ABSTRACT
The integrated Sachs–Wolfe (ISW) effect is caused by the decay of cosmological gravitational potential and is therefore a unique probe of dark energy. However, its robust detection is still ...problematic. Various tensions between different data sets, different large-scale structure (LSS) tracers, and between data and the ΛCDM theory prediction exist. We propose a novel method of ISW measurement by cross-correlating cosmic microwave background (CMB) and the LSS traced by ‘low-density position’ (LDP). It isolates the ISW effect generated by low-density regions of the universe but insensitive to selection effects associated with voids. We apply it to the DR8 galaxy catalogue of the DESI Legacy imaging surveys and obtain the LDPs at z ≤ 0.6 over ∼20 000 deg2 sky coverage. We then cross-correlate with the Planck temperature map and detect the ISW effect at 3.2σ. We further compare the measurement with numerical simulations of the concordance ΛCDM cosmology and find the ISW amplitude parameter AISW = 1.14 ± 0.38 when we adopt an LDP definition radius $R_\mathrm{ s}=3^{^{\prime }}$, fully consistent with the prediction of the standard ΛCDM cosmology (AISW = 1). This agreement with ΛCDM cosmology holds for all the galaxy samples and Rs that we have investigated. Furthermore, the S/N is comparable to that of galaxy ISW measurement. These results demonstrate the LDP method as a competitive alternative to existing ISW measurement methods and provide independent checks to existing tensions.
Abstract
We employ a set of high resolution
N
-body simulations to study the merger rate of dark matter halos. We define a specific merger rate by normalizing the average number of mergers per halo ...with the logarithmic mass growth change of the hosts at the time of accretion. Based on the simulation results, we find that this specific merger rate,
dN
merge
(
ξ
∣
M
,
z
)
/
d
ξ
/
d
log
M
(
z
)
, has a universal form, which is only a function of the mass ratio of merging halo pairs,
ξ
, and does not depend on the host halo mass,
M
, or redshift,
z
, over a wide range of masses (10
12
≲
M
≲ 10
14
M
⊙
h
−1
) and merger ratios (
ξ
≥ 1
e
− 2). We further test with simulations of different Ω
m
and
σ
8
, and get the same specific merger rate. The universality of the specific merger rate shows that halos in the universe are built up self-similarly, with a universal composition in the mass contributions and an absolute merger rate that grows in proportion to the halo mass growth. As a result, the absolute merger rate relates with redshift and cosmology only through the halo mass variable, whose evolution can be readily obtained from the universal mass accretion history (MAH) model of Zhao et al. Last, we show that this universal specific merger rate immediately predicts an universal unevolved subhalo mass function that is independent on the redshift, MAH or the final halo mass, and vice versa.
Abstract
We extend the halo-based group finder developed by Yang et al. (2005c) to use data
simultaneously
with either photometric or spectroscopic redshifts. A mock galaxy redshift survey ...constructed from a high-resolution
N
-body simulation is used to evaluate the performance of this extended group finder. For galaxies with magnitude z ≤ 21 and redshift 0 <
z
≤ 1.0 in the DESI legacy imaging surveys (the Legacy Surveys), our group finder successfully identifies more than 60% of the members in about 90% of halos with mass ≳10
12.5
h
−1
M
⊙
. Detected groups with mass ≳10
12.0
h
−1
M
⊙
have a purity (the fraction of true groups) greater than 90%. The halo mass assigned to each group has an uncertainty of about 0.2 dex at the high-mass end ≳10
13.5
h
−1
M
⊙
and 0.45 dex at the low-mass end. Groups with more than 10 members have a redshift accuracy of ∼0.008. We apply this group finder to the Legacy Surveys DR8 and find 6.4 million groups with at least three members. About 500,000 of these groups have at least 10 members. The resulting catalog containing 3D coordinates, richness, halo masses, and total group luminosities is made publicly available.
Abstract
The apparent shape of galaxy clustering depends on the adopted cosmology used to convert observed redshift to comoving distance, the
r
(
z
) relation, as it changes the line elements along ...and across the line of sight differently. The Alcock–Paczyński (AP) test exploits this property to constrain the expansion history of the universe. We present an extensive review of past studies on the AP test. We adopt an extended AP test method introduced by Park et al., which uses the full shape of redshift-space two-point correlation function (CF) as the standard shape, and apply it to the Sloan Digital Sky Survey DR7, BOSS, and eBOSS LRG samples covering the redshift range up to
z
= 0.8. We calibrate the test against the nonlinear cosmology-dependent systematic evolution of the CF shape using Multiverse simulations. We focus on examining whether or not the flat Lambda cold dark matter (ΛCDM)
concordance
model is consistent with observation. We constrain the flat
w
CDM model to have
w
=
−
0.892
−
0.050
+
0.045
and
Ω
m
=
0.282
−
0.023
+
0.024
from our AP test alone, which is significantly tighter than the constraints from the BAO or SNe Ia methods by a factor of 3–6. When the AP test result is combined with the recent BAO and SNe Ia results, we obtain
w
=
−
0.903
−
0.023
+
0.023
and
Ω
m
=
0.285
−
0.009
+
0.014
. This puts a strong tension with the flat ΛCDM model with
w
= −1 at the 4.2
σ
level. Consistency with
w
= −1 is obtained only when the Planck cosmic microwave background (CMB) observation is combined. It remains to be seen whether this tension between observations of galaxy distribution at low redshifts and CMB anisotropy at the decoupling epoch becomes greater in future studies and leads us to a new paradigm of cosmology.
Abstract
In certain cases of astronomical data analysis, the meaningful physical quantity to extract is the ratio
R
between two data sets. Examples include the lensing ratio, the interloper rate in ...spectroscopic redshift samples, and the decay rate of gravitational potential and
E
G
to test gravity. However, simply taking the ratio of the two data sets is biased, since it renders (even statistical) errors in the denominator into systematic errors in
R
. Furthermore, it is not optimal in minimizing statistical errors of
R
. Based on Bayesian analysis and the usual assumption of Gaussian error in the data, we derive an analytical expression of the posterior probability density function
P
(
R
). This result enables fast and unbiased
R
measurement, with minimal statistical errors. Furthermore, it relies on no underlying model other than the proportionality relation between the two data sets. Even more generally, it applies to cases where the proportionality relation holds for the underlying physics/statistics instead of the two data sets directly. It also applies to the case of multiple ratios (
R
→
R
= (
R
1
,
R
2
, ⋯ )). We take the lensing ratio as an example to demonstrate our method. We take lenses as DESI imaging survey galaxies, and sources as DECaLS cosmic shear and Planck cosmic microwave background (CMB) lensing. We restrict the analysis to the ratio between CMB lensing and cosmic shear. The resulting
P
(
R
) values, for multiple lens–shear pairs, are all nearly Gaussian. The signal-to-noise ratio of measured
R
ranges from 4.9 to 8.4. We perform several tests to verify the robustness of the above result.
There has been great interest in developing and designing economical, stable and highly active electrocatalysts for the hydrogen evolution reaction (HER) via water splitting in an aqueous solution at ...different pH values. Transition-metal dichalcogenides (TMDCs), e.g., MoS2, are identified to be promising catalysts for the HER due to the limited active sites at their edges, while the large basal plane of MoS2 is inert and shows poor performance in electrocatalytic hydrogen production. We theoretically propose orbital modulation to improve the HER performance of the basal plane of MoS2 through non-metal P doping. The substitutional doping of P provides empty 3pz orbitals, perpendicular to the basal plane, can enhance the hydrogen adsorption for acid HER and can promote water dissociation for alkaline HER, which creates significant active sites and enhances the electronic conductivity as well. In addition, 3P-doped MoS2 exhibits excellent HER catalytic activity with ideal free energy at acid media and low reaction-barrier energy in alkaline media. Thus, the doping of P could significantly boost the HER activity of MoS2 in such conditions. Our study suggests an effective strategy to tune HER catalytic activity of MoS2 through orbital engineering, which should also be feasible for other TMDC-based electrocatalysts.
Newly developed Ti–10Mo–6Zr–4Sn–3Nb has fascinating mechanical properties to be used as a biomedical material. However, there is still a lack of investigation focusing on the corrosion behavior of ...Ti–10Mo–6Zr–4Sn–3Nb. In this work, the microstructure and corrosion behavior of as-cast Ti–10Mo–6Zr–4Sn–3Nb was investigated by optical microscopy, X-ray diffraction, and electrochemical measurements. Hank’s solution was used as the electrolyte. A classical as-cast Ti–6Al–4V was used as reference. The results showed that Ti–10Mo–6Zr–4Sn–3Nb has a higher corrosion potential and a lower corrosion current density compared with Ti–6Al–4V, indicating better corrosion resistance. However, after applying anodic potentials, Ti–10Mo–6Zr–4Sn–3Nb shows larger passivation current density in both potentiodynamic polarization and potentiostatic polarization tests. This is because more alloying elements contained in Ti–10Mo–6Zr–4Sn–3Nb trigger the production of a larger number of oxygen vacancies, resulting in a higher flux of oxygen vacancy. This finding illustrates that the passive film on Ti–10Mo–6Zr–4Sn–3Nb is less protective compared with that on Ti–6Al–4V when applying an anodic potential in their passivation range.
In this work, we present a novel centroiding method based on Fourier space Phase Fitting (FPF) for Point Spread Function (PSF) reconstruction. We generate two sets of simulations to test our method. ...The first set is generated by GalSim with an elliptical Moffat profile and strong anisotropy that shifts the center of the PSF. The second set of simulations is drawn from CFHT i band stellar imaging data. We find non-negligible anisotropy from CFHT stellar images, which leads to ∼0.08 scatter in units of pixels using a polynomial fitting method (Vakili & Hogg). When we apply the FPF method to estimate the centroid in real space, the scatter reduces to ∼0.04 in S/N = 200 CFHT-like sample. In low signal-to-noise ratio (S/N; 50 and 100) CFHT-like samples, the background noise dominates the shifting of the centroid; therefore, the scatter estimated from different methods is similar. We compare polynomial fitting and FPF using GalSim simulation with optical anisotropy. We find that in all S/N (50, 100, and 200) samples, FPF performs better than polynomial fitting by a factor of ∼3. In general, we suggest that in real observations there exists anisotropy that shifts the centroid, and thus, the FPF method provides a better way to accurately locate it.
The amorphous alloys of Zr57Al10Cu15.4Ni12.6Nb5 and Zr55Cu30Ni5Al10 were melted and cast under the gaseous mixture of hydrogen and argon. The effects of hydrogen on glass plasticity of the two bulk ...metallic glasses were studied. Surprisingly, it was found that hydrogen addition can increase the glass plasticity significantly. The large plasticity of glass is attributed to the large amount of free volume induced by hydrogen addition, which could make this melt hydrogenation method to be a promising new way to improve plasticity of amorphous glass.
► A simple and efficient method to synthesize BMGs with hydrogen was presented. ► Hydrogen addition can increase the glass plasticity significantly. ► The large plasticity of glass is attributed to the large amount of free volume.
Abstract
The hidden order of atomic packing in amorphous structures and how this may provide the origin of plastic events have long been a goal in the understanding of plastic deformation in metallic ...glasses. To pursue this issue, we employ here molecular dynamic simulations to create three-dimensional models for a few metallic glasses where, based on the geometrical frustration of the coordination polyhedra, we classify the atoms in the amorphous structure into six distinct species, where “gradient atomic packing structure” exists. The local structure in the amorphous state can display a gradual transition from loose stacking to dense stacking of atoms, followed by a gradient evolution of atomic performance. As such, the amorphous alloy specifically comprises three discernible regions: solid-like, transition, and liquid-like regions, each one possessing different types of atoms. We also demonstrate that the liquid-like atoms correlate most strongly with fertile sites for shear transformation, the transition atoms take second place, whereas the solid-like atoms contribute the least because of their lowest correlation level with the liquid-like atoms. Unlike the “geometrically unfavored motifs” model which fails to consider the role of medium-range order, our model gives a definite structure for the so-called “soft spots”, that is, a combination of liquid-like atoms and their neighbors, in favor of quantifying and comparing their number between different metallic glasses, which can provide a rational explanation for the unique mechanical behavior of metallic glasses.
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