Author Correction: A massive core for a cluster of galaxies at a redshift of 4.3.

Change history: In this Letter, the Acknowledgements section should have included the following sentence: "The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.". This omission has been corrected online.

A massive core for a cluster of galaxies at a redshift of 4.3.

Massive galaxy clusters have been found that date to times as early as three billion years after the Big Bang, containing stars that formed at even earlier epochs1-3. The high-redshift progenitors of these galaxy clusters-termed 'protoclusters'-can be identified in cosmological simulations that have the highest overdensities (greater-than-average densities) of dark matter4-6. Protoclusters are expected to contain extremely massive galaxies that can be observed as luminous starbursts 7 . However, recent detections of possible protoclusters hosting such starbursts8-11 do not support the kind of rapid cluster-core formation expected from simulations 12 : the structures observed contain only a handful of starbursting galaxies spread throughout a broad region, with poor evidence for eventual collapse into a protocluster. Here we report observations of carbon monoxide and ionized carbon emission from the source SPT2349-56. We find that this source consists of at least 14 gas-rich galaxies, all lying at redshifts of 4.31. We demonstrate that each of these galaxies is forming stars between 50 and 1,000 times more quickly than our own Milky Way, and that all are located within a projected region that is only around 130 kiloparsecs in diameter. This galaxy surface density is more than ten times the average blank-field value (integrated over all redshifts), and more than 1,000 times the average field volume density. The velocity dispersion (approximately 410 kilometres per second) of these galaxies and the enormous gas and star-formation densities suggest that this system represents the core of a cluster of galaxies that was already at an advanced stage of formation when the Universe was only 1.4 billion years old. A comparison with other known protoclusters at high redshifts shows that SPT2349-56 could be building one of the most massive structures in the Universe today.

A dust-obscured massive maximum-starburst galaxy at a redshift of 6.34.

Massive present-day early-type (elliptical and lenticular) galaxies probably gained the bulk of their stellar mass and heavy elements through intense, dust-enshrouded starbursts--that is, increased rates of star formation--in the most massive dark-matter haloes at early epochs. However, it remains unknown how soon after the Big Bang massive starburst progenitors exist. The measured redshift (z) distribution of dusty, massive starbursts has long been suspected to be biased low in z owing to selection effects, as confirmed by recent findings of systems with redshifts as high as ~5 (refs 2-4). Here we report the identification of a massive starburst galaxy at z = 6.34 through a submillimetre colour-selection technique. We unambiguously determined the redshift from a suite of molecular and atomic fine-structure cooling lines. These measurements reveal a hundred billion solar masses of highly excited, chemically evolved interstellar medium in this galaxy, which constitutes at least 40 per cent of the baryonic mass. A 'maximum starburst' converts the gas into stars at a rate more than 2,000 times that of the Milky Way, a rate among the highest observed at any epoch. Despite the overall downturn in cosmic star formation towards the highest redshifts, it seems that environments mature enough to form the most massive, intense starbursts existed at least as early as 880 million years after the Big Bang.

Dusty starburst galaxies in the early Universe as revealed by gravitational lensing.

In the past decade, our understanding of galaxy evolution has been revolutionized by the discovery that luminous, dusty starburst galaxies were 1,000 times more abundant in the early Universe than at present. It has, however, been difficult to measure the complete redshift distribution of these objects, especially at the highest redshifts (z > 4). Here we report a redshift survey at a wavelength of three millimetres, targeting carbon monoxide line emission from the star-forming molecular gas in the direction of extraordinarily bright millimetre-wave-selected sources. High-resolution imaging demonstrates that these sources are strongly gravitationally lensed by foreground galaxies. We detect spectral lines in 23 out of 26 sources and multiple lines in 12 of those 23 sources, from which we obtain robust, unambiguous redshifts. At least 10 of the sources are found to lie at z > 4, indicating that the fraction of dusty starburst galaxies at high redshifts is greater than previously thought. Models of lens geometries in the sample indicate that the background objects are ultra-luminous infrared galaxies, powered by extreme bursts of star formation.

Neuronal activity related to memory in the intermediate and medial part of the hyperstriatum ventrale of the chick brain.

The intermediate and medial part of the hyperstriatum ventrale (IMHV) in the forebrain of the domestic chick Gallus gallus domesticus has been shown in previous studies to be critically involved in the learning process of imprinting. In the present study, 1-day-old chicks were imprinted by exposing them to one of two artificial stimuli. 24 h later each chick was given a preference test in which the two stimuli were presented in sequence. A preference score, an index of the strength of imprinting (i.e., of learning), was calculated from approach activity during the preference test. The chicks were divided into groups with low, medium and high preference scores (corresponding to weak, medium and strong learning respectively) and coded so that all subsequent procedures were performed blind. Each chick was then anaesthetized and spontaneous action potentials recorded extracellularly from groups of neurones in the left IMHV. The mean neuronal firing rate in chicks with medium and high preference scores was significantly greater than that in chicks with low preference scores. This relation between neuronal activity and preference score was not attributable to the chicks' locomotor activity per se. The results demonstrate an association between spontaneous electrical activity in the left IMHV and a measure of learning 1 day after the learning occurred.

Adaptation to telestereoscopic viewing measured by one-handed ball-catching performance.

A one-handed ball-catching task was used to study the disturbance of depth judgement induced by telestereoscopic viewing (ie viewing with increased effective interocular separation), the recovery of performance with experience in the telestereoscope, and the errors that subsequently arose when the telestereoscope was removed. The ball's trajectory was variable so that subjects had to control both the position and the timing of the grasp in order to catch the ball. On first wearing the telestereoscope, subjects closed the hand when the ball was approximately twice as far away from the eyes as the hand was. After fewer than twenty trials in the telestereoscope subjects were closing the hand at approximately the correct time and place, although rather more trials were needed for ball-catching performance to recover to normal. When the telestereoscope was removed there was an aftereffect, with subjects making the opposite errors to when they began the task. The existence of an aftereffect shows that the process of adaptation involves reevaluation rather than neglect of the misleading binocular information. Helmholtz's theory that telestereoscopes cause the world to be perceived as a scale model is considered. Initial misreaching is roughly consistent with this theory, but there are insufficient data to test it rigorously. Data from the aftereffect phase are clearly inconsistent with the theory. The results confirm the importance of binocular information in dynamic motor tasks, such as ball catching.