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A massive starburst galaxy with 100 billion solar masses of gas is identified at a redshift of 6.34; a ‘maximum starburst’ converts the gas into stars at a rate more than 2,000 times that of the Milky Way. A massive starburst galaxy unveiled The physical properties of the first massive starburst galaxies in the Universe provide important clues as to patterns of early cosmic structure formation. But as regions of intense star formation tend to be shrouded in dust, the search for such systems at very high redshift has been a major challenge. Now a massive starburst galaxy has been identified at a redshift z = 6.34, just 880 million years after the Big Bang when the Universe was one-sixteenth of its present age. Line-emission data reveal the presence of 100 billion solar masses of gas, equivalent to at least 40% of the galaxy's baryonic (visible matter) mass. The galaxy hosts an intense starburst, converting gas into stars at a rate more than 2,000 times that of the Milky Way. These findings are consistent with the theory that massive galaxies form via extreme starbursts in the early Universe. 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 1 , 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 5 , 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.

Dynamical modelling of the ultra-compact dwarf galaxy M60-UCD1 reveals the presence of a supermassive black hole; this suggests the object is a stripped galaxy nucleus and implies the existence of supermassive black holes in many other ultra-compact dwarf galaxies. Smallish galaxy hosts supermassive black hole The object M60-UCD1 is the brightest ultracompact dwarf galaxy (UCD) currently known and — at about 200 million solar masses — one of the most massive. Anil Seth et al . have used adaptive optics spectra to resolve the kinematics of M60-UCD1. They detect a supermassive black hole of 21 million solar masses at its centre. M60-UCD1 is thus the lowest-mass system known to host a supermassive black hole. The authors suggest that it may once have been at the centre of a larger galaxy that was later tidally torn apart by a massive neighbour. Their analysis also shows that M60-UCD1's stellar mass is consistent with its luminosity, implying that many other ultra-compact dwarf galaxies may contain previously unrecognized supermassive black holes. Ultra-compact dwarf galaxies are among the densest stellar systems in the Universe. These systems have masses of up to 2 × 10 8 solar masses, but half-light radii of just 3–50 parsecs 1 . Dynamical mass estimates show that many such dwarfs are more massive than expected from their luminosity 2 . It remains unclear whether these high dynamical mass estimates arise because of the presence of supermassive black holes or result from a non-standard stellar initial mass function that causes the average stellar mass to be higher than expected 3 , 4 . Here we report adaptive optics kinematic data of the ultra-compact dwarf galaxy M60-UCD1 that show a central velocity dispersion peak exceeding 100 kilometres per second and modest rotation. Dynamical modelling of these data reveals the presence of a supermassive black hole with a mass of 2.1 × 10 7 solar masses. This is 15 per cent of the object’s total mass. The high black hole mass and mass fraction suggest that M60-UCD1 is the stripped nucleus of a galaxy. Our analysis also shows that M60-UCD1’s stellar mass is consistent with its luminosity, implying a large population of previously unrecognized supermassive black holes in other ultra-compact dwarf galaxies 2 .

The hard spectra of X-ray binaries make them ineffective at heating primordial gas, which must have resulted in a delayed and spatially uniform heating during the epoch of reionization; this means that the signature of reionization in spectra of the 21-cm transition of atomic hydrogen will produce a more complex signal than has been predicted, including a distinct minimum at less than a millikelvin. A revised view of cosmic reionization The epoch of reionization was a one during which the neutral gas of the 'dark age' Universe became an ionized plasma. Simulations suggest that the 21-cm spectral transition of atomic hydrogen will show a clear fluctuation peak at a redshift and scale that mark the central stage of reionization. This prediction is based on the assumption that heating of the cosmic gas, which occurs before reionization, is caused by stellar remnants (particularly X-ray binaries) and reaches temperatures well above the cosmic microwave background of the time (30 kelvin). However, Rennan Barkana and colleagues report that the hard spectra of X-ray binaries (with more high-energy than low-energy photons) make such heating ineffective, resulting in a delayed and spatially uniform heating. In this new model, the 21-cm signature of reionization is modified to a more complex signal with a distinct minimum temperature (below 1 millikelvin) that marks the rise of the cosmic mean gas temperature above the microwave background. Models and simulations 1 , 2 , 3 , 4 of the epoch of reionization predict that spectra of the 21-centimetre transition of atomic hydrogen will show a clear fluctuation peak, at a redshift and scale, respectively, that mark the central stage of reionization and the characteristic size of ionized bubbles. This is based on the assumption 5 , 6 , 7 that the cosmic gas was heated by stellar remnants—particularly X-ray binaries—to temperatures well above the cosmic microwave background at that time (about 30 kelvin). Here we show instead that the hard spectra (that is, spectra with more high-energy photons than low-energy photons) of X-ray binaries 8 , 9 make such heating ineffective, resulting in a delayed and spatially uniform heating that modifies the 21-centimetre signature of reionization. Rather than looking for a simple rise and fall of the large-scale fluctuations (peaking at several millikelvin), we must expect a more complex signal also featuring a distinct minimum (at less than a millikelvin) that marks the rise of the cosmic mean gas temperature above the microwave background. Observing this signal, possibly with radio telescopes in operation today, will demonstrate the presence of a cosmic background of hard X-rays at that early time.

A study of the stellar kinematics of a large sample of early-type galaxies provides evidence that the stellar initial mass function depends on the galaxy’s stellar mass-to-light ratio and thus is strongly dependent on the galaxy’s formation history. Galactic influence on star formation For decades, the stellar initial mass function (IMF), which describes the mass distribution of stars at the time of their formation, had been assumed to be independent of the type of galaxy in which the stars formed. But in recent years, evidence in Nature ( http://go.nature.com/dh1vc5 ) and elsewhere has pointed to variation in IMF between galaxy types. Now, a survey of a mass-selected sample of 260 early galaxies reveals systematic variation in IMF. The authors suggest that models assuming a constant IMF need to be revised to explain how stars seem to sense what kind of galaxy they are creating. Much of our knowledge of galaxies comes from analysing the radiation emitted by their stars, which depends on the present number of each type of star in the galaxy. The present number depends on the stellar initial mass function (IMF), which describes the distribution of stellar masses when the population formed, and knowledge of it is critical to almost every aspect of galaxy evolution. More than 50 years after the first IMF determination 1 , no consensus has emerged on whether it is universal among different types of galaxies 2 . Previous studies indicated that the IMF and the dark matter fraction in galaxy centres cannot both be universal 3 , 4 , 5 , 6 , 7 , but they could not convincingly discriminate between the two possibilities. Only recently were indications found that massive elliptical galaxies may not have the same IMF as the Milky Way 8 . Here we report a study of the two-dimensional stellar kinematics for the large representative ATLAS 3D sample 9 of nearby early-type galaxies spanning two orders of magnitude in stellar mass, using detailed dynamical models. We find a strong systematic variation in IMF in early-type galaxies as a function of their stellar mass-to-light ratios, producing differences of a factor of up to three in galactic stellar mass. This implies that a galaxy’s IMF depends intimately on the galaxy's formation history.

Birth of a black-hole relativistic jet Two groups report observations of the X-ray source Swift J164449.3+573451, which was discovered when it triggered the Swift Burst Alert Telescope on 28 March 2011. Burrows et al . report that the source has increased in brightness in the X-ray band more than 10,000-fold since 1990, and by more than 100-fold since early 2010. They conclude that we are observing the onset of relativistic jet activity from a supermassive black hole. Zauderer et al . arrive at a similar conclusion based on their observation of a radio transient associated with the source, and extensive monitoring at centimetre to millimetre wavelengths during the first month of its evolution. They estimate the mass of the black hole at around 10 6 solar masses. Active galactic nuclei, which are powered by long-term accretion onto central supermassive black holes, produce 1 relativistic jets with lifetimes of at least one million years, and the observation of the birth of such a jet is therefore unlikely. Transient accretion onto a supermassive black hole, for example through the tidal disruption 2 , 3 of a stray star, thus offers a rare opportunity to study the birth of a relativistic jet. On 25 March 2011, an unusual transient source (Swift J164449.3+573451) was found 4 , potentially representing 5 , 6 such an accretion event. Here we report observations spanning centimetre to millimetre wavelengths and covering the first month of evolution of a luminous radio transient associated with Swift J164449.3+573451. The radio transient coincides 7 with the nucleus of an inactive galaxy. We conclude that we are seeing a newly formed relativistic outflow, launched by transient accretion onto a million-solar-mass black hole. A relativistic outflow is not predicted in this situation, but we show that the tidal disruption of a star naturally explains the observed high-energy properties and radio luminosity and the inferred rate of such events. The weaker beaming in the radio-frequency spectrum relative to γ-rays or X-rays suggests that radio searches may uncover similar events out to redshifts of z  ≈ 6.

Two nearby black holes are the most massive yet found, with masses—of around ten billion solar masses—considerably greater than predicted by conventional methods relating black-hole mass with the stellar velocity dispersion and bulge luminosity of the host galaxy. New 'record' for black hole size At 6.3 billion solar masses, the central black hole in the supergiant elliptical galaxy M87 has been regarded as the most massive known black hole in the Universe for more than three decades. This paper reports two galaxies containing black holes that exceed that figure. NGC 3842 has a central black hole of 9.7 billion solar masses, and NGC 4889 has one of comparable or greater mass. Indications of such massive black holes have existed in the early Universe from luminous quasars but have eluded our detection; these results are the first to connect these early massive black holes to host galaxies in the nearby Universe. Observational work conducted over the past few decades indicates that all massive galaxies have supermassive black holes at their centres. Although the luminosities and brightness fluctuations of quasars in the early Universe suggest that some were powered by black holes with masses greater than 10 billion solar masses 1 , 2 , the remnants of these objects have not been found in the nearby Universe. The giant elliptical galaxy Messier 87 hosts the hitherto most massive known black hole, which has a mass of 6.3 billion solar masses 3 , 4 . Here we report that NGC 3842, the brightest galaxy in a cluster at a distance from Earth of 98 megaparsecs, has a central black hole with a mass of 9.7 billion solar masses, and that a black hole of comparable or greater mass is present in NGC 4889, the brightest galaxy in the Coma cluster (at a distance of 103 megaparsecs). These two black holes are significantly more massive than predicted by linearly extrapolating the widely used correlations between black-hole mass and the stellar velocity dispersion or bulge luminosity of the host galaxy 5 , 6 , 7 , 8 , 9 . Although these correlations remain useful for predicting black-hole masses in less massive elliptical galaxies, our measurements suggest that different evolutionary processes influence the growth of the largest galaxies and their black holes.

A primitive star in the Galactic halo For theoretical reasons and because of an apparent absence of stars with low metallicities (abundance of elements heavier than helium), it has been suggested that low-mass stars cannot form until the interstellar medium has been enriched above a critical value of metallicity, Z , estimated as lying between 1.5 × 10 −8 and 1.5 × 10 −6 . Caffau et al . now describe a star with a primordial-type composition, suggesting that, in fact, long-lived low-mass stars can form when the concentration of complex nuclei is low. The star is in the Galactic halo, has very low metallicity ( Z ≤ 6.9 × 10 −7 ) and no enrichment of carbon, nitrogen or oxygen. Its chemical composition should provide clues as to how the first stars formed. The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium 1 ; almost all other elements were subsequently created in stars and supernovae. The mass fraction of elements more massive than helium, Z , is known as ‘metallicity’. A number of very metal-poor stars has been found 2 , 3 , some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen 4 , 5 , 6 . For theoretical reasons 7 , 8 and because of an observed absence of stars with Z  < 1.5 × 10 −5 , it has been suggested that low-mass stars cannot form from the primitive interstellar medium until it has been enriched above a critical value of Z , estimated to lie in the range 1.5 × 10 −8 to 1.5 × 10 −6 (ref. 8 ), although competing theories claiming the contrary do exist 9 . (We use ‘low-mass’ here to mean a stellar mass of less than 0.8 solar masses, the stars that survive to the present day.) Here we report the chemical composition of a star in the Galactic halo with a very low Z (≤ 6.9 × 10 −7 , which is 4.5 × 10 −5 times that of the Sun 10 ) and a chemical pattern typical of classical extremely metal-poor stars 2 , 3 —that is, without enrichment of carbon, nitrogen and oxygen. This shows that low-mass stars can be formed at very low metallicity, that is, below the critical value of Z . Lithium is not detected, suggesting a low-metallicity extension of the previously observed trend in lithium depletion 11 . Such lithium depletion implies that the stellar material must have experienced temperatures above two million kelvin in its history, given that this is necessary to destroy lithium.

The lesser lights of nearby galaxies The bulk of the stellar population is comprised of dwarf stars. This fact is reflected in the stellar initial mass function (IMF), which describes the mass distribution of stars at the time of their formation. The IMF is reasonably well constrained in the disk of the Milky Way, but we have little direct information on the IMF in other galaxies and at earlier cosmic epochs. Pieter van Dokkum and Charlie Conroy have now spectroscopically detected the signature of the many 'invisible' stars in the light of nearby elliptical galaxies by observing the Na I doublet and the Wing–Ford molecular FeH band, lines which are strong in stars with masses of less than a third that of the Sun. The data imply that these smaller stars account for more than 80% of the total number of stars and contribute more than 60% of total stellar mass in elliptical galaxies. The stellar initial mass function describes the mass distribution of stars at the time of their formation. This study reports observations of the Na I doublet and the Wing-Ford molecular FeH band in the spectra of elliptical galaxies. These lines are strong in stars with masses <0.3 solar masses and weak or absent in all other types of stars. The direct detection of the light of low-mass stars implies that they are very abundant in elliptical galaxies, making up >80% of the total number of stars and contributing >60% of the total stellar mass. The stellar initial mass function (IMF) describes the mass distribution of stars at the time of their formation and is of fundamental importance for many areas of astrophysics. The IMF is reasonably well constrained in the disk of the Milky Way 1 but we have very little direct information on the form of the IMF in other galaxies and at earlier cosmic epochs. Here we report observations of the Na  i doublet 2 , 3 and the Wing–Ford molecular FeH band 4 , 5 in the spectra of elliptical galaxies. These lines are strong in stars with masses less than 0.3 M ⊙ (where M ⊙ is the mass of the Sun) and are weak or absent in all other types of stars 5 , 6 , 7 . We unambiguously detect both signatures, consistent with previous studies 8 that were based on data of lower signal-to-noise ratio. The direct detection of the light of low-mass stars implies that they are very abundant in elliptical galaxies, making up over 80% of the total number of stars and contributing more than 60% of the total stellar mass. We infer that the IMF in massive star-forming galaxies in the early Universe produced many more low-mass stars than the IMF in the Milky Way disk, and was probably slightly steeper than the Salpeter form 9 in the mass range 0.1 M ⊙ to 1 M ⊙ .

Early galaxy revealed in Hubble data An ultra-deep search through the full Hubble Ultra Deep Field data set has uncovered a galaxy with a redshift of z ≈ 10, equivalent to an age of only 500 million years after the Big Bang. The data also provide strong constraints on the volume density of galaxies — and hence the star formation rate — at this time. The authors conclude that the star formation rate increased by a factor of ten in the time between z ≈ 10 and z ≈ 8, implying that this period in the heart of the reionization epoch was one in which galaxies were evolving very rapidly. Here, the full two-year Hubble Ultra Deep Field (HUDF09) data are used to conduct an ultra-deep search for z ≈10 galaxies in the heart of the reionization epoch, only 500 million years after the Big Bang. One possible z ≈10 galaxy candidate is found. It is also shown that regardless of source detections, the star formation rate density is much smaller (∼10%) at this time than it is just ∼200 million years later at z ≈8. The 100–200 million years prior to z ≈10 is clearly a crucial phase in the assembly of the earliest galaxies. Searches for very-high-redshift galaxies over the past decade have yielded a large sample of more than 6,000 galaxies existing just 900–2,000 million years (Myr) after the Big Bang (redshifts 6 >  z  > 3; ref. 1 ). The Hubble Ultra Deep Field (HUDF09) data 2 , 3 have yielded the first reliable detections of z  ≈ 8 galaxies 3 , 4 , 5 , 6 , 7 , 8 , 9 that, together with reports of a γ-ray burst at z  ≈ 8.2 (refs 10 , 11 ), constitute the earliest objects reliably reported to date. Observations of z  ≈ 7–8 galaxies suggest substantial star formation at z  > 9–10 (refs 12 , 13 ). Here we use the full two-year HUDF09 data to conduct an ultra-deep search for z  ≈ 10 galaxies in the heart of the reionization epoch, only 500 Myr after the Big Bang. Not only do we find one possible z  ≈ 10 galaxy candidate, but we show that, regardless of source detections, the star formation rate density is much smaller (∼10%) at this time than it is just ∼200 Myr later at z  ≈ 8. This demonstrates how rapid galaxy build-up was at z  ≈ 10, as galaxies increased in both luminosity density and volume density from z  ≈ 10 to z  ≈ 8. The 100–200 Myr before z  ≈ 10 is clearly a crucial phase in the assembly of the earliest galaxies.

Cosmic-scale test for general relativity Testing general relativity on the large scales of the Universe remains a fundamental challenge to modern cosmology. The theoretical framework of cosmology is defined by gravity, for which general relativity is the current model. Wojtak et al . now show that a classical test of general relativity — the gravitational redshift experienced by photons propagating outwards from a gravitational potential well — provides a direct means of testing gravity on scales of several megaparsecs, independent of cosmology. Their observations of the gravitational redshift of light coming from galaxies in clusters at the 99% confidence level agree with the predictions of general relativity, and are inconsistent with alternative models designed to avoid the presence of dark matter. The theoretical framework of cosmology is mainly defined by gravity, of which general relativity is the current model. Recent tests of general relativity within the Lambda Cold Dark Matter (ΛCDM) model have found a concordance between predictions and the observations of the growth rate and clustering of the cosmic web 1 , 2 . General relativity has not hitherto been tested on cosmological scales independently of the assumptions of the ΛCDM model. Here we report an observation of the gravitational redshift of light coming from galaxies in clusters at the 99 per cent confidence level, based on archival data 3 . Our measurement agrees with the predictions of general relativity and its modification created to explain cosmic acceleration without the need for dark energy (the f ( R ) theory 4 ), but is inconsistent with alternative models designed to avoid the presence of dark matter 5 , 6 .