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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.

Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 ± 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole.

The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained by observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we detected ubiquitous, large (150-kiloparsec) halos of ionized oxygen surrounding star-forming galaxies; we found much less ionized oxygen around galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. Our data indicate that it is a basic component of nearly all star-forming galaxies that is removed or transformed during the quenching of star formation and the transition to passive evolution.

The observation of a flare of radiation from the centre of an inactive galaxy fits a model of the tidal disruption of a helium-rich stellar core and its accretion onto a black hole of about three million solar masses. A flare for black holes Central supermassive black holes in distant galaxies are normally invisible to us, but sometimes their presence becomes evident in the form of flares produced by the tidal disruption of a star being accreted to the black hole. Such events are rare, and often we see only the later stages of the encounter — but here, Gezari et al . report detailed monitoring of an ultraviolet and optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696, which was first seen on 31 May 2010, peaked in July and was over by September. The observed continuum is cooler than expected for a simple accreting debris disk, but the well sampled rise and decline of the light curve follows the predicted mass-accretion rate. The black hole has about two million solar masses and the disrupted star had a helium-rich stellar core, as the authors deduced from the spectroscopic signature of ionized helium from the unbound debris. The flare of radiation from the tidal disruption and accretion of a star can be used as a marker for supermassive black holes that otherwise lie dormant and undetected in the centres of distant galaxies 1 . Previous candidate flares 2 , 3 , 4 , 5 , 6 have had declining light curves in good agreement with expectations, but with poor constraints on the time of disruption and the type of star disrupted, because the rising emission was not observed. Recently, two ‘relativistic’ candidate tidal disruption events were discovered, each of whose extreme X-ray luminosity and synchrotron radio emission were interpreted as the onset of emission from a relativistic jet 7 , 8 , 9 , 10 . Here we report a luminous ultraviolet–optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696. The observed continuum is cooler than expected for a simple accreting debris disk, but the well-sampled rise and decay of the light curve follow the predicted mass accretion rate and can be modelled to determine the time of disruption to an accuracy of two days. The black hole has a mass of about two million solar masses, modulo a factor dependent on the mass and radius of the star disrupted. On the basis of the spectroscopic signature of ionized helium from the unbound debris, we determine that the disrupted star was a helium-rich stellar core.

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 ⊙ .

Black holes generate collimated, relativistic jets, which have been observed in gamma-ray bursts (GRBs), microquasars, and at the center of some galaxies [active galactic nuclei (AGN)]. How jet physics scales from stellar black holes in GRBs to the supermassive ones in AGN is still unknown. Here, we show that jets produced by AGN and GRBs exhibit the same correlation between the kinetic power carried by accelerated particles and the gamma-ray luminosity, with AGN and GRBs lying at the low- and high-luminosity ends, respectively, of the correlation. This result implies that the efficiency of energy dissipation in jets produced in black hole systems is similar over 10 orders of magnitude in jet power, establishing a physical analogy between AGN and GRBs.

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.

Gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, but most MBHs are considered dormant. Occasionally, a star passing too near an MBH is torn apart by gravitational forces, leading to a bright tidal disruption flare (TDF). Although the high-energy transient Sw 1644+57 initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that observations suggest a sudden accretion event onto a central MBH of mass about 10 6 to 10 7 solar masses. There is evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 to a temporary smaller-scale blazar.

Magnetic fields in galaxies are produced via the amplification of seed magnetic fields of unknown nature. The seed fields, which might exist in their initial form in the intergalactic medium, were never detected. We report a lower bound B ≥ 3 x 10⁻¹⁶ gauss on the strength of intergalactic magnetic fields, which stems from the nonobservation of GeV gamma-ray emission from electromagnetic cascade initiated by tera-electron volt gamma rays in intergalactic medium. The bound improves as λB⁻¹/² if magnetic field correlation length, λB, is much smaller than a megaparsec. This lower bound constrains models for the origin of cosmic magnetic fields.

Early star formation: steady progress Recent observations suggest that the massive galaxies that were at the height of their star-forming activity in the young Universe ten billion years ago formed their stars at surprisingly high rates. While such rates are commonly attributed to violent galaxy mergers, many of these galaxies are rotating discs, as extended as today's Milky Way, a structure that is incompatible with such a history. A new cosmological simulation suggests that these galaxies were 'stream fed', acquiring the material that was needed to fuel star formation as a steady flow of cold gas from the extended dark-matter haloes surrounding the galaxies. It is the rarer submillimetre galaxies, which form stars even more intensely, that are largely merger-induced starbursts. Massive galaxies in the young universe (ten billion years ago) formed stars at surprising intensities. Although this is commonly attributed to violent mergers, the properties of many of these galaxies are incompatible with mergers. This paper reports that they are 'stream-fed galaxies', growing via steady, narrow, cold gas streams. Unlike destructive mergers, the smoother flows are likely to keep the rotating disc configuration intact. Massive galaxies in the young Universe, ten billion years ago, formed stars at surprising intensities 1 , 2 . Although this is commonly attributed to violent mergers, the properties of many of these galaxies are incompatible with such events, showing gas-rich, clumpy, extended rotating disks not dominated by spheroids 1 , 2 , 3 , 4 , 5 . Cosmological simulations 6 and clustering theory 6 , 7 are used to explore how these galaxies acquired their gas. Here we report that they are ‘stream-fed galaxies’, formed from steady, narrow, cold gas streams that penetrate the shock-heated media of massive dark matter haloes 8 , 9 . A comparison with the observed abundance of star-forming galaxies implies that most of the input gas must rapidly convert to stars. One-third of the stream mass is in gas clumps leading to mergers of mass ratio greater than 1:10, and the rest is in smoother flows. With a merger duty cycle of 0.1, three-quarters of the galaxies forming stars at a given rate are fed by smooth streams. The rarer, submillimetre galaxies that form stars even more intensely 2 , 12 , 13 are largely merger-induced starbursts. Unlike destructive mergers, the streams are likely to keep the rotating disk configuration intact, although turbulent and broken into giant star-forming clumps that merge into a central spheroid 4 , 10 , 11 . This stream-driven scenario for the formation of discs and spheroids is an alternative to the merger picture.