An 84-microG magnetic field in a galaxy at redshift z = 0.692

The magnetic field pervading our Galaxy is a crucial constituent of the interstellar medium: it mediates the dynamics of interstellar clouds, the energy density of cosmic rays, and the formation of stars. The field associated with ionized interstellar gas has been determined through observations of pulsars in our Galaxy. Radio-frequency measurements of pulse dispersion and the rotation of the plane of linear polarization, that is, Faraday rotation, yield an average value for the magnetic field of B approximately 3 microG (ref. 2). The possible detection of Faraday rotation of linearly polarized photons emitted by high-redshift quasars suggests similar magnetic fields are present in foreground galaxies with redshifts z > 1. As Faraday rotation alone, however, determines neither the magnitude nor the redshift of the magnetic field, the strength of galactic magnetic fields at redshifts z > 0 remains uncertain. Here we report a measurement of a magnetic field of B approximately 84 microG in a galaxy at z = 0.692, using the same Zeeman-splitting technique that revealed an average value of B = 6 microG in the neutral interstellar gas of our Galaxy. This is unexpected, as the leading theory of magnetic field generation, the mean-field dynamo model, predicts large-scale magnetic fields to be weaker in the past rather than stronger.     

The essential signature of a massive starburst in a distant quasar

Observations of carbon monoxide emission in high-redshift (zeta > 2) galaxies indicate the presence of large amounts of molecular gas. Many of these galaxies contain an active galactic nucleus powered by accretion of gas onto a supermassive black hole, and a key question is whether their extremely high infrared luminosities result from the active galactic nucleus, from bursts of massive star formation (associated with the molecular gas), or both. In the Milky Way, high-mass stars form in the dense cores of interstellar molecular clouds, where gas densities are n(H2) > 10(5) cm(-3) (refs 1, 2). Recent surveys show that virtually all galactic sites of high-mass star formation have similarly high densities. The bulk of the cloud material traced by CO observations, however, is at a much lower density. For galaxies in the local Universe, the HCN molecule is an effective tracer of high-density molecular gas. Here we report observations of HCN emission from the infrared-luminous 'Cloverleaf' quasar (at a redshift zeta = 2.5579). The HCN line luminosity indicates the presence of 10 billion solar masses of very dense gas, an essential feature of an immense starburst, which contributes, together with the active galactic nucleus it harbours, to its high infrared luminosity.     

Magnetic field surrounding the starburst nucleus of the galaxy M82 from polarized dust emission

Magnetic fields may play an important role in the star-formation process, especially in the central regions of 'starburst' galaxies where star formation is vigorous. But the field directions are very difficult to determine in the dense molecular gas out of which the stars form, so it has hitherto been impossible to test this hypothesis. Dust grains in interstellar clouds tend to be magnetically aligned, and it is possible to determine the alignment direction based on the polarization of optical light due to preferential extinction along the long axes of the aligned grains. This technique works, however, only for diffuse gas, not for the dense molecular gas. Here we report observations of polarized thermal emission from the aligned dust grains in the central region of M82, which directly traces the magnetic field structure (as projected onto the plane of the sky). Organized field lines are seen around the brightest star-forming regions, while in the dusty halo the field lines form a giant magnetic bubble possibly blown out by the galaxy's 'superwind'.     

分子天文学的发展与展望

介绍分子天文学及其在天文研究中的重要作用;综述近年来在分子云大尺度分布、分子云与恒星形成、拱星包层和恒星演化晚期、天体脉泽等方面的主要成果和进展;讨论了分子天体化学发展的情况及其重要意义;并提出对今后的发展趋势及主要研究课题的展望。     

A minimum column density of 1 g cm(-2) for massive star formation

Massive stars are very rare, but their extreme luminosities make them both the only type of young star we can observe in distant galaxies and the dominant energy sources in the Universe today. They form rarely because efficient radiative cooling keeps most star--forming gas clouds close to isothermal as they collapse, and this favours fragmentation into stars of one solar mass or lower. Heating of a cloud by accreting low-mass stars within it can prevent fragmentation and allow formation of massive stars, but the necessary properties for a cloud to form massive stars-and therefore where massive stars form in a galaxy--have not yet been determined. Here we show that only clouds with column densities of at least 1 g cm(-2) can avoid fragmentation and form massive stars. This threshold, and the environmental variation of the stellar initial mass function that it implies, naturally explain the characteristic column densities associated with massive star clusters and the difference between the radial profiles of Halpha and ultraviolet emission in galactic disks. The existence of a threshold also implies that the initial mass function should show detectable variation with environment within the Galaxy, that the characteristic column densities of clusters containing massive stars should vary between galaxies, and that star formation rates in some galactic environments may have been systematically underestimated.     

The central dusty torus in the active nucleus of NGC 1068

Active galactic nuclei (AGNs) display many energetic phenomena--broad emission lines, X-rays, relativistic jets, radio lobes--originating from matter falling onto a supermassive black hole. It is widely accepted that orientation effects play a major role in explaining the observational appearance of AGNs. Seen from certain directions, circum-nuclear dust clouds would block our view of the central powerhouse. Indirect evidence suggests that the dust clouds form a parsec-sized torus-shaped distribution. This explanation, however, remains unproved, as even the largest telescopes have not been able to resolve the dust structures. Here we report interferometric mid-infrared observations that spatially resolve these structures in the galaxy NGC 1068. The observations reveal warm (320 K) dust in a structure 2.1 parsec thick and 3.4 parsec in diameter, surrounding a smaller hot structure. As such a configuration of dust clouds would collapse in a time much shorter than the active phase of the AGN, this observation requires a continual input of kinetic energy to the cloud system from a source coexistent with the AGN.     

引力不稳定性在巨分子云聚合形成中的作用

　Two models of molecular cloud in disk galaxies are proposed to investigate the formation of giant molecular clouds (GMCs) under the gravitational instability and random collision using PP(Particle-Particle) simulation. Some conclusions can be drawn: 1) The gravitational instability can make small clouds form large clouds faster than random collision. 2) The differential rotation in the gravitational instability model plays a positive role in the agglomeration of molecular clouds.     

Internal structure of a cold dark molecular cloud inferred from the extinction of background starlight

Stars and planets form within dark molecular clouds, but little is understood about the internal structure of these clouds, and consequently about the initial conditions that give rise to star and planet formation. The clouds are primarily composed of molecular hydrogen, which is virtually inaccessible to direct observation. But the clouds also contain dust, which is well mixed with the gas and which has well understood effects on the transmission of light. Here we use sensitive near-infrared measurements of the light from background stars as it is absorbed and scattered by trace amounts of dust to probe the internal structure of the dark cloud Barnard 68 with unprecedented detail. We find the cloud's density structure to be very well described by the equations for a pressure-confined, self-gravitating isothermal sphere that is critically stable according to the Bonnor-Ebert criteria. As a result we can precisely specify the physical conditions inside a dark cloud on the verge of collapse to form a star.     

Contribution of cloud condensate to surface rainfall process

Contribution of cloud condensate to surface rainfall processes is investigated in a life span of tropical convection based on hourly data from a two-dimensional cloud resolving simulation. The model is forced by the large-scale vertical velocity, zonal wind and horizontal advections obtained from tropical ocean global atmosphere coupled ocean-atmosphere response experiment (TOGA COARE). The results show that during the genesis, development, and decay of tropical convection, calculations with water vapor overestimate surface rain rate, and cloud condensate plays an important role in correcting overestimation in surface rain rates. The analysis is carried out in deep convective clouds and anvil clouds during the development of tropical convection. The surface rain rates calculated with water vapor in deep convective clouds and anvil clouds have similar magnitudes, the large surface rain rate appears in deep convective clouds due to the consumption of water hydrometeors whereas the small surface rain rate occurs in anvil clouds because of the gain of ice hydrometeors. Further analysis of the grid data shows that the surface rain rates calculated with water vapor and with cloud condensate are negatively correlated with the correlation coefficient of -0.85, and the surface rain rate calculated with cloud condensate is mainly contributed to the water hydrometeors in the tropical deep convective regime.     

Generation of scaled protogalactic seed magnetic fields in laser-produced shock waves

The standard model for the origin of galactic magnetic fields is through the amplification of seed fields via dynamo or turbulent processes to the level consistent with present observations. Although other mechanisms may also operate, currents from misaligned pressure and temperature gradients (the Biermann battery process) inevitably accompany the formation of galaxies in the absence of a primordial field. Driven by geometrical asymmetries in shocks associated with the collapse of protogalactic structures, the Biermann battery is believed to generate tiny seed fields to a level of about 10(-21)?gauss (refs 7, 8). With the advent of high-power laser systems in the past two decades, a new area of research has opened in which, using simple scaling relations, astrophysical environments can effectively be reproduced in the laboratory. Here we report the results of an experiment that produced seed magnetic fields by the Biermann battery effect. We show that these results can be scaled to the intergalactic medium, where turbulence, acting on timescales of around 700 million years, can amplify the seed fields sufficiently to affect galaxy evolution.     

基于中点偏移算法的云的模型设计

自然景观的模拟在许多方面得到了应用，云是自然界中的一项重要景观，对云进行模型设计显得十分重要。针对目前大多数的云的模型设计方法难以在普通PC机上快速实现真实感较强的云的模型设计的问题，提出了一种自然景观中云的模型设计方法。该方法以分形理论为基础，设计并实现了一种云的纹理生成算法，并采用纹理映射技术，绘制云的图像模型，得到了较为逼真的模拟结果。     

引力不稳定性在巨分子云的形成和演化中的作用

为了研究引力不稳定性在巨分子云形成中的作用,通过计算机模拟技术建立了星系中分子云的较差自转模型.在演化中除了分子云之间的碰撞外,模型还考虑了分子云在恒星盘的背景引力场中所受的引力和分子云之间的相互自引力.其中分别考虑分子云之间的短程力(随机碰撞模型)和长程力(引力不稳定性模型).通过对计算机模拟的结果进行详细的分析和讨论,得到了以下结果:对于较差自转模型,引力不稳定性在巨分子云的聚合形成中起了关键的积极作用,它能够加速和加大分子云的成团.     

Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo

The Milky Way's halo contains clouds of neutral hydrogen with high radial velocities which do not follow the general rotational motion of the Galaxy. Few distances to these high-velocity clouds are known, so even gross properties such as total mass are hard to determine. As a consequence, there is no generally accepted theory regarding their origin. One idea is that they result from gas that has cooled after being ejected from the Galaxy through fountain-like flows powered by supernovae; another is that they are composed of gas, poor in heavy elements, which is falling onto the disk of the Milky Way from intergalactic space. The presence of molecular hydrogen, whose formation generally requires the presence of dust (and therefore gas, enriched in heavy elements), could help to distinguish between these possibilities. Here we report the discovery of molecular hydrogen absorption in a high-velocity cloud along the line of sight to the Large Magellanic Cloud. We also derive for the same cloud an iron abundance which is half of the solar value. From these data, we conclude that gas in this cloud originated in the disk of the Milky Way.     

Polarization of the prompt gamma-ray emission from the gamma-ray burst of 6 December 2002

Observations of the afterglows of gamma-ray bursts (GRBs) have revealed that they lie at cosmological distances, and so correspond to the release of an enormous amount of energy. The nature of the central engine that powers these events and the prompt gamma-ray emission mechanism itself remain enigmatic because, once a relativistic fireball is created, the physics of the afterglow is insensitive to the nature of the progenitor. Here we report the discovery of linear polarization in the prompt gamma-ray emission from GRB021206, which indicates that it is synchrotron emission from relativistic electrons in a strong magnetic field. The polarization is at the theoretical maximum, which requires a uniform, large-scale magnetic field over the gamma-ray emission region. A large-scale magnetic field constrains possible progenitors to those either having or producing organized fields. We suggest that the large magnetic energy densities in the progenitor environment (comparable to the kinetic energy densities of the fireball), combined with the large-scale structure of the field, indicate that magnetic fields drive the GRB explosion.     

A fossil origin for the magnetic field in A stars and white dwarfs

Some main-sequence stars of spectral type A are observed to have a strong (0.03-3 tesla), static, large-scale magnetic field, of a chiefly dipolar shape: they are known as 'Ap stars', such as Alioth, the fifth star in the Big Dipper. Following the discovery of these fields, it was proposed that they are remnants of the star's formation, a 'fossil' field. An alternative suggestion is that they could be generated by a dynamo process in the star's convective core. The dynamo hypothesis, however, has difficulty explaining high field strengths and the observed lack of a correlation with rotation. The weakness of the fossil-field theory has been the absence of field configurations stable enough to survive in a star over its lifetime. Here we report numerical simulations that show that stable magnetic field configurations, with properties agreeing with those observed, can develop through evolution from arbitrary, unstable initial fields. The results are applicable equally to Ap stars, magnetic white dwarfs and some highly magnetized neutron stars known as magnetars. This establishes fossil fields as the natural, unifying explanation for the magnetism of all these stars.     

Methane drizzle on Titan

Saturn's moon Titan shows landscapes with fluvial features suggestive of hydrology based on liquid methane. Recent efforts in understanding Titan's methane hydrological cycle have focused on occasional cloud outbursts near the south pole or cloud streaks at southern mid-latitudes and the mechanisms of their formation. It is not known, however, if the clouds produce rain or if there are also non-convective clouds, as predicted by several models. Here we show that the in situ data on the methane concentration and temperature profile in Titan's troposphere point to the presence of layered optically thin stratiform clouds. The data indicate an upper methane ice cloud and a lower, barely visible, liquid methane-nitrogen cloud, with a gap in between. The lower, liquid, cloud produces drizzle that reaches the surface. These non-convective methane clouds are quasi-permanent features supported by the global atmospheric circulation, indicating that methane precipitation occurs wherever there is slow upward motion. This drizzle is a persistent component of Titan's methane hydrological cycle and, by wetting the surface on a global scale, plays an active role in the surface geology of Titan.     

A magnetically collimated jet from an evolved star

Planetary nebulae often have asymmetric shapes, even though their progenitor stars were symmetric; this structure could be the result of collimated jets from the evolved stars before they enter the planetary nebula phase. Theoretical models have shown that magnetic fields could be the dominant source of jet-collimation in evolved stars, just as these fields are thought to collimate outflows in other astrophysical sources, such as active galactic nuclei and proto-stars. But hitherto there have been no direct observations of both the magnetic field direction and strength in any collimated jet. Here we report measurements of the polarization of water vapour masers that trace the precessing jet emanating from the asymptotic giant branch star W43A (at a distance of 2.6 kpc from the Sun), which is undergoing rapid evolution into a planetary nebula. The masers occur in two clusters at opposing tips of the jets, approximately 1,000 au from the star. We conclude from the data that the magnetic field is indeed collimating the jet.     

Influencing intramolecular motion with an alternating electric field

Analogues of mechanical devices that operate on the molecular level, such as shuttles, brakes, ratchets, turnstiles and unidirectional spinning motors, are current targets of both synthetic chemistry and nanotechnology. These structures are designed to restrict the degrees of freedom of submolecular components such that they can only move with respect to each other in a predetermined manner, ideally under the influence of some external stimuli. Alternating-current (a.c.) electric fields are commonly used to probe electronic structure, but can also change the orientation of molecules (a phenomenon exploited in liquid crystal displays), or interact with large-scale molecular motions, such as the backbone fluctuations of semi-rigid polymers. Here we show that modest a.c. fields can be used to monitor and influence the relative motion within certain rotaxanes, molecules comprising a ring that rotates around a linear 'thread' carrying bulky 'stoppers' at each end. We observe strong birefringence at frequencies that correspond to the rate at which the molecular ring pirouettes about the thread, with the frequency of maximum birefringence, and by inference also the rate of ring pirouetting giving rise to it, changing as the electric field strength is varied. Computer simulations and nuclear magnetic resonance spectroscopy show the ring rotation to be the only dynamic process occurring on a timescale corresponding to the frequency of maximum birefringence, thus confirming that mechanical motion within the rotaxanes can be addressed, and to some extent controlled, by oscillating electric fields.     

ASTER多角度影像云参数模型解算与云迹风反演

卫星资料具有覆盖范围广、时空密度大等优点,能有效地弥补缺测地区观测信息不足的现状,可为天气分析和数值预报模式等研究提供大量有用资料.采用青藏高原西藏阿里地区上空的Terra ASTER多角度影像立体成像像对,结合遥感和摄影测量理论,建立了基于摄影中心、影像点和实际物方点共线关系的云顶三维参数解算模型,用以求取云顶三维坐...     

The inner jet of an active galactic nucleus as revealed by a radio-to-gamma-ray outburst

Blazars are the most extreme active galactic nuclei. They possess oppositely directed plasma jets emanating at near light speeds from accreting supermassive black holes. According to theoretical models, such jets are propelled by magnetic fields twisted by differential rotation of the black hole's accretion disk or inertial-frame-dragging ergosphere. The flow velocity increases outward along the jet in an acceleration and collimation zone containing a coiled magnetic field. Detailed observations of outbursts of electromagnetic radiation, for which blazars are famous, can potentially probe the zone. It has hitherto not been possible to either specify the location of the outbursts or verify the general picture of jet formation. Here we report sequences of high-resolution radio images and optical polarization measurements of the blazar BL Lacertae. The data reveal a bright feature in the jet that causes a double flare of radiation from optical frequencies to TeV gamma-ray energies, as well as a delayed outburst at radio wavelengths. We conclude that the event starts in a region with a helical magnetic field that we identify with the acceleration and collimation zone predicted by the theories. The feature brightens again when it crosses a standing shock wave corresponding to the bright 'core' seen on the images.