Constraints on Dark Energy from the CSST Galaxy Clusters

We study the potential of the galaxy cluster sample expected from the Chinese Space Station Telescope(CSST)survey to constrain dark energy properties.By modeling the distribution of observed cluster mass for a given true mass to be log-normal and adopting a selection threshold in the observed mass M_200m≥0.836×10¹⁴ h^-1 M_⊙,we find about 4.1×10⁵ clusters in the redshift range 0≤z≤1.5 can be detected by the CSST.We construct the Fisher matrix fo...     

A star-based method for precise wavelength calibration of the Chinese Space Station Telescope(CSST) slitless spectroscopic survey

The Chinese Space Station Telescope(CSST)spectroscopic survey aims to deliver high-quality low-resolution(R>200)slitless spectra for hundreds of millions of targets down to a limiting magnitude of about 21 mag,distributed within a large survey area(17500 deg2)and covering a wide wavelength range(255-1000 nm by three bands GU,GV,and GI).As slitless spectroscopy precludes the usage of wavelength calibration lamps,wavelength calibration is one of the most challenging issues in the reduction of slitless spectra,yet it plays a key role in measuring precise radial velocities of stars and redshifts of galaxies.In this work,we propose a star-based method that can monitor and correct for possible errors in the CSST wavelength calibration using normal scientific observations,taking advantage of the facts that(ⅰ)there are about ten million stars with reliable radial velocities now available thanks to spectroscopic surveys like LAMOST,(ⅱ)the large field of view of CSST enables efficient observations of such stars in a short period of time,and(ⅲ)radial velocities of such stars can be reliably measured using only a narrow segment of CSST spectra.We demonstrate that it is possible to achieve a wavelength calibration precision of a few km s^-1 for the GU band,and about 10 to 20 kms^-1 for the GV and GI bands,with only a few hundred velocity standard stars.Implementations of the method to other surveys are also discussed.     

Integrated HI emission in galaxy groups and clusters

The integrated HI emission from hierarchical structures such as groups and clusters of galaxies can be detected by FAST at intermediate redshifts. Here we propose to use FAST to study the evolution of the global HI content of clusters and groups over cosmic time by measuring their integrated HI emissions. We use the Virgo Cluster as an example to estimate the detection limit of FAST, and have estimated the integration time to detect a Virgo type cluster at different redshifts(from z = 0.1 to z = 1.5). We have also employed a semi-analytic model(SAM) to simulate the evolution of HI contents in galaxy clusters. Our simulations suggest that the HI mass of a Virgo-like cluster could be 2–3 times higher and the physical size could be more than 50% smaller when redshift increases from z = 0.3 to z = 1. Thus the integration time could be reduced significantly and gas rich clusters at intermediate redshifts can be detected by FAST in less than 2 hours of integration time. For the local Universe, we have also used SAM simulations to create mock catalogs of clusters to predict the outcomes from FAST all sky surveys. Comparing with the optically selected catalogs derived by cross matching the galaxy catalogs from the SDSS survey and the ALFALFA survey, we find that the HI mass distribution of the mock catalog with 20 s of integration time agrees well with that of observations. However, the mock catalog with 120 s of integration time predicts many more groups and clusters that contain a population of low mass HI galaxies not detected by the ALFALFA survey. A future deep HI blind sky survey with FAST would be able to test such prediction and set constraints on the numerical simulation models. The observational strategy and sample selections for future FAST observations of galaxy clusters at high redshifts are also discussed.     

Limiting Magnitudes of the Wide Field Survey Telescope(WFST)

Expected to be of the highest survey power telescope in the northern hemisphere, the Wide Field Survey Telescope(WFST) will begin its routine observations of the northern sky since 2023. WFST will produce a lot of scientific data to support the researches of time-domain astronomy, asteroids and the solar system, galaxy formation and cosmology and so on. We estimated that the 5σ limiting magnitudes of WFST with 30 s exposure are u = 22.31mag, g = 23.42 mag, r = 22.95 mag, i = 22.43 mag, z = 21.50...     

The Hybrid Nature of 0846＋51W1： a BL Lac Object with a Narrow Line Seyfert 1 Nucleus

We found a NLS1 nucleus in the extensively studied eruptive BL Lac object, 0846+51W1, out of a large sample of NLS1s compiled from the spectroscopic dataset of SDSS DR1. Its optical spectrum can be well decomposed into three components: a power law component from the relativistic jet, a stellar component from the host galaxy, and a component from a typical NLS1 nucleus. The emission line properties of 0846+51W1, FWHM     

Photometric Redshifts and Galaxy Clusters for DES DR2, DESI DR9, and HSC-SSP PDR3 Data

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 s...     

A Chandra study of the massive,distant galaxy cluster SDSS J0150–1005

We present a study of a fossil cluster,SDSS J0150–1005（z 0.364）,with high spatial resolution based on the imaging spectroscopic analysis of Chandra observations.The Chandra X-ray image shows a relaxed and symmetric morphology,which indicates that SDSS J0150–1005 is a well-developed galaxy cluster with no sign of a recent merger.According to the isothermal model,its global gas temperature is 5.73±0.80 keV,and the virial mass is 6.23±1.34×1014M⊙.Compared with the polytropic temperature model,the mass calculated based on the isothermal model is overestimated by 49%±11.The central gas entropy,S0.1 r200=143.9±18.3 keV cm2,is significantly lower than the average value of normal galaxy clusters with similar temperatures.Our results indicate that SDSS J0150–1005 formed during an early epoch.     

Resolved SZE Cluster Count

We study the counts of resolved SZE(Sunyaev-Zel‘dovich effect)clusters expected from an interferometric survey in different cosmological models under different conditions.The self-similar universal gas model and Press-Schechter mass function are used.We take the observing frequency to be 90GHz,and consider two dish diameters,1.2m and 2.5m.We calculate the number density of the galaxy clusters dN/(dΩdz)at a high flux limit Sv^lim=100mJy and at a relative low Sv^lim=10mJy.The total numbers of SZE clusters N in two low-Ω0 models are compared.The results show that the inθuence of the resolved effect depends not only on D,but also on Sv^lim;at a given D,the effect is more significant for a high than for a low Sv^lim.Also,the resolved effect for a flat universe is more impressive than tnat for an open universe.ForD=1.2m and Sv^lim=10mJy,the resolved effect is very weak.Considering the designed interferometers which will be used to survey SZE clusters,we find that the resolved effect is insignificant when estimating the expected yield of the SZE cluster surveys.     

A New Consistency Test for ΛCDM Cosmology Using Galaxy Surveys

We propose a new consistency test for the ΛCDM cosmology using baryonic acoustic oscillations(BAO) and redshift space distortion(RSD) measurements from galaxy redshift surveys. Specifically, we determine the peak position of fσ8(z) in redshift z offered by an RSD measurement, and compare it to the one predicted by the BAO observables assuming a flat ΛCDM cosmology. We demonstrate this new test using the simulated data for the DESI galaxy survey, and argue that this test complements those using t...     

High resolution deep imaging of a bright radio quiet QSO at z～3

We have obtained deep J and Ks-band images centered on a bright radio quiet QSO UM 402(zem= 2.856) using the IRCS camera and adaptive optics systems that are part of the Subaru Telescope, as well as retrieved WFC3/F140 W archive images of this object. A faint galaxy(mk= 23.32 ± 0.05 in the Vega magnitude system)that lies ～ 2.4′′north of the QSO sightline has been clearly resolved in all three deep high resolution datasets, and appears as an irregular galaxy with two close components in the Ks-band images(separation ～ 0.3′′). Given the small impact parameter(b = 19.6 kpc, at zlls= 2.531), as well as the red color of(J- K s)Vega～ 1.6,it might be a candidate galaxy giving rise to the Lyman Limit system absorption at zabs= 2.531 seen in the QSO spectrum. After carefully subtracting the point spread function from the QSO images, the host galaxy of this bright radio quiet QSO at z ～ 3was marginally revealed. We placed a lower limit on the host component of mk～ 23.3according to our analyses.     

Stellar Population Analysis of Galaxies based on Genetic Algorithms

We present a new method for determining the age and relative contribution of different stellar populations in galaxies based on the genetic algorithm. We apply this method to the barred spiral galaxy NGC 3384, using CCD images in U, B, V, R and I bands. This analysis indicates that the galaxy NGC 3384 is mainly inhabited by old stellar population (age >109yr). Some problems were encountered when numerical simulations are used for determining the contribution of different stellar populations in the integrated color of a galaxy. The results show that the proposed genetic algorithm can search efficiently through the very large space of the possible ages.     

Local Environmental Dependence of Galaxy Properties in a Volume-Limited Sample of Main Galaxies

Using a volume-limited sample of Main Galaxies from SDSS Data Release 5,we investigate the dependence of galaxy properties on local environment.For each galaxy,a local three-dimensional density is calculated.We find that the galaxy morphological type depends strongly on the local environment:galaxies in dense environments have predominantly early type morphologies.Galaxy colors have only a weak dependence on the environment.This puts an important constraint on the process of galaxy formation.     

Automated Separation os Stars and Normal Galaxies Based on Statistical Mixture Modeling with RBF Neural Net—works

For LAMOST,the largest sky survey program in China,the solution of the problem of automatic discrimination of stars from galaxies by spectra has shown that the results of the PSF test can be significantly refined.However,the problem is made worse when the redshifts of galaxies are not available.We present a new automatic method of star/(normal)galaxy separation,which is based on Statistical Mixture Modeling with Radial Basis Function Neural netrworks(SMM-RBFNN).This work is a continuation of our previous one,where active and non-active celestial objects were successfully segregated.By combining the method in this paper and the previous one,stars can now be effectively separated from galaxies and AGNs by their spectra-a major goal of LAMOST,and an indispensable step in any automatic spectrum classification system.In our work,the training set includes standard stellar spectra from Jacoby‘s spectrum library and simulated galaxy spectra of EO,SO,Sa,Sb types with redshift ranging from 0 to 1.2,and the test set of stellar spectra from Pickles‘atlas and SDSS spectra of normal galaxies with SNR of 13.Experiments show that our SMM-RBFNN is more efficient in both the training and testing stages than the BPNN(back propagation neural networks),and more importantly,it can achieve a good classification accuracy of 99.22% and 96.52%,respectively for stars and normal galaxies.     

Color-Redshift Relations and Photometric Redshift Estimations of Quasars in Large Sky Surveys

With a recently constructed composite quasar spectrum and the X2 minimization technique, we describe a general method for estimating the photometric redshifts of a large sample of quasars by deriving theoretical color-redshift relations and comparing the theoretical colors with the observed ones. We estimated the photometric redshifts from the 5-band SDSS photometric data of 18678 quasars in the first major data release of SDSS and compared them with their spectroscopic redshifts. The difference is less than 0.1 for 47% of the quasars and less than 0.2 for 68%. Based on the calculation of the theoretical color-color diagrams of stars, galaxies and quasars both on the SDSS system and on the BATC system, we expect that we would be able to select candidates of high redshift quasars more efficaciously with the latter than with the former, provided the BATC survey can detect objects with magnitudes fainter than 21.     

Automated Separation of Stars and Normal Galaxies Based on Statistical Mixture Modeling with RBF Neural Networks

For LAMOST, the largest sky survey program in China, the solution of the problem of automatic discrimination of stars from galaxies by spectra has shown that the results of the PSF test can be significantly refined. However, the problem is made worse when the redshifts of galaxies are not available. We present a new automatic method of star/(normal) galaxy separation, which is based on Statistical Mixture Modeling with Radial Basis Function Neural Networks (SMM-RBFNN). This work is a continuation of our previous one, where active and non-active celestial objects were successfully segregated. By combining the method in this paper and the previous one, stars can now be effectively separated from galaxies and AGNs by their spectra-a major goal of LAMOST, and an indispensable step in any automatic spectrum classification system. In our work, the training set includes standard stellar spectra from Jacoby's spectrum library and simulated galaxy spectra of EO, SO, Sa, Sb types with redshift ranging from 0 to 1     

Mapping the Milky Way with LAMOST Ⅰ: method and overview

We present a statistical method to derive the stellar density profiles of the Milky Way from spectroscopic survey data,taking into account selection effects.We assume the selection function,which can be altered during observations and data reductions,of the spectroscopic survey is based on photometric colors and magnitude.Then the underlying selection function for a line-of-sight can be recovered well by comparing the distribution of the spectroscopic stars in a color-magnitude plane with that of the photometric dataset.Subsequently,the stellar density profile along a line-of-sight can be derived from the spectroscopically measured stellar density profile multiplied by the selection function.The method is validated using Galaxia mock data with two different selection functions.We demonstrate that the derived stellar density profiles reconstruct the true ones well not only for the full set of targets,but also for sub-populations selected from the full dataset.Finally,the method is applied to map the density profiles for the Galactic disk and halo,using the LAMOST RGB stars.The Galactic disk extends to about R=19 kpc,where the disk still contributes about 10% to the total stellar surface density.Beyond this radius,the disk smoothly transitions to the halo without any truncation,bending or breaking.Moreover,no over-density corresponding to the Monoceros ring is found in the Galactic anti-center direction.The disk shows moderate north–south asymmetry at radii larger than 12 kpc.On the other hand,the R–Z tomographic map directly shows that the stellar halo is substantially oblate within a Galactocentric radius of 20 kpc and gradually becomes nearly spherical beyond 30 kpc.     

The AGN Feedback in Compact Galaxies: On the Impact of a More Massive Central Black Hole

We conduct high-resolution hydrodynamical simulations using the MACER framework to investigate the interplay between the interstellar medium,active galactic nuclei(AGN) feedback and black hole(BH) feeding in a massive compact galaxy,with an emphasis on the impact of different central BH masses.We find that with a more massive central BH,high-speed outflows are more prominent,and the gas fraction in the compact galaxy is reduced.Due to the lower gas density and higher gas temperature,the compact ...     

Two-fluid Dynamics in Clusters of Galaxies

We develop a theoretical formulation for the large-scale dynamics of galaxy clusters involving two spherical ‘isothermal fluids‘ coupled by their mutual gravity and derive asymptotic similarity solutions analytically. One of the fluids roughly approximates the massive dark matter halo, while the other describes the hot gas, the relatively small mass contribution from the galaxies being subsumed in the gas. By properly choosing the self-similar variables, it is possible to consistently transform the set of time-dependent two-fluid equations of spherical symmetry with serf-gravity into a set of coupled nonlinear ordinary differential equations (ODEs). We focus on the analytical analysis and discuss applications of the solutions to galaxy clusters.     

Statistical Cluster-QSO Weak Lensing in the Sloan Digital Sky Survey

We investigate the cross-correlation between galaxy clusters and QSOs using Sloan Digital Sky Survey (SDSS) DR4 - 5000 deg2 data. With photometric redshifts of galaxies, we select galaxy clusters based on the local projected densities of LRGs brighter than Mr' = -22. The QSOs are from the main sample of SDSS QSO spectroscopic survey to i' - 19. A significant positive correlation is found between the clusters and QSOs. Under the assumption that the signal is caused by gravitational leasing, we fit the signal with singular isothermal sphere (SIS) model and NFW profile halo model. The velocity dispersionσv = 766 km s-1 is derived for the best-fit of SIS model. Best-fit for the NFW model requires the dark matter halo mass within 1.5 h-1 Mpc to be 4.6×1014 h-1 M⊙. The mass parameterΩcl of the cluster sample is deduced as 0.077 with the SIS model and 0.083 with the NFW model. Our results ofΩcl are smaller than those given by Croom & Shanks and by Myers et al.     

Machine Learning to Search for Accreting Neutron Star Binary Candidates Using Chinese Space Station Telescope Photometric System

Accreting neutron star binary(ANSB) systems can provide some important information about neutron stars(NSs),especially on the structure and the equation of state of NSs. However, only a few ANSBs are known so far. The upcoming Chinese Space Station Telescope(CSST) provides an opportunity to search for a large number of ANSB candidates. We aim to investigate whether or not a machine learning method may efficiently search for ANSBs based on CSST photometric system. In this paper, we generate some ...