Resolving the H1 in damped Lyman alpha systems that power star formation

Reservoirs of dense atomic gas (primarily hydrogen) contain approximately 90 per cent of the neutral gas at a redshift of 3, and contribute to between 2 and 3 per cent of the total baryons in the Universe(1-4). These 'damped Lyman alpha systems'-so called because they absorb Lyman alpha photons within and from background sources-have been studied for decades, but only through absorption lines present in the spectra of background quasars and gamma-ray bursts(5-10). Such pencil beams do not constrain the physical extent of the systems. Here we report integral-field spectroscopy of a bright, gravitationally lensed galaxy at a redshift of 2.7 with two foreground damped Lyman alpha systems. These systems are greater than 238 kiloparsecs squared in extent, with column densities of neutral hydrogen varying by more than an order of magnitude on scales of less than 3 kiloparsecs. The mean column densities are between 10(20.46) and 10(20.84) centimetres squared and the total masses are greater than 5.5 x 10(8)-1.4 x 10(9) times the mass of the Sun, showing that they contain the necessary fuel for the next generation of star formation, consistent with relatively massive, low-luminosity primeval galaxies at redshifts greater than 2.