LAMOST恒星光谱巡天与银河系恒星晕研究

星系的形成和演化问题是当今天体物理的前沿和核心。由于身居其中,而且可以对单个的恒星进行仔细的观测,银河系因此成为了研究星系的形成和演化乃至宇宙学的一个极重要样本。银河系的晕(包括恒星晕和暗物质晕)被证实具有非常复杂的结构,其中所蕴藏的信息使人们有可能揭示银河系的结构、形成和丰富的演化历史,其中的一些特殊天体可以用来探求银河系中央黑洞等当下天体物理的热点问题。LAMOST银河系恒星低分辨光谱巡天是迄今为止最大的同类观测项目,其形成的数据库非常适宜人们开展许多银河系结构和演化方面的工作。文章介绍了LAMOST银河系巡天在银晕和相关结构上正在开展的研究工作,并展示了一些早期的科学研究成果。     

相同质量的矮星系

试想象一下：把100个块头各不相同的人放到秤上,却发现他们体重一模一样!在观测与银河系相邻的一群微型星系时,天文学家们发现了跟上述情况同样莫名其妙的现象：不管各星系包含多少恒星,每个星系的质量都是相同的。这项发现或许可以揭示有关星系的形成,甚至有关被称为“暗物质”的那种神秘物质的特性。     

宇宙射线跟随银河系的旋转

高能宇宙射线来自天空中某个特定的区域，并与银河系中我们所在的旋臂共同旋转．这些数据或许能够帮助我们理解我们的银河系如何与太阳磁场相互作用．     

银河、星系和大宇宙

银河系是我们生活的地球和太阳所在的恒星系统,其质量的1.4×10^11太阳质量,其中恒星约占90%,气体和尘埃组成的星际物质约占10%。平时我们在晚间看到的所有恒星几乎全部属于银河系,由于大量恒星投影在天球上形成乳白色光带。     

走近天文之五 从银河到银河系、从星云到星系

正1导语如果说宇宙像一个无边无际的大海,那么星系就像大海中的一座座小岛。宇宙中有几千亿个星系,而我们的银河系是其中普普通通的一员。身在庐山中,难识真面目。那身在银河系中的我们,是如何分析出我们所处银河系的样子呢?我们对银河系之外的河外星系又有怎样的认知历程呢?接下来,请随笔者一起简要回溯我们对星系的认识历程,从银河到银河系,从银河系到河外星系。     

银河系可能诞生于宇宙早期

中国科技大学天体物理中心孔旭教授与他的欧、日、美的合作者，对形成于宇宙早期的高红移星系的研究取得重要进展．他们认为，类似于银河系这样的大质量旋涡星系在宇宙形成的早期就已经存在．这一研究结果发表在近期的美国《天体物理学报》和8月17日英国的《自然》杂志上。     

万亿电子伏的γ射线源自银河

最近位于新墨西哥州的高山上的Mila铲。探测器观察到源自我们居住的银河系的最高能的电磁辐射1万亿电子伏（TeV）的γ射线．这种高能的光子被认为是当能量更高的宇宙射线撞击银河系的高物质密度核心时产生的碎片的一部分。     

宇宙线的百年研究

二十年的大气电离之谜导致了宇宙线的发现．宇宙线的研究打开了粒子物理学的大门．而今天，宇宙线又为银河系外的天体物理学开启了一扇窗口．     

非旋转银河系的重力不稳定性

银河系中星体分布的重力不稳定性在天文物理是个熟知的问题.尝试以加速器物理的方法初步分析了这个现象, 得到非旋转银河系在它的大小超过某阈值时会呈现不稳定现象的结论,而此阈值与银河系的质量密度有关.     

宇宙磁场

磁场在宇宙中无处不在,贯穿各种天体。磁场对天体形成、辐射和演化起重要作用,也是宇宙线研究的基础。以前人们对宇宙磁场的理解主要是来自对地球和太阳磁场的测量和物理解释。近20年来,对宇宙更大尺度天体的磁场测量取得很大进展：发现了银河系具备沿着银河旋臂的几万光年尺度磁场,强度为2—4 μG;发现了银河系晕里具有环向磁场,在银道面上下方向相反;发现了银河系中心有极向磁场的观测证据;发现了邻近旋涡星系可能具有万光年尺度磁场的证据;发现了星系团几千万光年范围内磁场存在的证据。宇宙学模拟也显示在宇宙超大纤维结构中应该有磁场存在。这些磁场可能都是宇宙早期产生并在天体形成和演化过程中得以发扬光大。     

Cosmological evolution of supermassive black holes in galactic centers unveiled by hard X-ray observations

We review the current understanding of the cosmological evolution of supermassive black holes in galactic centers elucidated by X-ray surveys of active galactic nuclei (AGNs). Hard X-ray observations at energies above 2 keV are the most efficient and complete tools to find “obscured” AGNs, which are dominant populations among all AGNs. Combinations of surveys with various flux limits and survey area have enabled us to determine the space number density and obscuration properties of AGNs as a function of luminosity and redshift. The results have essentially solved the origin of the X-ray background in the energy band below ∼10 keV. The downsizing (or anti-hierarchical) evolution that more luminous AGNs have the space-density peak at higher redshifts has been discovered, challenging theories of galaxy and black hole formation. Finally, we summarize unresolved issues on AGN evolution and prospects for future X-ray missions.     

黑洞照亮宇宙——银河系中心黑洞及其物理意义

2020年度诺贝尔物理学奖颁发给为黑洞和超大质量致密天体做出突出贡献的三位科学家,他们分别从理论和观测上提供了令人信服的证明和证据。他们的工作打开了理解宇宙中大质量天体命运的窗口。人们普遍相信超大质量黑洞存在于每一个星系的中心,是这些黑洞照亮了再电离时期的宇宙,也是它们为揭开宇宙膨胀历史、暗能量宇宙演化性质、纳赫兹低频引力波等诸多谜团提供了十分强大的工具。预计未来5年内,反响映射和GRAVITY/VLTI联合观测将在以黑洞研究为支撑的领域取得重大进展。     

Evolution of primordial magnetic fields

A new mechanism for the evolution of primordial magnetic fields is described and analysed. The field evolution is followed from the time of its creation until the epoch of structure and galaxy formation. The mechanism takes into account the turbulent behaviour of the early universe plasma, whose properties determine strongly the evolution of the field configuration. A number of other related issues such as the case of an electroweak plasma are also considered. Finally, as an example, the mechanism is applied to specific models.     

Percolation analysis of stochastic models of galactic evolution

The stochastic star formation model of galactic evolution can be cast as a problem of directed percolation, the time dimension being that along which the directed bonds exist. We study various aspects of this percolation, those of general interest for the percolation phase transition and those of particular importance for the astrophysical application. Both analytical calculations and computer simulations are provided and the results compared. Among the properties are: value of the percolation threshold, critical indices, percolation probability (star density) near and away from the critical point, local density, cluster sizes, effects of rotation (for disk galaxy models) on the percolation threshold. Astrophysical consequences of some of these properties are discussed, in particular the way in which general phase transition behavior contributes to spiral arm morphology. We look at 1 (space) + 1 (time), 2 + 1 and + 1 dimensions, the 2 + 1 case being of interest for disk galaxies.     

On the Distribution of Radio Pulsars Above the Galaxy Plane

A mechanism of pulsar accumulation over the galaxy plane, caused by deceleration in the gravitational field and stopping at a certain distance of z from the plane is proposed. For pulsars with velocities close to the average observed velocity of about 300 km/s, the consideration of kinematics of such motion estimates the age of stopped pulsars of the order of ten millions years and the average distance from the galaxy plane of about 2 kpc. The consistency in the pulsar longitude distribution at |z| = 0.45–4.5 kpc and the distribution of the formation regions of massive stars in the galaxy disk was found.     

Distinctive aspects of the evolution of galactic magnetic fields

We perform an in-depth analysis of the evolution of galactic magnetic fields within a semi-analytic galaxy formation and evolution framework, determine various distinctive aspects of the evolution process, and obtain analytic solutions for a wide range of possible evolution scenarios.     

Radio emission from interstellar neutral hydrogen

The emission line for neutral hydrogen at 1420 Mc/s is the only line so far discovered in radioastronomy. Since its mechanism of origin is completely understood, observations of this line provide direct information about conditions in interstellar space such as temperatures, densities and velocities. Extensive investigations of our own Milky Way system have shown clearly that it is a spiral galaxy similar to, but rather smaller than, the great spiral nebula in Andromeda. Our knowledge of the spiral structure of galaxies is far from complete; hydrogen-line measures of high-speed expansions in the centre of the Milky Way system may provide a clue to the understanding of this problem. In addition, determinations of the hydrogen content of different types of galaxy reinforce current theories which suggest that irregular galaxies evolve through the spiral form to elliptical galaxies.     

Galactic cosmic ray propagation: sub-PeV diffuse gamma-ray and neutrino emission

The Tibet ASγ experiment just reported their measurement of sub-PeV diffuse gamma-ray emission from the Galactic disk, with the highest energy up to 957 TeV. These diffuse gamma rays are most likely the hadronic origin by cosmic ray (CR) interaction with interstellar gas in the galaxy. This measurement provides direct evidence to the hypothesis that the Galactic Cosmic Rays (GCRs) can be accelerated beyond PeV energies. In this work, we try to explain the sub-PeV diffuse gamma-ray spectrum with different CR propagation models. We find that there is a tension between the sub-PeV diffuse gamma-ray and the local CR spectrum. To describe the sub-PeV diffuse gamma-ray flux, it generally requires larger local CR flux than measurement in the knee region. We further calculate the PeV neutrino flux from the CR propagation model. Even all of these sub-PeV diffuse gamma rays originate from the propagation, the Galactic Neutrinos (GNs) only account for less than ~15% of observed flux, most of which are still from extragalactic sources.     

The Coevolution of Galaxies and Supermassive Black Holes in the Near Universe

A fundamental role is attributed to supermassive black holes (SMBH), and the feedback they generate, in the evolution of galaxies. But theoretical models trying to reproduce the M_SMBH vs. sigma relation (between the SMBH mass and stellar velocity dispersion of the galaxy bulge) make broad assumptions about the physical processes involved. These assumptions are needed due to the scarcity of observational constraints on the relevant physical processes which occur when the SMBH is being fed via mass accretion in active galactic nuclei (AGN). In search for these constraints, our group—AGN integral field spectroscopy (AGNIFS)—has been mapping the gas kinematics as well as the stellar population properties of the inner few hundred parsecs of a sample of nearby AGN hosts. In this contribution, I report on results obtained so far which show gas inflows along nuclear spirals and compact disks in the inner tens to hundreds of pc in nearby AGN hosts which seem to be the sources of fuel to the AGN. As the inflow rates are much larger than the AGN accretion rate, the excess gas must be depleted via formation of new stars in the bulge. Indeed, in many cases, we find ～100 pc circumnuclear rings of recent star formation (ages ～10–500 Myr) that can be interpreted as a signature of coevolution of the host galaxy and its AGN. I also report on the mapping of outflows in ionized gas, which are ubiquitous in Seyfert galaxies, and discuss mass outflow rates and powers.     

Evolution of the Plasma Universe: II. The Formation of Systems of Galaxies

The model of the plasma universe, inspired by totally unexpected phenomena observed with the advent and application of fully three-dimensional electromagnetic particle-in-cell simulations to filamentary plasmas, consists of studying the interaction between field-aligned current-conducting, galactic-dimensioned plasma sheets or filaments (Birkeland currents). In a preceding paper, the evolution of the interaction spanned some 108-109 years, where simulational analogs of synchrotron-emitting double radio galaxies and quasars were discovered. This paper reports the evolution through the next 109-5 × 109 years. In particular, reconfiguration and compression of tenuous cosmic plasma due to the self-consistent magnetic fields from currents conducted through the filaments leads to the formation of elliptical, peculiar, and barred and normal spiral galaxies. The importance of the electromagnetic pinch in producing condense states and initiating gravitational collapse of dusty galactic plasma to stellisimals, then stars, is discussed. Simulation data are directly compared to galaxy morphology types, synchrotron flux, Hi distributions, and fine detail structure in rotational velocity curves. These comparisons suggest that knowledge obtained from laboratory, simulation, and magnetospheric plasmas offers not only to enhance our understanding of the universe, but also to provide feedback information to laboratory plasma experiments from the unprecedented source of plasma data provided by the plasma universe.