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Magnetic fields are fundamental to the evolution of galaxies, playing a key role in the astrophysics of the interstellar medium and star formation. Large-scale ordered magnetic fields have been mapped in the Milky Way and nearby galaxies, but it is not known how early in the Universe such structures form. Here we report the detection of linearly polarized thermal emission from dust grains in a strongly lensed, intrinsically luminous galaxy that is forming stars at a rate more than a thousand times that of the Milky Way at redshift 2.6, within 2.5 Gyr of the Big Bang. The polarized emission arises from the alignment of dust grains with the local magnetic field. The median polarization fraction is of order one per cent, similar to nearby spiral galaxies. Our observations support the presence of a 5 kiloparsec-scale ordered magnetic field with a strength of around 500uG or lower, orientated parallel to the molecular gas disk. This confirms that such structures can be rapidly formed in galaxies, early in cosmic history., Comment: Published in Nature. Online version available at https://www.nature.com/articles/s41586-023-06346-4

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

We present new observations with the Atacama Large Millimeter/sub-millimeter Array of the 122um and 205um fine-structure line emission of singly-ionised nitrogen in a strongly lensed starburst galaxy at z=2.6. The 122/205um [NII] line ratio is sensitive to electron density, n_e, in the ionised interstellar medium, and we use this to measure n_e~300cm^-3 averaged across the galaxy. This is over an order of magnitude higher than the Milky Way average, but comparable to localised Galactic star-forming regions. Combined with observations of the atomic carbon (CI(1-0)) and carbon monoxide (CO(4-3)) in the same system, we reveal the conditions in this intensely star-forming system. The majority of the molecular interstellar medium has been driven to high density, and the resultant conflagration of star formation produces a correspondingly dense ionised phase, presumably co-located with myriad HII regions that litter the gas-rich disk., Comment: 9 pages, 4 figures. Accepted for publication in ApJ