Galaxy mass deduced from the structure of Einstein ring MG1654+1346

ELLIPTICAL galaxies acting as gravitational lenses occasionally produce spectacular images-Einstein rings-of distant objects. Giant arcs(1) and radio rings(2) have been observed. A wide variety of image morphologies is possible, the generation of which is qualitatively understood in terms of large magnifications at caustic and critical lines in the lensing geometry(3-5). From the angular size of the image, and with knowledge of the distances of the lensed and lensing object, rough estimates of the mass of the lensing galaxy can be obtained. We have made high-resolution radio-interferometric observations of MG1654 + 1346 (ref. 6) a radio quasar in near-perfect alignment with an elliptical galaxy, and show that radio emission is distorted into a narrow Einstein ring that lies within a diamond-shaped region bounded by caustic lines. From the details of the ring structure, a new model for the lensing geometry is deduced which leads to a more accurate estimate of the mass of the lensing galaxy.     

An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy

The unusual morphology of the Andromeda galaxy (Messier 31, the closest spiral galaxy to the Milky Way) has long been an enigma. Although regarded for decades as showing little evidence of a violent history, M31 has a well-known outer ring of star formation at a radius of ten kiloparsecs whose centre is offset from the galaxy nucleus. In addition, the outer galaxy disk is warped, as seen at both optical and radio wavelengths. The halo contains numerous loops and ripples. Here we report the presence of a second, inner dust ring with projected dimensions of 1.5 x 1 kiloparsecs and offset by about half a kiloparsec from the centre of the galaxy (based upon an analysis of previously-obtained data). The two rings appear to be density waves propagating in the disk. Numerical simulations indicate that both rings result from a companion galaxy plunging through the centre of the disk of M31. The most likely interloper is M32. Head-on collisions between galaxies are rare, but it appears nonetheless that one took place 210 million years ago in our Local Group of galaxies.     

Gravitational detection of a low-mass dark satellite galaxy at cosmological distance

The mass function of dwarf satellite galaxies that are observed around Local Group galaxies differs substantially from simulations based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard. A massive dark satellite in an early-type lens galaxy at a redshift of 0.222 was recently found using a method based on gravitational lensing, suggesting that the mass fraction contained in substructure could be higher than is predicted from simulations. The lack of very low-mass detections, however, prohibited any constraint on their mass function. Here we report the presence of a (1.9?±?0.1)?×?10(8) M dark satellite galaxy in the Einstein ring system JVAS B1938+666 (ref. 11) at a redshift of 0.881, where M denotes the solar mass. This satellite galaxy has a mass similar to that of the Sagittarius galaxy, which is a satellite of the Milky Way. We determine the logarithmic slope of the mass function for substructure beyond the local Universe to be 1.1(+0.6)(-0.4), with an average mass fraction of 3.3(+3.6)(-1.8) per cent, by combining data on both of these recently discovered galaxies. Our results are consistent with the predictions from cold dark matter simulations at the 95 per cent confidence level, and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter.     

A massive, cooling-flow-induced starburst in the core of a luminous cluster of galaxies

In the cores of some clusters of galaxies the hot intracluster plasma is dense enough that it should cool radiatively in the cluster's lifetime, leading to continuous 'cooling flows' of gas sinking towards the cluster centre, yet no such cooling flow has been observed. The low observed star-formation rates and cool gas masses for these 'cool-core' clusters suggest that much of the cooling must be offset by feedback to prevent the formation of a runaway cooling flow. Here we report X-ray, optical and infrared observations of the galaxy cluster SPT-CLJ2344-4243 (ref. 11) at redshift z = 0.596. These observations reveal an exceptionally luminous (8.2?×?10(45)?erg?s(-1)) galaxy cluster that hosts an extremely strong cooling flow (around 3,820 solar masses a year). Further, the central galaxy in this cluster appears to be experiencing a massive starburst (formation of around 740 solar masses a year), which suggests that the feedback source responsible for preventing runaway cooling in nearby cool-core clusters may not yet be fully established in SPT-CLJ2344-4243. This large star-formation rate implies that a significant fraction of the stars in the central galaxy of this cluster may form through accretion of the intracluster medium, rather than (as is currently thought) assembling entirely via mergers.     

A filament of dark matter between two clusters of galaxies

It is a firm prediction of the concordance cold-dark-matter cosmological model that galaxy clusters occur at the intersection of large-scale structure filaments. The thread-like structure of this 'cosmic web' has been traced by galaxy redshift surveys for decades. More recently, the warm–hot intergalactic medium (a sparse plasma with temperatures of 10(5) kelvin to 10(7) kelvin) residing in low-redshift filaments has been observed in emission and absorption. However, a reliable direct detection of the underlying dark-matter skeleton, which should contain more than half of all matter, has remained elusive, because earlier candidates for such detections were either falsified or suffered from low signal-to-noise ratios and unphysical misalignments of dark and luminous matter. Here we report the detection of a dark-matter filament connecting the two main components of the Abell 222/223 supercluster system from its weak gravitational lensing signal, both in a non-parametric mass reconstruction and in parametric model fits. This filament is coincident with an overdensity of galaxies and diffuse, soft-X-ray emission, and contributes a mass comparable to that of an additional galaxy cluster to the total mass of the supercluster. By combining this result with X-ray observations, we can place an upper limit of 0.09 on the hot gas fraction (the mass of X-ray-emitting gas divided by the total mass) in the filament.     

Are some BL Lacs artefacts of gravitational lensing?

WE suggested in 1985 that a significant fraction of BL Lacertae objects, a kind of lineless quasar, seen in nearby galaxies are in fact images, gravitationally lensed and substantially amplified by stars in the nearby galaxy, of background objects, optically violent variable (OVV) quasars at redshifts z > 1 (ref. 1). This hypothesis was made on the basis of certain general similarities between BL Lacs and O Ws, but for two recently observed BL Lacs(2,3) a strong case can be made that the accompanying elliptical galaxy is a foreground object. In addition, we argue that the distribution of BL Lac redshifts is hard to understand without gravitational lensing, unless we happen to be at a very local maximum of the spatial cosmic distribution of BL Lacs. Our analysis also indicates that the galaxies whose stars are likely to act as microlenses will be found in two peaks, one nearby, with redshift 0.05-0.10, and the other near the distant quasar.     

Rapid evolution of the most luminous galaxies during the first 900 million years

The first 900 million years (Myr) to redshift z approximately 6 (the first seven per cent of the age of the Universe) remains largely unexplored for the formation of galaxies. Large samples of galaxies have been found at z approximately 6 (refs 1-4) but detections at earlier times are uncertain and unreliable. It is not at all clear how galaxies built up from the first stars when the Universe was about 300 Myr old (z approximately 12-15) to z approximately 6, just 600 Myr later. Here we report the results of a search for galaxies at z approximately 7-8, about 700 Myr after the Big Bang, using the deepest near-infrared and optical images ever taken. Under conservative selection criteria we find only one candidate galaxy at z approximately 7-8, where ten would be expected if there were no evolution in the galaxy population between z approximately 7-8 and z approximately 6. Using less conservative criteria, there are four candidates, where 17 would be expected with no evolution. This demonstrates that very luminous galaxies are quite rare 700 Myr after the Big Bang. The simplest explanation is that the Universe is just too young to have built up many luminous galaxies at z approximately 7-8 by the hierarchical merging of small galaxies.     

An actively accreting massive black hole in the dwarf starburst galaxy Henize 2-10

Supermassive black holes are now thought to lie at the heart of every giant galaxy with a spheroidal component, including our own Milky Way. The birth and growth of the first 'seed' black holes in the earlier Universe, however, is observationally unconstrained and we are only beginning to piece together a scenario for their subsequent evolution. Here we report that the nearby dwarf starburst galaxy Henize?2-10 (refs 5 and 6) contains a compact radio source at the dynamical centre of the galaxy that is spatially coincident with a hard X-ray source. From these observations, we conclude that Henize?2-10 harbours an actively accreting central black hole with a mass of approximately one million solar masses. This nearby dwarf galaxy, simultaneously hosting a massive black hole and an extreme burst of star formation, is analogous in many ways to galaxies in the infant Universe during the early stages of black-hole growth and galaxy mass assembly. Our results confirm that nearby star-forming dwarf galaxies can indeed form massive black holes, and that by implication so can their primordial counterparts. Moreover, the lack of a substantial spheroidal component in Henize?2-10 indicates that supermassive black-hole growth may precede the build-up of galaxy spheroids.     

A gravitationally lensed quasar with quadruple images separated by 14.62 arcseconds

Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures (that is, clusters of galaxies and even larger assemblies) predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004 + 4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model.     

Direct detection of a microlens in the Milky Way.

The nature of dark matter remains mysterious, with luminous material accounting for at most approximately 25 per cent of the baryons in the Universe. We accordingly undertook a survey looking for the microlensing of stars in the Large Magellanic Cloud (LMC) to determine the fraction of Galactic dark matter contained in massive compact halo objects (MACHOs). The presence of the dark matter would be revealed by gravitational lensing of the light from an LMC star as the foreground dark matter moves across the line of sight. The duration of the lensing event is the key observable parameter, but gives non-unique solutions when attempting to estimate the mass, distance and transverse velocity of the lens. The survey results to date indicate that between 8 and 50 per cent of the baryonic mass of the Galactic halo is in the form of MACHOs (ref. 3), but removing the degeneracy by identifying a lensing object would tighten the constraints on the mass in MACHOs. Here we report a direct image of a microlens, revealing it to be a nearby low-mass star in the disk of the Milky Way. This is consistent with the expected frequency of nearby stars acting as lenses, and demonstrates a direct determination of a lens mass from a microlensing event. Complete solutions such as this for halo microlensing events will probe directly the nature of the MACHOs.     

基于平面模板的摄像机标定新方法

提出的一种基于平面模板摄像机标定的新方法是先通过假设图像中心坐标为模型的成像中心,由摄像机线性模型求解中间变量然后分解独立的摄像机参数,并将求得的参数作为带畸变的非线性模型初始值进行优化求解,最终得到摄像机模型所有参数的精确值。通过仿真实验和真实实验对本方法与其他方法进行测试与比较,结果表明算法有相似的收敛性,在噪声环境中有更好鲁棒性,并且具有更小反投影误差。     

Rapid growth of black holes in massive star-forming galaxies

The tight relationship between the masses of black holes and galaxy spheroids in nearby galaxies implies a causal connection between the growth of these two components. Optically luminous quasars host the most prodigious accreting black holes in the Universe, and can account for greater than or approximately equal to 30 per cent of the total cosmological black-hole growth. As typical quasars are not, however, undergoing intense star formation and already host massive black holes (> 10(8)M(o), where M(o) is the solar mass), there must have been an earlier pre-quasar phase when these black holes grew (mass range approximately (10(6)-10(8))M(o)). The likely signature of this earlier stage is simultaneous black-hole growth and star formation in distant (redshift z > 1; >8 billion light years away) luminous galaxies. Here we report ultra-deep X-ray observations of distant star-forming galaxies that are bright at submillimetre wavelengths. We find that the black holes in these galaxies are growing almost continuously throughout periods of intense star formation. This activity appears to be more tightly associated with these galaxies than any other coeval galaxy populations. We show that the black-hole growth from these galaxies is consistent with that expected for the pre-quasar phase.     

A close nuclear black-hole pair in the spiral galaxy NGC 3393

The current picture of galaxy evolution advocates co-evolution of galaxies and their nuclear massive black holes, through accretion and galactic merging. Pairs of quasars, each with a massive black hole at the centre of its galaxy, have separations of 6,000 to 300,000 light years (refs 2 and 3; 1 parsec = 3.26 light years) and exemplify the first stages of this gravitational interaction. The final stages of the black-hole merging process, through binary black holes and final collapse into a single black hole with gravitational wave emission, are consistent with the sub-light-year separation inferred from the optical spectra and light-variability of two such quasars. The double active nuclei of a few nearby galaxies with disrupted morphology and intense star formation (such as NGC 6240 with a separation of about 2,600 light years and Mrk 463 with a separation of about 13,000 light years between the nuclei) demonstrate the importance of major mergers of equal-mass spiral galaxies in this evolution; such mergers lead to an elliptical galaxy, as in the case of the double-radio-nucleus elliptical galaxy 0402+379 (with a separation of about 24 light years between the nuclei). Minor mergers of a spiral galaxy with a smaller companion should be a more common occurrence, evolving into spiral galaxies with active massive black-hole pairs, but have hitherto not been seen. Here we report the presence of two active massive black holes, separated by about 490 light years, in the Seyfert galaxy NGC 3393 (50 Mpc, about 160 million light years). The regular spiral morphology and predominantly old circum-nuclear stellar population of this galaxy, and the closeness of the black holes embedded in the bulge, provide a hitherto missing observational point to the study of galaxy/black hole evolution. Comparison of our observations with current theoretical models of mergers suggests that they are the result of minor merger evolution.     

A size of approximately 1 au for the radio source Sgr A* at the centre of the Milky Way

Although it is widely accepted that most galaxies have supermassive black holes at their centres, concrete proof has proved elusive. Sagittarius A* (Sgr A*), an extremely compact radio source at the centre of our Galaxy, is the best candidate for proof, because it is the closest. Previous very-long-baseline interferometry observations (at 7 mm wavelength) reported that Sgr A* is approximately 2 astronomical units (au) in size, but this is still larger than the 'shadow' (a remarkably dim inner region encircled by a bright ring) that should arise from general relativistic effects near the event horizon of the black hole. Moreover, the measured size is wavelength dependent. Here we report a radio image of Sgr A* at a wavelength of 3.5 mm, demonstrating that its size is approximately 1 au. When combined with the lower limit on its mass, the lower limit on the mass density is 6.5 x 10(21)M(o) pc(-3) (where M(o) is the solar mass), which provides strong evidence that Sgr A* is a supermassive black hole. The power-law relationship between wavelength and intrinsic size (size proportional, variantwavelength(1.09)) explicitly rules out explanations other than those emission models with stratified structure, which predict a smaller emitting region observed at a shorter radio wavelength.     

Formation of massive black holes through runaway collisions in dense young star clusters

A luminous X-ray source is associated with MGG 11--a cluster of young stars approximately 200 pc from the centre of the starburst galaxy M 82 (refs 1, 2). The properties of this source are best explained by invoking a black hole with a mass of at least 350 solar masses (350 M(o)), which is intermediate between stellar-mass and supermassive black holes. A nearby but somewhat more massive cluster (MGG 9) shows no evidence of such an intermediate-mass black hole, raising the issue of just what physical characteristics of the clusters can account for this difference. Here we report numerical simulations of the evolution and motion of stars within the clusters, where stars are allowed to merge with each other. We find that for MGG 11 dynamical friction leads to the massive stars sinking rapidly to the centre of the cluster, where they participate in a runaway collision. This produces a star of 800-3,000 M(o) which ultimately collapses to a black hole of intermediate mass. No such runaway occurs in the cluster MGG 9, because the larger cluster radius leads to a mass segregation timescale a factor of five longer than for MGG 11.     

A giant stream of metal-rich stars in the halo of the galaxy M31.

Recent observations have revealed streams of gas and stars in the halo of the Milky Way that are the debris from interactions between our Galaxy and some of its dwarf companion galaxies; the Sagittarius dwarf galaxy and the Magellanic clouds. Analysis of the material has shown that much of the halo is made up of cannibalized satellite galaxies, and that dark matter is distributed nearly spherically in the Milky Way. It remains unclear, however, whether cannibalized substructures are as common in the haloes of galaxies as predicted by galaxy-formation theory. Here we report the discovery of a giant stream of metal-rich stars within the halo of the nearest large galaxy, M31 (the Andromeda galaxy). The source of this stream could be the dwarf galaxies M32 and NGC205, which are close companions of M31 and which may have lost a substantial number of stars owing to tidal interactions. The results demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that tidal streams may be a generic feature of galaxy haloes.     

Magnetic support of the optical emission line filaments in NGC 1275

The giant elliptical galaxy NGC 1275, at the centre of the Perseus cluster, is surrounded by a well-known giant nebulosity of emission-line filaments, which are plausibly in excess of 10(8) years old. The filaments are dragged out from the centre of the galaxy by radio-emitting 'bubbles' rising buoyantly in the hot intracluster gas, before later falling back. They act as markers of the feedback process by which energy is transferred from the central massive black hole to the surrounding gas. The mechanism by which the filaments are stabilized against tidal shear and dissipation into the surrounding extremely hot (4 x 10(7) K) gas has been unclear. Here we report observations that resolve thread-like structures in the filaments. Some threads extend over 6 kpc, yet are only 70 pc wide. We conclude that magnetic fields in the threads, in pressure balance with the surrounding gas, stabilize the filaments, so allowing a large mass of cold gas to accumulate and delay star formation.     

The massive binary companion star to the progenitor of supernova 1993J

The massive star that underwent a collapse of its core to produce supernova (SN)1993J was subsequently identified as a non-variable red supergiant star in images of the galaxy M81 taken before explosion. It showed an excess in ultraviolet and B-band colours, suggesting either the presence of a hot, massive companion star or that it was embedded in an unresolved young stellar association. The spectra of SN1993J underwent a remarkable transformation from the signature of a hydrogen-rich type II supernova to one of a helium-rich (hydrogen-deficient) type Ib. The spectral and photometric peculiarities were best explained by models in which the 13-20 solar mass supergiant had lost almost its entire hydrogen envelope to a close binary companion, producing a 'type IIb' supernova, but the hypothetical massive companion stars for this class of supernovae have so far eluded discovery. Here we report photometric and spectroscopic observations of SN1993J ten years after the explosion. At the position of the fading supernova we detect the unambiguous signature of a massive star: the binary companion to the progenitor.     

Suppression of dwarf galaxy formation by cosmic reionization

A large number of faint galaxies, born less than a billion years after the Big Bang, have recently been discovered. Fluctuations in the distribution of these galaxies contributed to a scatter in the ionization fraction of cosmic hydrogen on scales of tens of megaparsecs, as observed along the lines of sight to the earliest known quasars. Theoretical simulations predict that the formation of dwarf galaxies should have been suppressed after cosmic hydrogen was reionized, leading to a drop in the cosmic star-formation rate. Here we report evidence for this suppression. We show that the post-reionization galaxies that produced most of the ionizing radiation at a redshift z approximately 5.5 must have had a mass in excess of approximately 10(10.9 +/- 0.5) solar masses (M(o)) or else the aforementioned scatter would have been smaller than observed. This limiting mass is two orders of magnitude larger than the galaxy mass that is thought to have dominated the reionization of cosmic hydrogen (approximately 10(8) M(o)). We predict that future surveys with space-based infrared telescopes will detect a population of smaller galaxies that reionized the Universe at an earlier time, before the epoch of dwarf galaxy suppression.