The Jiao Tong University Spectroscopic Telescope Project

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=4000-5000)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~100000)designed to identify Jupiter analogs and Earth-like planets,with the capability to characterize the atmospheres of hot exoplanets.

CosmicGrowth Simulations——Cosmological simulations for structure growth studies

I present a large set of high resolution simulations, called CosmicGrowth Simulations, which were generated with either 8.6 billion or 29 billion particles. As for the nominal cosmological model that can match nearly all observations on cosmological scales, I have adopted a flat Cold Dark Matter(CDM) model with a cosmological constant Λ(ΛCDM). The model parameters have been taken either from the latest result of the WMAP satellite(WMAP ΛCDM) or from the first year's result of the Planck satellite(Planck ΛCDM). Six simulations are produced in the ΛCDM models with two in the Planck model and the others in the WMAP model. In order for studying the non-linear evolution of the clustering, four simulations were also produced with 8.6 billion particles for the scale-free models of an initial power spectrum P(k) ∝ k^n with n = 0,-1,-1.5 or-2.0. Furthermore, two radical CDM models(XCDM) are simulated with 8.6 billion particles each. Since the XCDM have some of the model parameters distinct from those of the ΛCDM models, they must be unable to match the observations, but are very useful for studying how the clustering properties depend on the model parameters. The Friends-of-Friends(FoF) halos were identified for each snapshot and subhalos were produced by the Hierarchical Branch Tracing(HBT) algorithm. These simulations form a powerful database to study the growth and evolution of the cosmic structures both in theory and in observation.

Hunting for the dark sector by searching for dark photons with the newly proposed DarkSHINE experiment

The existence of dark matter(DM) is inferred by observing its gravitational effects. Its origin from the thermal equilibrium in the early universe is proposed by the “freeze-out” mechanism. To satisfy about 27% of the DM density in the universe,the DM in the “freeze-out” is typically restricted to have a mass ranging from MeVs to 10 s of TeV. In the past decays, this has motivated various underground and collider experiments to search for weakly interacting massive particles.

Elucidating galaxy assembly bias in SDSS

We investigate the level of galaxy assembly bias in the Sloan Digital Sky Survey (SDSS) main galaxy sample using ELUCID,a state-of-the-art constrained simulation that accurately reconstructed the initial density perturbations within the SDSS volume.On top of the ELUCID haloes,we develop an extended HOD model that includes the assembly bias of central and satellite galaxies,parameterized as Q_(cen)and Q_(sat),respectively,to predict a suite of one-and two-point observables.In particular,our fiducial constraint employs the probability distribution of the galaxy number counts measured on 8 h^(-1)Mpc scales N_8^(g) and the projected cross-correlation functions of quintiles of galaxies selected by N_(8)^(g) with our entire galaxy sample.We perform extensive tests of the efficacy of our method by fitting the same observables to mock data using both constrained and non-constrained simulations.We discover that in many cases the level of cosmic variance between the two simulations can produce biased constraints that lead to an erroneous detection of galaxy assembly bias if the non-constrained simulation is used.When applying our method to the SDSS data,the ELUCID reconstruction effectively removes an otherwise strong degeneracy between cosmic variance and galaxy assembly bias in SDSS,enabling us to derive an accurate and stringent constraint on the latter.Our fiducial ELUCID constraint,for galaxies above a stellar mass threshold M_(*)=10^(10.2)h^(-2)M_(⊙),is Q_(cen)=-0.09±0.05 and Q_(sat)=0.09±0.10,indicating no evidence for a significant (>2σ) galaxy assembly bias in the local Universe probed by SDSS.Finally,our method provides a promising path to the robust modelling of the galaxy-halo connection within future surveys like DESI and PFS.

A deeper distinct view of the Milky Way by FAST

Many bright stars are visible in the sky. They gather and form a Milky Way, looking like a mysterious “silver river” in the sky.People wonder what property, structure, and content are in the Milky Way.With the development of modern astronomy, especially with optical telescopes, it has been known that the stars are concentrated together and form spiral arms.

New connection between dark matter direct detections,astrophysical and cosmological observations with self-interacting dark matter

The self-interacting dark matter(SIDM)model is an ideal candidate for explaining the discrepancy between small-scale structure observations and predictions by the prevailing collisionless cold dark matter(CDM)model.SIDM indicates the existence of a light mediator with a typical mass of 10 MeV.Searching for SIDM particles has therefore become one important alternative to the traditional weakly interacting massive particles(WIMPs)in direct detection experiments such as PandaX.Of note,the constraints obtained from direct detections are associated with the unknown mixing parameter(such as photon kinetic mixing)that describes the strength of the coupling between the mediator and ordinary particles。

The dawn of a new era of pulsar discoveries by Chinese radio telescope FAST

Pulsars are rapidly rotating compact stars subjected to physical conditions far beyond the reach of any laboratory on Earth.These astrophysical laboratories could help answer many fundamental physical questions. The Fermi Large Area Telescope(LAT) has discovered >250 γ-ray pulsars since its launch in 2008. Targeted radio searches of pulsar-like Fermi-LAT γ-ray sources have led to the discovery ~100 millisecond pulsars(MSPs).

A giant central red disk galaxy at redshift z=0.76:Challenge to theories of galaxy formation

We report a giant red central disk galaxy in the XMM-LSS north region. The region is covered with a rich variety of multi-band photometric and spectroscopic observations. Using the photometric data of the Canada-France-Hawaii Telescope Legacy Survey(CFHTLS) and spectroscopic observation of the baryon oscillation spectroscopic survey(BOSS), we find that the galaxy has a stellar mass of ~1011.6 M⊙. The galaxy has a red color and has an old stellar population, and thus its star formation has stopped.With the photometric image data of the Hyper Suprime-Cam(HSC) Subaru Strategic Program, we demonstrate that its luminosity profile is perfectly described by a Sérsic form with n=1.22 consistent with that of a disk. We also analyze its environment based on the VIMOS Public Extragalactic Redshift Survey(VIPERS) photometric catalog, and find that its close neighbors are all less massive, indicating that our observed galaxy is sitting at the center of its dark matter host halo. Existence of such a giant red central disk galaxy seriously challenges the current standard paradigm of galaxy formation, as there is no known physical mechanism to explain the quenching of its star formation. This conclusion is supported by state-of-the-art hydrodynamical simulations of galaxy formation.

The clustering of galaxies in the DESI imaging legacy surveys DR8:I. The luminosity and color dependent intrinsic clustering

In a recent study,we developed a method that models the impact of photometric redshift uncertainty on the two-point correlation function (2PCF).Using this method,we simultaneously obtained both the intrinsic clustering strength and the photometric redshift errors by fitting the projected 2PCF with two integration depths along the line-of-sight.Herein,we apply this method to the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys Data Release 8 (LS DR8),which is the largest galaxy sample currently available.We separate the galaxies into 20 samples in eight redshift bins,from z=0.1 to z=1.0,and several z-band absolute magnitude bins,with M_(z)≤-20.These galaxies are further divided into red and blue subsamples according to their M_(r)^(0.5)-M_(z)^(0.5) colors.We measure the projected 2PCFs for all these galaxy subsamples and fit them using our photometric redshift 2PCF model.We find that the photometric redshift errors are smaller in the red subsamples than in the overall population.In contrast,some systematic photometric redshift errors exist in the blue subsamples,such that some of the subsamples show a significantly enhanced 2PCF on large scales.Therefore,separately focusing only on the red subsamples and on all the subsamples,we find that the biases of the galaxies in these subsamples exhibit clear color,redshift,and luminosity dependencies;the brighter red galaxies at higher redshift are more biased than their bluer and low-redshift counterparts.Apart from the best-fit set of parameters,σz and b,from this state-of-the-art photometric redshift survey,we obtain high-precision intrinsic clustering measurements for these 40 red-and all-galaxy subsamples.These measurements,on large and small scales,hold important information regarding cosmology and galaxy formation that will be used in our subsequent probes in this series.

New footprints of the Gaia-Sausage-Enceladus galaxy found in the LAMOST survey

Merging is an important process during the formation and evolution of galaxies and structures in the Universe.The massive ancient merger event of the Gaia-Sausage-Enceladus (GSE) has significantly reshaped our Galaxy.Recently,a research group led by Prof.Gang Zhao from the National Astronomical Observatories,Chinese Academy of Sciences,found two footprints of the GSE merger event [1],and detected for the first time a metal-rich component with a low-alpha ratio in the LAMOST survey [2,3].

A fundamental step towards the cosmological 21 cm signal

The expansion history and the nature of dark energy are of fundamental importance for understanding the universe.Among various dark energy probes are included the type Ia supernovae(SNe Ia),baryon acoustic oscillations(BAO),cosmic microwave background(CMB),weak lensing,and galaxy clusters.Compared with the conventional optical BAO measurement that requires millions of galaxy spectra,the newly developed 21 cm intensity mapping(IM)technique enables efficient measurements of large-scale structures inaccessible to any other known techniques.