We have detected strong 21-cm absorption by cold hydrogen at a redshift of z = 0.656 (a distance of 7.3 billion light-years, or when the Universe was 44% its present age). The absorption is seen towards the background radio quasar/optical Quasi-Stellar Object (QSO) 3C336, which is at z = 0.927.

Absorption by warmer hydrogen gas, through the ultra-violet Lyman-alpha line, has previously been detected with the Hubble Space Telescope. However, the galaxy responsible for this absorption is somewhat of a mystery, since the only z = 0.656 galaxy in the field is located at a projected distance of 220,000 light-years from the QSO (Fig 1). This raises two distinct possibilities:

1. The ultra-violet absorption is due to other, very dim, galaxies, unseen in this very deep Hubble Space Telescope image, lying close to the QSO sight-line. There is a lot of space between us and the QSO (9.7 billion light-years, or 91,000,000,000,000,000,000,000 kilometres), and so to be at the same distance these must be part of a cluster in which the z = 0.656 galaxy is the only visible member.

2. The visible ‘galaxy’ at z = 0.656 is in fact just the central bulge of a much larger (spiral) galaxy which is surrounded by a large invisible disk of hydrogen spanning over most of the image (Fig. 1), which is too cold to emit any detectable radiation.

Either way, the shallow, wide absorption (Fig. 2) of the radiation originating from the background quasar's radio lobes indicates a vast amount of cold (< 60 Kelvin) gas. If the profile is the result of gravitationally bound gas, the total mass is around 1 trillion solar masses, similar to a large spiral such as our own Milky Way or the near-by Andromeda galaxy, although the profile could be result of a bound group of smaller galaxies totalling a similar mass. This gives a mass-to-light ratio of M/L > 400, (cf.  M/L = 1 for the Sun (by definition) and M/L > 10, for a typical spiral, which is 90% dark matter). This means that this large mass of cold absorbing gas is apparently devoid of stars.

Fig. 1: An HST optical image of the field towards 3C336. The overlaid contours show the radio emission at 8.4 and 1.4 GHz (outer contours). In our rest frame the absorption occurs at 0.86 GHz and so we expect the emission at this frequency to be more extended.
Fig. 2: The 21-cm hydrogen spectrum obtained with the Westerbork Synthesis Radio Telescope in the Netherlands. The absorption is relative to the background flux density of 3.55 x 10-26 W/m2 emitted by the background quasar.