Modeling the Newtonian dynamics for rotation curve analysis of thin-disk galaxies

We present an efficient, robust computational method for modeling the Newtonian dynamics for rotation curve analysis of thin-disk galaxies. With appropriate mathematical treatments, the apparent numerical difficulties associated with singularities in computing elliptic integrals are completely removed. Using a boundary element discretization procedure, the governing equations are transformed into a linear algebra matrix equation that can be solved by straightforward Gauss elimination in one step without further iterations. The numerical code implemented according to our algorithm can accurately determine the surface mass density distribution in a disk galaxy from a measured rotation curve （or vice versa）. For a disk galaxy with a typical flat rotation curve, our modeling results show that the surface mass density monotonically decreases from the galactic center toward the periphery, according to Newtonian dynamics. In a large portion of the galaxy, the surface mass density follows an approximately exponential law of decay with respect to the galactic radial coordinate. Yet the radial scale length for the surface mass density seems to be generally larger than that of the measured brightness distribution, suggesting an increasing mass-tolight ratio with the radial distance in a disk galaxy. In a nondimensionalized form, our mathematical system contains a dimensionless parameter which we call the ＂galactic rotation number＂ that represents the gross ratio of centrifugal force and gravitational force. The value of this galactic rotation number is determined as part of the numerial solution. Through a systematic computational analysis, we have illustrated that the galactic rotation number remains within 4-10% of 1.70 for a wide variety of rotation curves. This implies that the total mass in a disk galaxy is proportional to V02 Rg, with V0 denoting the characteristic rotation velocity （such as the ＂flat＂ value in a typical ro- tation curve） and Rg the radius of the galactic disk. The predicted total galactic mass of the Milky Way is in good agreement with the star-count data.

The formation and evolution of massive galaxies

The discovery of massive galaxies at high redshifts,especially the passive ones,poses a big challenge for the current standard galaxy formation models.Here we use the semi-analytic galaxy formation model developed by Henriques et al.to explore the formation and evolution of massive galaxies(MGs,stellar-mass M*>1011 M⊙).Different from previous works,we focus on the ones just formed(e.g.just reach?1011 M⊙).We find that most of the MGs are formed around z=0.6,with the earliest formation at z>4.Interestingly,although most of the MGs in the local Universe are passive,we find that only 13%of the MGs are quenched at the formation time.Most of the quenched MGs at formation already host a very massive supermassive black hole(SMBH)which could power the very effective AGN feedback.For the star-forming MGs,the ones with more massive SMBH prefer to quench in shorter timescales;in particular,those with MSMBH>107.5 M⊙have a quenching timescale of~0.5 Gyr and the characteristic MSMBH depends on the chosen stellar mass threshold in the definition of MGs as a result of their co-evolution.We also find that the"in-situ"star formation dominates the stellar mass growth of MGs until they are formed.Over the whole redshift range,we find the quiescent MGs prefer to stay in more massive dark matter halos,and have more massive SMBH and less cold gas masses.Our results provide a new angle on the whole life of the growth of MGs in the Universe.

Stellar dynamical modeling-accuracy of 3D density estimation for edge-on axisymmetric galaxies

From Rybicki’s analysis using the Fourier slice theorem,mathematically it is possible to reproduce uniquely an edge-on axisymmetric galaxy’s 3D light distribution from its 2D surface brightness.Utilizing galaxies from a cosmological simulation,we examine the ability of Syer and Tremaine’s madeto-measure method and Schwarzschild’s method for stellar dynamical modeling to do so for edge-on oblate axisymmetric galaxies.Overall,we find that the methods do not accurately recover the 3D distributions,with the made-to-measure method producing more accurate estimates than Schwarzschild’s method.Our results have implications broader than just luminosity density,and affect other luminosity-weighted distributions within galaxies,for example,age and metallicity.

Empirical formulae to describe some physical properties of small groups of protogalaxies with multiplicity

By means of identical cubic elements,we generate a partition of a volume in which a particle-based cosmological simulation is carried out.In each cubic element,we determine the gas particles with a normalized density greater than an arbitrarily chosen density threshold.By using a proximity parameter,we calculate the neighboring cubic elements and generate a list of neighbors.By imposing dynamic conditions on the gas particles,we identify gas clumps and their neighbors,so that we calculate and fit some properties of the groups so identified,including the mass,size and velocity dispersion,in terms of their multiplicity(here defined simply as the number of member galaxies).Finally,we report the value of the ratio of kinetic energy to gravitational energy of such dense gas clumps,which will be useful as initial conditions in simulations of gravitational collapse of gas clouds and clusters of gas clouds.

Research on magnetic fields of galaxies using RZ-model

The generation of magnetic fields of galaxies is usually described by the dynamo mechanism.This process is characterized by the Steenbeck-Krause-Radler equation,which is the result of averaging the magnetohydrodynamics equations by distances which are associated with the size of turbulent cells in the interstellar medium.This equation is quite difficult to solve both from an analytical and numerical point of view.For galaxies,the no-z approximation is widely used.It describes the magnetic fields in thin discs.For such objects,where it is important to study the vertical structure of the field,it is not very applicable,so it is quite useful to adopt the RZ-model,which takes into account the dependence of the distance from the equatorial plane.During our research we have obtained the critical values of the dynamo number for galaxies with large half-thickness.We have also described typical z-structure for the magnetic field.Moreover,we have demonstrated that it is possible to generate dipolar magnetic fields.

Recovering 3D structural properties of galaxies from SDSS-like photometry

Because of the 3D nature of galaxies, an algorithm for constructing spatial density distribution models of galaxies on the basis of galaxy images has many advan- tages over approximations of the surface density distribution. We present a method for deriving the spatial structure and overall parameters of galaxies from images and estimate its accuracy and derived parameter degeneracies on a sample of idealised model galaxies. The test galaxies consist component with varying proportions and of a disc-like component and a spheroidal properties. Both components are assumed to be axially symmetric and coplanar. We simulate these test galaxies as if they had been observed in the SDSS project through ugriz filters, thus gaining a set of realis- tically imperfect images of galaxies with known intrinsic properties. These artificial SDSS galaxies were thereafter remodelled by approximating the surface brightness distribution with a 2D projection of a bulge＋disc spatial distribution model and the restored parameters were compared to the initial ones. Down to the r-band limiting magnitude of 18, errors in the restored integral luminosities and colour indices re- main within 0.05 mag and errors in the luminosities of individual components within 0.2 mag. Accuracy of the restored bulge-to-disc luminosity ratio （B/D） is within 40% in most cases, and becomes worse for galaxies with low B/D, but the general balance between bulges and discs is not shifted systematically. Assuming that the intrinsic disc axial ratio is ≤ 0.3, then the inclination angles can be estimated with errors 〈 5° for most of the galaxies with B/D 〈 2 and with errors 〈 15° up to B/D = 6. Errors in the recovered sizes of the galactic components are below 10% in most cases. The axial ratios and the shape parameter N of Einasto＇s distribution （similar to the Sersic index） are relatively inaccurate, but can provide statistical estimates for large samples. In general, models of disc components are more accurate than models of spheroidal components for geometrical reasons.

Probing satellite galaxies in the Local Group by using FAST

The abundance of neutral hydrogen（HI）in satellite galaxies in the local group is important for studying the formation history of our local group.In this work,we generated mock HI satellite galaxies in the Local Group using the high mass-resolution hydrodynamic APOSTLE simulation.The simulated HI mass function agrees with the ALFALFA survey very well above 10~6M_⊙,although there is a discrepancy below this scale because of the observed flux limit.After carefully checking various systematic elements in the observations,including fitting of line width,sky coverage,integration time and frequency drift due to uncertainty in a galaxy’s distance,we predicted the abundance of HI in galaxies in a future survey that will be conducted by FAST.FAST has a larger aperture and higher sensitivity than the Arecibo telescope.We found that the HI mass function could be estimated well around 10~5M_⊙ if the integration time is 40 minutes.Our results indicate that there are 61 HI satellites in the Local Group and 36 in the FAST field above 10~5M_⊙.This estimation is one order of magnitude better than the current data,and will put a strong constraint on the formation history of the Local Group.Also more high resolution simulated samples are needed to achieve this target.

Chemo-dynamical modelling with Schwarzschild's method

We extend Schwarzschild’s dynamical modelling method to model absorption line strength data as well as the more usual luminosity and kinematic data.Our approach draws on earlier published work by the first author with the Syer & Tremaine made-to-measure(M2M) dynamical modelling method and uses similar ideas to create a chemo-Schwarzschild method.We apply our extended Schwarzschild method to the same four early type galaxies(NGC 1248, NGC 3838, NGC 4452,NGC 4551) as the chemo-M2M work, and are able to recover successfully the 2D absorption line strength for the three lines we model(Hβ, Fe5015, Mg b).We believe that this is the first time Schwarzschild’s method has been used in this way.The techniques developed can be applied to modelling other aspects of galaxies, for example age and metallicity data coming from stellar population modelling, and are not specific to absorption line strength data.

星系NGC5018的星族合成研究

利用星团谱样本的星族合成方法,研究了邻近巨椭圆星系NGC5018中的星族成分和其内部的恒星形成历史,给出了星系中不同年龄和金属丰度星族的成分占有比.星族合成结果表明,NGC5018中不仅存在大量金属丰度低的年老恒星成分,而且较年轻的星族成分(T=5×108yr)对星系光度贡献也很重要.星系吞并和相互作用过程可能是触发这些较年轻星族形成的物理原因,椭圆星系内部的恒星形成历史可能是2次爆发或者多次爆发过程.这些结果可以很好地解释NGC5018颜色偏蓝、Mg2谱指数强度偏弱等观测特征.

Modeling the dynamical friction timescale of a sinking satellite

When a satellite galaxy falls into a massive dark matter halo, it suffers from the dynamical friction force which drags it into the halo＇s center, where it finally merges with the central galaxy. The time interval between entry and merger is called the dynamical friction timescale （Tdf）. Many studies have been dedicated to deriving Tdf using analytical models or N-body simulations. These studies have obtained qualitative agreements on how Zdf depends on the orbital parameters, and the mass ratio between the satellite and the host＇s halo. However, there are still disagreements on deriving an accurate form for Tdf. We present a semi-analytical model to predict Tdf and we focus on interpreting the discrepancies among different studies. We find that the treatment of mass loss from the satellite by tidal stripping dominates the behavior of Tdf. We also identify other model parameters which affect the predicted Tdf.

Made-to-measure galaxy modelling utilising absorption line strength data

We enhance the Syer ＆ Tremaine made-to-measure （M2M） particle method of stellar dynamical modelling to model simultaneously both kinematic data and absorption line strength data, thus creating a ＇chemo-M2M＇ modelling scheme. We apply the enhanced method to four galaxies （NGC 1248, NGC 3838, NGC 4452, NGC 4551） observed using the SAURON integral-field spectrograph as part of the ATLAS3D programme. We are able to reproduce successfully the 2D line strength data achieving mean X2 per bin values of ≈ 1 with 〉 95% of particles having converged weights. Because M2M uses a 3D particle system, we are also able to examine the underlying 3D line strength distributions. The extent to which these dis- tributions are plausible representations of real galaxies requires further consideration. Overall, we consider the modelling exercise to be a promising first step in developing a ＇chemo-M2M＇ modelling system and in understanding some of the issues to be addressed. While the made-to-measure techniques developed have been applied to absorption line strength data, they are in fact general and may be of value in modelling other aspects of galaxies.

The nature of the γ-ray flare associated with blazar 3C 454.3

We have studied the simultaneous spectral energy distributions （SEDs） of the 2009 December flare and those of the quiescent state of blazar 3C 454.3 by con- structing a multi-component model We find that all six SEDs can be explained by a one-zone leptonic model involving synchrotron self-Compton （SSC） plus external Compton emission from an accretion disk （ECD） and that from a broad-line region （ECC）. X-ray emission is dominated by the SSC mechanism, and the γ-ray spectrum is well represented by a combination of ECD and ECC processes. Our results indicate that the energy density of the magnetic field and electrons decrease with distance from the central engine, and the Doppler factor increases with the blob moving outward in the development of the 2009 December flare. The increase in the observed flux density is possibly due to the increase in the Doppler factor of the blob. The relation between the Doppler factor σb and the distance from the central black hole suggests the magnetically driven jets span a sub-pc scale, and the relation between the magnetic field Bt and the dimension of the emission region R＇b is in good agreement with what is required by conservation of magnetic flux. The weak ＂harder-when-brighter＂ behavior of the γ-ray spectrum could be a result of the increase in Doppler factor during the outward motion of the blob. The parameters during the quiescent state obviously deviate from those during the flare state. We propose that the flare was likely caused by the ejection of a new blob. The gamma-ray emissions in different states are associated with the evolution of the blob.

盘星系并合的数值模拟

使用一系列的模拟,包括了不同的轨道参数、主并合与小并合,以及不同的恒星盘自旋角动量与轨道角动量的耦合方式,来研究盘星系的并合过程中,顺行交会和逆行交会对并合过程会产生怎样的影响.这些影响包括并合时标、轨道形态、星系的恒星及暗物质成分剥离效率、星系的形态变化,以及形成的潮汐结构等.研究发现,由于在并合过程的前期,逆行并合的恒星剥离效率要明显小于顺行并合,因此会形成小得多的潮汐结构.但与通常认为的相反的是,与顺行并合相比,逆行并合并没有显著改变轨道形态和增大并合时标,对恒星成分的剥离效率也没有显著的影响.与顺行与否相比,潮汐半径在小并合时,卫星星系恒星成分被潮汐剥离的过程中扮演着更重要的角色,潮汐半径与恒星盘标长相等的时刻可以被认为是卫星星系恒星盘瓦解的时刻.

PHoToNs–A parallel heterogeneous and threads oriented code for cosmological N-body simulation

We introduce a new code for cosmological simulations, PHo To Ns, which incorporates features for performing massive cosmological simulations on heterogeneous high performance computer(HPC) systems and threads oriented programming. PHo To Ns adopts a hybrid scheme to compute gravitational force, with the conventional Particle-Mesh(PM) algorithm to compute the long-range force,the Tree algorithm to compute the short range force and the direct summation Particle-Particle(PP) algorithm to compute gravity from very close particles. A self-similar space filling a Peano-Hilbert curve is used to decompose the computing domain. Threads programming is advantageously used to more flexibly manage the domain communication, PM calculation and synchronization, as well as Dual Tree Traversal on the CPU+MIC platform. PHo To Ns scales well and efficiency of the PP kernel achieves68.6% of peak performance on MIC and 74.4% on CPU platforms. We also test the accuracy of the code against the much used Gadget-2 in the community and found excellent agreement.

赛弗特星系恒星速度弥散度的测量

星系中心黑洞质量和核球恒星速度弥散度的紧密关系揭示出准确测量恒星速度弥散度对测定星系中心黑洞质量尤为重要.文中提供了一种利用SDSS(Sloan Digital SkySurvey)光谱测定速度弥散度及其不确定性的方法.通过对像素空间包含显著特征吸收线的4个不同谱区的拟合,得到准确测量恒星速度弥散度σ的光谱区域.文中4个拟合波段主要包含的吸收线为CaⅡK,MgⅠb三重线(波长5167.5,5172.7,5183.6(?))和CaT(CaⅡ三重线,波长8498.0,8542.1,8662.1(?)).不同区域结果表明,MgⅠb区由于受到铁族发射线影响,拟合的σ值偏低;CaⅡK线区谱线强度很弱,易受限于最小二乘法搜索算法;CaT+CaⅡK联合区得出的速度弥散度和只计算CaT区域的结果相当.利用该方法,测试了一个红移小于0.05的赛弗特星系样本,发现CaT区是测速度弥散度的最佳谱区.

银盘径向丰度梯度及冷气体的演化

为了理解银盘的径向丰度梯度形成机制及冷气体演化,构建了银盘化学演化模型.模型中采用了与分子氢相关的恒星形成律,分开计算了分子氢和原子氢的质量面密度演化,并将模型预言和观测到的各物理量径向轮廓进行了比较.结果表明,模型预言对所采用的内落时标非常敏感,采用与分子氢相关的恒星形成律的模型能够较好地符合银盘基本的观测特征,尤其是能够自然地得到银盘的径向氧丰度梯度和冷气体的径向面密度轮廓.采用瞬时或非瞬时循环假设对冷气体的演化影响不大,在气体密度较低的情况下更是如此.

The role of mergers and gas accretion in black hole growth and galaxy evolution

We use a semi-analytic galaxy formation model to study the co-evolution of supermassive black holes(SMBHs) with their host galaxies.Although the coalescence of SMBHs is not important,the quasarmode accretion induced by mergers plays a dominant role in the growth of SMBHs.Mergers play a more important role in the growth of SMBH host galaxies than in the SMBH growth.It is the combined contribution from quasar mode accretion and mergers to the SMBH growth and the combined contribution from starburst and mergers to their host galaxy growth that determine the observed scaling relation between the SMBH masses and their host galaxy masses.We also find that mergers are more important in the growth of SMBH host galaxies compared to normal galaxies which share the same stellar mass range as the SMBH host galaxies.

星系形成的超大型计算机模拟

扼要地就数值模拟在星系形成研究中的作用、数值模拟与解析模型之间的关系、数值模拟的局限性等几方面的问题作了综述；然后介绍了马普小组与日本东京大学最近的合作情况，包括合作得到的模拟样本、研究成果和正在开展的工作。

对N体数值模拟中暗物质晕的卫星星系空间分布的研究

对N体数值模拟中暗物质晕的卫星星系的空间分布进行三轴椭球拟合,以拟合椭球的轴比来衡量该分布的扁平程度,通过比较不同条件下的轴比分布分析样本数量、样本选取方式以及对样本径向分布的归一化对计算结果的影响,并考察了暗晕所在的大尺度结构的空间方向与拟合椭球的空间取向之间的关系.发现对样本径向分布的归一化对计算结果具有较大影响,同时发现大尺度丝状结构中的暗晕的拟合椭球的短轴更趋于与丝状结构的方向垂直,而大尺度片状结构中的暗晕的拟合椭球的短轴则更趋于与片状结构的法线方向平行.

SHORT-RANGE FORCE IN THE NUMERICAL SIMULATION OF LARGE-SCALE STRUCTURE IN THE UNIVERSE

In this paper, an improved CPU time-saving method for the calculation of the short-rangeforce in P^3M scheme of the N-body simulation of the large-scale structure in the universe has been proposed. The main point is that, by rearranging the particles according to the cells they belongto, the particle pairs needed to be calculated for the short-range force are greatly decreased, and then the calculation time is much saved. A comparison is given between this method and that of Efstathiou et al., which shows that by giving the same initial conditions and running on the same kind of computers, the former is 1-5 times faster than the latter.