Abstract
We present a spectroscopic survey of Ly
α
emitters (LAEs) at
z
∼ 3.7 and
z
∼ 4.8. The LAEs are selected using the narrowband technique based on the combination of deep narrowband and ...broadband imaging data in two deep fields, and then spectroscopically confirmed with the MMT multifiber spectrograph Hectospec. The sample consists of 71 LAEs at
z
∼ 3.7 and 69 LAEs at
z
∼ 4.8 over ∼1.5 deg
2
, making it one of the largest spectroscopically confirmed samples of LAEs at the two redshifts. Their Ly
α
luminosities are measured using the secure redshifts and deep photometric data, and span a range of ∼10
42.5
–10
43.6
erg s
−1
, so these LAEs represent the most luminous galaxies at the redshifts in terms of Ly
α
luminosity. We estimate and correct sample incompletenesses and derive reliable Ly
α
luminosity function (LFs) at
z
∼ 3.7 and 4.8 based on the two spectroscopic samples. We find that our Ly
α
LFs are roughly consistent (within a factor of 2−3) with previous measurements at similar redshifts that were derived from either photometric samples or spectroscopic samples. By comparing with previous studies in different redshifts, we find that the Ly
α
LFs decrease mildly from
z
∼ 3.1 to
z
∼ 5.7, supporting the previous claim of the slow LF evolution between
z
∼ 2 and
z
∼ 6. At
z
> 5.7, the LF declines rapidly toward higher redshift, partly due to the effect of cosmic reionization.
We present the HST WFC3/F275W UV imaging observations of A2218-Flanking, a lensed compact dwarf galaxy at redshift . The stellar mass of A2218-Flanking is and SFR is yr−1 after correcting the ...magnification. This galaxy has a young galaxy age of 127 Myr and a compact galaxy size of . The HST UV imaging observations cover the rest-frame Lyman continuum (LyC) emission (∼800 ) from A2218-Flanking. We firmly detect ( ) the LyC emission in A2218-Flanking in the F275W image. Together with the HST F606W images, we find that the absolute escape fraction of LyC is based on the flux density ratio between 1700 and 800 ( ). The morphology of the LyC emission in the F275W images is extended and follows the morphology of the UV continuum morphology in the F606W images, suggesting that the f800 is not from foreground contaminants. We find that the region with a high star formation rate surface density has a lower (higher ) ratio than the diffused regions, suggesting that LyC photons are more likely to escape from the region with the intensive star-forming process. We compare the properties of galaxies with and without LyC detections and find that LyC photons are easier to escape in low-mass galaxies.
We present results from a spectroscopic survey of z ∼ 5 quasars in the CFHT Legacy Survey. Using both optical color selection and a likelihood method, we select 97 candidates over an area of 105 deg2 ...to a limit of iAB < 23.2, and 7 candidates in the range 23.2 < iAB < 23.7 over an area of 18.5 deg2. Spectroscopic observations for 43 candidates were obtained with Gemini, MMT, and Large Binocular Telescope, of which 37 are z > 4 quasars. This sample extends measurements of the quasar luminosity function ∼1.5 mag fainter than our previous work in Sloan Digital Sky Survey Stripe 82. The resulting luminosity function is in good agreement with our previous results, and suggests that the faint end slope is not steep. We perform a detailed examination of our survey completeness, particularly the impact of the Ly emission assumed in our quasar spectral models, and find hints that the observed Ly emission from faint z ∼ 5 quasars is weaker than for z ∼ 3 quasars at a similar luminosity. Our results strongly disfavor a significant contribution of faint quasars to the hydrogen-ionizing background at z = 5.
The commercial LiFePO4 cathode has attracted great attention for large-scale energy storage because of its high safety, low cost, low toxicity and excellent thermal stability. Unfortunately, the poor ...electronic conductivity of LiFePO4 leads to bad lithium storage property. In this work, the nitrogen-doped graphene-decorated LiFePO4 was prepared via a facile hydrothermal method followed by a conventional solid-state route. Benefiting from the advantages of nitrogen-doped graphene, the conductivity of LiFePO4 is greatly improved. Therefore, the obtained composite shows excellent electrochemical performances, including high reversible capacity (163.1 mAh g−1 at 0.1 C), good rate capability (113.7 mAh g−1 at 10 C) and stable cycle-life (111.6 mAh g−1 after 150 cycles at 10 C). These results reveal that the designed nitrogen-doped graphene-decorated LiFePO4 composite is a promising cathode material for rechargeable lithium-ion batteries.
•LiFePO4 nanocrystals embedded in nitrogen-doped graphene sheets have been successfully fabricated through a simple sol-gel approach followed by the solid-state reaction.•The conductive nitrogen-doped graphene sheets can greatly enhance the apparent electronic conductivity of LiFePO4 material.•The as-prepared LiFePO4-NG electrode exhibits outstanding high-rate capability and long cycle-life for lithium-ion batteries.
•Different temperature histories were set to determine the chloride diffusion coefficient of HVFA concrete.•Variations of chloride diffusion coefficient with temperature histories and age were ...revealed.•Relationships between chloride diffusion coefficient with Maturity were obtained.
This paper presents the influence of temperature history on chloride diffusion of high volume fly ash (HVFA) concrete. Four different curing temperature histories, referenced as SDC (standard curing), SMC (steam curing), OC (outdoor curing) and TMC (massive concrete temperature matched curing), were set. Concretes with different replacement levels of fly ash were cast and cured under the four temperature histories, respectively. The variations of compressive strength, porosity and chloride diffusion coefficient with different ages were determined by experiment. SEM and XRD were used to analyze the microstructure and compositions of hydrated pastes. The results show that the performances of fly ash concrete were influenced by temperature histories. And the influence was more obvious with the addition of replacement level of fly ash. For ordinary concrete, higher accumulated temperature at early age got higher compressive strength, better pore structure and lower chloride diffusion coefficient, but had negative effects at later age compared to SDC. For HVFA concrete, it was beneficial for the development of performances. Although the chloride diffusion coefficient was higher than ordinary concrete at early age, it would become small even lower at later age. And higher accumulated temperature promoted this process. In addition, by utilizing the concept of maturity, good power function relations between chloride diffusion coefficient of fly ash concrete and Maturity were obtained at early age.
The fabrication of Mg–Al layered double hydroxides (LDHs) intercalated with NO
3
−
, NO
2
−
and
p
-aminobenzoate and comparative investigation on their chloride absorption in aqueous solutions have ...been performed in this paper. The Mg–Al-LDHs intercalated with NO
2
−
and
p
-aminobenzoate were synthesized by anion exchange in the host materials, Mg–Al–NO
3
LDH, which was prepared by a coprecipitation method. Also, the equilibrium isotherms of chloride adsorption on the as-obtained LDHs were measured. Besides, their morphologies and microstructures were observed using scanning electron microscopy, X-ray diffraction instrument, infrared spectroscopy and thermal analysis. It has been found that the as-obtained LDHs are well crystallized and have a very large distribution of size. The basal spacing of hexagonal plates is dependent on the types of intercalated anions. Langmuir isotherm is more suitable to fit the experimental data of chloride adsorption than Freundlich isotherm. Compared to Mg–Al–NO
3
LDH, the maximum adsorption capacity of chloride is slightly reduced for Mg–Al–NO
2
LDH, but remarkably decreased for the LDH intercalated with
p
-aminobenzoate anions.
The durability of the concrete in underground and marine engineering is affected by the underground and ocean environment. Chloride diffusion coefficient under hydrostatic pressure is a key parameter ...of concrete durability design under corresponding conditions. Therefore, this paper studies the diffusion behavior of chloride in different diffusion source solutions by experiment and simulation. Based on the experimental results, this paper proposes a new chloride diffusion model under the coupling effect of diffusion and convection. The interaction of ions and compounds in the diffusion source solutions, concrete pore fluid, and concrete material are considered in the new chloride diffusion model. The experimental results show that chloride diffusion rate is significantly affected by hydrostatic pressure, which increases with the increase of hydrostatic pressure. The chloride diffusion coefficient shows a certain difference in difference diffusion source solutions. The chloride diffusion coefficient in divalent cationic diffusion source solutions is the largest, the chloride diffusion coefficient in the divalent and monovalent cationic compound ones is in the middle, and the chloride diffusion coefficient in the monovalent cationic ones is the smallest. There is a linear relationship between the chloride diffusion coefficient and the hydrostatic pressure whether in single or combined cationic diffusion source solutions.
Abstract
Distant quasars are unique tracers to study the formation of the earliest supermassive black holes (SMBHs) and the history of cosmic reionization. Despite extensive efforts, only two quasars ...have been found at
z
≥ 7.5, due to a combination of their low spatial density and the high contamination rate in quasar selection. We report the discovery of a luminous quasar at
z
= 7.642, J0313−1806, the most distant quasar yet known. This quasar has a bolometric luminosity of 3.6 × 10
13
L
⊙
. Deep spectroscopic observations reveal a SMBH with a mass of (1.6 ± 0.4) × 10
9
M
⊙
in this quasar. The existence of such a massive SMBH just ∼670 million years after the big bang challenges significantly theoretical models of SMBH growth. In addition, the quasar spectrum exhibits strong broad absorption line (BAL) features in C
iv
and Si
iv
, with a maximum velocity close to 20% of the speed of light. The relativistic BAL features, combined with a strongly blueshifted C
iv
emission line, indicate that there is a strong active galactic nucleus (AGN)-driven outflow in this system. Atacama Large Millimeter/submillimeter Array observations detect the dust continuum and C
ii
emission from the quasar host galaxy, yielding an accurate redshift of 7.6423 ± 0.0013 and suggesting that the quasar is hosted by an intensely star-forming galaxy, with a star formation rate of ∼200
M
⊙
yr
−1
and a dust mass of ∼7 × 10
7
M
⊙
. Follow-up observations of this reionization-era BAL quasar will provide a powerful probe of the effects of AGN feedback on the growth of the earliest massive galaxies.
Abstract We present a study of seven spectroscopically confirmed (Ly α -emitting) galaxies at redshift z ≃ 6 using the JWST/NIRCam imaging data. These galaxies, with a wide range of Ly α ...luminosities, were recently observed in a series of NIRCam broad and medium bands. We constrain the rest-frame UV/optical continua and measure the H α line emission of the galaxies using the combination of JWST/NIRCam and archival HST/WFC3 infrared photometry. We further estimate the escape fractions of their Ly α photons ( f esc Ly α ) and the production efficiency of ionizing photons ( ξ ion ). Among the sample, six out of seven galaxies have Ly α escape fractions of ≲10%, which might be the status for most of the star-forming galaxies at z ≃ 6. One UV-faint Ly α galaxy with an extremely blue UV slope has a large value of f esc Ly α reaching ≃50%. These galaxies have a broad range of ξ ion over log 10 ξ ion,0 (Hz erg −1 ) ∼ 25.0–26.5. We find that UV-fainter galaxies with bluer UV-continuum slopes likely have higher escape fractions of Ly α photons. We also find that galaxies with higher Ly α line emission tend to produce ionizing photons more efficiently. The most Ly α -luminous galaxy in the sample has a very high ξ ion,0 of log 10 ξ ion,0 (Hz erg −1 ) > 26. Our results support the scenario that Ly α galaxies may have served as an important contributor to cosmic reionization. Blue and bright Ly α galaxies are excellent targets for JWST follow-up spectroscopic observations.
The rare case of changing-look (CL) active galactic nuclei (AGNs), with the appearance or disappearance of broad Balmer emission lines within a few years, challenges our understanding of the AGN ...unified model. We present a sample of 21 new CL AGNs at 0.08 < z < 0.58 , which doubles the number of such objects known to date. These new CL AGNs were discovered in various ways, from (1) repeat spectra in the SDSS, (2) repeat spectra in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and SDSS, and (3) photometric variability and new spectroscopic observations. We use the photometric data from surveys, including the SDSS imaging survey, the Pan-STARRS1, the DESI Legacy imaging survey, the Wide-field Infrared Survey Explorer (WISE), the Catalina Real-time Transient Survey, and the Palomar Transient Factory. The estimated upper limits of the transition timescale of the CL AGNs in this sample spans from 0.9 to 13 years in the rest-frame. The continuum flux in the optical and mid-infrared becomes brighter when the CL AGNs turn on, or vice versa. Variations of more than 0.2 mag in the W1 band were detected in 15 CL AGNs during the transition. The optical and mid-infrared variability is not consistent with the scenario of variable obscuration in 10 CL AGNs at more than the 3 confidence level. We confirm a bluer-when-brighter trend in the optical. However, the mid-infrared WISE colors W1-W2 become redder when the objects become brighter in the W1 band, possibly due to a stronger hot dust contribution in the W2 band when the AGN activity becomes stronger. The physical mechanism of type transition is important for understanding the evolution of AGNs.
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