z=2.6

We report the detection of the ground state rotational emission of ammonia, ortho-NH ₃ (J _K = 1 ₀ → 0 ₀) in a gravitationally lensed intrinsically hyperluminous star-bursting galaxy at z = 2.6. The integrated line profile is consistent with other molecular and atomic emission lines which have resolved kinematics well modelled by a 5 kpc-diameter rotating disc. This implies that the gas responsible for NH ₃ emission is broadly tracing the global molecular reservoir, but likely distributed in pockets of high density (n ≳ 5 × 10 ⁴ cm ⁻³). With a luminosity of 2.8 × 10 ⁶ L _☉, the NH ₃ emission represents 2.5 × 10 ⁻⁷ of the total infrared luminosity of the galaxy, comparable to the ratio observed in the Kleinmann-Low nebula in Orion and consistent with sites of massive star formation in the Milky Way. If L _NH3/L _IR serves as a proxy for the 'mode' of star formation, this hints that the nature of star formation in extreme starbursts in the early Universe is similar to that of Galactic star-forming regions, with a large fraction of the cold interstellar medium in this state, plausibly driven by a storm of violent disc instabilities in the gas-dominated disc. This supports the 'full of Orions' picture of star formation in the most extreme galaxies seen close to the peak epoch of stellar mass assembly.