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nature, 29 february 2024, vol 626, issue 8001
Nature, February 29, 2024, Vol. 626, No. 8001
Astronomy
most of the photons that reionized the universe came from dwarf galaxies
Most of the photons that reionize the universe come from dwarf galaxies
Authors: Hakim Atek, Ivo Labbé, Lukas J furtak, iryna chemerynska, seiji fujimoto, d**id j. setton, et al.
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Summary:
Cosmic reionization is the major phase transition from neutral hydrogen to ionized plasma about 600 800 myr after the universe is large, but there has been controversy about determining what drives cosmic reionization. Some models suggest that the quasar's high ionizing emissivity and escape fraction (FeSC) support its role in driving cosmic reionization.
Others have proposed that high fesc values in bright galaxies generate enough ionizing radiation to drive this process. Finally, some studies have shown that the number density of faint galaxies can effectively control cosmic reionization when combined with the ionization efficiency and the stellar mass-dependent model of FESC. To date, however, low-mass galaxies have not been fully spectroscopically studied due to their extremely weak nature.
The research group reported an analysis of 8 ultra-faint galaxies (in a very small field) during the reionization period, with absolute magnitudes between MUV 17 and 15 (as low as 0.).005l⋆）。It was found that during the first billion years of the universe, faint galaxies produced log[ ion (hz erg 1)] = 2580±0.14 ionized photons, which is 4 times higher than the value usually assumed.
If this field represents a large-scale distribution of faint galaxies, then the rate of ionization of photons exceeds the rate required for reionization, even at a FeSC of the order of 5%.
▲ abstract：
the identification of sources driving cosmic reionization, a major phase transition from neutral hydrogen to ionized plasma around 600–800 myr after the big bang, has been a matter of debate. some models suggest that high ionizing emissivity and escape fractions (fesc) from quasars support their role in driving cosmic reionization. others propose that the high fesc values from bright galaxies generate sufficient ionizing radiation to drive this process. finally, a few studies suggest that the number density of faint galaxies, when combined with a stellar-mass-dependent model of ionizing efficiency and fesc, can effectively dominate cosmic reionization. however, so far, comprehensive spectroscopic studies of low-mass galaxies h**e not been done because of their extreme faintness. here we report an analysis of eight ultra-faint galaxies (in a very small field) during the epoch of reionization with absolute magnitudes between muv ≈ 17 mag and −15 mag (down to 0.005l⋆).we find that faint galaxies during the first thousand million years of the universe produce ionizing photons with log[ξion (hz erg−1)]= 25.80±0.14, a factor of 4 higher than commonly assumed values. if this field is representative of the large-scale distribution of faint galaxies, the rate of ionizing photons exceeds that needed for reionization, even for escape fractions of the order of 5%.
Materials Science
high fatigue resistance in a titanium alloy via near-void-free 3d printing
The high fatigue resistance of titanium alloys is achieved through a 3D printing process that is virtually porous
Authors: Zhan Qu, Zhenjun Zhang, Rui Liu, Ling Xu, Yining Zhang, Xiaotao Li, et al
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Summary:
The advantages of 3D printing, i.e., additive manufacturing of structural materials, are severely compromised by their poor fatigue properties. In general, poor fatigue properties appear to be caused by porosity during the current printing process. Therefore, the research group questioned whether the elimination of these porosity could provide a viable solution to significantly improve the fatigue resistance of non-porous AM (NET-AM) alloys.
By understanding the asynchronicity of phase transitions and grain growth, the research group further developed the NET-AM process technology and successfully reconstructed the almost porosity-free AM microstructure in Ti-6Al-4V titanium alloy. They determined the fatigue resistance of this AM microstructure and showed that its high fatigue limit of about 1 GPA surpasses the fatigue resistance of all AM and wrought titanium alloys, as well as other metallic materials.
The results confirm the high fatigue resistance of the NET-AM microstructure and the potential advantages of the AM process in the production of structural components with maximum fatigue strength, which is conducive to the further application of AM technology in the field of engineering.
▲ abstract：
the advantage of 3d printing—that is, additive manufacturing (am) of structural materials—has been severely compromised by their disappointing fatigue properties. commonly, poor fatigue properties appear to result from the presence of microvoids induced by current printing process procedures. accordingly, the question that we pose is whether the elimination of such microvoids can provide a feasible solution for marked enhancement of the fatigue resistance of void-free am (net-am) alloys. here we successfully rebuild an approximate void-free am microstructure in ti-6al-4v titanium alloy by development of a net-am processing technique through an understanding of the asynchronism of phase transformation and grain growth. we identify the fatigue resistance of such am microstructures and show that they lead to a high fatigue limit of around 1 gpa, exceeding the fatigue resistance of all am and forged titanium alloys as well as that of other metallic materials. we confirm the high fatigue resistance of net-am microstructures and the potential advantages of am processing in the production of structural components with maximum fatigue strength, which is beneficial for further application of am technologies in engineering fields.
site-specific reactivity of stepped pt surfaces driven by stress release
Site-specific reactivity of step-type PT surfaces driven by stress relief
Authors: Guangdong Liu, Arthur J shih, huiqiu deng, kasinath ojha, xiaoting chen, mingchuan luo, et al.
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Summary:
Heterogeneous catalysts are widely used to facilitate chemical reactions. While chemical reactions are known to typically occur at the catalyst surface, only specific surface sites have high catalytic activity. Therefore, identifying the active sites and maximizing their effectiveness is at the heart of catalysis research, where the classical model is to classify the active sites according to different surface configurations, such as plateaus and steps.
However, this simple classification often leads to orders of magnitude errors in catalyst activity and qualitative uncertainty in the active site, limiting the opportunities for catalyst design.
Taking the stepped PT(111) surface and electrochemical oxygen reduction (ORR) reaction as an example, the research group proved that the root cause of the large error and uncertainty is the simplification of the classification and the neglect of the atomic site-specific reactivity driven by the release of surface stress. Specifically, the surface stress relief of the individual steps introduces an uneven strain field with a compression ratio of up to 55%, resulting in different electronic structures and reactivity of plateau atoms with the same local coordination, and resulting in enhanced atomic site-specific ORR activity.
For the plateau atoms on either side of the step edge, the activity enhancement is 50 times higher than that of the atoms in the middle of the plateau, which enables the control of ORR reactivity by changing the plateau width or controlling the external stresses. Therefore, the discovery of the above synergistic effects provides a new perspective for the basic understanding of catalytic active atomic sites and the design principles of heterogeneous catalysts.
▲ abstract：
heterogeneous catalysts are widely used to promote chemical reactions. although it is known that chemical reactions usually happen on catalyst surfaces, only specific surface sites h**e high catalytic activity. thus, identifying active sites and maximizing their presence lies at the heart of catalysis research, in which the classic model is to categorize active sites in terms of distinct surface motifs, such as terraces and steps. however, such a **categorization often leads to orders of magnitude errors in catalyst activity predictions and qualitative uncertainties of active sites, thus limiting opportunities for catalyst design. here, using stepped pt(111) surfaces and the electrochemical oxygen reduction reaction (orr) as examples, we demonstrate that the root cause of larger errors and uncertainties is a simplified categorization that overlooks atomic site-specific reactivity driven by surface stress release. specifically, surface stress release at steps introduces inhomogeneous strain fields, with up to 5.5% compression, leading to distinct electronic structures and reactivity for terrace atoms with identical local coordination, and resulting in atomic site-specific enhancement of orr activity. for the terrace atoms flanking both sides of the step edge, the enhancement is up to 50 times higher than that of the atoms in the middle of the terrace, which permits control of orr reactivity by either varying terrace widths or controlling external stress. thus, the discovery of the above synergy provides a new perspective for both fundamental understanding of catalytically active atomic sites and design principles of heterogeneous catalysts.
supramolecular polymers form tactoids through liquid–liquid phase separation
Supramolecular polymers are separated by liquid-liquid phase to form crystalline clusters
Authors: Hailin Fu, Jingyi Huang, Joost J b. van der tol, lu su, yuyang wang, swayandipta dey, et al.
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Summary:
Liquid-liquid phase separation (LLPS) of biopolymers has recently been shown to play a central role in the formation of membrane-free organelles with multiple biological functions. The interaction between LLPS and macromolecule coagulation is part of ongoing research. Synthetic supramolecular polymers are non-covalent crowds of macromolecules, but the occurrence of LLPS has rarely been reported.
The research group showed that the continuously growing fibrils obtained by supramolecular polymerization of the synthetic components are responsible for phase separation into highly anisotropic aqueous droplets (metamorphic clusters) through an entropy-driven pathway. The crowded environment regulated by dextroside concentration not only affects the supramolecular polymerization kinetics, but also affects the properties of LLPS, including phase separation kinetics, morphology, internal ordering, fluidity, and mechanical properties of the final crystal-like clusters.
In addition, the substrate-liquid and liquid-liquid interfaces have been shown to accelerate the LLPS of supramolecular polymers, resulting in a myriad of three-dimensional ordered structures, including micron-long clusters arrays with highly ordered surfaces. Several supramolecular polymers demonstrate the ubiquity and many possibilities of supramolecular polymerization to control emerging forms, opening up a new field of matter from highly structured aqueous solutions that stabilize LLPs to nanoscale soft matter.
▲ abstract：
liquid–liquid phase separation (llps) of biopolymers has recently been shown to play a central role in the formation of membraneless organelles with a multitude of biological functions. the interplay between llps and macromolecular condensation is part of continuing studies. synthetic supramolecular polymers are the non-covalent equivalent of macromolecules but they are not reported to undergo llps yet. here we show that continuously growing fibrils, obtained from supramolecular polymerizations of synthetic components, are responsible for phase separation into highly anisotropic aqueous liquid droplets (tactoids) by means of an entropy-driven pathway. the crowding environment, regulated by dextran concentration, affects not only the kinetics of supramolecular polymerizations but also the properties of llps, including phase-separation kinetics, morphology, internal order, fluidity and mechanical properties of the final tactoids. in addition, substrate–liquid and liquid–liquid interfaces proved capable of accelerating llps of supramolecular polymers, allowing the generation of a myriad of three-dimensional-ordered structures, including highly ordered arrays of micrometre-long tactoids at surfaces. the generality and many possibilities of supramolecular polymerizations to control emerging morphologies are demonstrated with several supramolecular polymers, opening up a new field of matter ranging from highly structured aqueous solutions by means of stabilized llps to nanoscopic soft matter.
Chemistry
identifying general reaction conditions by bandit optimization
The bandit optimization method was used to determine the general reaction conditions
Author: Jason YWang, Jason M stevens, st**ros k. kariofillis, mai-jan tom, dung l. golden, jun li, et al.
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Summary:
There is an urgent need for reaction conditions that can be universally applied to a wide range of substrates, especially in the pharmaceutical and chemical industries. While there are many methods that can be used to assess the general applicability of the conditions developed, there are few general methods that effectively identify these conditions during optimization.
The research group reported on the design, implementation, and application of the reinforcement learning bandit optimization model, which identified the universally applicable conditions through effective condition sampling and evaluation of experimental feedback. The performance benchmark statistics of the existing dataset show that the accuracy of the model in identifying common conditions is high, and it is 31% higher than the baseline that mimics the state-of-the-art optimization method.
The research group conducted experimental studies on palladium-catalyzed imidazole c-h arylation, anilamide coupling and phenol alkylation to evaluate the application and function of the bandit optimization model in practice. After investigating less than 15% of the reaction spaces designed by experts, the study identified the reaction conditions that are most universally applicable in the above three cases but have not been fully studied.
▲ abstract：
reaction conditions that are generally applicable to a wide variety of substrates are highly desired, especially in the pharmaceutical and chemical industries. although many approaches are **ailable to evaluate the general applicability of developed conditions, a universal approach to efficiently discover these conditions during optimizations is rare. here we report the design, implementation and application of reinforcement learning bandit optimization models to identify generally applicable conditions by efficient condition sampling and evaluation of experimental feedback. performance benchmarking on existing datasets statistically showed high accuracies for identifying general conditions, with up to 31% improvement over baselines that mimic state-of-the-art optimization approaches. a palladium-catalysed imidazole c–h arylation reaction, an aniline amide coupling reaction and a phenol alkylation reaction were investigated experimentally to evaluate use cases and functionalities of the bandit optimization model in practice. in all three cases, the reaction conditions that were most generally applicable yet not well studied for the respective reaction were identified after surveying less than 15% of the expert-designed reaction space.
Sociology
online images amplify gender bias
Web images amplify gender bias
Authors: Douglas Guilbeault, Solène Delecourt, Tasker Hull, Bharg** Srinivasa Desikan, Mark Chu & Ethan Nadler
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Summary:
Every year, people spend less and less time reading and more and more time on the proliferation of images on the web. Millions of people interact every day from platforms such as Google and Wikipedia, and millions more interact through social networks such as Instagram and TikTok, mainly in exchange for visual content. At the same time, news organizations and digital advertisers are increasingly engaging online with images that are processed faster, more subtle, and easier to remember than text.
The research team showed that the rise of online images has greatly exacerbated gender bias, both in terms of statistical universality and psychological impact. They looked at gender associations in 3,495 social categories (such as "**" or "banker") from more than 1 million ** from Google, Wikipedia, and the Internet Movie Database (IMDB), as well as billions of words from these platforms.
It was found that gender bias was consistently more prevalent in images than in texts in social categories of both sexes. At the same time, documented networks of women are underrepresented, a deficiency that is much more acute in images than in text, public**, and U.S. Census data. Finally, a nationally representative pre-registration experiment showed that searching for occupation** rather than textual descriptions on Google amplified gender bias in participants' beliefs.
Addressing the social impact of this massive shift to visual communication is critical to developing an equitable and inclusive Internet future.
▲ abstract：
each year, people spend less time reading and more time viewing images, which are proliferating online. images from platforms such as google and wikipedia are downloaded by millions every day, and millions more are interacting through social media, such as instagram and tiktok, that primarily consist of exchanging visual content. in parallel, news agencies and digital advertisers are increasingly capturing attention online through the use of images, which people process more quickly, implicitly and memorably than text. here we show that the rise of images online significantly exacerbates gender bias, both in its statistical prevalence and its psychological impact. we examine the gender associations of 3,495 social categories (such as ‘nurse’ or ‘banker’) in more than one million images from google, wikipedia and internet movie database (imdb), and in billions of words from these platforms. we find that gender bias is consistently more prevalent in images than text for both female- and male-typed categories. we also show that the documented underrepresentation of women online is substantially worse in images than in text, public opinion and us census data. finally, we conducted a nationally representative, preregistered experiment that shows that googling for images rather than textual descriptions of occupations amplifies gender bias in participants’ beliefs. addressing the societal effect of this large-scale shift towards visual communication will be essential for developing a fair and inclusive future for the internet.