Low-Mach-number turbulence in interstellar gas revealed by radio polarization gradients

The interstellar medium of the Milky Way is multiphase, magnetized and turbulent. Turbulence in the interstellar medium produces a global cascade of random gas motions, spanning scales ranging from 100 parsecs to 1,000 kilometres (ref. 4). Fundamental parameters of interstellar turbulence such as the sonic Mach number (the speed of sound) have been difficult to determine, because observations have lacked the sensitivity and resolution to image the small-scale structure associated with turbulent motion. Observations of linear polarization and Faraday rotation in radio emission from the Milky Way have identified unusual polarized structures that often have no counterparts in the total radiation intensity or at other wavelengths, and whose physical significance has been unclear. Here we report that the gradient of the Stokes vector (Q, U), where Q and U are parameters describing the polarization state of radiation, provides an image of magnetized turbulence in diffuse, ionized gas, manifested as a complex filamentary web of discontinuities in gas density and magnetic field. Through comparison with simulations, we demonstrate that turbulence in the warm, ionized medium has a relatively low sonic Mach number, M(s)???2. The development of statistical tools for the analysis of polarization gradients will allow accurate determinations of the Mach number, Reynolds number and magnetic field strength in interstellar turbulence over a wide range of conditions.     

Molecular gas in the host galaxy of a quasar at redshift z = 6.42

Observations of molecular hydrogen in quasar host galaxies at high redshifts provide fundamental constraints on galaxy evolution, because it is out of this molecular gas that stars form. Molecular hydrogen is traced by emission from the carbon monoxide molecule, CO; cold H2 itself is generally not observable. Carbon monoxide has been detected in about ten quasar host galaxies with redshifts z > 2; the record-holder is at z = 4.69 (refs 1-3). Here we report CO emission from the quasar SDSS J114816.64 + 525150.3 (refs 5, 6) at z = 6.42. At that redshift, the Universe was only 1/16 of its present age, and the era of cosmic reionization was just ending. The presence of about 2 x 1010 M\circ of H2 in an object at this time demonstrates that molecular gas enriched with heavy elements can be generated rapidly in the youngest galaxies.     

Investigating Galactic cosmic rays with γ-ray astronomy

Cosmic rays (CRs) are one of the most important components in the interstellar medium (ISM), and the origin of CRs remains a mystery. The diffusion of CRs in turbulent magnetic fields erases the information on the distribution of CR accelerators to a large extent. The energy dependent diffusion of CRs also significantly modifies the initial (acceleration) spectra of CRs. In this regard, γ-rays, the secondary products of interactions of CRs with gas and photons in the ISM, provide us with more information about the origin of CRs. More specifically, the γ-ray emissions associated with gas, can be used to study the distribution of CRs throughout the Galaxy; discrete γ-ray sources can elucidate the locations of individual CR accelerators. Here, the current status and prospects in these fields are reviewed.     

The alignment of molecular cloud magnetic fields with the spiral arms in M33

The formation of molecular clouds, which serve as stellar nurseries in galaxies, is poorly understood. A class of cloud formation models suggests that a large-scale galactic magnetic field is irrelevant at the scale of individual clouds, because the turbulence and rotation of a cloud may randomize the orientation of its magnetic field. Alternatively, galactic fields could be strong enough to impose their direction upon individual clouds, thereby regulating cloud accumulation and fragmentation, and affecting the rate and efficiency of star formation. Our location in the disk of the Galaxy makes an assessment of the situation difficult. Here we report observations of the magnetic field orientation of six giant molecular cloud complexes in the nearby, almost face-on, galaxy M33. The fields are aligned with the spiral arms, suggesting that the large-scale field in M33 anchors the clouds.     

稀磁半导体Cd＿（1－x）Mn＿xTe，Cd＿（1－x）Fe＿xTe的巨法拉第效应

用消光法、交替磁场法和磁光调制法测量了不同温度下、不同组分的稀磁半导体Ｃｄ＿（１－ｘ）Ｍｎ＿ｘＴｅ和Ｃｄ＿（１－ｘ）Ｆｅ＿ｘＴｅ的巨法拉第效应。实验表明：对较高组分的，法拉第旋转角为负，用单振子模型能很好地描述实验结果。稀磁半导体Ｃｄ＿（１－ｘ）Ｆｅ＿ｘＴｅ表现出与Ｃｄ＿（１－ｘ）Ｍｎ＿ｘＴｅ同量级的巨法拉第效应，当组分较低时，法拉第旋转角随入射光子能量出现由正到负的变化，必须用多振子模型才能很好解释实验结果。当样品很薄或磁场较低时，磁光调制法以其很高的测量精度显示出巨大的优越性。     

A median redshift of 2.4 for galaxies bright at submillimetre wavelengths

A significant fraction of the energy emitted in the early Universe came from very luminous galaxies that are largely hidden at optical wavelengths (because of interstellar dust grains); this energy now forms part of the cosmic background radiation at wavelengths near 1 mm (ref. 1). Some submillimetre (submm) galaxies have been resolved from the background radiation, but they have been difficult to study because of instrumental limitations. This has impeded the determination of their redshifts (z), which is a crucial element in understanding their nature and evolution. Here we report spectroscopic redshifts for ten submm galaxies that were identified using high-resolution radio observations. The median redshift for our sample is 2.4, with a quartile range of 1.9-2.8. This population therefore coexists with the peak activity of quasars, suggesting a close relationship between the growth of massive black holes and luminous dusty galaxies. The space density of submm galaxies at redshifts over 2 is about 1,000 times greater than that of similarly luminous galaxies in the present-day Universe, so they represent an important component of star formation at high redshifts.     

A massive cloud of cold atomic hydrogen in the outer Galaxy.

A large fraction of the mass of the interstellar medium in our Galaxy is in the form of warm (103-104 K) and cool (50-100 K) atomic hydrogen (H i) gas. Cold (10-30 K) regions are thought to be dominated by dense clouds of molecular hydrogen. Cold H i is difficult to observe, and therefore our knowledge of its abundance and distribution in the interstellar medium is poor. The few known clouds of cold H i are much smaller in size and mass than typical molecular clouds. Here we report the discovery that the H i supershell GSH139-03-69 is very cold (10 K). It is about 2 kiloparsecs in size and as massive as the largest molecular complexes. The existence of such an immense structure composed of cold atomic hydrogen in the interstellar medium runs counter to the prevailing view that cold gas resides almost exclusively in clouds dominated by molecular hydrogen.     

反铁磁质中右旋和左旋的园偏振波

计算在反铁磁性中沿垂直外磁场方向同时传播的右旋和左旋的园偏振波的磁导率，分析右旋和左旋的园偏振波的磁导率及其色散关系，并计算和讨论在垂直外磁场下，反铁磁质中的法拉第旋转效应。     

A magnetic torsional wave near the Galactic Centre traced by a 'double helix' nebula

The magnetic field in the central few hundred parsecs of the Milky Way has a dipolar geometry and is substantially stronger than elsewhere in the Galaxy, with estimates ranging up to a milligauss (refs 1-6). Characterization of the magnetic field at the Galactic Centre is important because it can affect the orbits of molecular clouds by exerting a drag on them, inhibit star formation, and could guide a wind of hot gas or cosmic rays away from the central region. Here we report observations of an infrared nebula having the morphology of an intertwined double helix about 100 parsecs from the Galaxy's dynamical centre, with its axis oriented perpendicular to the Galactic plane. The observed segment is about 25 parsecs in length, and contains about 1.25 full turns of each of the two continuous, helically wound strands. We interpret this feature as a torsional Alfvén wave propagating vertically away from the Galactic disk, driven by rotation of the magnetized circumnuclear gas disk. The direct connection between the circumnuclear disk and the double helix is ambiguous, but the images show a possible meandering channel that warrants further investigation.     

Unusual spectral energy distribution of a galaxy previously reported to be at redshift 6.68

Observations of distant galaxies are important both for understanding how galaxies form and for probing the physical conditions of the Universe at early times. It is, however, very difficult to identify galaxies at redshifts z > 5, because they are so faint and have few spectral characteristics. We previously reported the probable identification of a galaxy at z = 6.68, based on one line and an apparent break in the spectrum just shortwards of that, which we interpreted as Lyman alpha emission and the Lyman alpha break, where photons with shorter wavelengths are absorbed by the intervening neutral hydrogen gas. Here we present optical photometry that shows moderate detections of light in the B- and V-band images, which are inconsistent with the expected absence of flux shortwards of the Lyman alpha break for alpha galaxy at z > 5, and inconsistent with the previous flux measurement. Moreover, the spectral energy distribution for this object cannot readily be fitted by any known galaxy spectral template at any redshift, so the redshift is undetermined.     

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'.     

The far-ultraviolet signature of the 'missing' baryons in the Local Group of galaxies

The number of baryons detected in the low-redshift (z < 1) Universe is far smaller than the number detected in corresponding volumes at higher redshifts. Simulations of the formation of structure in the Universe show that up to two-thirds of the 'missing' baryons may have escaped detection because of their high temperature and low density. One of the few ways to detect this matter directly is to look for its signature in the form of ultraviolet absorption lines in the spectra of background sources such as quasars. Here we show that the amplitude of the average velocity vector of 'high velocity' O vi (O5+) absorption clouds detected in a survey of ultraviolet emission from active galactic nuclei decreases significantly when the vector is transformed to the frames of the Galactic Standard of Rest and the Local Group of galaxies. At least 82 per cent of these absorbers are not associated with any 'high velocity' atomic hydrogen complex in our Galaxy, and are therefore likely to result from a primordial warm-hot intergalactic medium pervading an extended corona around the Milky Way or the Local Group. The total mass of baryons in this medium is estimated to be up to approximately 10(12) solar masses, which is of the order of the mass required to dynamically stabilize the Local Group.     

低温强磁场下NdF_3法拉第效应的计算

用量子理论得出求解NdF3磁光效应的近似方法。计算得到了在温度为15 K、18 K和25K时,外磁场C轴磁化的法拉第旋转角与外磁场的变化关系,其结果与实验值吻合较好。     

法拉第效应的光谱测量法

设计了一种在室温和低磁场条件下．用光谱法测量法拉第效应的实验方法．利用多通道分光光度计,测量自然光通过顺磁性玻璃材料时的法拉第旋转谱线．得到费尔德常数与不同波长光线的关系曲线．从而方便、直观地揭示出自然光的旋光色散现象．     

The cosmological density of baryons from observations of 3He+ in the Milky Way.

Primordial nucleosynthesis after the Big Bang can be constrained by the abundances of the light elements and isotopes 2H, 3He, 4He and 7Li (ref. 1). The standard theory of stellar evolution predicts that 3He is also produced by solar-type stars, so its abundance is of interest not only for cosmology, but also for understanding stellar evolution and the chemical evolution of the Galaxy. The 3He abundance in star-forming (H II) regions agrees with the present value for the local interstellar medium, but seems to be incompatible with the stellar production rates inferred from observations of planetary nebulae, which provide a direct test of stellar evolution theory. Here we develop our earlier observations, which, when combined with recent theoretical developments in our understanding of light-element synthesis and destruction in stars, allow us to determine an upper limit for the primordial abundance of 3He relative to hydrogen: 3He/H = (1.1 +/- 0.2) x 10(-5). The primordial density of all baryons determined from the 3He data is in excellent agreement with the densities calculated from other cosmological probes. The previous conflict is resolved because most solar-mass stars do not produce enough 3He to enrich the interstellar medium significantly.     

A photometric redshift of z = 6.39 +/- 0.12 for GRB 050904

Gamma-ray bursts (GRBs) and their afterglows are the most brilliant transient events in the Universe. Both the bursts themselves and their afterglows have been predicted to be visible out to redshifts of z approximately 20, and therefore to be powerful probes of the early Universe. The burst GRB 000131, at z = 4.50, was hitherto the most distant such event identified. Here we report the discovery of the bright near-infrared afterglow of GRB 050904 (ref. 4). From our measurements of the near-infrared afterglow, and our failure to detect the optical afterglow, we determine the photometric redshift of the burst to be z = 6.39 - 0.12 + 0.11 (refs 5-7). Subsequently, it was measured spectroscopically to be z = 6.29 +/- 0.01, in agreement with our photometric estimate. These results demonstrate that GRBs can be used to trace the star formation, metallicity, and reionization histories of the early Universe.     

稀土石榴石晶体磁光效应微观机制研究

本文用量子理论计算了RE:YIG(RE=Ce,Pr)晶体薄膜中RE离子经晶场和交换作用劈裂后的能级和波函数及4f~n→4f~(n-1)5d~1电偶极跃迁几率,进而计算了该晶体的Faraday旋转,计算结果与实验完全符合.结果表明,RE:YIG晶体的磁光效应主要来自RE离子4f~n→4f~(n-1)5d~1的电偶极跃迁;晶场与交换作用起着重要作用.     

The mass of the missing baryons in the X-ray forest of the warm-hot intergalactic medium

Recent cosmological measurements indicate that baryons comprise about four per cent of the total mass-energy density of the Universe, which is in accord with the predictions arising from studies of the production of the lightest elements. It is also in agreement with the actual number of baryons detected at early times (redshifts z > 2). Close to our own epoch (z < 2), however, the number of baryons detected add up to just over half (approximately 55 per cent) of the number seen at z > 2 (refs 6-11), meaning that about approximately 45 per cent are 'missing'. Here we report a determination of the mass-density of a previously undetected population of baryons, in the warm-hot phase of the intergalactic medium. We show that this mass density is consistent, within the uncertainties, with the mass density of the missing baryons.