In response to the problems of the current practice-oriented project-based courses, such as the s... more In response to the problems of the current practice-oriented project-based courses, such as the single form of curriculum politics evaluation and the subjective way of curriculum politics evaluation, the CIPP model suitable for curriculum politics evaluation is adopted, and the evaluation system is constructed from four aspects: context evaluation, input evaluation, process evaluation and product evaluation. The CIPP model is used to construct an evaluation system with 4 level-1 indicators and 11 level-2 indicators. Analytic hierarchy process was applied to derive the weights of each index. According to the weights, the teaching implementation of the project-based course was reconstructed to enhance the openness of project selection, to connect the in-class and out-of-class, and to deeply involve students in the construction of the resource library. The questionnaire survey before and after the improvement of the implementation plan showed that the implementation plan led to a signi...
The presence of an extra radio background besides the cosmic microwave background has important i... more The presence of an extra radio background besides the cosmic microwave background has important implications for the observation of the 21-cm signal during the cosmic Dark Ages, Cosmic Dawn, and epoch of Reionization. The strong absorption trough found in the 21-cm global spectrum measured by the EDGES experiment, which has a much greater depth than the standard model prediction, has drawn great interest to this scenario, but more generally it is still of great interest to consider such a cosmic radio background (CRB) in the early Universe. To be effective in affecting the 21-cm signal at early time, such a radio background must be produced by sources which can emit strong radio signals but modest amount of X-rays, so that the gas is not heated up too early. We investigate the scenario that such a radio background is produced by the primordial black holes (PBHs). For PBH with a single mass, we find that if the PBHs' abundance log(fPBH) (ratio of total PBH mass density to total matter density) and mass satisfy the relation log(fPBH) ∼ −1.8 log(M•/M⊙) − 3.5 for 1 M⊙ M• 300M⊙, and have jet emission, they can generate a CRB required for reproducing the 21-cm absorption signal seen by the EDGES. The accretion rate can be boosted if the PBHs are surrounded by dark matter halos, which permits lower fPBH value to satisfy the EDGES observation. In the latter scenario, since the accretion rate can evolve rapidly during the Cosmic Dawn, the frequency (redshift) and depth of the absorption trough can determine the mass and abundance of the PBHs simultaneously. For absorption trough redshift ∼ 17 and depth ∼ −500 mK, it corresponds to M• ∼ 1.05 M⊙ and fPBH ∼ 1.5 × 10 −4 .
A Semiblind PCA-based Foreground Subtraction Method for 21 cm Intensity Mapping
The Astrophysical Journal
The principal component analysis (PCA) method and the singular value decomposition (SVD) method a... more The principal component analysis (PCA) method and the singular value decomposition (SVD) method are widely used for foreground subtraction in 21 cm intensity mapping experiments. We show their equivalence, and point out that the condition for completely clean separation of foregrounds and cosmic 21 cm signal using the PCA/SVD is unrealistic. We propose a PCA-based foreground subtraction method, dubbed the “singular vector projection (SVP)” method, which exploits a priori information of the left and/or right singular vectors of the foregrounds. We demonstrate with simulation tests that this new, semiblind method can reduce the error of the recovered 21 cm signal by orders of magnitude, even if only the left and/or right singular vectors in the largest few modes are exploited. The SVP estimators provide a new, effective approach for 21 cm observations to remove foregrounds and uncover the physics in the cosmic 21 cm signal.
We theoretically investigate the recovery of global spectrum (monopole) from visibilities (cross-... more We theoretically investigate the recovery of global spectrum (monopole) from visibilities (cross-correlation only) measured by the interferometer array and the feasibility of extracting 21 cm signal of cosmic dawn. In our approach, the global spectrum is obtained by solving the monopole and higher-order components simultaneously from the visibilities measured with up to thousands of baselines. Using this algorithm, the monopole of both foreground and the 21 cm signal can be correctly recovered in a broad range of conditions. We find that a 3D baseline distribution can have much better performance than a 2D (planar) baseline distribution, particularly when there is a lack of shorter baselines. We simulate for ground-based 2D and 3D array configurations, and a cross-shaped space array located at the Sun-Earth L2 point that can form 3D baselines through orbital precession. In all simulations we obtain good recovered global spectrum, and successfully extract the 21 cm signal from it, with reasonable number of antennas and observation time.
This paper introduces the first results of observations with the Ultra-Long-Wavelength (ULW)-Low ... more This paper introduces the first results of observations with the Ultra-Long-Wavelength (ULW)-Low Frequency Interferometer and Spectrometer (LFIS) on board the selenocentric satellite Longjiang-2. We present a brief description of the satellite and focus on the LFIS payload. The in-orbit commissioning confirmed a reliable operational status of the instrumentation. We also present results of a transition observation, which offers unique measurements on several novel aspects. We estimate the RFI suppression required for such a radio astronomy instrumentation at the Moon distances from Earth to be of the order of 80 dB. We analyse a method of separating Earth-and satellite-originated radio frequency interference (RFI). It is found that the RFI level at frequencies lower than a few MHz is smaller than the receiver noise floor.
ABSTRACTLine intensity mapping (LIM) is a promising probe to study star formation, the large-scal... more ABSTRACTLine intensity mapping (LIM) is a promising probe to study star formation, the large-scale structure of the Universe, and the epoch of reionization (EoR). Since carbon monoxide (CO) is the second most abundant molecule in the Universe except for molecular hydrogen H2, it is suitable as a tracer for LIM surveys. However, just like other LIM surveys, CO intensity mapping also suffers strong foreground contamination that needs to be eliminated for extracting valuable astrophysical and cosmological information. In this work, we take 12CO($\it J$ = 1-0) emission line as an example to investigate whether deep learning method can effectively recover the signal by removing the foregrounds. The CO(1-0) intensity maps are generated by N-body simulations considering CO luminosity and halo mass relation, and we discuss two cases with median and low CO signals by comparing different relations. We add foregrounds generated from real observations, including thermal dust, spinning dust, fre...
A redshifted 21 cm line absorption signature is commonly expected from the cosmic dawn era, when ... more A redshifted 21 cm line absorption signature is commonly expected from the cosmic dawn era, when the first stars and galaxies formed. The detailed traits of this signal can provide important insight on the cosmic history. However, high-precision measurement of this signal is hampered by ionosphere refraction and absorption, as well as radio frequency interference (RFI). Space observation can solve the problem of the ionosphere, and the Moon can shield the RFI from Earth. In this paper, we present simulations of the global spectrum measurement in the 30–120 MHz frequency band on the lunar orbit from the proposed Discovering the Sky at the Longest wavelength project. In particular, we consider how the measured signal varies as the satellite moves along the orbit and take into account the blockage of different parts of the sky by the Moon and the antenna response. We estimate the sensitivity for such a 21 cm global spectrum experiment. An rms noise level of ≤0.05 K is expected at 75 MH...
The free-free absorption of low frequency radio waves by thermal electrons in the warm ionized me... more The free-free absorption of low frequency radio waves by thermal electrons in the warm ionized medium of our Galaxy becomes very significant at 10 MHz (ultralong-wavelength), and the absorption strength depends on the radio frequency. Upcoming space experiments such as the Discovering Sky at the Longest wavelength (DSL) and Farside Array for Radio Science Investigations of the Dark ages and Exoplanets (FARSIDE) will produce high-resolution multi-frequency sky maps at the ultralong-wavelength, providing a new window to observe the Universe. In this paper we propose that from these ultralong-wavelength multi-frequency maps, the threedimensional distribution of the Galactic electrons can be reconstructed. This novel and robust reconstruction of the Galactic electron distribution will be a key science case of those space missions. Ultralong-wavelength observations will be a powerful tool for studying the astrophysics relevant to the Galactic electron distribution, for example, the impacts of supernova explosions on electron distribution, and the interaction between interstellar atoms and ionizing photons escaped from the HII regions around massive stars. An animation shows the reconstructed results using NE2001 model as input test. On ArXiv, it is given in the directory: Ancillary files. In the paper the animation is linked to Fig. 5.
International Journal of Environmental Research and Public Health
Moral sensitivity helps individuals resolve moral dilemmas as a precursor to moral decision-makin... more Moral sensitivity helps individuals resolve moral dilemmas as a precursor to moral decision-making. Intensive care unit (ICU) nurses are at high risk for encountering moral dilemmas and should have the moral sensitivity to recognize moral issues. The activities of ICU nurses in moral decision-making are guided by moral sensitivity but are also based on emotional intelligence (EI). EI, be recognized as an integral part of moral sensitivity with long-standing theoretical foundations. It is necessary to explicate the true role of EI in moral sensitivity through empirical research. To measure the level of moral sensitivity of ICU nurses and determine the relationship between moral sensitivity and EI. We recruited 467 ICU nurses of ten hospitals from March to June 2021 in Hunan Province, China for a cross-sectional questionnaire survey. The moral sensitivity and EI were measured using the Moral Sensitivity Questionnaire-Revised Version into Chinese (MSQ-R-CV) and the Wong and Law Emotion...
We search for the dark flow induced diffuse kinetic Sunyaev Zel'dovich (kSZ) effect through CMBga... more We search for the dark flow induced diffuse kinetic Sunyaev Zel'dovich (kSZ) effect through CMBgalaxy cross correlation. Such angular correlation is anisotropic, with a unique cos(θ DF) angular dependence and hence can be distinguished from other components. Here, θ DF is the angle between the opposite dark flow direction and the direction of the sky where the correlation is measured. We analyze the KIAS-VAGC galaxy catalog of SDSS-DR7 and the WMAP seven-year temperature maps, applying an unbiased optimal weighting scheme to eliminate any statistically isotropic components and to enhance the dark flow detection signal. Non-zero weighted cross correlations are detected at 3.5σ for the redshift bin z < 0.1 and at 3σ for the bin 0.1 < z < 0.2, implying the existence of statistically anisotropic components in CMB. However, further analysis does not support the dark flow explanation. The observed directional dependence deviates from the ∝ cos θ DF relation expected, and hence can not be explained by the presence of a single dark flow, and if the observed cross correlation is generated by the dark flow induced kSZ effect, the velocity would be too high (6000 km/s). We report this work as the first attempt to search for dark flow through weighted CMB-galaxy cross correlation and to draw the attention on the sources of the detected anomalous CMB-galaxy cross correlation.
Monthly Notices of the Royal Astronomical Society, 2020
We investigate the dependence of the galaxy properties on cosmic web environments using the most ... more We investigate the dependence of the galaxy properties on cosmic web environments using the most up-to-date hydrodynamic simulation: Evolution and Assembly of Galaxies and their Environments (EAGLE). The baryon fractions in haloes and the amplitudes of the galaxy luminosity function decrease going from knots to filaments to sheets to voids. Interestingly, the value of L* varies dramatically in different cosmic web environments. At z = 0, we find a characteristic halo mass of $10^{12}\, {\rm h}^{-1}\rm M_{\odot }$, below which the stellar-to-halo mass ratio is higher in knots, while above which it reverses. This particular halo mass corresponds to a characteristic stellar mass of $1.8\times 10^{10} \,{\rm h}^{-1}\rm M_{\odot }$. Below the characteristic stellar mass, central galaxies have redder colours, lower sSFRs, and higher metallicities in knots than those in filaments, sheets and voids, while above this characteristic stellar mass, the cosmic web environmental dependences eithe...
Within a large class of exact solutions of the Einstein equations describing a black hole embedde... more Within a large class of exact solutions of the Einstein equations describing a black hole embedded in a Friedmann universe it is shown that, under certain assumptions, only those with comoving Hawking-Hayward quasi-local mass are generic, in the sense that they are late-time attractors.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2020
Due to ionosphere absorption and the interference of natural and artificial radio emissions, astr... more Due to ionosphere absorption and the interference of natural and artificial radio emissions, astronomical observation from the ground becomes very difficult at the wavelengths of decametre or longer, which we shall refer to as the ultralong wavelengths. This unexplored part of the electromagnetic spectrum has the potential for great discoveries, notably in the study of cosmic dark ages and dawn, but also in heliophysics and space weather, planets and exoplanets, cosmic ray and neutrinos, pulsar and interstellar medium (ISM), extragalactic radio sources, and so on. The difficulty of the ionosphere can be overcome by space observation, and the Moon can shield the radio frequency interferences (RFIs) from the Earth. A lunar orbit array can be a practical first step to opening up the ultralong wave band. Compared with a lunar surface observatory on the far side, the lunar orbit array is simpler and more economical, as it does not need to make the risky and expensive landing, can be easi...
We use galaxy groups at redshifts between 0.4 and 1.0 selected from the Great Observatories Origi... more We use galaxy groups at redshifts between 0.4 and 1.0 selected from the Great Observatories Origins Deep Survey (GOODS) to study the color-morphological properties of satellite galaxies, and investigate possible alignment between the distribution of the satellites and the orientation of their central galaxy. We confirm the bimodal color and morphological type distribution for satellite galaxies at this redshift range: the red and blue classes corresponds to the early and late morphological types respectively, and the early-type satellites are on average brighter than the late-type ones. Furthermore, there is a morphological conformity between the central and satellite galaxies: the fraction of early-type satellites in groups with an early-type central is higher than those with a late-type central galaxy. This effect is stronger at smaller separations from the central galaxy. We find a marginally significant signal of alignment between the major axis of the early-type central galaxy and its satellite system, while for the late-type centrals no significant alignment signal is found. We discuss the alignment signal in the context of shape evolution of groups.
In 21 cm intensity mapping, the spectral smoothness of the foreground is exploited to separate it... more In 21 cm intensity mapping, the spectral smoothness of the foreground is exploited to separate it from the much weaker 21 cm signal. However, the non-smooth frequency response of the instrument complicates this process. Reflections and standing waves generate modulations on the frequency response. Here we report the analysis of the standing waves in the bandpass of the signal channels of the Tianlai Cylinder Array. By Fourier transforming the bandpass into the delay time domain, we find various standing waves generated on the telescope. A standing wave with time delay at ∼142 ns is most clearly identified which is produced in the 15-meter feed cable. We also identify a strong peak at a shorter delay of τ < 50 ns, which may be a mix of the standing wave between the reflector and feed, and the standing wave on the 4 m intermediate frequency (IF) cable. We also show that a smoother frequency response could be partially recovered by removing the reflection-inducted modulations. Howev...
arXiv: Cosmology and Nongalactic Astrophysics, 2019
The Dark Ages are the period between the last scattering of the cosmic microwave background and t... more The Dark Ages are the period between the last scattering of the cosmic microwave background and the appearance of the first luminous sources, spanning approximately 1100 < z < 30. The only known way to measure fluctuations in this era is through the 21-cm line of neutral hydrogen. Such observations have enormous potential for cosmology, because they span a large volume while the fluctuations remain linear even on small scales. Observations of 21-cm fluctuations during this era can therefore constrain fundamental aspects of our Universe, including inflation and any exotic physics of dark matter. While the observational challenges to these low-frequency 21-cm observations are enormous, especially from the terrestrial environment, they represent an important goal for cosmology.
I introduce the Tianlai project, which is a 21cm intensity mapping experiment designed to probe t... more I introduce the Tianlai project, which is a 21cm intensity mapping experiment designed to probe the dark energy by measure the BAO peaks.
Uploads
Papers by Xuelei Chen