Compact [C II] emitters around a C IV absorption complex at redshift 5.7.

The physical conditions of the circumgalactic medium are investigated by means of intervening absorption-line systems in the spectrum of background quasi-stellar objects (QSOs) out to the epoch of cosmic reionization1-4. A correlation between the ionization state of the absorbing gas and the nature of the nearby galaxies has been suggested by the sources detected in either Lyα or [C II] 158 µm near to, respectively, highly ionized and neutral absorbers5,6. This is also probably linked to the global changes in the incidence of absorption systems of different types and the process of cosmic reionization7-12. Here we report the detection of two [C II]-emitting galaxies at redshift z ≈ 5.7 that are associated with a complex, high-ionization C IV absorption system. These objects are part of an overdensity of galaxies and have compact sizes (<2.4 kpc) and narrow linewidths (full width at half maximum (FWHM) ≈ 62-64 km s-1). Hydrodynamic simulations predict that similar narrow [C II] emission may arise from the heating of small (≲3 kpc) clumps of cold neutral medium or a compact photodissociation region13,14. The lack of counterparts in the rest-frame ultraviolet (UV) indicates severe obscuration of the sources that are exciting the [C II] emission. These results may suggest a connection between the properties of the [C II] emission, the rare overdensity of galaxies and the unusual high ionization state of the gas in this region.

Resolving the H I in damped Lyman α 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 Universe1-4. These 'damped Lyman α systems'-so called because they absorb Lyman α photons within and from background sources-have been studied for decades, but only through absorption lines present in the spectra of background quasars and γ-ray bursts5-10. 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 α 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 1020.46 and 1020.84 centimetres squared and the total masses are greater than 5.5 × 108-1.4 × 109 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.

The low density and magnetization of a massive galaxy halo exposed by a fast radio burst.

Present-day galaxies are surrounded by cool and enriched halo gas extending for hundreds of kiloparsecs. This halo gas is thought to be the dominant reservoir of material available to fuel future star formation, but direct constraints on its mass and physical properties have been difficult to obtain. We report the detection of a fast radio burst (FRB 181112), localized with arcsecond precision, that passes through the halo of a foreground galaxy. Analysis of the burst shows that the halo gas has low net magnetization and turbulence. Our results imply predominantly diffuse gas in massive galactic halos, even those hosting active supermassive black holes, contrary to some previous results.