The neighbourhoods of extremely bright astronomical objects called quasars in the early Universe have been incompletely probed. Observations suggest that these regions harbour some of the most massive known galaxies. See Letter p.457
Quasars are the brightest non-transient emitters of light in the Universe. These remarkable objects have been discovered to exist as early as 750 million years after the Big Bang1,2 and are thought to be powered by black holes that are at least a billion times more massive than the Sun3,4. Since the discovery of such quasars in the early Universe, major questions have persisted regarding how these extreme objects formed in such a relatively short time. On page 457, Decarli et al.5 report the discovery of four massive star-forming galaxies around distant quasars. The results improve our understanding of the formation of quasars, and provide astronomers with a potentially powerful approach to studying the most massive galaxies in the early Universe.
The black holes that power quasars probably started their lives in miniature and grew exponentially by accretion6 — whereby matter close to a black hole cannot escape the strong gravitational field and is ultimately pulled into the black hole. To have assembled such a huge mass so quickly, the bright quasars discovered in the early Universe1,2 are thought to have resided in regions that had a particularly high density of matter6. Such an environment not only would have fuelled the rapid growth of the black holes powering these quasars, but also would have spurred the growth of galaxies in the quasars' immediate vicinity.
There have been many attempts to test this general model for early-quasar growth by looking for galaxies around luminous quasars in the early Universe. However, such efforts have been either limited by the poor spatial (line-of-sight) resolution of standard search methods for distant galaxies7,8,9,10,11, or restricted by techniques that are sensitive to only low-mass galaxies around quasars12,13. These studies have provided support for the idea that luminous quasars formed in high-density environments of the Universe, but the evidence has not been overwhelming14.
The Atacama Large Millimeter/submillimeter Array (ALMA) observatory in Chile has provided astronomers with a powerful tool with which to search for spectral-line emission from massive star-forming galaxies in the immediate vicinity of bright quasars. ALMA can achieve this feat because of its immense radiation-collecting area and design as an interferometer — an array of antennas that are linked to combine astronomical observations. The latter allows ALMA to obtain an emission spectrum at every position in space over its 25-arcsecond field of view (about 1.5% of the full Moon's angular diameter).
Decarli et al. used the unique capabilities of ALMA to observe 25 luminous quasars that existed less than 900 million years after the Big Bang — at redshifts larger than 6 (the higher the redshift of a cosmological object, the younger the Universe was when the object emitted its light). They then carried out a blind search for additional sources in the same volume of space as a quasar, with a separation of less than 70 kiloparsecs in the plane of the sky and 2 megaparsecs along Earth's line of sight.
The authors targeted an emission line associated with singly ionized carbon that is particularly prominent in spectra of the interstellar medium of galaxies in the present-day Universe and of massive star-forming galaxies in the early Universe15,16. In addition to finding strong emission from the studied quasars, Decarli et al. discovered four bright line-emitting sources in the same observations (Fig. 1). The authors justifiably interpreted these sources as massive star-forming galaxies.
Decarli and colleagues' identification of bright galaxies in the vicinity of distant quasars is important for at least two reasons. First, it provides definitive and convincing evidence that luminous high-redshift quasars formed in particularly dense environments, and gives us a concrete and salient answer to how rich these environments are in massive galaxies. By probing luminous quasars at redshifts greater than 6, Decarli et al. have substantially extended the results from previous studies that presented related findings at redshifts of 4.8 (ref. 17) and 5.3 (ref. 18).
Second, on the basis of a blind search around bright quasars, the authors have detected what are likely to be three of the most massive star-forming galaxies discovered so far at redshifts larger than 6. Remarkably, their efforts seem to have outstripped dedicated searches for such galaxies that have much larger fields of view (several times the full Moon's angular diameter)16,19. The authors' galaxies have high luminosities and lack contaminating light from their neighbouring quasars, potentially allowing a detailed characterization of galaxies in the early Universe to be made.
The discovered sources seem certain to be targets for ALMA, the James Webb Space Telescope and other facilities in the immediate future. Such follow-up observations are needed to make sense of the full scope of Decarli and colleagues' findings, because our understanding of the sources' properties is limited by the current data. Nevertheless, the authors have presented an extremely valuable pathfinding study that could bring about a fundamental change not only in our probing of the regions surrounding bright high-redshift quasars, but also in how astronomers look for the most massive galaxies in the early Universe.