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FC641050-E0FB-4722-8D74-BED0F650885D

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

M. J. Doherty, J. E. Geach, R. J. Ivison, K. M. Menten, A. M. Jacob, J. Forbrich, S. Dye

Submitted on 19 September 2022

## Abstract

We report the detection of the ground state rotational emission of ammonia, ortho-NH${}_{3}$ $\left({J}_{K}={1}_{0}\to {0}_{0}\right)$ 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-diametre 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 ($n\gtrsim 5×{10}^{4}$ cm${}^{-3}$). With a luminosity of $2.8×{10}^{6}$ ${L}_{\odot }$, 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 ${L}_{\mathrm{N}{\mathrm{H}}_{3}}/{L}_{\mathrm{I}\mathrm{R}}$ 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.

## Preprint

Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letters

Subject: Astrophysics - Astrophysics of Galaxies