A cold, massive, rotating disk galaxy 1.5 billion years after the Big Bang
Massive disk galaxies like the Milky Way are expected to form at late times in traditional models of galaxy formation 1 , 2 , but recent numerical simulations suggest that such galaxies could form as early as a billion years after the Big Bang through the accretion of cold material and mergers 3 , 4 . Observationally, it has been difficult to identify disk galaxies in emission at high redshift 5 , 6 in order to discern between competing models of galaxy formation. Here we report imaging, with a resolution of about 1.3 kiloparsecs, of the 158-micrometre emission line from singly ionized carbon, the far-infrared dust continuum and the near-ultraviolet continuum emission from a galaxy at a redshift of 4.2603, identified by detecting its absorption of quasar light. These observations show that the emission arises from gas inside a cold, dusty, rotating disk with a rotational velocity of about 272 kilometres per second. The detection of emission from carbon monoxide in the galaxy yields a molecular mass that is consistent with the estimate from the ionized carbon emission of about 72 billion solar masses. The existence of such a massive, rotationally supported, cold disk galaxy when the Universe was only 1.5 billion years old favours formation through either cold-mode accretion or mergers, although its large rotational velocity and large content of cold gas remain challenging to reproduce with most numerical simulations 7 , 8 . A massive rotating disk galaxy was formed a mere 1.5 billion years after the Big Bang, a surprisingly short time after the origin of the Universe.