A hybrid Fast Multipole Method for cosmological N-body simulations

We investigate a hybrid numerical algorithm aimed at large-scale cosmological N-body simulation for on-going and future high precision sky surveys.It makes use of a truncated Fast Multiple Method(FMM)for short-range gravity,incorporating a Particle Mesh(PM)method for long-range potential,which is applied to deal with extremely large particle number.In this work,we present a specific strategy to modify a conventional FMM by a Gaussian shaped factor and provide quantitative expressions for the interaction kernels between multipole expansions.Moreover,a proper Multipole Acceptance Criterion for the hybrid method is introduced to solve potential precision loss induced by the truncation.Such procedures reduce the amount of computation compared to an original FMM and decouple the global communication.A simplified version of code is introduced to verify the hybrid algorithm,accuracy and parallel implementation.

The formation and evolution of massive galaxies

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

Blind search for 21-cm absorption systems using a new generation of Chinese radio telescopes

Neutral hydrogen clouds are known to exist in the Universe, however their spatial distributions and physical properties are poorly understood. Such missing information can be studied by the new generation of Chinese radio telescopes through a blind search of 21-cm absorption systems. We forecast the capabilities of surveys of 21-cm absorption systems by two representative radio telescopes in China - the Five-hundred-meter Aperture Spherical radio Telescope （FAST） and Tianlai 21-cm cosmology experiment （Tianlai）. Facilitated by either the high sensitivity （FAST） or wide field of view （Tianlai） of these telescopes, more than a thousand 21-cm absorption systems can be discovered in a few years, representing orders of magnitude improvement over the cumulative discoveries in the past half a century.

The Spin Measurement of MAXI J0637-430:a Black Hole Candidate with High Disk Density

The Galactic black hole candidate MAXI J0637-430 was first discovered by MAXI/GSC on 2019 November 2.We study the spectral properties of MAXI J0637-430 by using the archived NuSTAR data and Swift/XRT data.After fitting the eight spectra by using a disk component and a powerlaw component model with absorption,we select the spectra with relatively strong reflection components for detailed X-ray reflection spectroscopy.Using the most state-of-art reflection model,relxillCp,the spectral fitting measures a black hole spin a_(*)>0.72 and the inclination angle of the accretion disk i=46.1_(-5.3)^(+4.0)degrees,at a 90%confidence level.In addition,the fitting results show an extreme supersolar iron abundance.Combined with the fitting results of reflection model reflionx_hd,we consider that this unphysical iron abundance may be caused by a very high-density accretion disk(n_(e)>2.34×10^(21)cm^(-3))or a strong Fe K_(α) emission line.The soft excess is found in the soft state spectral fitting results,which may be an extra free-free heating effect caused by high density of the accretion disk.Finally,we discuss the robustness of black hole spin obtained by X-ray reflection spectroscopy.The result of relatively high spin is self-consistent with broadened Fe K_(α) line.Iron abundance and disk density have no effect on the spin results.

PhotoNs-GPU:A GPU accelerated cosmological simulation code

We present a GPU-accelerated cosmological simulation code,PhotoNs-GPU,based on an algorithm of Particle Mesh Fast Multipole Method(PM-FMM),and focus on the GPU utilization and optimization.A proper interpolated method for truncated gravity is introduced to speed up the special functions in kernels.We verify the GPU code in mixed precision and different levels of the interpolated method on GPU.A run with single precision is roughly two times faster than double precision for current practical cosmological simulations.But it could induce an unbiased small noise in power spectrum.Compared with the CPU version of PhotoNs and Gadget-2,the efficiency of the new code is significantly improved.Activated all the optimizations on the memory access,kernel functions and concurrency management,the peak performance of our test runs achieves 48%of the theoretical speed and the average performance approaches to~35%on GPU.

The design and implementation of a ROACH2+GPU based correlator on the Tianlai dish array

A digital correlator is a crucial element in a modern radio telescope.In this paper,we describe a scalable design for the correlator system of the Tianlai pathfinder array,which is an experiment dedicated to testing key technologies for conducting a 21cm intensity mapping survey.The correlator implements the FX design,which firstly performs a fast Fourier transform(FFT) including polyphase filter bank(PFB) computation using a Collaboration for Astronomy Signal Processing and Electronics Research(CASPER) Reconfigurable Open Architecture Computing Hardware-2(ROACH2) board,then computes cross-correlations by employing Graphics Processing Units(GPUs).The design has been tested both in laboratory and in actual observation.

GABE: Galaxy Assembly with Binary Evolution

We developed a new semi-analytic galaxy formation model: Galaxy Assembly with Binary Evolution(GABE). For the first time, we introduce binary evolution into semi-analytic models of galaxy formation by using the Yunnan-Ⅱ stellar population synthesis model, which includes various binary interactions. When implementing our galaxy formation model onto the merger trees extracted from the Millennium simulation, it can reproduce a large body of observational results. We find that in the local universe, the model including binary evolution reduces the luminosity at optical and infrared wavelengths slightly, but it increases the luminosity at ultraviolet wavelengths significantly, especially in FUV band. The resulting luminosity function does not change very much over SDSS optical bands and infrared band, but the predicted colors are bluer, especially when the FUV band is under consideration. The new model allows us to explore the physics of various high energy events related to the remnants of binary stars, such as type Ia supernovae, short gamma-ray bursts and gravitational wave events, and their relation with host galaxies in a cosmological context.

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

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

Hydrodynamical simulations of the triggering of nuclear activities by minor mergers of galaxies

Major mergers of galaxies are considered to be an efficient way to trigger Active Galactic Nuclei and are thought to be responsible for the phenomenon of quasars. This has however recently been challenged by observations of a large number of low luminosity Active Galactic Nuclei at low redshift(z■1) without obvious major merger signatures. Minor mergers are frequently proposed to explain the existence of these Active Galactic Nuclei. In this paper, we perform nine high resolution hydrodynamical simulations of minor galaxy mergers, and investigate whether nuclear activities can be efficiently triggered by minor mergers, by setting various properties for the progenitor galaxies of those mergers. We find that minor galaxy mergers can activate the massive black hole in the primary galaxy with an Eddington ratio of f Edd > 0.01 and> 0.05(or a bolometric luminosity > 10^43 and > 10^44 erg s^-1) with a duration of 2.71 and 0.49 Gyr(or 2.69 and 0.19 Gyr), respectively. The nuclear activity of the primary galaxy strongly depends on the nucleus separation, such that the nucleus is more active as the two nuclei approach each other. Dual Active Galactic Nuclei systems can still possibly be formed by minor mergers of galaxies, though the time duration for dual Active Galactic Nuclei is only ~ 0.011 Gyr and ~ 0.017 Gyr with Eddington ratio of f Edd > 0.05 and bolometric luminosity > 10^44 erg s^-1. This time period is typically shorter than that of dual Active Galactic Nuclei induced by major galaxy mergers.

Variations of broad emission lines from periodicity QSOs under the interpretation of supermassive binary black holes with misaligned circumbinary broad line regions

Quasars with periodic light curves are considered as candidates of supermassive binary black hole(BBH)systems.One way for further confirmations may be searching for other characteristic signatures,such as those in their broad emission lines(BELs),if any,which require a thorough understanding on the response of BELs to the BBH systems.In Ji et al.(2021),we have investigated the response of circumbinary broad line region(BLR)to the central active secondary black hole under the relativistic Doppler boosting(BBH-DB)and intrinsic variation(BBH-IntDB)dominant mechanisms for continuum variation by assuming the middle plane of the BLR aligned with the BBH orbital plane.In this paper,we explore how the BEL profiles vary when the BLR is misaligned from the BBH orbital plane with different offset angles under both the BBH-DB and BBH-IntDB scenarios.Given a fixed inclination angle of the BBH orbital plane viewed in edge-on and similar continuum light curves produced by the two scenarios,increasing offset angles make the initial opening angle of the circumbinary BLR enlarged due to orbital precession caused by the BBH system,especially for clouds in the inner region,which result in Lorentzlike BEL profiles for the BBH-DB model but still Gaussion-like profiles for the BBH-IntDB model at the vertical BLR case.The amplitude of profile variations decreases with increasing offset angles for the BBHDB scenario,while keeps nearly constant for the BBH-IntDB scenario,since the Doppler boosting effect is motion direction preferred but the intrinsic variation is radiated isotropically.If the circumbinary BLR is composed of a coplanar and a vertical components with their number of clouds following the mass ratio of the BBHs,then the bi-BLR features are more significant for the BBH-IntDB model that requires larger mass ratio to generate similar continuum variation than the BBH-DB model.

Cosmological constraints on ultra-light axion fields

Ultra-light axions(ULAs)with mass less than 10-20 eV have interesting behaviors that may contribute to either dark energy or dark matter at different epochs of the Universe.Their properties can be explored by cosmological observations,such as expansion history of the Universe,cosmic large-scale structure,cosmic microwave background,etc.In this work,we study the ULAs with mass around 10-33 eV,which means that the ULA field still rolls slowly at present with the equation of state w=-1 as dark energy.To investigate the mass and other properties of this kind of ULA field,we adopt the measurements of Type Ia supernova(SN Ia),baryon acoustic oscillation(BAO),and Hubble parameter H(z).The Markov Chain Monte Carlo(MCMC)technique is employed to perform the constraints on the parameters.Finally,by exploring four cases of the model,we find that the mass of this ULA field is about 3×10-33 eV if assuming the initial axion fieldφi=Mpl.We also investigate a general case by assumingφi≤Mpl,and find that the fitting results ofφi/Mpl are consistent with or close to 1 for the datasets that we use.

Predicting Supermassive Black Hole Mass with Machine Learning Methods

It is crucial to measure the mass of supermassive black holes(SMBHs)in understanding the co-evolution between the SMBHs and their host galaxies.Previous methods usually require spectral data which are expensive to obtain.We use the AGN catalog from the Sloan Digital Sky Survey project Data Release 7(DR7)to investigate the correlations between SMBH mass and their host galaxy properties.We apply the machine learning algorithms,such as Lasso regression,to establish the correlation between the SMBH mass and various photometric properties of their host galaxies.We find an empirical formula that can predict the SMBH mass according to galaxy luminosity,colors,surface brightness,and concentration.The root-mean-square error is 0.5 dex,comparable to the intrinsic scatter in SMBH mass measurements.The 1σscatter in the relation between the SMBH mass and the combined galaxy properties relation is 0.48 dex,smaller than the scatter in the SMBH mass versus galaxy stellar mass relation.This relation could be used to study the SMBH mass function and the AGN duty cycles in the future.

Investigating the relationship between cosmic curvature and dark energy models with the latest supernova sample

We investigate the relationship between cosmic curvature and model of dark energy(hereafter DE)with recent TypeⅠa supernovae(hereafter SNeⅠa)data,i.e.,the Pantheon sample including 1048 SNeⅠa with 0.01<z<2.3.We obtain measurements of the dimensionless spatial curvature density today,i.e.,Ωk0=-0.062-0.169+0.189,-0.004-0.134+0.228,0.127-0.276+0.280 and 0.422-0.338+0.213 at 68%confidence level(CL),respectively,in the scenarios of ACDM,φCDM(i.e.,scalar field DE),ωCDM andω0ωaCDM models.In the scenario of ACDM model,a closed universe is preferred by the Pantheon sample,which is consistent with that from the Planck CMB spectra.However,the uncertainty of Qko from the Pantheon SNe sample is about 8 times larger than that from the Planck data,so the former one supports a closed universe at a much lower CL than that from the latter one.An open unverse is supported by the Pantheon sample at^32%and^78%CLs,respectively,in theωCDM andω0ωaCDM models.Among these models,theφCDM model is the one which supports the flat universe most strongly.It shows that Qk0 is significantly dependent on the adopted model of DE,and there is a negative correlation betweenΩk0 and the equation of state of DE.

The effect of hydrodynamics alone on the subhalo population in aΛCDM rich cluster sized dark matter halo

We perform a set of non-radiative hydro-dynamical(NHD)simulations of a rich cluster sized dark matter halo from the Phoenix Project with three different numerical resolutions,to investigate the effect of hydrodynamics alone on the subhalo population in the halo.Compared to dark matter only(DMO)simulations of the same halo,subhaloes are less abundant for relatively massive subhaloes(Msub>2.5×109h 1M⊙,or Vmax>70 km s 1)but more abundant for less massive subhaloes in the NHD simulations.This results in different shapes in the subhalo mass/Vmax function in two different sets of simulations.At given subhalo mass,the subhaloes less massive than 1010h 1M⊙have larger Vmax in the NHD than DMO simulations,while Vmax is similar for the subhaloes more massive than the mass value.This is mainly because the progenitors of present day low mass subhaloes have larger concentration parameters in the NHD than DMO simulations.The survival number fraction of the accreted low mass progenitors of the main halo at redshift 2 is about 50 percent higher in the NHD than DMO simulations.

The temperature of IGM at high redshifts:shock heating and high mach problem

The thermal history of cosmic gas in the dark ages remains largely unknown.It is important to quantify the impact of relevant physics on the IGM temperature between z=10 and z^30,in order to interpret recent and oncoming observations,including results reported by EDGES.We revisit the gas heating due to structure formation shocks in this era,using a set of fixed grid cosmological hydrodynamical simulations performed by three different codes.In all our simulations,the cosmic gas is predicted to be in multiphase state since z>30.The gas surrounding high density peaks gradually develops a relation more sharp than T∝ρ2/3,approximately T∝ρ2,from z=30 to z=11,might be due to shock heating.Meanwhile,the gas in void region tends to have a large local Mach number,and their thermal state varies significantly from code to code.In the redshift range 11-20,the mass fraction of gas shock heated above the CMB temperature in our simulations is larger than previous semi-analytical results by a factor of 2 to 8.At z=15,the fraction varies from^19%to 52%among different codes.Between z=11 and z=20,the gas temperature<1/TK>M-1 is predicted to be^10-20 K by two codes,much higher than the adiabatic cooling model and some previous works.However,in our simulations performed by RAMSES,<1/TK>M-1is predicted to be even below the temperature required to explain result of the EDGES.Given the fact that different codes give different predictions,currently,it seems a challenge to make solid prediction on the temperature of gas at z^17 in simulations.

Are pulsar giant pulses induced by re-emission of cyclotron resonance absorption?

It is conjectured that coherent re-emission of cyclotron resonance absorption could result in pulsar giant pulses.This conjecture seems reasonable as it can naturally explain the distribution of pulsars with giant pulses on the P-P diagram.

The Halo Concentration and Mass Relation Traced by Satellite Galaxies

We study the relation between halo concentration and mass(c-M relation) using galaxy catalogs of the Seventh and Eighth Data Releases of the Sloan Digital Sky Survey(SDSS DR7 and DR8). Assuming that the satellite galaxies follow the distribution of dark matter, we derive the halo concentration by fitting the satellite radial profile with a Nararro Frank and White(NFW) format. The derived c-M relation covers a wide halo mass range from10^(11.6)to 10^(14.1)M_(⊙). We confirm the anti-correlation between the halo mass and concentration as predicted in cosmological simulations. Our results are in good agreement with those derived using galaxy dynamics and gravitational lensing for halos of 10^(11.6)– 10^(12.9)M_(⊙), while they are slightly lower for halos of 10^(12.9)– 10^(14.1)M_(⊙).This is because blue satellite galaxies are less concentrated, especially in the inner regions. Instead of using all satellite galaxies, red satellites could be better tracers of the underlying dark matter distribution in galaxy groups.

Cosmological neutrino simulations at extreme scale

Constraining neutrino mass remains an elusive challenge in modern physics.Precision measurements are expected from several upcoming cosmological probes of large-scale structure.Achieving this goal relies on an equal level of precision from theoretical predictions of neutrino clustering.Numerical simulations of the non-linear evolution of cold dark matter and neutrinos play a pivotal role in this process.We incorporate neutrinos into the cosmological N-body code CUBEP3M and discuss the challenges associated with pushing to the extreme scales demanded by the neutrino problem.We highlight code optimizations made to exploit modern high performance computing architectures and present a novel method of data compression that reduces the phase-space particle footprint from 24 bytes in single precision to roughly 9 bytes.We scale the neutrino problem to the Tianhe-2 supercomputer and provide details of our production run,named Tian Nu,which uses 86%of the machine(13 824 compute nodes).With a total of 2.97 trillion particles,Tian Nu is currently the world’s largest cosmological N-body simulation and improves upon previous neutrino simulations by two orders of magnitude in scale.We finish with a discussion of the unanticipated computational challenges that were encountered during the Tian Nu runtime.

Pulsar giant pulse: Coherent instability near light cylinder

Giant pulses(GPs) are extremely bright individual pulses of radio pulsar. In microbursts of Crab pulsar, which is an active GP emitter, zebra-pattern-like spectral structures are observed, which are reminiscent of the "zebra bands" that are observed in type Ⅳ solar radio flares. However, band spacing linearly increases with the band center frequency of ～5-30 GHz. In this study, we propose that the Crab pulsar GP can originate from the coherent instability of plasma near a light cylinder. Further, the growth of coherent instability can be attributed to the resonance observed between the cyclotron-resonant-excited wave and the background plasma oscillation. The particles can be injected into the closed-field line regions owing to magnetic reconnection near a light cylinder. These particles introduce a large amount of free energy that further causes cyclotron-resonant instability, which grows and amplifies radiative waves at frequencies close to the electron cyclotron harmonics that exhibit zebra-pattern-like spectral band structures. Further, these structures can be modulated by the resonance between the cyclotron-resonant-excited wave and the background plasma oscillation. In this scenario, the band structures of the Crab pulsar can be well fitted by a coherent instability model, where the plasma density of a light cylinder should be ～10^(13-15) cm^(-3), with an estimated gradient of >5.5 × 10~5 cm^(-4). This process may be accompanied by high-energy emissions. Similar phenomena are expected to be detected in other types of GP sources that have magnetic fields of ? 106 G in a light cylinder.

Investigating the co-evolution of massive black holes in dual active galactic nuclei and their host galaxies via galaxy merger simulations

Major galaxy mergers can trigger nuclear activities and are responsible for high-luminosity quasi-stellar objects/active galactic nuclei(QSOs/AGNs). In certain circumstances, such mergers may cause dual active galactic nuclei(dAGN) phenomenon. This study investigates dAGN triggering and evolution of massive black holes(MBHs) during the merging processes using hydrodynamic code GADGET-2 to simulate several gas-rich major mergers at redshift z = 2 and 3, respectively. Results reveal that gas-rich major mergers can trigger significant nuclear activities after the second and third pericentric passages and the formation of dAGN with significant time duration(~10-390 Myr). During the merging processes, galactic bulge evolves with time because of the rapid star formation in each(or both) galactic centers and initial mixing of stars in galactic disks due to violent relaxation.MBHs grow substantially due to accretion and finally merge into a bigger black hole. The growth of galactic bulges and corresponding increases of its velocity dispersions predate the growth of MBHs in the d AGN stages. The MBHs in these stages deviate below the relation between MBH mass and bulge mass(or velocity dispersion), and they revert to the relation after the final mergers due to the significant accretion that occurs mostly at a separation less than a few kpc. Then, the two MBHs merge with each other.