分类： 天文学 >> 天文仪器与技术 提交时间： 2024-02-07 合作期刊: 《天文技术与仪器（英文）》
JUST Team
Chengze Liu
Ying Zu
Fabo Feng
Zhaoyu Li
Yu Yu
Hua Bai
Xiangqun Cui
Bozhong Gu
Yizhou Gu
Jiaxin Han
Yonghui Hou
Zhongwen Hu
Hangxin Ji
Yipeng Jing
Wei Li
Zhaoxiang Qi
Xianyu Tan
Cairang Tian
Dehua Yang
摘要：The Jiao Tong University Spectroscopic Telescope (JUST) is a 4.4-meter f/6.0 segmented-mirror telescope dedicated to spectroscopic observations. The JUST primary mirror is composed of 18 hexagonal segments, each with a diameter of 1.1 m. JUST provides two Nasmyth platforms for placing science instruments. One Nasmyth focus fits a field of view of 10 ′ and the other has an extended field of view of 1.2° with correction optics. A tertiary mirror is used to switch between the two Nasmyth foci. JUST will be installed at a site at Lenghu in Qinghai Province, China, and will conduct spectroscopic observations with three types of instruments to explore the dark universe, trace the dynamic universe, and search for exoplanets: (1) a multi-fiber (2000 fibers) medium-resolution spectrometer (R=4 000−5 000) to spectroscopically map galaxies and large-scale structure; (2) an integral field unit (IFU) array of 500 optical fibers and/or a long-slit spectrograph dedicated to fast follow-ups of transient sources for multi-messenger astronomy; (3) a high-resolution spectrometer (R~100 000) designed to identify Jupiter analogs and Earth-like planets, with the capability to characterize the atmospheres of hot exoplanets.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Cluster cosmology depends critically on how the optical clusters are selected from imaging surveys. We compare the conditional luminosity function (CLF) and weak lensing halo masses between two different cluster samples at fixed richness, detected within the same volume ($0.1{<}z{<}0.34$) using the red-sequence and halo-based methods. After calibrating our CLF deprojection method against mock galaxy samples, we measure the 3D CLFs by cross-correlating clusters with SDSS photometric galaxies. As expected, the CLFs of the red-sequence and halo-based finders exhibit redder and bluer populations, respectively. We also find significant shape discrepancies between the two CLFs at the faint end, where the red-sequence clusters show a strong deficit of faint galaxies but a bump at $M_r{\sim}-20.5$, while the halo-based clusters host an increasing number of blue satellites. By comparing the subsamples of clusters that have a match between the two catalogues to those without matches, we discover that the CLF shape depends sensitively on the cluster centroiding. However, the average weak lensing halo mass between the matched and non-matched clusters are roughly consistent with each other in either cluster sample. Since the colour preferences of the two cluster finders are almost orthogonal, such a consistency indicates that the scatter in the mass-richness relation of either cluster sample is close to random. Therefore, while the choice of how optical clusters are identified impacts the satellite content, our result suggests that it should not introduce strong systematics biases in cluster cosmology.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We investigate the importances of various dynamical features in predicting the dynamical state (DS) of galaxy clusters, based on the Random Forest (RF) machine learning approach. We use a large sample of galaxy clusters from the Three Hundred Project of hydrodynamical zoomed-in simulations, and construct dynamical features from the raw data as well as from the corresponding mock maps in the optical, X-ray, and Sunyaev-Zel'dovich (SZ) channels. Instead of relying on the impurity based feature importance of the RF algorithm, we directly use the out-of-bag (OOB) scores to evaluate the importances of individual features and different feature combinations. Among all the features studied, we find the virial ratio, $\eta$, to be the most important single feature. The features calculated directly from the simulations and in 3-dimensions carry more information on the DS than those constructed from the mock maps. Compared with the features based on X-ray or SZ maps, features related to the centroid positions are more important. Despite the large number of investigated features, a combination of up to three features of different types can already saturate the score of the prediction. Lastly, we show that the most sensitive feature $\eta$ is strongly correlated with the well-known half-mass bias in dynamical modelling. Without a selection in DS, cluster halos have an asymmetric distribution in $\eta$, corresponding to an overall positive half-mass bias. Our work provides a quantitative reference for selecting the best features to discriminate the DS of galaxy clusters in both simulations and observations.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Recent observations reveal that, at a given stellar mass, blue galaxies tend to live in haloes with lower mass while red galaxies live in more massive host haloes. The physical driver behind this is still unclear because theoretical models predict that, at the same halo mass, galaxies with high stellar masses tend to live in early-formed haloes which naively leads to an opposite trend. Here, we show that the {\sc Simba} simulation quantitatively reproduces the colour bimodality in SHMR and reveals an inverse relationship between halo formation time and galaxy transition time. It suggests that the origin of this bimodality is rooted in the intrinsic variations of the cold gas content due to halo assembly bias. {\sc Simba}'s SHMR bimodality quantitatively relies on two aspects of its input physics: (1) Jet-mode AGN feedback, which quenches galaxies and sets the qualitative trend; and (2) X-ray AGN feedback, which fully quenches galaxies and yields better agreement with observations. The interplay between the growth of cold gas and the AGN quenching in {\sc Simba} results in the observed SHMR bimodality.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Ziwen Zhang
Huiyuan Wang
Wentao Luo
H. J. Mo
Zhixiong Liang
Ran Li
Xiaohu Yang
Tinggui Wang
Hongxin Zhang
Hui Hong
Xiaoyu Wang
Enci Wang
Pengfei Li
JingJing Shi
摘要： Based on the spectroscopic and shear catalogs for SDSS galaxies in the local Universe, we compare optically-selected active galactic nuclei (AGNs) with control star-forming and quiescent galaxies on galactic, inter-halo and larger scales. We find that AGNs are preferentially found in two specific stages of galaxy evolution: star-burst and `green valley' phases, and that the stellar population of their host galaxies is quite independent of stellar mass, different from normal galaxies. Combining galaxy-galaxy lensing and galaxy clustering on large scales, we measure the mass of AGN host halos. The typical halo mass is about $10^{12}h^{-1}\rm M_{\odot}$, similar to the characteristic mass in the stellar mass-halo mass relation (SHMR). For given stellar mass, AGN host galaxies and star-forming galaxies share the same SHMR, while quiescent galaxies have more massive halos. Clustering analysis on halo scales reveals that AGNs are surrounded by a larger number of satellites (with stellar mass down to 1/1000 of the mass of the central galaxy) than star-forming galaxies, and that galaxies with larger stellar velocity dispersion have more satellites. The number of satellites also increase with halo mass, reaching unity around $10^{12}h^{-1}\rm M_{\odot}$. Our results suggest a scenario, in which the interaction of the central galaxy with the satellites triggers an early episode of star burst and AGN activities, followed by multiple AGN cycles driven by the non-axisymmetric structure produced by the interaction. The feedback from the starburst and AGN reduces the amount of cold gas for fueling the central black hole, producing a characteristic halo mass scale, $\sim 10^{12}h^{-1}\rm M_{\odot}$, where the AGN fraction peaks.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We measure the cross-correlation between galaxy groups constructed from DESI
Legacy Imaging Survey DR8 and \emph{Planck} CMB lensing, over overlapping sky
area of 16876 $\rm deg^2$. The detections are significant and consistent with
the expected signal of the large-scale structure of the universe, over group
samples of various redshift, mass, richness $N_{\rm g}$ and over various scale
cuts. The overall S/N is 40 for a conservative sample with $N_{\rm g}\geq 5$,
and increases to $50$ for the sample with $N_{\rm g}\geq 2$. Adopting the
\emph{Planck} 2018 cosmology, we constrain the density bias of groups with
$N_{\rm g}\geq 5$ as $b_{\rm g}=1.31\pm 0.10$, $2.22\pm 0.10$, $3.52\pm 0.20$
at $0.1
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Ying Zu
Huanyuan Shan
Jun Zhang
Sukhdeep Singh
Zhiwei Shao
Xiaokai Chen
Ji Yao
Jesse B. Golden-Marx
Weiguang Cui
Eric Jullo
Jean-Paul Kneib
Pengjie Zhang
Xiaohu Yang
摘要： Concentration is one of the key dark matter halo properties that could drive
the scatter in the stellar-to-halo mass relation of massive clusters. We derive
robust photometric stellar masses for a sample of brightest central galaxies
(BCGs) in SDSS redMaPPer clusters at $0.17
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Wentao Luo
Jiajun Zhang
Vitali Halenka
Xiaohu Yang
Surhud More
Chris Miller
Tomomi Sunayama
Lei Liu
Feng Shi
摘要： We test the Emergent Gravity(EG) theory using the galaxy-galaxy lensing technique based on SDSS DR7 data. In the EG scenario, we do not expect color dependence of the galaxy sample in the 'apparent dark matter' predicted by EG, which is exerted only by the baryonic mass. If the baryonic mass is similar, then the predicted lensing profiles from the baryonic mass should be similar according to the EG, regardless of the color of the galaxy sample. We use the stellar mass of the galaxy as a proxy of its baryonic mass. We divide our galaxy sample into 5 stellar mass bins, and further classify them as red and blue subsamples in each stellar mass bin. If we set halo mass and concentration as free parameters, $\Lambda$CDM is favored by our data in terms of the reduced $\chi^2$ while EG fails to explain the color dependence of ESDs from the galaxy-galaxy lensing measurement.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Calibrating the redshift distributions of photometric galaxy samples is essential in weak lensing studies. The self-calibration method combines angular auto- and cross-correlations between galaxies in multiple photometric redshift (photo-$z$) bins to reconstruct the scattering rates matrix between redshift bins. In this paper, we test a recently proposed self-calibration algorithm using the DECaLS Data Release 9 and investigate to what extent the scattering rates are determined. We first mitigate the spurious angular correlations due to imaging systematics by a machine learning based method. We then improve the algorithm for $\chi^2$ minimization and error estimation. Finally, we solve for the scattering matrices, carry out a series of consistency tests and find reasonable agreements: (1) finer photo-$z$ bins return a high-resolution scattering matrix, and it is broadly consistent with the low-resolution matrix from wider bins; (2) the scattering matrix from the Northern Galactic Cap is almost identical to that from Southern Galactic Cap; (3) the scattering matrices are in reasonable agreement with those constructed from the power spectrum and the weighted spectroscopic subsample. We also evaluate the impact of cosmic magnification. Although it changes little the diagonal elements of the scattering matrix, it affects the off-diagonals significantly. The scattering matrix also shows some dependence on scale cut of input correlations, which may be related to a known numerical degeneracy between certain scattering pairs. This work demonstrates the feasibility of the self-calibration method in real data and provides a practical alternative to calibrate the redshift distributions of photometric samples.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present an algorithm to extend subhalo merger trees in a low-resolution dark-matter-only simulation by conditionally matching them to those in a high-resolution simulation. The algorithm is general and can be applied to simulation data with different resolutions using different target variables. We instantiate the algorithm by a case in which trees from ELUCID, a constrained simulation of $(500h^{-1}{\rm Mpc})^3$ volume of the local universe, are extended by matching trees from TNGDark, a simulation with much higher resolution. Our tests show that the extended trees are statistically equivalent to the high-resolution trees in the joint distribution of subhalo quantities and in important summary statistics relevant to modeling galaxy formation and evolution in halos. The extended trees preserve certain information of individual systems in the target simulation, including properties of resolved satellite subhalos, and shapes and orientations of their host halos. With the extension, subhalo merger trees in a cosmological scale simulation are extrapolated to a mass resolution comparable to that in a higher-resolution simulation carried out in a smaller volume, which can be used as the input for (sub)halo-based models of galaxy formation. The source code of the algorithm, and halo merger trees extended to a mass resolution of $\sim 2 \times 10^8 h^{-1}M_\odot$ in the entire ELUCID simulation, are available.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We carry out a thermal energy census of hot baryons at $z < 1$, by cross-correlating the \emph{Planck} MILCA y-map with 0.8 million clusters/groups selected from the Yang et.al (2021) catalog. The thermal Sunyaev-Zel'dovich (tSZ) effect around these clusters/groups are reliably obtained, which enables us to make our model constraints based on one-halo (1h) and two-halo (2h) contributions, respectively. (1) The total measurement S/N of the one-halo term is 63. We constrain the $Y$-$M$ relation over the halo mass range of $10^{13}$-$10^{15} M_\odot/h$, and find $Y\propto M^{\alpha}$ with $\alpha= 1.8$ at $z=0.14$ ($\alpha=2.1$ at $z=0.75$). The total thermal energy of gas bound to clusters/groups increases from $0.1\ \rm meV/cm^3$ at $z=0.14$ to $0.22\ \rm meV/cm^3$ at $z=0.75$. (2) The two-halo term is used to constrain the bias-weighted electron pressure $\langle b_yP_e \rangle$. We find that $\langle b_yP_e \rangle$ (in unit of $\rm meV/cm^3$) increases from $0.24\pm 0.02$ at $z=0.14$ to $0.45\pm 0.02$ at $z=0.75$. These results lead to several implications. (i) The hot gas fraction $f_{\rm gas}$ in clusters/groups monotonically increase with halo mass, where $f_{\rm gas}$ of a $10^{14} M_\odot/h$ halo is $\sim 50\%$ ($25\%$) of the cosmic mean at $z=0.14\ (0.75)$. (ii) By comparing the 1h- and 2h-terms, we obtain tentative constraint on the thermal energy of unbound gas. (iii) The above results lead to significant suppression of matter and weak lensing power spectrum at small scales. These implications are important for astrophysics and cosmology, and we will further investigate them with improved data and gas modeling.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Qingyang Li
Xiaohu Yang
Chengze Liu
Yipeng Jing
Min He
Jiasheng Huang
Y. Sophia Dai
Marcin Sawicki
Stephane Arnouts
Stephen Gwyn
Thibaud Moutard
H. J. Mo
Kai Wang
Antonios Katsianis
Weiguang Cui
Jiaxin Han
I-Non Chiu
Yizhou Gu
Haojie Xu
摘要： Using the extended halo-based group finder developed by Yang et al. (2021), which is able to deal with galaxies via spectroscopic and photometric redshifts simultaneously, we construct galaxy group and candidate protocluster catalogs in a wide redshift range ($0 2.0$. By checking the galaxy number distributions within a $5-7\ h^{-1}\mathrm{Mpc}$ projected separation and a redshift difference $\Delta z \le 0.1$ around those richest groups at redshift $z>2$, we identified a list of 761, 343 and 43 protocluster candidates in the redshift bins $2\leq z<3$, $3\leq z<4$ and $z \geq 4$, respectively. In general, these catalogs of galaxy groups and protocluster candidates will provide useful environmental information in probing galaxy evolution along the cosmic time.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present an algorithm to extend subhalo merger trees in a low-resolution dark-matter-only simulation by conditionally matching them to those in a high-resolution simulation. The algorithm is general and can be applied to simulation data with different resolutions using different target variables. We instantiate the algorithm by a case in which trees from ELUCID, a constrained simulation of $(500h^{-1}{\rm Mpc})^3$ volume of the local universe, are extended by matching trees from TNGDark, a simulation with much higher resolution. Our tests show that the extended trees are statistically equivalent to the high-resolution trees in the joint distribution of subhalo quantities and in important summary statistics relevant to modeling galaxy formation and evolution in halos. The extended trees preserve certain information of individual systems in the target simulation, including properties of resolved satellite subhalos, and shapes and orientations of their host halos. With the extension, subhalo merger trees in a cosmological scale simulation are extrapolated to a mass resolution comparable to that in a higher-resolution simulation carried out in a smaller volume, which can be used as the input for (sub)halo-based models of galaxy formation. The source code of the algorithm, and halo merger trees extended to a mass resolution of $\sim 2 \times 10^8 h^{-1}M_\odot$ in the entire ELUCID simulation, are available.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We carry out a thermal energy census of hot baryons at $z < 1$, by cross-correlating the \emph{Planck} MILCA y-map with 0.8 million clusters/groups selected from the Yang et.al (2021) catalog. The thermal Sunyaev-Zel'dovich (tSZ) effect around these clusters/groups are reliably obtained, which enables us to make our model constraints based on one-halo (1h) and two-halo (2h) contributions, respectively. (1) The total measurement S/N of the one-halo term is 63. We constrain the $Y$-$M$ relation over the halo mass range of $10^{13}$-$10^{15} M_\odot/h$, and find $Y\propto M^{\alpha}$ with $\alpha= 1.8$ at $z=0.14$ ($\alpha=2.1$ at $z=0.75$). The total thermal energy of gas bound to clusters/groups increases from $0.1\ \rm meV/cm^3$ at $z=0.14$ to $0.22\ \rm meV/cm^3$ at $z=0.75$. (2) The two-halo term is used to constrain the bias-weighted electron pressure $\langle b_yP_e \rangle$. We find that $\langle b_yP_e \rangle$ (in unit of $\rm meV/cm^3$) increases from $0.24\pm 0.02$ at $z=0.14$ to $0.45\pm 0.02$ at $z=0.75$. These results lead to several implications. (i) The hot gas fraction $f_{\rm gas}$ in clusters/groups monotonically increase with halo mass, where $f_{\rm gas}$ of a $10^{14} M_\odot/h$ halo is $\sim 50\%$ ($25\%$) of the cosmic mean at $z=0.14\ (0.75)$. (ii) By comparing the 1h- and 2h-terms, we obtain tentative constraint on the thermal energy of unbound gas. (iii) The above results lead to significant suppression of matter and weak lensing power spectrum at small scales. These implications are important for astrophysics and cosmology, and we will further investigate them with improved data and gas modeling.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Renjie Li
Huiyuan Wang
H. J. Mo
Shuiyao Huang
Neal Katz
Xiong Luo
Weiguang Cui
Hao Li
Xiaohu Yang
Ning Jiang
Yuning Zhang
摘要： Using reconstructed initial conditions in the SDSS survey volume, we carry out constrained hydrodynamic simulations in three regions representing different types of the cosmic web: the Coma cluster of galaxies; the SDSS great wall; and a large low-density region at $z\sim 0.05$. These simulations, which include star formation and stellar feedback but no AGN formation and feedback, are used to investigate the properties and evolution of intergalactic and intra-cluster media. About half of the warm-hot intergalactic gas is associated with filaments in the local cosmic web. Gas in the outskirts of massive filaments and halos can be heated significantly by accretion shocks generated by mergers of filaments and halos, respectively, and there is a tight correlation between gas temperature and the strength of the local tidal field. The simulations also predict some discontinuities associated with shock fronts and contact edges, which can be tested using observations of the thermal SZ effect and X-rays. A large fraction of the sky is covered by Ly$\alpha$ and OVI absorption systems, and most of the OVI systems and low-column density HI systems are associated with filaments in the cosmic web. The constrained simulations, which follow the formation and heating history of the observed cosmic web, provide an important avenue to interpret observational data. With full information about the origin and location of the cosmic gas to be observed, such simulations can also be used to develop observational strategies.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We develop a model to establish the interconnection between galaxies and their dark matter halos. We use Principal Component Analysis (PCA) to reduce the dimensionality of both the mass assembly histories of halos/subhalos and the star formation histories of galaxies, and Gradient Boosted Decision Trees (GBDT) to transform halo/subhalo properties into galaxy properties. We use two sets of hydrodynamic simulations to motivate our model architecture and to train the transformation. We then apply the two sets of trained models to dark matter only (DMO) simulations to show that the transformation is reliable and statistically accurate. The model trained by a high-resolution hydrodynamic simulation, or by a set of such simulations implementing the same physics of galaxy formation, can thus be applied to large DMO simulations to make "mock" copies of the hydrodynamic simulation. The model is both flexible and interpretable, which paves the way for future applications in which we will constrain the model using observations at different redshifts simultaneously and explore how galaxies form and evolve in dark matter halos empirically.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： By linking galaxies in Sloan Digital Sky Survey (SDSS) to subhaloes in the ELUCID simulation, we investigate the relation between subhalo formation time and the galaxy properties, and the dependence of galaxy properties on the cosmic web environment. We find that central and satellite subhaloes have different formation time, where satellite subhaloes are older than central subhaloes at fixed mass. At fixed mass, the galaxy stellar-to-subhalo mass ratio is a good proxy of the subhalo formation time, and increases with the subhalo formation redshifts, especially for massive galaxies. The subhalo formation time is dependent on the cosmic web environment. For central subhaloes, there is a characteristic subhalo mass of $\sim 10^{12} \msun$, below which subhaloes in knots are older than subhaloes of the same mass in filaments, sheets, or voids, while above which it reverses. The cosmic web environmental dependence of stellar-to-subhalo mass ratio is similar to that of the subhalo formation time. For centrals, there is a characteristic subhalo mass of $\sim 10^{12} \msun$, below which the stellar-to-subhalo mass ratio is higher in knots than in filaments, sheets and voids, above which it reverses. Galaxies in knots have redder colors below $10^{12} \msun$, while above $10^{12} \msun$, the environmental dependence vanishes. Satellite fraction is strongly dependent on the cosmic web environment, and decreases from knots to filaments to sheets to voids, especially for low-mass galaxies.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Calibrating the redshift distributions of photometric galaxy samples is essential in weak lensing studies. The self-calibration method combines angular auto- and cross-correlations between galaxies in multiple photometric redshift (photo-$z$) bins to reconstruct the scattering rates matrix between redshift bins. In this paper, we test a recently proposed self-calibration algorithm using the DECaLS Data Release 9 and investigate to what extent the scattering rates are determined. We first mitigate the spurious angular correlations due to imaging systematics by a machine learning based method. We then improve the algorithm for $\chi^2$ minimization and error estimation. Finally, we solve for the scattering matrices, carry out a series of consistency tests and find reasonable agreements: (1) finer photo-$z$ bins return a high-resolution scattering matrix, and it is broadly consistent with the low-resolution matrix from wider bins; (2) the scattering matrix from the Northern Galactic Cap is almost identical to that from Southern Galactic Cap; (3) the scattering matrices are in reasonable agreement with those constructed from the power spectrum and the weighted spectroscopic subsample. We also evaluate the impact of cosmic magnification. Although it changes little the diagonal elements of the scattering matrix, it affects the off-diagonals significantly. The scattering matrix also shows some dependence on scale cut of input correlations, which may be related to a known numerical degeneracy between certain scattering pairs. This work demonstrates the feasibility of the self-calibration method in real data and provides a practical alternative to calibrate the redshift distributions of photometric samples.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： In the local Universe, the efficiency for converting baryonic gas into stars is very low. In dark matter halos where galaxies form and evolve, the average efficiency varies with galaxy stellar mass and has a maximum of about twenty percent for Milky-Way-like galaxies. The low efficiency at higher mass is believed to be produced by some quenching processes, such as the feedback from active galactic nuclei. We perform an analysis of weak lensing and satellite kinematics for SDSS central galaxies. Our results reveal that the efficiency is much higher, more than sixty percent, for a large population of massive star-forming galaxies around $10^{11}M_{\odot}$. This suggests that these galaxies acquired most of the gas in their halos and converted it into stars without being affected significantly by quenching processes. This population of galaxies is not reproduced in current galaxy formation models, indicating that our understanding of galaxy formation is incomplete. The implications of our results on circumgalactic media, star formation quenching and disc galaxy rotation curves are discussed. We also examine systematic uncertainties in halo-mass and stellar-mass measurements that might influence our results.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yiming Wu
Chengze Liu
Eric W. Peng
Youkyung Ko
Patrick Côté
Rashi Jain
Laura Ferrarese
Xiaohu Yang
Ariane Lançon
Thomas Puzia
Sungsoon Lim
摘要： Combining data from the ACS Virgo Cluster Survey (ACSVCS) and the Next Generation Virgo cluster Survey (NGVS), we extend previous studies of color gradients of the globular cluster (GC) systems of the two most massive galaxies in the Virgo cluster, M87 and M49, to radii of $\sim 15~R_e$ ($\sim 200$ kpc for M87 and $\sim 250$ kpc for M49). We find significant negative color gradients, i.e., becoming bluer with increasing distance, out to these large radii. The gradients are driven mainly by the outwards decrease of the ratio of red to blue GC numbers. The color gradients are also detected out to $\sim 15~R_e$ in the red and blue sub-populations of GCs taken separately. In addition, we find a negative color gradient when we consider the satellite low-mass elliptical galaxies as a system, i.e., the satellite galaxies closer to the center of the host galaxy usually have redder color indices, both for their stars and GCs. According to the "two phase" formation scenario of massive early-type galaxies, the host galaxy accretes stars and GCs from low-mass satellite galaxies in the second phase. So the accreted GC system naturally inherits the negative color gradient present in the satellite population. This can explain why the color gradient of the GC system can still be observed at large radii after multiple minor mergers.
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