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

Abstract

We report the detection of the ground state rotational emission of ammonia, ortho-NH₃ (J_K = 1₀ → 0₀) 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 NH₃ emission is broadly tracing the global molecular reservoir, but likely distributed in pockets of high density (n ≳ 5 × 10⁴ cm^-3). With a luminosity of 2.8 × 10⁶ L_⊙, the NH₃ 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 $L_{\rm NH_3}/L_{\rm IR}$ 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.