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-NH$_3$ $(J_K=1_0\rightarrow0_0)$ 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-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 ($n\gtrsim5\times10^4$ cm$^{-3}$). With a luminosity of $2.8\times10^{6}$ $L_\odot$, the NH$_3$ emission represents $2.5\times10^{-7}$ 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.