Spectroscopic measurements of a galaxy that shines brightly at submillimetre wavelengths place it in the middle of a nascent galaxy cluster at a scant one billion years after the Big Bang. See Letter p.233
As in many scientific fields, but perhaps more than in most, advances in technology frequently drive progress in astronomy. The deployment of a new instrument or a capability can afford a completely different view of the Universe, opening a window onto an aspect of reality that was previously unsuspected or only theorized. One such advance occurred in the late 1990s with the advent of large-format submillimetre-wave cameras — particularly SCUBA, the Submillimetre Common-User Bolometer Array mounted on the James Clerk Maxwell Telescope in Mauna Kea, Hawaii — and the resulting discovery of a class of luminous yet elusive galaxies. On page 233 of this issue, Walter et al.1 describe an analysis that advances our understanding of this family of galaxies and closes a chapter on the story of their origins.
Early on, images taken with SCUBA revealed a population of galaxies that shine brightly at submillimetre wavelengths, prosaically named submillimetre galaxies (SMGs). One of the first deep SCUBA images to uncover SMGs was obtained2 in 1998 through observations of the Hubble Deep Field (Fig. 1), which is perhaps the most emblematic patch of sky observed by the Hubble Space Telescope. Surprisingly, SMGs were very faint, or even invisible, in optical images of this and other fields. Moreover, the SCUBA data provided no spectroscopic information, and thus no direct knowledge of the objects' distance or redshift, which are necessary to establish most of a galaxy's properties. What were these mysterious submillimetre sources?
The poster child for this question is HDF 850.1, the brightest SMG in the Hubble Deep Field. Walter and colleagues determine the galaxy's redshift, providing an important piece of the SMG puzzle. Analyses of the galaxies present in optical images at positions coincident, within errors, with those of SCUBA sources suggested from the outset that these sources are located at cosmological distances from Earth, corresponding to a time when the Universe was a fraction of its present age. The analyses also indicated that SMGs are tremendously powerful systems — with energy outputs of the order of 1012 times that of the Sun2,3. Such luminosities imply that, if they are powered mostly by star formation, SMGs are forming stars at rates several hundred times that of our Milky Way. Yet dust hides these starbursts from optical observations, which could therefore be missing an important fraction of the star-formation activity occurring in the early Universe3.
Although there are indirect indications of how far away SMGs are located, spectroscopy of these sources — and so direct measurement of their redshifts — has remained difficult. One successful indirect technique relies on the correlation between an object's radio and far-infrared luminosities, and uses radio interferometers to pinpoint its precise sky coordinates. Measurement of the source's redshift then requires long-exposure spectroscopy of its optical counterparts using the largest optical telescopes. The results from studies based on this indirect method indicate that most SMGs exist at redshifts of about 1.5–3, around the peak of star-formation activity in the Universe4. Because of the built-in observational selection effects of this technique, the question of how many SMGs lie at redshifts beyond 3 is still open. SMGs could shine in the submillimetre range at much greater distances than in the radio range, and so this approach might fail to detect distant SMGs.
In their study, Walter and collaborators used the IRAM Plateau de Bure Interferometer near Montmaur, France, to obtain direct spectroscopic measurements of HDF 850.1, pinpointing its redshift at 5.183 — a scant 1.1 billion years after the Big Bang. The technique relies on detecting the spectral lines associated with two rotational transitions of the carbon monoxide (CO) molecule to unambiguously determine the redshift. The method has recently become feasible thanks to the increasing information-transmission speed and computing power of (sub)millimetre-wave interferometric spectrographs, which can provide instantaneous spectra spanning tens of gigahertz. Determination of redshifts at millimetre and submillimetre wavelengths is a powerful tool, and will be used increasingly for the study of the early Universe with new advanced observatories, particularly the Atacama Large Millimetre Array in Chile's Atacama Desert.
The authors' spectroscopic analysis also revealed a spectral line associated with ionized carbon. This detection not only confirms the SMG redshift, but also supports the idea that the radiation previously detected by SCUBA stems from a monstrous starburst producing stars 800 times faster than the present-day Milky Way. Yet the source is completely invisible in the deepest optical images, in which the radiation from young stars is not detected because of obscuration by dust present in the source. This obscuring dust absorbs ultraviolet and optical light and re-emits it at far-infrared wavelengths, which are subsequently 'redshifted' (stretched) by the expansion of the Universe and so fall in the submillimetre band observed by SCUBA.
The authors find that the abundance of dust relative to gas in HDF 850.1 is similar to that found in the Milky Way. Dust is predominantly composed of heavy elements (elements other than hydrogen and helium), most of which originate from stellar explosions known as supernovae, which are caused by the death of massive stars. The unexpectedly high abundance of dust relative to gas at this early cosmic epoch indicates that heavy-element enrichment of the interstellar medium of HDF 850.1 proceeded at an extremely fast pace. The mechanism responsible for creating this much dust so quickly is not entirely understood.
A remarkable finding is that HDF 850.1 is not alone. It is sitting in the middle of a clear 'overdensity' of sources that also includes a rare quasar (an active galactic nucleus powered by a supermassive black hole). Twelve other sources have been identified within a redshift interval of about 0.03 from the galaxy's location. This interval corresponds to a local depth in the sky of about 2.5 megaparsecs (8 million light years), which is also the approximate extent of the structure in the sky. This overdensity is one of two5 spectroscopically verified candidate protoclusters at such high redshift, and thus represents one of the earliest structures known in the Universe — occurring after the epoch at which relic radiation from the Big Bang, known as the cosmic microwave background radiation, emerged. This discovery lends further support to the idea that the most luminous SMGs are signposts for rare overdensities in the overall structure of the Universe. Such extremely overdense regions are thought to have evolved into present-day massive clusters, and the brightest SMGs in these regions probably became today's giant elliptical galaxies at the centres of the clusters' potential well6. The conclusion of the story for HDF 850.1, then, is that this SMG is nothing less than an infant giant undergoing a growth spurt.