In order to find the physical parameters which determine the accuracy of pho- tometric redshifts, we compare the spectroscopic and photometric redshifts （photo-z＇s） for a large sample of -80000 SDSS-2MASS galaxies....In order to find the physical parameters which determine the accuracy of pho- tometric redshifts, we compare the spectroscopic and photometric redshifts （photo-z＇s） for a large sample of -80000 SDSS-2MASS galaxies. Photo-z＇s in this paper are estimated by using the artificial neural network photometric redshift method （ANNz）. For a subset of -40000 randomly selected galaxies, we find that the photometric redshift recovers the spectroscopic redshift distribution very well with rms of 0.016. Our main results are as follows： （1） Using magnitudes directly as input parameters produces more accurate photo-z＇s than using colors; （2） The inclusion of 2MASS （J, H, Ks） bands does not improve photo-z＇s significantly, which indicates that near infrared data might not be important for the low-redshift sample; （3） Adding the concentration index （essentially the steepness of the galaxy brightness profile） as an extra input can improve the photo-z＇s estimation up to -10 percent; （4） Dividing the sample into early- and late-type galaxies by using the concentration index, normal and abnormal galaxies by using the emission line flux ratios, and red and blue galaxies by using color index （g - r）, we can improve the accuracy of photo-z＇s significantly; （5） Our analysis shows that the outliers （where there is a big difference between the spectroscopic and photometric redshifts） are mainly correlated with galaxy types, e.g., most outliers are late-type （blue） galaxies.展开更多
The Intergalactic Medium (IGM) is commonly thought to be occupied by approximately one atom of Hydrogen per cubic metre of space either as neutral Hydrogen or partially/fully ionised. This cannot be true as galaxies w...The Intergalactic Medium (IGM) is commonly thought to be occupied by approximately one atom of Hydrogen per cubic metre of space either as neutral Hydrogen or partially/fully ionised. This cannot be true as galaxies will “boil off” electrons from their outer surfaces by the photo-electric effect and so the IGM must be filled with electrons. UV and X-ray photons, as they leave the galaxy, can remove an electron from a Hydrogen atom at the surface of the galaxy, give it sufficient energy to escape the gravitational pull of the galaxy and go on to fill the IGM. A typical galaxy emits approximately 5×1047 X-ray photons each second. All of which pass through the outer surface of the galaxy and have sufficient energy to eject an electron and send it off to the IGM. Adding to these photons in the UV and gamma, we can see that galaxies are ejecting large amounts of electrons each second that go on to fill the IGM. Data from FRB 121102 give the value for the electron number density in the IGM as ne ≈ 0.5 m-3. Under certain conditions, an electron gas will crystallise into a Wigner-Seitz crystal. Here the electrical potential energy of repulsion between the electrons dominates their kinetic energy and the electrons form on a BCC lattice structure. The electrons oscillate, performing SHM about their lattice positions. With ne ≈ 0.5 m-3 the electrons in the IGM satisfy the energy criteria for crystallisation to occur when interacting with other electrons within a sphere far less in radius than the corresponding Debye sphere. Thus, the conditions are met for the electrons to form an “electron glass.” Since the electrons in their BCC formation are spatially coherent, light will travel through the crystals in a straight line and thus objections to “Tired Light” theories are now removed since images will neither be destroyed nor “blurred.” Charges are not created but separated, if the electrons are removed from the galaxy and sent to fill the IGM;the remaining protons are left behind. These are “thermal” and will not have sufficient energy to escape but will be held gravitationally to that galaxy. Could these too form a spherical Wigner-Seitz sphere around that galaxy? Since the structure would be transparent, light would pass through in straight lines and thus we would not see it. They would however, interact gravitationally with the galaxy and have an effect on the rotation curves of single galaxies and on the motion of galactic clusters. Just as we cannot see the clear water in a fish tank when we look at the fish, the transparent, crystalline sphere of protons around galaxies would be “dark”.展开更多
Gravity as a fundamental force plays a dominant role in the formation and evolution of cosmic objects and leaves its effect in the emergence of symmetric and asymmetric structures.Thus,analyzing the symmetry criteria ...Gravity as a fundamental force plays a dominant role in the formation and evolution of cosmic objects and leaves its effect in the emergence of symmetric and asymmetric structures.Thus,analyzing the symmetry criteria allows us to uncover mechanisms behind the gravity interaction and understand the underlying physical processes that contribute to the formation of large-scale structures such as galaxies.We use a segmentation process using intensity thresholding and the k-means clustering algorithm to analyze radio galaxy images.We employ a symmetry criterion and explore the relation between morphological symmetry in radio maps and host galaxy properties.Optical properties(stellar mass,black hole mass,optical size(R_(50)),concentration,stellar mass surface density(μ_(50)),and stellar age)and radio properties(radio flux density,radio luminosity,and radio size)are considered.We found that there is a correlation between symmetry and radio size,indicating larger radio sources have smaller symmetry indices.Therefore,size of radio sources should be considered in any investigation of symmetry.Weak correlations are also observed with other properties,such as R_(50)for FRI galaxies and stellar age.We compare the symmetry differences between FRI and FRII radio galaxies.FRII galaxies show higher symmetry in 1.4 GHz and 150 MHz maps.Investigating the influence of radio source sizes,we discovered that this result is independent of the sizes of radio sources.These findings contribute to our understanding of the morphological properties and analyses of radio galaxies.展开更多
The star formation rate function(SFRF) and specific star formation rate function(s SFRF) from observations are impacted by the Eddington bias, due to uncertainties in the estimated star formation rate(SFR). We develop...The star formation rate function(SFRF) and specific star formation rate function(s SFRF) from observations are impacted by the Eddington bias, due to uncertainties in the estimated star formation rate(SFR). We develop a novel method to correct the Eddington bias and obtain the intrinsic SFRF and sSFRF from the Sloan Digital Sky Survey(SDSS) Data Release 7. The intrinsic SFRF is in good agreement with measurements from previous data in the literature that relied on UV SFRs but its high star-forming end is slightly lower than the corresponding IR and radio tracers. We demonstrate that the intrinsic sSFRF from SDSS has a bimodal form with one peak found at sSFR ~10-9.7 yr^-1 representing the star-forming objects while the other peak is found at sSFR ~10-12 yr^-1 representing the quenched population. Furthermore, we compare our observations with the predictions from the Illustris TNG and Illustris simulations and affirm that the "TNG" model performs much better than its predecessor. However,we show that the simulated SFRF and CSFRD of TNG simulations are highly dependent on resolution,reflecting the limitations of the model and today’s state-of-the-art simulations. We demonstrate that the bimodal, two peaked s SFRF implied by the SDSS observations does not appear in TNG regardless of the adopted box-size or resolution. This tension reflects the need for inclusion of an additional efficient quenching mechanism in the TNG model.展开更多
Utilizing a BzK-selecfion technique, we obtain 14 550 star-forming galaxies （sBzKs） and 1763 passive galaxies （pBzKs） at z - 2 from the K-selected （KAB 〈 22.5） catalog in the COSMOS/UltraVISTA field. The differ...Utilizing a BzK-selecfion technique, we obtain 14 550 star-forming galaxies （sBzKs） and 1763 passive galaxies （pBzKs） at z - 2 from the K-selected （KAB 〈 22.5） catalog in the COSMOS/UltraVISTA field. The differential number counts of sBzKs and pBzKs are consistent with the results from the literature. Compared to the observed results, semi-analytic models of galaxy formation and evolution provide too few （many） galaxies at the high （low） mass end. Moreover, we find that the star formation rate and stellar mass of sBzKs follow the relation of the main sequence. Based on HST/Wide Field Camera 3 F160W imaging, we find a wide range of morphological diversities for sBzKs, from diffuse to early-type spiral structures, with relatively high M20, large size and low G, while pBzKs have elliptical-like compact morphologies with lower M20, smaller size and higher G, indicating a more concentrated and symmetric spatial extent of stellar population distribution in pBzKs than sBzKs. Furthermore, the sizes of pBzKs （sBzKs） at z - 2 are on average two to three （one to two） times smaller than those of local early-type （late-type） galaxies with similar stellar mass. Our findings imply that the two classes have different evolution models and mass assembly histories.展开更多
Photometric redshift(photoz)is a fundamental parameter for multi-wavelength photometric surveys,while galaxy clusters are important cosmological probes and ideal objects for exploring the dense environmental impact on...Photometric redshift(photoz)is a fundamental parameter for multi-wavelength photometric surveys,while galaxy clusters are important cosmological probes and ideal objects for exploring the dense environmental impact on galaxy evolution.We extend our previous work on estimating photoz and detecting galaxy clusters to the latest data releases of the Dark Energy Spectroscopic Instrument(DESI)imaging surveys,Dark Energy Survey(DES)and Hyper Suprime-Cam Subaru Strategic Program(HSC-SSP)imaging surveys and make corresponding catalogs publicly available for more extensive scientific applications.The photoz catalogs include accurate measurements of photoz and stellar mass for about 320,293and 134 million galaxies with r<23,i<24 and i<25 in DESI DR9,DES DR2 and HSC-SSP PDR3 data,respectively.The photoz accuracy is about 0.017,0.024 and 0.029 and the general redshift coverage is z<1,z<1.2 and z<1.6,respectively for those three surveys.The uncertainty of the logarithmic stellar mass that is inferred from stellar population synthesis fitting is about 0.2 dex.With the above photoz catalogs,galaxy clusters are detected using a fast cluster-finding algorithm.A total of 532,810,86,963 and 36,566 galaxy clusters with the number of members larger than 10 is discovered for DESI,DES and HSC-SSP,respectively.Their photoz accuracy is at the level of 0.01.The total mass of our clusters is also estimated by using the calibration relations between the optical richness and the mass measurement from X-ray and radio observations.The photoz and cluster catalogs are available at Science DB(https://www.doi.org/10.11922/sciencedb.o00069.00003)and Paper Data Repository(https://doi.org/10.12149/101089).展开更多
Based on a large sample of massive(M* 1010 M⊙) compact galaxies at 1.0 < z < 3.0 in five 3 D-HST/CANDELS fields, we quantify the fractional abundance and comoving number density of massive compact galaxies as a...Based on a large sample of massive(M* 1010 M⊙) compact galaxies at 1.0 < z < 3.0 in five 3 D-HST/CANDELS fields, we quantify the fractional abundance and comoving number density of massive compact galaxies as a function of redshift. The samples of compact quiescent galaxies(cQGs) and compact star-forming galaxies(cSFGs) are constructed by various selection criteria of compact galaxies in the literature, and the effect of compactness definition on abundance estimate has proven to be remarkable,particularly for the cQGs and cSFGs at high redshifts. Regardless of the compactness criteria adopted,their overall redshift evolutions of fractional abundance and number density are found to be rather similar.Large samples of the cQGs exhibit a sustained increase in number density from z ~ 3 to 2 and a plateau at 1 < z < 2. For massive cSFGs, a plateau in the number density at 2 < z < 3 can be found, as well as a continuous drop from z ~ 2 to 1. The evolutionary trends of the cQG and cSFG abundances support the scenario that the cSFGs at z≥ 2 may have been rapidly quenched into quiescent phase via violent dissipational processes, such as major merger and disk instabilities. The rarity of the cSFGs at lower redshifts(z < 1) can be interpreted by the decrease of gas reservoirs in dark matter halos and the consequent low efficiency of gas-rich dissipation.展开更多
This report is about the graviton redshift theory (GRST) which hypothesises the redshift of the energy of gravitons traveling in fields. A new source of energy loss in galaxy dynamics is introduced. Due to the hypothe...This report is about the graviton redshift theory (GRST) which hypothesises the redshift of the energy of gravitons traveling in fields. A new source of energy loss in galaxy dynamics is introduced. Due to the hypothetical interactions of gravitons with the expansion of the universe, which causes an energy loss of the gravitons due to cosmological redshift, the rotation equation for galaxies, which previously had the Newtonian potential energy and the graviton gravitational redshift energy loss, is now updated with the graviton cosmological redshift energy loss. From the galaxy rotation equation, the baryonic Tully-Fisher relation (BTFR) and the modified Newtonian dynamics (MOND) are defined in radial distribution form. Fits to galaxy rotation motion are detailed. A cosmic connection for the BTFR is defined. The result is that galaxy rotation curves are fully accounted for with the GRST rotation equation and the BTFR and MOND theories are incorporated into a unified framework.展开更多
Through the analysis of representative samples of field galaxies,both local(z~0.027)and distant(z~0.7),we explore the barred galaxy fraction and its dependence on stellar mass,color,and morphology,aiming to understand...Through the analysis of representative samples of field galaxies,both local(z~0.027)and distant(z~0.7),we explore the barred galaxy fraction and its dependence on stellar mass,color,and morphology,aiming to understand the evolution of these structural components through cosmic time.To this end,two complementary bar detection techniques were employed:elliptical isophote fitting and two-dimensional Fourier analysis,both applied to deep optical images.The observational samples were drawn from previously established and calibrated catalogs to ensure a homogeneous selection in stellar mass,enabling a robust comparison between local galaxies(z~0.027)and those in the distant Universe(z~0.7).This study systematically applies both isophotal fitting and Fourier decomposition across a wide redshift range,offering a comprehensive view of the evolution of bar incidence as a function of stellar mass and morphology.The results indicate that the fraction of barred galaxies is significantly higher in the local Universe than at earlier epochs,particularly among spiral galaxies.Furthermore,a clear correlation is observed between the presence of bars and stellar mass,especially in the high-mass regime(log(M_*/M☉)>10.5).In distant galaxies,this fraction is lower across all mass ranges,which may be related to more active dynamical processes.Overall,the findings reinforce the idea that stellar bars emerge as a consequence of dynamical cooling and the progressive stabilization of galactic disks,playing a key role in gas transport and the internal structural evolution of galaxies from z~1 to the present day.展开更多
We determined HI parameters for eleven nearby late-type dwarf galaxies using FASHI data cubes,despite the fact that the first version of the FAS HI catalog does not list any radio sources that could correspond to thes...We determined HI parameters for eleven nearby late-type dwarf galaxies using FASHI data cubes,despite the fact that the first version of the FAS HI catalog does not list any radio sources that could correspond to these galaxies.Four of them are probable peripheral satellites of the bright spiral galaxies:NGC 3556,NGC 4258,NGC 4274 and NGC 4490,while others are isolated objects.The considered sample has the following median parameters:a heliocentric velocity of V_(h)=542 km s^(-1),an HI-line width of W_(50)=28 km s^(-1),a hydrogen mass of log(M_(HI)/M_(⊙))=6.83,a stellar mass of log(M_(*)/M_(⊙))=7.19,and a specific star formation rate of sSFR=-10.17 yr^(-1).展开更多
基于CANDELS(Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey)5个深场巡天的多波段测光数据和HST WFC3(Hubble Space Telescope Wide Field Camera 3)近红外(F125W和F160W)高分辨率观测图像,利用质量限(恒星质量M_*&g...基于CANDELS(Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey)5个深场巡天的多波段测光数据和HST WFC3(Hubble Space Telescope Wide Field Camera 3)近红外(F125W和F160W)高分辨率观测图像,利用质量限(恒星质量M_*>10^(10)M_⊙)选取了8002个红移分布在1<z<3范围内的星系样本,并对这些大质量星系的形态和结构性质进行了定量分析研究.通过星系形态的神经网络分类方法(Conv Nets),将样本中的星系划分为4类:椭球星系(SPHeroids,SPH)、早型盘星系(Early-Type Disks,ETD)、晚型盘星系(Late-Type Disks,LTD)和不规则星系(IRRegulars,IRR).结果发现星系的形态和结构随红移发生演化,在高红移宇宙星系主要表现出不规则形态,但到低红移处椭球和盘主导的哈勃星系形态序列已经形成.在相同红移区间内,不同类型星系的物理尺寸(r_e)中值从大到小的排序是IRR、LTD、ETD和SPH,而对应的Sersic指数(n)中值大小排序却相反.另外,不同类型星系的re与红移之间存在明显的演化关系,但这样的现象在平均轴比(b/a)和Sersic指数与红移的关系中并没有被发现.展开更多
We investigate two training-set methods; support vector machines （SVMs） and Kernel Regression （KR） for photometric redshift estimation with the data from the databases of Sloan Digital Sky Survey Data Release 5 an...We investigate two training-set methods; support vector machines （SVMs） and Kernel Regression （KR） for photometric redshift estimation with the data from the databases of Sloan Digital Sky Survey Data Release 5 and Two Micron All Sky Survey. We probe the performances of SVMs and KR for different input patterns. Our experiments show that with more parameters considered, the accuracy does not always increase, and only when appropriate parameters are chosen, the accuracy can improve. For different approaches, the best input pattern is different. With different parameters as input, the optimal bandwidth is dissimilar for KR. The rms errors of photometric redshifts based on SVM and KR methods are less than 0.03 and 0.02, respectively. Strengths and weaknesses of the two approaches are summarized. Compared to other methods of estimating photometric redshifts, they show their superiorities, especially KR, in terms of accuracy.展开更多
We study the growth of black holes and stellar population in spheroids at high redshift using several （sub）mm-loud QSO samples. Applying the same criteria established in an earlier work, we find that, similar to IR ...We study the growth of black holes and stellar population in spheroids at high redshift using several （sub）mm-loud QSO samples. Applying the same criteria established in an earlier work, we find that, similar to IR QSOs at low redshift, the far-infrared emission of these （sub）mm-loud QSOs mainly originates from dust heated by starbursts. By combining low-z IR QSOs and high-z （sub）mm-loud QSOs, we find a trend that the star formation rate （M＊） increases with the accretion rate （Mace）. We compare the values of M＊/Macc for submm emitting galaxies （SMGs）, far-infrared ultraluminous/hypeduminous QSOs and typical QSOs, and construct a likely evolution scenario for these objects. The （sub）mm-loud QSO transition phase has both high Macc and M＊ and hence is important for establishing the correlation between the masses of black holes and spheroids.展开更多
We analyse the possibility that the observed cosmological redshift may be cumulatively due to the expansion of the universe and the tired light phenomenon. Since the source of both the redshifts is the same, they both...We analyse the possibility that the observed cosmological redshift may be cumulatively due to the expansion of the universe and the tired light phenomenon. Since the source of both the redshifts is the same, they both independently relate to the same proper distance of the light source. Using this approach we have developed a hybrid model combining the Einstein de Sitter model and the tired light model that yields a slightly better fit to Supernovae Ia redshift data using one parameter than the standard ΛCDM model with two parameters. We have shown that the ratio of tired light component to the Einstein de Sitter component of redshift has evolved from 2.5 in the past, corresponding to redshift 1000, to its present value of 1.5. The hybrid model yields Hubble constant H0 =69.11(±0.53)km·s-1 ·Mpc-1 and the deceleration parameter q0 =-0.4. The component of Hubble constant responsible for expansion of the universe is 40% of H0 and for the tired light is 60% of H0. Consequently, the critical density is only 16% of its currently accepted value;a lot less dark matter is needed to make up the critical density. In addition, the best data fit yields the cosmological constant density parameter =0. The tired light effect may thus be considered equivalent to the cosmological constant in the hybrid model.展开更多
The presence of the dark energy allows both the acceleration and the expansion of the universe. In the case of a constant equation of state for dark energy we derived an analytical solution for the Hubble radius in te...The presence of the dark energy allows both the acceleration and the expansion of the universe. In the case of a constant equation of state for dark energy we derived an analytical solution for the Hubble radius in terms of the hypergeometric function. An approximate Taylor expansion of order seven is derived for both the constant and the variable equation of state for dark energy. In the case of the Cardassian cosmology, we also derived an analytical solution for the Hubble radius in terms of the hypergeometric function. The astronomical samples of the distance modulus for Supernova (SN) of type Ia allows the derivation of the involved cosmological in the case of constant equation of state, variable equation of state and Cardassian cosmology.展开更多
Hypervelocity stars are believed to be ejected out from the Galactic center through dynamical interactions between（binary） stars and the central supermassive black hole（s）. In this paper, we report 19 low mass F/G...Hypervelocity stars are believed to be ejected out from the Galactic center through dynamical interactions between（binary） stars and the central supermassive black hole（s）. In this paper, we report 19 low mass F/G/K type hypervelocity star candidates from over one million stars found in the first data release of the LAMOST regular survey. We determine the unbound probability for each candidate using a MonteCarlo simulation by assuming a non-Gaussian proper-motion error distribution, and Gaussian heliocentric distance and radial velocity error distributions. The simulation results show that all the candidates have unbound possibilities over 50% as expected,and one of them may even exceed escape velocity with over 90% probability. In addition, we compare the metallicities of our candidates with the metallicity distribution functions of the Galactic bulge, disk, halo and globular clusters, and conclude that the Galactic bulge or disk is likely the birth place for our candidates.展开更多
The massive photometric data collected from multiple large-scale sky surveys offer significant opportunities for measuring distances of celestial objects by photometric redshifts. However, catastrophic failure is an u...The massive photometric data collected from multiple large-scale sky surveys offer significant opportunities for measuring distances of celestial objects by photometric redshifts. However, catastrophic failure is an unsolved problem with a long history and it still exists in the current photometric redshift estimation approaches （such as the k-nearest neighbor （KNN） algorithm）. In this paper, we propose a novel two-stage approach by integration of KNN and support vector machine （SVM） methods together. In the first stage, we apply the KNN algorithm to photometric data and estimate their corresponding Zphot. Our analysis has found two dense regions with catastrophic failure, one in the range of Zphot E [0.3, 1.2] and the other in the range of Zphot E [1.2, 2.1]. In the second stage, we map the photometric input pattern of points falling into the two ranges from their original attribute space into a high dimensional feature space by using a Gaussian kernel function from an SVM. In the high dimensional feature space, many outliers resulting from catastrophic failure by simple Euclidean distance computation in KNN can be identified by a classification hyperplane of SVM and can be further corrected. Experimental results based on the Sloan Digital Sky Survey （SDSS） quasar data show that the two-stage fusion approach can significantly mitigate catastrophic failure and improve the estimation accuracy of photometric redshifts of quasars. The percents in different /△z/ ranges and root mean square （rms） error by the integrated method are 83.47%, 89.83%, 90.90% and 0.192, respectively, compared to the results by KNN （71.96%, 83.78%, 89.73% and 0.204）.展开更多
We present a sample of about 120 000 red clump candidates selected from the LAMOST DR2 catalog based on the empirical distribution model in the effective temperature vs. surface gravity plane. Although, in general, re...We present a sample of about 120 000 red clump candidates selected from the LAMOST DR2 catalog based on the empirical distribution model in the effective temperature vs. surface gravity plane. Although, in general, red clump stars are considered as standard candles, they do not exactly stay in a narrow range of absolute magnitude, but may have a range of more than one magnitude depending on their initial mass. Consequently, conventional oversimplified distance estimations with the assumption of a fixed luminosity may lead to systematic bias related to the initial mass or age, which can potentially affect the study of the evolution of the Galaxy with red clump stars. We therefore employ an isochrone-based method to estimate the absolute magnitude of red clump stars from their observed surface gravities, effective temperatures and metallicities. We verify that the estimation removes the systematics well and provides initial mass/age estimates that are independent of distance with accuracy better than 10%.展开更多
With the recent observational evidence in extragalactic astronomy,the interpretation of the nature of quasar redshift continues to be a research interest.Very high redshifts are being detected for extragalactic object...With the recent observational evidence in extragalactic astronomy,the interpretation of the nature of quasar redshift continues to be a research interest.Very high redshifts are being detected for extragalactic objects that are presumably very distant and young while also exhibiting properties that are characteristic of a more mature galaxy such as ours.According to Halton Arp and Geoffrey Burbidge,redshift disparities consist of an intrinsic component and are related to an evolutionary process.Karlsson observed redshift periodicity at integer multiples of0.089 in log scale and Burbidge observed redshift periodicity at integer multiples of 0.061 in linear scale.Since Singular Value Decomposition based periodicity estimation is known to be superior for noisy data sets,especially when the data contain multiple harmonics and overtones,mainly irregular in nature,we have chosen it to be our primary tool for analysis of the quasar-galaxy pair redshift data.We have observed a fundamental periodicity of0.051 with a confidence interval of 95%in linear scale with the site-available Sloan Digital Sky Survey Data Release 7(SDSS DR7)quasar-galaxy pair data set.We have independently generated quasar-galaxy pair data sets from both 2d F and SDSS and found fundamental periodicities of 0.077 and 0.089,respectively,in log scale with a confidence interval of 95%.展开更多
An optical photometric observation with the Beijing-Arizona-Taiwan- Connecticut （BATC） multicolor system is carried out for A98 （z =0.104）, a galaxy cluster with two large enhancements in X-ray surface brightness....An optical photometric observation with the Beijing-Arizona-Taiwan- Connecticut （BATC） multicolor system is carried out for A98 （z =0.104）, a galaxy cluster with two large enhancements in X-ray surface brightness. Spectral energy distributions （SEDs） covering 15 intermediate bands are obtained for all sources detected down to V - 20 mag in a field of 58′× 58′. After star-galaxy separation with colorcolor diagrams, a photometric redshift technique is applied to the galaxy sample for further membership determination. The color-magnitude relation is taken as a further restriction of the early-type cluster galaxies. As a result, a list of 198 faint member galaxies is achieved. Based on the newly generated sample of member galaxies, the dynamical substructures, A98N, A98S, and A98W, are investigated in detail. A separate galaxy group, A98X, is also found to the south of the main concentration of A98, which is gravitationally unbound to A98. For 74 spectroscopically confirmed member galaxies, the environmental effect on the star formation history is investigated. The bright galaxies in the core region are found to have shorter time scales of star formation, longer mean stellar ages, and higher interstellar medium metallicities, which can be interpreted in the context of the hierarchical cosmological scenario.展开更多
基金Supported by the National Natural Science Foundation of China
文摘In order to find the physical parameters which determine the accuracy of pho- tometric redshifts, we compare the spectroscopic and photometric redshifts （photo-z＇s） for a large sample of -80000 SDSS-2MASS galaxies. Photo-z＇s in this paper are estimated by using the artificial neural network photometric redshift method （ANNz）. For a subset of -40000 randomly selected galaxies, we find that the photometric redshift recovers the spectroscopic redshift distribution very well with rms of 0.016. Our main results are as follows： （1） Using magnitudes directly as input parameters produces more accurate photo-z＇s than using colors; （2） The inclusion of 2MASS （J, H, Ks） bands does not improve photo-z＇s significantly, which indicates that near infrared data might not be important for the low-redshift sample; （3） Adding the concentration index （essentially the steepness of the galaxy brightness profile） as an extra input can improve the photo-z＇s estimation up to -10 percent; （4） Dividing the sample into early- and late-type galaxies by using the concentration index, normal and abnormal galaxies by using the emission line flux ratios, and red and blue galaxies by using color index （g - r）, we can improve the accuracy of photo-z＇s significantly; （5） Our analysis shows that the outliers （where there is a big difference between the spectroscopic and photometric redshifts） are mainly correlated with galaxy types, e.g., most outliers are late-type （blue） galaxies.
文摘The Intergalactic Medium (IGM) is commonly thought to be occupied by approximately one atom of Hydrogen per cubic metre of space either as neutral Hydrogen or partially/fully ionised. This cannot be true as galaxies will “boil off” electrons from their outer surfaces by the photo-electric effect and so the IGM must be filled with electrons. UV and X-ray photons, as they leave the galaxy, can remove an electron from a Hydrogen atom at the surface of the galaxy, give it sufficient energy to escape the gravitational pull of the galaxy and go on to fill the IGM. A typical galaxy emits approximately 5×1047 X-ray photons each second. All of which pass through the outer surface of the galaxy and have sufficient energy to eject an electron and send it off to the IGM. Adding to these photons in the UV and gamma, we can see that galaxies are ejecting large amounts of electrons each second that go on to fill the IGM. Data from FRB 121102 give the value for the electron number density in the IGM as ne ≈ 0.5 m-3. Under certain conditions, an electron gas will crystallise into a Wigner-Seitz crystal. Here the electrical potential energy of repulsion between the electrons dominates their kinetic energy and the electrons form on a BCC lattice structure. The electrons oscillate, performing SHM about their lattice positions. With ne ≈ 0.5 m-3 the electrons in the IGM satisfy the energy criteria for crystallisation to occur when interacting with other electrons within a sphere far less in radius than the corresponding Debye sphere. Thus, the conditions are met for the electrons to form an “electron glass.” Since the electrons in their BCC formation are spatially coherent, light will travel through the crystals in a straight line and thus objections to “Tired Light” theories are now removed since images will neither be destroyed nor “blurred.” Charges are not created but separated, if the electrons are removed from the galaxy and sent to fill the IGM;the remaining protons are left behind. These are “thermal” and will not have sufficient energy to escape but will be held gravitationally to that galaxy. Could these too form a spherical Wigner-Seitz sphere around that galaxy? Since the structure would be transparent, light would pass through in straight lines and thus we would not see it. They would however, interact gravitationally with the galaxy and have an effect on the rotation curves of single galaxies and on the motion of galactic clusters. Just as we cannot see the clear water in a fish tank when we look at the fish, the transparent, crystalline sphere of protons around galaxies would be “dark”.
文摘Gravity as a fundamental force plays a dominant role in the formation and evolution of cosmic objects and leaves its effect in the emergence of symmetric and asymmetric structures.Thus,analyzing the symmetry criteria allows us to uncover mechanisms behind the gravity interaction and understand the underlying physical processes that contribute to the formation of large-scale structures such as galaxies.We use a segmentation process using intensity thresholding and the k-means clustering algorithm to analyze radio galaxy images.We employ a symmetry criterion and explore the relation between morphological symmetry in radio maps and host galaxy properties.Optical properties(stellar mass,black hole mass,optical size(R_(50)),concentration,stellar mass surface density(μ_(50)),and stellar age)and radio properties(radio flux density,radio luminosity,and radio size)are considered.We found that there is a correlation between symmetry and radio size,indicating larger radio sources have smaller symmetry indices.Therefore,size of radio sources should be considered in any investigation of symmetry.Weak correlations are also observed with other properties,such as R_(50)for FRI galaxies and stellar age.We compare the symmetry differences between FRI and FRII radio galaxies.FRII galaxies show higher symmetry in 1.4 GHz and 150 MHz maps.Investigating the influence of radio source sizes,we discovered that this result is independent of the sizes of radio sources.These findings contribute to our understanding of the morphological properties and analyses of radio galaxies.
基金supported by the National Natural Science Foundation of China(Nos.11833005,11890692 and 11621303)111 project(No.B20019)+1 种基金Shanghai Natural Science Foundation(No.15ZR1446700)the support of the Key Laboratory for Particle Physics,Astrophysics and Cosmology,Ministry of Educationthe Tsung-Dao Lee Institute。
文摘The star formation rate function(SFRF) and specific star formation rate function(s SFRF) from observations are impacted by the Eddington bias, due to uncertainties in the estimated star formation rate(SFR). We develop a novel method to correct the Eddington bias and obtain the intrinsic SFRF and sSFRF from the Sloan Digital Sky Survey(SDSS) Data Release 7. The intrinsic SFRF is in good agreement with measurements from previous data in the literature that relied on UV SFRs but its high star-forming end is slightly lower than the corresponding IR and radio tracers. We demonstrate that the intrinsic sSFRF from SDSS has a bimodal form with one peak found at sSFR ~10-9.7 yr^-1 representing the star-forming objects while the other peak is found at sSFR ~10-12 yr^-1 representing the quenched population. Furthermore, we compare our observations with the predictions from the Illustris TNG and Illustris simulations and affirm that the "TNG" model performs much better than its predecessor. However,we show that the simulated SFRF and CSFRD of TNG simulations are highly dependent on resolution,reflecting the limitations of the model and today’s state-of-the-art simulations. We demonstrate that the bimodal, two peaked s SFRF implied by the SDSS observations does not appear in TNG regardless of the adopted box-size or resolution. This tension reflects the need for inclusion of an additional efficient quenching mechanism in the TNG model.
基金Supported by the National Natural Science Foundation of China
文摘Utilizing a BzK-selecfion technique, we obtain 14 550 star-forming galaxies （sBzKs） and 1763 passive galaxies （pBzKs） at z - 2 from the K-selected （KAB 〈 22.5） catalog in the COSMOS/UltraVISTA field. The differential number counts of sBzKs and pBzKs are consistent with the results from the literature. Compared to the observed results, semi-analytic models of galaxy formation and evolution provide too few （many） galaxies at the high （low） mass end. Moreover, we find that the star formation rate and stellar mass of sBzKs follow the relation of the main sequence. Based on HST/Wide Field Camera 3 F160W imaging, we find a wide range of morphological diversities for sBzKs, from diffuse to early-type spiral structures, with relatively high M20, large size and low G, while pBzKs have elliptical-like compact morphologies with lower M20, smaller size and higher G, indicating a more concentrated and symmetric spatial extent of stellar population distribution in pBzKs than sBzKs. Furthermore, the sizes of pBzKs （sBzKs） at z - 2 are on average two to three （one to two） times smaller than those of local early-type （late-type） galaxies with similar stellar mass. Our findings imply that the two classes have different evolution models and mass assembly histories.
基金supported by the National Natural Science Foundation of China(NSFC,Grant No.12120101003)Beijing Municipal Natural Science Foundation under grant 1222028+2 种基金the China Manned Space Project with Nos.CMS-CSST-2021-A02 and CMS-CSST-2021-A04supported by NSFC under grants 11890691,11890693,11873053,12073035,12133010,11733007the National Key R&D Program of China under grant 2019YFA0405501。
文摘Photometric redshift(photoz)is a fundamental parameter for multi-wavelength photometric surveys,while galaxy clusters are important cosmological probes and ideal objects for exploring the dense environmental impact on galaxy evolution.We extend our previous work on estimating photoz and detecting galaxy clusters to the latest data releases of the Dark Energy Spectroscopic Instrument(DESI)imaging surveys,Dark Energy Survey(DES)and Hyper Suprime-Cam Subaru Strategic Program(HSC-SSP)imaging surveys and make corresponding catalogs publicly available for more extensive scientific applications.The photoz catalogs include accurate measurements of photoz and stellar mass for about 320,293and 134 million galaxies with r<23,i<24 and i<25 in DESI DR9,DES DR2 and HSC-SSP PDR3 data,respectively.The photoz accuracy is about 0.017,0.024 and 0.029 and the general redshift coverage is z<1,z<1.2 and z<1.6,respectively for those three surveys.The uncertainty of the logarithmic stellar mass that is inferred from stellar population synthesis fitting is about 0.2 dex.With the above photoz catalogs,galaxy clusters are detected using a fast cluster-finding algorithm.A total of 532,810,86,963 and 36,566 galaxy clusters with the number of members larger than 10 is discovered for DESI,DES and HSC-SSP,respectively.Their photoz accuracy is at the level of 0.01.The total mass of our clusters is also estimated by using the calibration relations between the optical richness and the mass measurement from X-ray and radio observations.The photoz and cluster catalogs are available at Science DB(https://www.doi.org/10.11922/sciencedb.o00069.00003)and Paper Data Repository(https://doi.org/10.12149/101089).
基金the 3D-HST Treasury Program (GO 12177 and 12328) with the NASA/ESA HSTsupported by the National Natural Science Foundation of China (Nos. 11673004, 11873032 and 11433005)by the Research Fund for the Doctoral Program of Higher Education of China (No. 20133207110006)he support from Yunnan young and middleaged academic and technical leaders reserve talent program (No. 201905C160039)
文摘Based on a large sample of massive(M* 1010 M⊙) compact galaxies at 1.0 < z < 3.0 in five 3 D-HST/CANDELS fields, we quantify the fractional abundance and comoving number density of massive compact galaxies as a function of redshift. The samples of compact quiescent galaxies(cQGs) and compact star-forming galaxies(cSFGs) are constructed by various selection criteria of compact galaxies in the literature, and the effect of compactness definition on abundance estimate has proven to be remarkable,particularly for the cQGs and cSFGs at high redshifts. Regardless of the compactness criteria adopted,their overall redshift evolutions of fractional abundance and number density are found to be rather similar.Large samples of the cQGs exhibit a sustained increase in number density from z ~ 3 to 2 and a plateau at 1 < z < 2. For massive cSFGs, a plateau in the number density at 2 < z < 3 can be found, as well as a continuous drop from z ~ 2 to 1. The evolutionary trends of the cQG and cSFG abundances support the scenario that the cSFGs at z≥ 2 may have been rapidly quenched into quiescent phase via violent dissipational processes, such as major merger and disk instabilities. The rarity of the cSFGs at lower redshifts(z < 1) can be interpreted by the decrease of gas reservoirs in dark matter halos and the consequent low efficiency of gas-rich dissipation.
文摘This report is about the graviton redshift theory (GRST) which hypothesises the redshift of the energy of gravitons traveling in fields. A new source of energy loss in galaxy dynamics is introduced. Due to the hypothetical interactions of gravitons with the expansion of the universe, which causes an energy loss of the gravitons due to cosmological redshift, the rotation equation for galaxies, which previously had the Newtonian potential energy and the graviton gravitational redshift energy loss, is now updated with the graviton cosmological redshift energy loss. From the galaxy rotation equation, the baryonic Tully-Fisher relation (BTFR) and the modified Newtonian dynamics (MOND) are defined in radial distribution form. Fits to galaxy rotation motion are detailed. A cosmic connection for the BTFR is defined. The result is that galaxy rotation curves are fully accounted for with the GRST rotation equation and the BTFR and MOND theories are incorporated into a unified framework.
基金financial support from the Secretaría Nacional de Ciencia,Tecnologia e Innovacion(SENACYT),through the Departamento de Fortalecimiento a los Posgrados Nacionalesthe coordination of the program"MOVILIDAD DE INVESTIGACION."+3 种基金the financial support provided by PAPIIT projects IN108323 and IN111825 from DGAPA-UNAMsupport from the Agencia Nacional de Investigación y Desarrollo(ANID)through Basal project FB210003FONDECYT Regular projects 1241426 and 123044Millennium Science Initiative Program NCN2024_112。
文摘Through the analysis of representative samples of field galaxies,both local(z~0.027)and distant(z~0.7),we explore the barred galaxy fraction and its dependence on stellar mass,color,and morphology,aiming to understand the evolution of these structural components through cosmic time.To this end,two complementary bar detection techniques were employed:elliptical isophote fitting and two-dimensional Fourier analysis,both applied to deep optical images.The observational samples were drawn from previously established and calibrated catalogs to ensure a homogeneous selection in stellar mass,enabling a robust comparison between local galaxies(z~0.027)and those in the distant Universe(z~0.7).This study systematically applies both isophotal fitting and Fourier decomposition across a wide redshift range,offering a comprehensive view of the evolution of bar incidence as a function of stellar mass and morphology.The results indicate that the fraction of barred galaxies is significantly higher in the local Universe than at earlier epochs,particularly among spiral galaxies.Furthermore,a clear correlation is observed between the presence of bars and stellar mass,especially in the high-mass regime(log(M_*/M☉)>10.5).In distant galaxies,this fraction is lower across all mass ranges,which may be related to more active dynamical processes.Overall,the findings reinforce the idea that stellar bars emerge as a consequence of dynamical cooling and the progressive stabilization of galactic disks,playing a key role in gas transport and the internal structural evolution of galaxies from z~1 to the present day.
基金supported by the Russian Science Foundation grant No 24-12-00277。
文摘We determined HI parameters for eleven nearby late-type dwarf galaxies using FASHI data cubes,despite the fact that the first version of the FAS HI catalog does not list any radio sources that could correspond to these galaxies.Four of them are probable peripheral satellites of the bright spiral galaxies:NGC 3556,NGC 4258,NGC 4274 and NGC 4490,while others are isolated objects.The considered sample has the following median parameters:a heliocentric velocity of V_(h)=542 km s^(-1),an HI-line width of W_(50)=28 km s^(-1),a hydrogen mass of log(M_(HI)/M_(⊙))=6.83,a stellar mass of log(M_(*)/M_(⊙))=7.19,and a specific star formation rate of sSFR=-10.17 yr^(-1).
文摘基于CANDELS(Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey)5个深场巡天的多波段测光数据和HST WFC3(Hubble Space Telescope Wide Field Camera 3)近红外(F125W和F160W)高分辨率观测图像,利用质量限(恒星质量M_*>10^(10)M_⊙)选取了8002个红移分布在1<z<3范围内的星系样本,并对这些大质量星系的形态和结构性质进行了定量分析研究.通过星系形态的神经网络分类方法(Conv Nets),将样本中的星系划分为4类:椭球星系(SPHeroids,SPH)、早型盘星系(Early-Type Disks,ETD)、晚型盘星系(Late-Type Disks,LTD)和不规则星系(IRRegulars,IRR).结果发现星系的形态和结构随红移发生演化,在高红移宇宙星系主要表现出不规则形态,但到低红移处椭球和盘主导的哈勃星系形态序列已经形成.在相同红移区间内,不同类型星系的物理尺寸(r_e)中值从大到小的排序是IRR、LTD、ETD和SPH,而对应的Sersic指数(n)中值大小排序却相反.另外,不同类型星系的re与红移之间存在明显的演化关系,但这样的现象在平均轴比(b/a)和Sersic指数与红移的关系中并没有被发现.
基金Supported by the National Natural Science Foundation of China.
文摘We investigate two training-set methods; support vector machines （SVMs） and Kernel Regression （KR） for photometric redshift estimation with the data from the databases of Sloan Digital Sky Survey Data Release 5 and Two Micron All Sky Survey. We probe the performances of SVMs and KR for different input patterns. Our experiments show that with more parameters considered, the accuracy does not always increase, and only when appropriate parameters are chosen, the accuracy can improve. For different approaches, the best input pattern is different. With different parameters as input, the optimal bandwidth is dissimilar for KR. The rms errors of photometric redshifts based on SVM and KR methods are less than 0.03 and 0.02, respectively. Strengths and weaknesses of the two approaches are summarized. Compared to other methods of estimating photometric redshifts, they show their superiorities, especially KR, in terms of accuracy.
基金Supported by the National Natural Science Foundation of China.
文摘We study the growth of black holes and stellar population in spheroids at high redshift using several （sub）mm-loud QSO samples. Applying the same criteria established in an earlier work, we find that, similar to IR QSOs at low redshift, the far-infrared emission of these （sub）mm-loud QSOs mainly originates from dust heated by starbursts. By combining low-z IR QSOs and high-z （sub）mm-loud QSOs, we find a trend that the star formation rate （M＊） increases with the accretion rate （Mace）. We compare the values of M＊/Macc for submm emitting galaxies （SMGs）, far-infrared ultraluminous/hypeduminous QSOs and typical QSOs, and construct a likely evolution scenario for these objects. The （sub）mm-loud QSO transition phase has both high Macc and M＊ and hence is important for establishing the correlation between the masses of black holes and spheroids.
文摘We analyse the possibility that the observed cosmological redshift may be cumulatively due to the expansion of the universe and the tired light phenomenon. Since the source of both the redshifts is the same, they both independently relate to the same proper distance of the light source. Using this approach we have developed a hybrid model combining the Einstein de Sitter model and the tired light model that yields a slightly better fit to Supernovae Ia redshift data using one parameter than the standard ΛCDM model with two parameters. We have shown that the ratio of tired light component to the Einstein de Sitter component of redshift has evolved from 2.5 in the past, corresponding to redshift 1000, to its present value of 1.5. The hybrid model yields Hubble constant H0 =69.11(±0.53)km·s-1 ·Mpc-1 and the deceleration parameter q0 =-0.4. The component of Hubble constant responsible for expansion of the universe is 40% of H0 and for the tired light is 60% of H0. Consequently, the critical density is only 16% of its currently accepted value;a lot less dark matter is needed to make up the critical density. In addition, the best data fit yields the cosmological constant density parameter =0. The tired light effect may thus be considered equivalent to the cosmological constant in the hybrid model.
文摘The presence of the dark energy allows both the acceleration and the expansion of the universe. In the case of a constant equation of state for dark energy we derived an analytical solution for the Hubble radius in terms of the hypergeometric function. An approximate Taylor expansion of order seven is derived for both the constant and the variable equation of state for dark energy. In the case of the Cardassian cosmology, we also derived an analytical solution for the Hubble radius in terms of the hypergeometric function. The astronomical samples of the distance modulus for Supernova (SN) of type Ia allows the derivation of the involved cosmological in the case of constant equation of state, variable equation of state and Cardassian cosmology.
基金supported by the National Natural Science Foundation of China (Grant Nos. 11303036, 11390371/4 and 11233004)
文摘Hypervelocity stars are believed to be ejected out from the Galactic center through dynamical interactions between（binary） stars and the central supermassive black hole（s）. In this paper, we report 19 low mass F/G/K type hypervelocity star candidates from over one million stars found in the first data release of the LAMOST regular survey. We determine the unbound probability for each candidate using a MonteCarlo simulation by assuming a non-Gaussian proper-motion error distribution, and Gaussian heliocentric distance and radial velocity error distributions. The simulation results show that all the candidates have unbound possibilities over 50% as expected,and one of them may even exceed escape velocity with over 90% probability. In addition, we compare the metallicities of our candidates with the metallicity distribution functions of the Galactic bulge, disk, halo and globular clusters, and conclude that the Galactic bulge or disk is likely the birth place for our candidates.
基金supported by the National Natural Science Foundation of China (Grant Nos. 61272272 and U1531122)the Natural Science Foundation of Hubei province (Grant2015CFA058)+1 种基金the National Key Basic Research Program of China (2014CB845700)the NSFC-Texas A&M University Joint Research Program (No.11411120219)
文摘The massive photometric data collected from multiple large-scale sky surveys offer significant opportunities for measuring distances of celestial objects by photometric redshifts. However, catastrophic failure is an unsolved problem with a long history and it still exists in the current photometric redshift estimation approaches （such as the k-nearest neighbor （KNN） algorithm）. In this paper, we propose a novel two-stage approach by integration of KNN and support vector machine （SVM） methods together. In the first stage, we apply the KNN algorithm to photometric data and estimate their corresponding Zphot. Our analysis has found two dense regions with catastrophic failure, one in the range of Zphot E [0.3, 1.2] and the other in the range of Zphot E [1.2, 2.1]. In the second stage, we map the photometric input pattern of points falling into the two ranges from their original attribute space into a high dimensional feature space by using a Gaussian kernel function from an SVM. In the high dimensional feature space, many outliers resulting from catastrophic failure by simple Euclidean distance computation in KNN can be identified by a classification hyperplane of SVM and can be further corrected. Experimental results based on the Sloan Digital Sky Survey （SDSS） quasar data show that the two-stage fusion approach can significantly mitigate catastrophic failure and improve the estimation accuracy of photometric redshifts of quasars. The percents in different /△z/ ranges and root mean square （rms） error by the integrated method are 83.47%, 89.83%, 90.90% and 0.192, respectively, compared to the results by KNN （71.96%, 83.78%, 89.73% and 0.204）.
基金supported by the Strategic Priority Research Program "The Emergence of Cosmological Structures" of the Chinese Academy of Sciences (Grant No. XDB09000000)the National Key Basic Research Program of China (2014CB845700)CL acknowledges the National Natural Science Foundation of China (NSFC, Grant Nos. 11373032, 11333003 and U1231119)
文摘We present a sample of about 120 000 red clump candidates selected from the LAMOST DR2 catalog based on the empirical distribution model in the effective temperature vs. surface gravity plane. Although, in general, red clump stars are considered as standard candles, they do not exactly stay in a narrow range of absolute magnitude, but may have a range of more than one magnitude depending on their initial mass. Consequently, conventional oversimplified distance estimations with the assumption of a fixed luminosity may lead to systematic bias related to the initial mass or age, which can potentially affect the study of the evolution of the Galaxy with red clump stars. We therefore employ an isochrone-based method to estimate the absolute magnitude of red clump stars from their observed surface gravities, effective temperatures and metallicities. We verify that the estimation removes the systematics well and provides initial mass/age estimates that are independent of distance with accuracy better than 10%.
文摘With the recent observational evidence in extragalactic astronomy,the interpretation of the nature of quasar redshift continues to be a research interest.Very high redshifts are being detected for extragalactic objects that are presumably very distant and young while also exhibiting properties that are characteristic of a more mature galaxy such as ours.According to Halton Arp and Geoffrey Burbidge,redshift disparities consist of an intrinsic component and are related to an evolutionary process.Karlsson observed redshift periodicity at integer multiples of0.089 in log scale and Burbidge observed redshift periodicity at integer multiples of 0.061 in linear scale.Since Singular Value Decomposition based periodicity estimation is known to be superior for noisy data sets,especially when the data contain multiple harmonics and overtones,mainly irregular in nature,we have chosen it to be our primary tool for analysis of the quasar-galaxy pair redshift data.We have observed a fundamental periodicity of0.051 with a confidence interval of 95%in linear scale with the site-available Sloan Digital Sky Survey Data Release 7(SDSS DR7)quasar-galaxy pair data set.We have independently generated quasar-galaxy pair data sets from both 2d F and SDSS and found fundamental periodicities of 0.077 and 0.089,respectively,in log scale with a confidence interval of 95%.
基金funded by the National Natural Science Foundation of China (NSFC) (Grant Nos. 10778618 and 10633020)the National Basic Research Program of China (973 Program) (Grant No. 2007CB815403)
文摘An optical photometric observation with the Beijing-Arizona-Taiwan- Connecticut （BATC） multicolor system is carried out for A98 （z =0.104）, a galaxy cluster with two large enhancements in X-ray surface brightness. Spectral energy distributions （SEDs） covering 15 intermediate bands are obtained for all sources detected down to V - 20 mag in a field of 58′× 58′. After star-galaxy separation with colorcolor diagrams, a photometric redshift technique is applied to the galaxy sample for further membership determination. The color-magnitude relation is taken as a further restriction of the early-type cluster galaxies. As a result, a list of 198 faint member galaxies is achieved. Based on the newly generated sample of member galaxies, the dynamical substructures, A98N, A98S, and A98W, are investigated in detail. A separate galaxy group, A98X, is also found to the south of the main concentration of A98, which is gravitationally unbound to A98. For 74 spectroscopically confirmed member galaxies, the environmental effect on the star formation history is investigated. The bright galaxies in the core region are found to have shorter time scales of star formation, longer mean stellar ages, and higher interstellar medium metallicities, which can be interpreted in the context of the hierarchical cosmological scenario.
