第七章河外星系 (Chapter 24) 7.1 星系的形态和分类 7.2 测量星系 7.3 星系集团 7.4 星系的演化
7.1 星系的形态和分类 1. 河外星系的发现 1750 年, 英国教士赖特提出银河是恒星系统 1755 年, 康德指出旋涡星云的扁平形态是由于转动引起的, 它们是和银河类似的 宇宙岛 (island universes)
星云的观测 1781 年法国天文学家 Charles Messier 发表了包含 110 个星云 ( 其中 40 个实际上是星系 ) 的 梅西耶星表, 1800 年英国天文学家 William Herschel 发表包含 2500 个天体的星表 1864 年 John Herschel 发表 The General Catalogue of Nebulae and Clusters of Stars, 后来演变为包含超过 10,000 个星系的 New General Catalogue. 如仙女星系 :M 31, NGC 224. C. Messier W. Herschel
1920 年, Shapley 发现球状星团的空间球对称分布, 通过观测球状星团内的天琴座 RR 型变星, 确定银河系的大小 (100 kpc) 和太阳系到银心的距离 (16 kpc), 建立了直径达 100 kpc 的银河系模型
The Great Debate 1920 年沙普利 - 柯蒂斯关于宇宙尺度的大辩论 辩论焦点 (1) 旋涡星云 的距离是多远? (2) 旋涡星云 是恒星系统还是气体云? 旋涡星云 是银河系内气体云, 银河系就是整个宇宙 Harlow Shapley 宇宙是由无数类似银河系的星系构成的, 旋涡星云 实际上是星系 Heber D. Curtis
(3) 旋涡星云在天球上的分布为什么有 隐带 (the zone of avoidance)? 为什么星云的谱线出现红移? 沙普利 : 银河系对旋涡星云施加了一种未知的排斥力 旋涡星云的隐带分布和退行现象 柯蒂斯 : 有些旋涡星云 ( 实际是侧视旋涡星系 ) 的中心面有一条暗带 如果银河系也有类似的暗带, 如果我们位于银河系的暗带中, 如果旋涡星云是河外天体, 旋涡星云的分布出现隐带
The Zone of Avoidance in X-rays Stars within the Galaxies Active galaxies X-Rays from the Galactic Plane
哈勃的裁决 1924 年, 哈勃 (Edwin Hubble) 分解出 仙女座大星云 (M31) 中的造父变星, 证实它确实是恒星系统 由造父变星周光关系哈勃估计 M31 的距离 285 kpc( 实际距离 770 kpc) > 最远的球状星团的距离 (100 kpc) 因此 仙女座大星云 必定是河外星系!
2. 星系的哈勃分类 根据星系形态的不同, 哈勃首先提出星系可以分为椭圆星系 透镜状星系 旋涡星系 棒旋星系和不规则星系 5 种类型, 称为哈勃分类 The Hubble Sequence, or The Hubble Tuning Fork
(1) 椭圆星系 (Elliptical galaxies) 椭球形的星系, 符号为 E. 按椭率大小的增加分为 E0 E1...E7 八个次型 n=10(a-b)/a 主要由星族 Ⅱ 恒星构成, 没有星系盘, 没有或仅有少量星际气体和尘埃, 颜色偏红 恒星作无规则的椭圆轨道运动 中心区域最亮, 亮度向边缘递减 大小和质量相差悬殊 巨椭圆星系 ~10 6 pc, 10 13 M 矮椭圆星系 ~ 10 3 pc, 10 7 M -22 < M V < -18
Elliptical Galaxies Type E0 Type E3 Type E7
Example Spectrum: Elliptical
(2) 旋涡星系 (Spiral galaxies) 具有旋涡结构的星系, 符号为 S 中心是球状或椭球状的核球, 外面是扁平的星系盘 从核球两端延伸出两条或两条以上螺旋状旋臂叠加在星系盘上, 盘外面是球状的星系晕 星系盘颜色偏蓝, 星系晕和核偏红 在星系盘 特别是旋臂上主要是星族 I 恒星以及气体和尘埃, 核球和星系晕主要由星族 Ⅱ 恒星组成 -21 < M V < -17
按照核球的大小和旋臂的缠卷程度, 旋涡星系可以分为 Sa, Sb, Sc 三个次型 Sa 型核球最大, 旋臂缠卷最紧,Sc 型核球最小, 旋臂缠卷最松 ~0.3 L bulge /L disk ~0.05 ~6 Pitch angle ~18
Example Spectrum: Spiral
(3) 棒旋星系 (Barred spiral galaxies) 中心有棒状结构的旋涡星系, 符号为 SB 旋臂源于棒的两端 按照棒的大小和旋臂的缠卷程度, 棒旋星系可以分为 SBa, SBb, SBc 三个次型 其中 SBa 型棒最大, 旋臂缠卷最紧 银河系可能是一个 SBb 或 SBc 型星系
Bars of stars run through the nuclear bulges of barred spiral galaxies Type SBa Type SBb Type SBc
(4) 透镜状星系 (Lenticular galaxies) 介于椭圆星系和旋涡星系之间的 无旋臂的盘星系, 根据核心是否有棒状结构, 符号相应为 S0 或 SB0 在形态上, 透镜状星系与旋涡星系的主要差别是没有旋臂 ; 与椭圆星系的主要差别是有星系盘 主要由年老恒星组成, 气体很少
(5) 不规则星系 (Irregular galaxies) 外型或结构无明显对称性的星系, 符号为 Irr 无旋臂和中心核区 富含星际气体 尘埃和年轻恒星 -18 < M V < -10 IC5152 M82
Example Spectrum: Irregular
旋涡 / 棒旋星系 (S, SB) 小结 椭圆星系 (E) 不规则星系 (Irr) 由恒星和气体构成的扁盘 ( 包含旋臂和核球 ) 和星系晕 棒旋星系的核心有棒状结构 盘包含年轻和年老的恒星, 晕只有年老的恒星 盘包含大量气体和尘埃, 晕中的气体和尘埃很少 旋臂中有恒星形成过程 盘中的恒星和气体绕星系核心作圆轨道运动, 晕中的恒星绕绕星系核心作无规则轨道运动 球形或椭球形, 除中心核区外无其他结构 只有年老的恒星 没有或很少气体和尘埃 近 10 10 yr 没有明显的恒星形成过程 恒星绕绕星系核心作无规则轨道运动 无明显结构 包含年轻和年老的恒星 富含气体和尘埃 强烈的恒星形成过程 恒星和气体作无规则运动
Gaseous Galaxies This radio image shows a galaxy with a lot of hydrogen gas but very few stars. This image shows the same galaxy in visible light
3. 星系的相对数目 目前可观测宇宙中大约有 4 10 10 个星系 不规则星系数目最多, 其次是旋涡星系和椭圆星系
7.2 测量星系 1. 星系距离的测量 (1) 利用造父变星的周光关系测量星系距离最远距离 :~20 Mpc (2) 标准烛光法 (the standard candle) 通过比较星系中可证认的某些标准 ( 明亮 ) 天体的视星等和绝对星等来确定星系的距离 特点 : 光度高且基本恒定 星系 M100 中的造父变星
标准烛光源球状星团行星状星云 O, B 型超巨星新星 HII 区 Ia 型超新星明亮星系 最远距离 (Mpc) ~5 ~20 ~30 ~60 ~80 ~650 ~1500
The Luminosity Function of Globular Clusters and Planetary Nebulae LF-the fraction of objects per magnitude having absolute magnitudes in the interval (M, M+dM).
(3) 星系的速度弥散 光度经验关系 Tully-Fisher 关系对旋涡星系, 速度弥散 ΔV = 220 (L /L ) 0.22 H 原子 21 厘米谱线宽度 速度弥散 光度 ~ 200 Mpc 距离 Faber-Jackson 关系对椭圆星系, 速度弥散 ΔV = 220 (L /L ) 0.25
星系的速度弥散 光度经验关系 星系质量越大 恒星运动速度越快 速度弥散越大 谱线越宽星系质量越大 光度越高 光度 谱线宽度
宇宙距离阶梯
(4) 红移法 1912-1920 年,V. M. Slipher 通过测量旋涡星系谱线的 Doppler 位移发现绝大多数的星系具有谱线红移, 即它们正在远离银河系 一些星系的距离 CaII 的 H K 谱线和退行速度
哈勃定律 1929 年 Hubble 与 Humason 发现由星系谱线红移得到的星系退行速度 V 与星系的距离 D 成正比, 称为哈勃定律 V=H 0 D 其中哈勃常数 H 0 =72±7 kms -1 Mpc -1 Hubble Humason
哈勃定律的意义 哈勃定律反映了宇宙的膨胀由宇宙膨胀引起的星系的谱线红移称为宇宙学红移 (cosmological redshift) 星系的距离 D=V/H 0 如果宇宙的膨胀是均匀的, 可以确定 : 宇宙的年龄 t=d/v=1/h 0 星系的退行表明在过去它们必定离得很近 宇宙膨胀的起点是什么?
利用星系红移测量星系的距离 10019 27101 1.0 100 10029 30096 1.0 9754 21022 0.984 10.0 6483 8815 0.6 1.0 1336 1401 0.095 0.1 0.0 0.0 0.0 0.0 回溯时间 (Myr) 目前的距离 (Mly) V/c 红移 z 1 1) ( 1 1) ( 2 2 0 + + + z z H c D
2. 星系质量的测量 (1) 旋涡星系的自转曲线谱线位移 自转速度 质量 星系 NGC 247: 蓝色和红色分别表示恒星和 HII 区的辐射 自转曲线 : 实心和空心点分别代表 HII 区和 HI 区, 实线代表只考虑可见物质的自转曲线
(2) 椭圆星系中恒星的无规运动谱线位移 平均无规运动速度 质量类似地, 对星系团谱线位移 星系的运动速度 + 星系间的距离 星系团 ( 引力 ) 质量
(3) 星系际气体 (intergalactic gas) 的 X 射线辐射 X 射线卫星观测发现在部分椭圆星系周围存在大量的热气体 ( 温度 ~ 10 7-10 8 K, 密度 ~ 1 cm -3 ) X 射线辐射 气体温度 热运动速度 (< 当地逃逸速度 ) 引力质量 室女星系团 M87, M86, M84
Mass-Luminosity Ratio The M-L ratio indicates the average mass per unit energy output from the galaxy. Usually in units of solar masses and solar luminosities Types of galaxies E S0 Sa Sb Sc Irr M/L 20-40 10 10 10 <10 <10
(4) 测量结果 正常漩涡星系与椭圆星系质量 ~ 10 11-10 12 M 不规则星系质量 ~ 10 8-10 10 M 矮椭圆星系质量 ~ 10 6-10 7 M 星系团质量 ~ 10 13-10 14 M 星系和星系团的引力质量大约是可见质量的 10 倍
小结 : 星系的基本性质 旋涡星系 不规则星系 矮椭圆星系 巨椭圆星系 直径 (ly) 90 10 3 20 10 3 30 10 3 150 10 3 质量 (M 光度 ) (L ) 10 11-10 12 10 10 10 6 10 9 10 5-10 7 10 8 10 13 10 11 颜色 蓝 ( 盘 ) 红 ( 晕与核 ) 蓝 红 红 气体百分比 5% 15% <1% <1% 恒星类型 质光比 年轻 ( 盘 ) 年轻 年老 年老 年老 ( 晕与核 ) (M/L) Irr <(M/L) S <(M/L) E, (M/L) =1
7.3 星系集团 Galaxies are not distributed uniformly throughout space -- most are evidently held together by gravity due to dark matter between the galaxies. Systems of galaxies Binary galaxies( 双重星系 ) Multiple galaxies( 多重星系 ) Groups of galaxies( 星系群 ) Clusters of galaxies( 星系团 ) Superclusters of galaxies( 超星系团 ) The larger a given system, the less its density exceeds the mean density of the Universe.
1. The Local Group( 本星系群 ) The galaxy cluster includes the Milky Way galaxy, with size of ~1.2 Mpc. There are at least 40 galaxies in the Local Group, three of them are spiral galaxies, four are irregulars and more than 20 elliptical galaxies. The Milky Way Galaxy and Andromeda (M31) are the largest members, and most of the smaller galaxies are gravitationally bound to one or the other of them.
Spatial Distribution of the Local Group
Spatial Distribution of the Local Group
(1) The Magellanic Clouds( 麦哲伦云 )
The Large/Small Magellanic Clouds( 大 小麦哲伦云 ) First recorded by F. Magellan in 1519. Physical prameters LMC: D = 50 kpc, M = 2 10 10 M, d = 10 kpc. SMC: D = 60 kpc, M = 4 10 9 M, d = 6 kpc. Both are irregular galaxies.orbiting our Milky Way galaxy. Abundant young stars and neutral hydrogen gas. SN1987A occurred in LMC.
The Magellanic Stream It is thought that approximately 5 10 9 yrs ago, the Magellanic Clouds passes the Galaxy at a distance of about 10-15 kpc, leaving behind the Magellanic stream, a 180 long thin stream of neutral hydrogen clouds.
42,000 light years Canis Major Dwarf: A New Closest Galaxy
(2) The Andromeda galaxy (M 31) The largest galaxy in the Local group, with distance of ~770 kpc and diameter of ~60 kpc. Type Sb spiral galaxy It has seven satellites, all are elliptical galaxies. The Andromeda Galaxy and two of its satellite galaxies, M32 (left) and M110 (bottom right.)
X-ray Census in Andromeda Supermassive BH X-ray binaries
(3) The M33 galaxy The third largest galaxy in the Local Group, with distance of ~720 kpc and diameter of ~18 kpc. Type Sc spiral galaxy. Large amounts of Pop I objects.
2. Clusters of Galaxies A collection of galaxies containing at least 50 bright galaxies. CL1358+62
Irregular Cluster of Galaxies Virgo Cluster, the nearest cluster of galaxies. Distance around 18 Mpc, diameter ~3 Mpc. Consists of more than 2500 galaxies. A dense central region containing early type galaxies surrounded by a more extended distribution of mainly spiral galaxies. The central region of the Virgo cluster
Regular Clusters of Galaxies Coma Cluster Distance ~ 90 Mpc, diameter ~ 3 Mpc. Contains ~6700 galaxies. Most galaxies in Coma are spirals and others are ellipticals.
Rich and Poor Clusters of Galaxies There are thousands of galaxies in the Coma cluster. There are less than 100 galaxies in the Herculus cluster.
Masses of clusters of galaxies The motion of galaxies in a cluster prevents the collapse of the cluster. M ~ RV 2 /G For a typical rich cluster: R ~ 1 Mpc, V ~ 1000 km/s M ~ 2 10 14 M A cluster usually contains ~ 1000 galaxies, each galaxy has a typical luminosity ~ 10 10 L The mass-light ratio M/L ~ 20 M /L Dark matter contributes ~ 95% of the cluster s mass!
Hot gas in and around elliptical galaxies is taken as solid evidence for the presence of dark matter. 星系 Abell 2390( 上 ) 和 MS2137.3-2353 ( 下 ) 的 X 射线 ( 左 ) 与光学 ( 右 ) 像 气体与恒星的质量仅为束缚气体所需质量的 13%
Dark Matter and Gravitational Lensing CL0025+1654
3. Galaxy interaction Objects Planets in the Solar System Stars in the Galaxy Galaxies in clusters Distance/ Diameter 10 5 10 6 10 2 denser distribution of galaxies in clusters more likely to collide
Hubble deep field observations found lots of young, irregular, small galaxies at distance of 10 3 Mpc. This means that galaxy collisions were very popular 10 billions years ago.
Galaxy Interactions and Evolution (1) Distortion and starburst Tidal force intergalactic bridges and tails Gas compression rapid star formation Stefan's Quintet
M51: Spiral Structure, Dust and Stars Numerical simulation results
NGC 4676a/b
The Cartwheel Galaxy
NGC 4038/9 (Antennae)
Numerical Simulation
Starburst Galaxy M 82 Large clusters of stars and luminous X-ray sources Optical X-ray
Ultraluminous Infrared Galaxies
Hubble's Advanced Camera Unveils a Panoramic New View of the Universe The Tadpole Galaxy, UGC 10214 The Mice, NGC 4676
Animations
(2) Merger of galaxies formation of giant elliptical galaxies Observational evidence mass ratio of giant and dwarf elliptical galaxies ~10 7 Few spiral galaxies in the dense, central regions of clusters. Features of giant ellipticals (i) extended halos (ii) multi-neculei (ii) Usually in the central region of the clusters 星系团 Abell 2199 中心星系核区的小星系
Building an Bigger Galaxy NGC 1700
NGC 6240 A composite optical (Hubble) and x-ray (Chandra) photo of the inner regions of galaxy NGC 6240. X-rays from the two supermassive black holes appear blue in this false-color image.
Hubble Watches Galaxies Engage in Dance of Destruction
星系团 MS1054-93 中的星系碰撞过程
Numerical Simulation of Galaxy Mergers
4. Superclusters Consisting of tens or hundreds of clusters of galaxies. Size roughly of 100 Mpc, masses around 10 16 M. Expanding or loosely bound because of relatively weak mutual gravitation. Distribute as a continuous network, where the large clusters are connected by walls and strings formed by smaller systems.
The Local Supercluster The supercluster that include the Local Group. Centered around the Virgo cluster, radius of 30 Mpc.
The Local Supercluster Mass ~ 10 15 M Flat shape The Local Group is Moving towards the Virgo cluster at a speed of 250-350 kms -1 Distribution of 9325 galaxies
The 2MASS Galaxy Sky
The Large Scale Structure in the Universe Obtained through large scale (>100 Mpc)galaxy redshift survey. Simultaneously observe the spectra of thousands of galaxies to measure their distances. The galaxies are distributed in a kind of "foamy" texture consisting of huge sheets and filaments of galaxies (occupying 1%-2% space) separated by large "voids" where very few galaxies are found. The typical diameters of the voids are 50-100 million light years.
The first slice of a survey of the universe, covering 1057 galaxies out to an approximate distance of 200 Mpc The Great Wall
Two Degree Field (2dF) Redshift Survey 250 Mly
The SDSS 3D Universe Map
不同距离处星系密度的变化 对 300Mly 尺度的数值模拟
Hierarchy of Cosmic Structures
7.4 星系的演化 1. 星系的形成 What were the first sources of light in the Universe? How were luminous parts of galaxies assembled? How did the Hubble sequence of galaxy morphologies form? How do galaxies interact with their environment? What are the global histories of starformation, metal enrichment, and gas consumption? What is the relationship between active galactic nuclei and their host galaxies? z = 1000 z = 0
处于婴儿期的 ( 高红移 ) 星系
CDM - Hierarchical scenario Springel et al, 2001, MNRAS, 228, 726: high resolution N-body simulation of the evolution of clusters of galaxies
两种模型 (1) 自下而上 (bottomup) 模型较小的 (~10 6 M ) 不规则星系首先形成 星系合并形成较大的 (~10 9-10 11 M ) 星系 在引力的作用下聚集成星系团和超星系团, 产生星系团间的巨洞
观测证据 Hubble 空间望远镜的深场观测发现位于 2000 Mpc 距离之外存在大量的不规则的小星系 ( 超过同类星系在近距离星系中的比例 )
Gravitational Lens Helps Hubble and Keck Discover Galaxy Building Block
Galactic Baby Boom The Hubble Space Telescope bears witness to incredibly intense star formation in this image of NGC 7673 in Pegasus. The blue knots are regions where millions of new stars are igniting.
More Galaxies that Jump the Gun The galaxies in the image above are probably only the brightest tips of an iceberg; the region likely contains thousands more galaxies that are too faint to show, as depicted in this artist's concept.
Old Galaxies in the Young Universe
(2) 由上而下模型 (top-down) 原始气体的坍缩首先产生巨大的 (~10 14 M ) 薄饼状的云块 ( 超星系团 ) 云块分裂成星系团和星系
When did galaxy formation stop? Some maintain that there was a fairly well defined time in the past given by the age of the globular clusters in our own Galaxy, for example by which most galaxy formation was over. Others point out that many galaxies show evidence of mergers and the accumulation of smaller satellite galaxies over an extended period of time even up to the present day.
New Evidence About the Formation of Galaxies
2. 星系的演化 (1) 星系的形态与恒星形成星系形成于气体云的坍缩 星系的形态与星系中的恒星形成有关 如果恒星形成较快, 星系内的气体很快被用光, 没有星系盘形成, 椭圆星系 如果恒星形成较慢, 星系内有大量的气体形成星系盘, 盘内的恒星形成, 旋涡星系
(2) 内在因素 椭圆星系形成于初始涨落中的较密的区域, 所有的气体在 10 亿年内耗尽用于形成恒星
Early Supernovae Seeded the Universe With Elements In the early universe, small protogalaxies clustered together into vast filamentary structures. Within these glowing galactic building blocks, supernovae exploded like firecrackers as the first, "greatest generation" of stars rapidly used up their fuel and died. Credit: Christine Lafon (CfA), Volker Springel (Max Planck Institute for Astrophysics), and Lars Hernquist (CfA). Supercomputer simulations by Volker Bromm and colleagues show that the first supernovae were incredibly energetic, spewing tremendous amounts of iron, carbon, oxygen, and other elements across thousands of light-years of space. These first supernovae seeded the universe with the "stuff of life." Credit: Volker Bromm (CfA), Naoki Yoshida (National Astronomical Observatory of Japan), Lars Hernquist (CfA), and Christine Lafon (CfA).
旋涡星系形成于相对较稀的区域, 在湍动的影响下产生旋转, 早期恒星形成过程产生晕和核中的恒星, 剩余的气体形成盘, 维持持续的恒星形成
(3) 外部环境因素 观测线索质量最大的星系是椭圆星系, 星系间的合并 碰撞, 小星系的碰撞导致恒星的快速形成, 形成椭圆星系