旋涡星系颜色梯度的研究进展

旋涡星系的颜色梯度反映了其星族构成沿径向的分布,包含了星系恒星形成历史的信息.因此,对旋涡星系颜色梯度的研究有助于理解星系的形成和演化过程.大部分旋涡星系存在负的颜色梯度,其主要原因是旋涡星系存在星族梯度.颜色梯度与星系的面亮度之间存在内禀的相关,表明质量面密度在星系的形成和演化过程中具有重要作用.     

星系团Abell 2199的恒星形成性质

通过对近邻星系团Abell 2199中290颗成员星系进行形态分类,研究这些星系的恒星形成率及其与形态和相关物理特性之间的关系.该星系团中星系的特征恒星形成率与Ha等值宽度、星系光谱在4000A处的跃变程度以及星系所包含的恒星质量之间有较强的相关性.这些星系的恒星形成活动没有表现出明显的环境效应,表明该星系团仍处在剧烈的动力学演化阶段,远没有达到动力学平衡.     

高红移大质量星系的形成与演化

星系形成及演化一直是天体物理研究中最重要的领域之一.近10 yr来星系形成及演化的研究取得了突破性进展,主要包括:(1)近邻星系在颜色-星等图上呈现双峰分布,早型星系普遍颜色较红,而晚型星系颜色偏蓝;并且近邻宇宙红星系的总质量相对于红移z=1时至少增长了1倍,意味着存在蓝星系到红星系的转化过程.这一转化过程是如何发生的成为现代星系形成演化领域最重要的问题之一;(2)星系中心超大质量黑洞质量跟核球质量有很强的相关,意味着黑洞演化跟星系演化有着紧密的关系.黑洞的活动(活动星系核)如何影响星系演化也成为了亟待解决的问题之一.然而近邻宇宙中的大质量星系基本都已停止了剧烈的恒星形成活动和黑洞活动,因此,要回答这些问题,我们需要仔细研究宇宙早期红移在z≈2的星系性质.在这一红移处,星系中的恒星形成和黑洞增长均处于高峰期.     

后发团盘星系自转轴方向的分布

本文对后发团中心２．６３°×２．６３°范围内１３８个盘星系（９４个Ｓ０星系，４４个Ｓ和Ｉｒ星系）的自转轴方向的分布作了分析，结果表明，后发团中盘星系自转轴方向的分布是非均匀的．相对于假设的均匀分布，有较多的Ｓ０星系的自转轴平行于团平面，并且Ｓ０星系的自转轴在团平面上的投影倾向于沿着偏离团中心方向约４５°的方向；对于Ｓ和Ｉｒｒ星系，自转轴倾向于平行或垂直于团平面，自转轴在团平面上的投影则倾向于平行团中心方向．本文证实了后发团中盘星系自转轴方向分布的非均匀性及其与星系形态类型的相关，而且可能与星系光度也有关．样本越深，自转轴指向的分布似乎越不均匀．本文结果说明后发团存在一个优势平面（可能的团平面），在各种星系团及星系形成理论中，有支持“薄饼”（ｐａｎｃａｋｅ）模型的迹象．     

主并合星系对SFR的增幅与其他参数关系的研究

主并合星系对是研究星系同时受到本身与外部环境影响的绝佳实验对象,而星系恒星形成率的变化可以示踪这些影响产生的作用.星系的恒星质量、星系对之间的投影距离与相对倾角都是影响恒星形成率的几个重要因素.研究结果表明,更大恒星质量星系倾向于有更大的恒星形成率增幅,相对倾角接近平行的星系同样趋于有更大的恒星形成率增幅,而投影距离在研究范围内与恒星形成率没有相关性.     

星系内禀指向对弱引力透镜剪切场信号污染的去除方法

星系的内禀指向(intrinsic alignment, IA)的关联性是弱引力透镜观测中剪切场信号的一个重要系统误差,人们在之前的弱引力透镜研究中已经提出了许多修正该误差的方法。从数据处理方面,人们可以剔除物理距离比较近的星系对,但是这种方法只能近似消除星系内禀指向自相关带来的污染项,并不能消除星系内禀指向与周围物质密度场的相关性,并且这种方法也会丢失很多星系的信息。而目前弱引力透镜观测中广泛使用的IA模型与实际的IA模型可能相差甚远,使用不同的IA模型得到的宇宙学参数会存在很大差别。虽然零调(nulling)技术不用假设IA模型,但是这种技术仅能消除星系内禀指向与周围物质密度场的相关性。另外,由于这种技术须对红移设置不同的权重,所以会失去IA对红移的依赖性。Zhang^[1, 2]提出的自修正方法,在不假设任何IA模型的情况下,利用多种观测量以及几个物理量之间的比例关系就能够把弱引力透镜中的IA信号很好地消除。此自修正方法可望广泛应用于即将开始的第四代弱引力透镜巡天中。     

不同光度星系大尺度分布的交叉相关分析

在对不同光度星系大尺度分布进行空间两点相关函数分析的基础上,仍以CfA红移巡天资料为样本,对不同光度星系分布进行了交叉相关分析。结果表明,不同光度星系间的交叉相关函数仍可近似地以幂函数表示,说明不同光度星系在空间是一起成团的。但在较小尺度上((?)4—6Mpc),光度较高的星系间相关更强,而在更大一些尺度上光度较高的星系间相关减弱更快,甚至变得比与光度较低星系间的相关更弱。结合前面对自相关函数分析的结果可以看到,统计上看来,星系分布形成群和团。群或团中亮的星系形成更致密的分布而较暗的星系则在这些群和团中分布较弥散。此结果表明星系光度和其环境(密度)有关,从而从观测上为Biased星系形成理论提供了一个可能的证据。     

星系的基本面

基本面最早是在早型星系中发现的一种经验标度律,它是一个关于星系的有效半径、中心速度弥散度和有效半径内的平均面亮度之间的紧密相关关系.基本面对早型星系的形成机制、动力学演化理论等提出了很强的观测约束;不仅如此,它还可作为独立的星系距离测定方法,用于哈勃常数的定标和星系本动速度场的测量.对早型星系基本面的研究进展做了简要评...     

E+A星系的研究进展

E+A星系的光谱具有很强的巴耳末吸收线,缺乏与恒星形成相关的发射线,将典型的椭圆星系(E)和A型恒星的光谱进行线性组合就能够拟合出这类星系的光谱.它们的颜色、形态、星族年龄等参数介于典型的早型和晚型星系之间.E+A星系近期经历了星暴活动,在星系演化进程中,它们处于晚型到早型的过渡阶段,可能在演变过程中扮演着重要角色.介...     

高红移恒星形成星系的研究

<正>星系是组成宇宙的基石,其形成与演化是天体物理研究的重要内容.星系中的恒星形成活动是星系成长和演化的主要驱动力之一.已有的星系巡天给出比较一致的宇宙恒星形成历史:宇宙的恒星形成密度从高红移一直增加到红移z~2,随后按指数率下降直到z=0.系统地研究宇宙恒星形成峰值时期恒星形成星系的性质对我们理解并限制星系形成与演化的理论模型至关重要.     

The fate of substructures in cold dark matter haloes

We use the Millennium Simulation, a large, high-resolution N -body simulation of the evolution of structure in a Λ cold dark matter cosmology, to study the properties and fate of substructures within a large sample of dark matter haloes. We find that the subhalo mass function departs significantly from a power law at the high-mass end. We also find that the radial and angular distributions of substructures depend on subhalo mass. In particular, high-mass subhaloes tend to be less radially concentrated and to have angular distributions closer to the direction perpendicular to the spin of the host halo than their less massive counterparts. We find that mergers between subhaloes occur. These tend to be between substructures that were already dynamically associated before accretion into the main halo. For subhaloes larger than 0.001 times the mass of the host halo, it is more likely that the subhalo will merge with the central or main subhalo than with another subhalo larger than itself. For lower masses, subhalo–subhalo mergers become equally likely to mergers with the main subhalo. Our results have implications for the variation of galaxy properties with environment and for the treatment of mergers in galaxy formation models.     

Environment and mass dependencies of galactic λ spin parameter: cosmological simulations and observed galaxies compared

We use a sample of galaxies from the Sloan Digital Sky Survey (SDSS) to search for correlations between the λ spin parameter and the environment and mass of galaxies. In order to calculate the total value of λ for each observed galaxy, we employed a simple model of the dynamical structure of the galaxies, which allows a rough estimate of the value of λ using only readily obtainable observables from the luminous galaxies. Use of a large volume-limited sample (upwards of 11 000) allows reliable inferences of mean values and dispersions of λ distributions. We find, in agreement with some N -body cosmological simulations, no significant dependence of λ on the environmental density of the galaxies. For the case of mass, our results show a marked correlation with λ, in the sense that low-mass galaxies present both higher mean values of λ and associated dispersions, than high-mass galaxies. These results provide interesting constrain on the mechanisms of galaxy formation and acquisition of angular momentum, a valuable test for cosmological models.     

Feedback and the formation of dwarf galaxy stellar haloes

Stellar population studies show that low-mass galaxies in all environments exhibit stellar haloes that are older and more spherically distributed than the main body of the galaxy. In some cases, there is a significant intermediate age component that extends beyond the young disc. We examine a suite of Smoothed Particle Hydrodynamic simulations and find that elevated early star formation activity combined with supernova feedback can produce an extended stellar distribution that resembles these haloes for model galaxies ranging from   v ₂₀₀= 15  to 35 km s⁻¹, without the need for accretion of subhaloes.     

The growth and assembly of a massive galaxy at z∼ 2

We study the stellar mass assembly of the Spiderweb galaxy  (MRC 1138−262)  , a massive   z = 2.2  radio galaxy in a protocluster and the probable progenitor of a brightest cluster galaxy. Nearby protocluster galaxies are identified and their properties are determined by fitting stellar population models to their rest-frame ultraviolet to optical spectral energy distributions. We find that within 150 kpc of the radio galaxy the stellar mass is centrally concentrated in the radio galaxy, yet most of the dust-uncorrected, instantaneous star formation occurs in the surrounding low-mass satellite galaxies. We predict that most of the galaxies within 150 kpc of the radio galaxy will merge with the central radio galaxy by   z = 0  , increasing its stellar mass by up to a factor of ≃2. However, it will take several hundred Myr for the first mergers to occur, by which time the large star formation rates are likely to have exhausted the gas reservoirs in the satellite galaxies. The tidal radii of the satellite galaxies are small, suggesting that stars and gas are being stripped and deposited at distances of tens of kpc from the central radio galaxy. These stripped stars may become intracluster stars or form an extended stellar halo around the radio galaxy, such as those observed around cD galaxies in cluster cores.     

Black hole spin and radio loudness in a Λ cold dark matter universe

We use a combination of a cosmological N -body simulation of the concordance Λ cold dark matter paradigm and a semi-analytic model of galaxy formation to investigate the spin development of central supermassive black holes (BHs) and its relation to the BH host galaxy properties. In order to compute BH spins, we use the α model of Shakura & Sunyaev and consider the King et al. warped disc alignment criterion. The orientation of the accretion disc is inferred from the angular momentum of the source of accreted material, which bears a close relationship to the large-scale structure in the simulation. We find that the final BH spin depends almost exclusively on the accretion history and only weakly on the warped disc alignment. The main mechanisms of BH spin-up are found to be gas cooling processes and disc instabilities, a result that is only partially compatible with Monte Carlo models where the main spin-up mechanisms are major mergers and disc instabilities; the latter results are reproduced when implementing randomly oriented accretion discs in our model. Regarding the BH population, we find that more massive BHs, which are hosted by massive ellipticals, have higher spin values than less massive BHs, hosted by spiral galaxies. We analyse whether gas accretion rates and BH spins can be used as tracers of the radio loudness of active galactic nuclei (AGN). We find that the current observational indications of an increasing trend of radio-loud AGN fractions with stellar and BH mass can be easily obtained when placing lower limits on the BH spin, with a minimum influence from limits on the accretion rates; a model with random accretion disc orientations is unable to reproduce this trend. Our results favour a scenario where the BH spin is a key parameter to separate the radio-loud and radio-quiet galaxy populations.     

The image separation distribution of strong lenses: halo versus subhalo populations

We present a halo model prediction of the image separation distribution of strong lenses. Our model takes into account the subhalo population, which has been ignored in previous studies, as well as the conventional halo population. Haloes and subhaloes are linked to central and satellite galaxies by adopting a universal scaling relation between masses of (sub)haloes and luminosities of galaxies. Our model predicts that 10–20 per cent of lenses should be caused by the subhalo population. The fraction of lensing by satellite galaxies (subhaloes) peaks at ∼1 arcsec and decreases rapidly with increasing image separations. We compute fractions of lenses which lie in groups and clusters and find them to be ∼14 and ∼4 per cent, respectively; nearly half of such lenses are expected to be produced by satellite galaxies, rather than central parts of haloes. We also study mass distributions of lensing haloes and find that, even at image separations of ∼3 arcsec, the deviation of lens mass distributions from isothermal profiles is large; at or beyond ∼3 arcsec, image separations are enhanced significantly by surrounding haloes. Our model prediction agrees reasonably well with observed image separation distributions from galaxy to cluster scales.     

The clustering and host haloes of galaxy mergers at high redshift

High-resolution simulations of cosmological structure formation indicate that dark matter substructure in dense environments, such as groups and clusters, may survive for a long time. These dark matter subhaloes are the likely hosts of galaxies. We examine the small-scale spatial clustering of subhalo major mergers at high redshift using high-resolution N -body simulations of cosmological volumes. Recently merged, massive subhaloes exhibit enhanced clustering on scales  ∼100–300  h ⁻¹ kpc  , relative to all subhaloes of the same infall mass, for a short time after a major merger (<500 Myr). The small-scale clustering enhancement is smaller for lower mass subhaloes, which also show a deficit on scales just beyond the excess. Haloes hosting recent subhalo mergers tend to have more subhaloes; for massive subhaloes, the excess is stronger and it tends to increase for the most massive host haloes. The subhalo merger fraction is independent of halo mass for the scales we probe. In terms of satellite and central subhaloes, the merger increase in small-scale clustering for massive subhaloes arises from recently merged massive central subhaloes having an enhanced satellite population. Our mergers are defined via their parent infall mass ratios. Subhaloes experiencing major mass gains also exhibit a small-scale clustering enhancement, but these correspond to two-body interactions leading to two final subhaloes, rather than subhalo coalescence.     

Star clusters around recoiled black holes in the Milky Way halo

Gravitational wave emission by coalescing black holes (BHs) kicks the remnant BH with a typical velocity of hundreds of  km s⁻¹  . This velocity is sufficiently large to remove the remnant BH from a low-mass galaxy but is below the escape velocity from the Milky Way (MW) galaxy. If central BHs were common in the galactic building blocks that merged to make the MW, then numerous BHs that were kicked out of low-mass galaxies should be freely floating in the MW halo today. We use a large statistical sample of possible merger tree histories for the MW to estimate the expected number of recoiled BH remnants present in the MW halo today. We find that hundreds of BHs should remain bound to the MW halo after leaving their parent low-mass galaxies. Each BH carries a compact cluster of old stars that populated the core of its original host galaxy. Using the time-dependent Fokker–Planck equation, we find that the present-day clusters are  ≲1 pc  in size, and their central bright regions should be unresolved in most existing sky surveys. These compact systems are distinguishable from globular clusters by their internal (Keplerian) velocity dispersion greater than 100 km s⁻¹ and their high mass-to-light ratio owing to the central BH. An observational discovery of this relic population of star clusters in the MW halo would constrain the formation history of the MW and the dynamics of BH mergers in the early Universe. A similar population should exist around other galaxies and may potentially be detectable in M31 and M33.     

Low-mass cluster galaxies: A cornerstone of galaxy evolution

Low-Mass cluster galaxies are the most common galaxy type in the universe and are important objects for understanding galaxy formation, luminosity functions, dark matter and the formation of large scale structure. In this short summary I describe the properties and likely origins of low-mass cluster galaxies and what they reveal about broader cosmological issues. This revised version was published online in August 2006 with corrections to the Cover Date.