Published Works

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A two-phase model of galaxy formation: III. The formation of globular clusters
Yangyao Chen, et al.
arXiv 2024
We develop a model of globular cluster (GC) formation within the cosmological hierarchy of structure formation. The model is rooted in the `two-phase' scenario of galaxy formation developed in Paper-I, where the fast accretion of dark matter halos at high redshift leads to the formation of self-gravitating, turbulent gas clouds that subsequently fragment into dynamically hot systems of dense sub-clouds with masses $\sim 10^6$--$10^7 M_\odot$. Here we elaborate on the formation, evolution, and fate of these sub-clouds, and show that some of the sub-clouds can be compactified via two distinctive channels into a `supernova-free' regime to form two distinct populations of GCs. The model is simple, characterized by a small number of free parameters underpinned by physical considerations, and can be efficiently implemented into cosmological N-body simulations to generate a coherent sample of halos, galaxies, and GCs. Our model can reproduce a range of observations on GCs, including the mass function, the size-mass relation, the frequency per unit host galaxy/halo mass, the bimodal metallicity distribution, and the spatial profile. Predictions for GCs are made for both the local Universe and for redshift up to $z \approx 10$, and can be tested by upcoming observations.
ELUCID VIII: Simulating the Coma Galaxy Cluster to Calibrate Model and Understand Feedback
Xiong Luo, et al.
arXiv 2024
We conducted an investigation of the Coma cluster of galaxies by running a series of constrained hydrodynamic simulations with GIZMO-SIMBA and GADGET-3, based on initial conditions reconstructed from the SDSS survey volume in the ELUCID project. We compared simulation predictions and observations for galaxies, ICM and IGM in and around the Coma cluster to constrain galaxy formation physics. Our results demonstrate that this type of constrained investigation allows us to probe in more detail the implemented physical processes, because the comparison between simulations and observations is free of cosmic variance and hence can be conducted in a "one-to-one" manner. We found that an increase in the earlier star formation rate and the supernova feedback of the original GIZMO-SIMBA model is needed to match observational data on stellar, ISM and ICM metallicity. The simulations without AGN feedback can well reproduce the observational ICM electron density, temperature, and entropy profiles, ICM substructures, and the IGM temperature-density relation, while the ones with AGN feedback usually fail. However, one requires something like AGN feedback to reproduce a sufficiently large population of quiescent galaxies, particularly in low-density regions. The constrained simulations of the Coma cluster thus provide a test bed to understand processes that drive galaxy formation and evolution.
The true number density of massive galaxies in the early Universe revealed by JWST/MIRI
Tao Wang, et al.
arXiv 2024
One of the main challenges in galaxy formation that has emerged recently is the early assembly of massive galaxies. The observed number density and the maximum stellar mass ($M_{\star}$) of massive galaxies in the early Universe appear to be higher than model predictions, which may pose a serious problem to the LCDM cosmology. A major limitation in many previous studies is the large uncertainty in estimating $M_{\star}$ due to the lack of constraints in the rest-frame near-infrared part of the spectral energy distribution, which is critical to determining $M_{\star}$ accurately. Here we use data from a large JWST/MIRI survey in the PRIMER program to carry out a systematic analysis of massive galaxies at $z \sim 3-8$, leveraging photometric constraints at rest-frame $rsim 1 \mu$m. We find a significant reduction in the number and mass densities of massive galaxies at $z > 5$ compared to earlier results that did not use the MIRI photometry. Within the standard $\Lambda$CDM cosmology, our results require a moderate increase in the baryon-to-star conversion efficiency ($\epsilon$) towards higher redshifts and higher $M_{\star}$. For the most massive galaxies at $z\sim 8$, the required $\epsilon$ is $\sim 0.3$, in comparison to $\epsilon \sim 0.14$ for typical low-redshift galaxies. Our findings are consistent with models assuming suppressed stellar feedback due to the high gas density and the associated short free-fall time expected for massive halos at high redshift.
A two-phase model of galaxy formation: II. The size-mass relation of dynamically hot galaxies
Yangyao Chen, et al.
arXiv 2023
In Paper-I we developed a two-phase model to connect dynamically hot galaxies (such as ellipticals and bulges) with the formation of self-gravitating, turbulent gas clouds (SGC) associated with the fast assembly of dark matter halos. Here we explore the implications of the model for the size-stellar mass relation of dynamically hot galaxies. Star-forming sub-clouds produced by the fragmentation of the SGC inherit its spatial structure and dynamical hotness, which produces a tight and 'homologous' relation, $r_{\rm f}\approx\, 100 r_{\rm bulge}$, between the size of a dynamically hot galaxy ($r_{\rm bulge}$) and that of its host halo assembled in the fast assembly regime ($r_{\rm f}$), independent of redshift and halo mass. This relation is preserved by the 'dry' expansion driven by dynamical heating when a galaxy becomes gas-poor due to inefficient cooling, and is frozen during the slow assembly regime when the bulge stops growing in mass and dynamical heating is no longer effective. The size-stellar mass relation is thus a simple combination of the galaxy-halo homology and the non-linear relation between stellar mass and halo mass. Using a set of halo assembly histories we demonstrate that this model can reproduce all properties in the observed size-mass relation of dynamically hot galaxies, including the flattening of the relation in the low-mass end and the upturn in the very massive end. The predicted evolution of this relation matches observational data currently available to redshift $z \approx 4$, and can be tested in the future at higher $z$. Our results indicate that the sizes of dynamically hot galaxies are produced by the dissipation and collapse of gas in dark matter halos to establish self-gravitating systems of sub-clouds in which stars form.
A two-phase model of galaxy formation: I. The growth of galaxies and supermassive black holes
Houjun Mo, et al.
arXiv 2023
We develop a model for galaxy formation and the growth of supermassive black holes (SMBHs), based on the fact that cold dark matter (CDM) halos form their gravitational potential wells through a fast phase with rapid change in the potential, and that the high universal baryon fraction makes cooled gas in halos self-gravitating and turbulent before it can form rotation-supported disks. Gas fragmentation produces sub-clouds so dense that cloud-cloud collision and drag on clouds are not significant, producing a dynamically hot system of sub-clouds that form stars and move ballistically to feed the central SMBH. Active galactic nucleus (AGN) and supernova (SN) feedback is effective only in the fast phase, and the cumulative effects are to regulate star formation and SMBH growth, as well as to reduce the amount of cold gas in halos to allow the formation of globally stable disks. Using a set of halo assembly histories, we demonstrate that the model can reproduce a number of observations, including correlations among SMBH mass, stellar mass of galaxies and halo mass, the number densities of galaxies and SMBH, as well as their evolution over the cosmic time.
An efficient and robust method to estimate halo concentration
Kai Wang, et al.
MNRAS, 2024

We propose an efficient and robust method to estimate the halo concentration based on the first moment of the density distribution, which is $R_1\equiv \int_0^{r_{\rm vir}}4\pi r^3\rho(r)dr/M_{\rm vir}/r_{\rm vir}$. We find that $R_1$ has a monotonic relation with the concentration parameter of the NFW profile, and that a cubic polynomial function can fit the relation with an error $\lesssim 3\%$. Tests on ideal NFW halos show that the conventional NFW profile fitting method and the $V_{\rm max}/V_{\rm vir}$ method produce biased halo concentration estimation by $\approx 10\%$ and $\approx 30\%$, respectively, for halos with 100 particles. In contrast, the systematic error for our $R_1$ method is smaller than $0.5\%$ even for halos containing only 100 particles. Convergence tests on realistic halos in $N$-body simulations show that the NFW profile fitting method underestimates the concentration parameter for halos with $\lesssim 300$ particles by $\gtrsim 20\%$, while the error for the $R_1$ method is $\lesssim 8\%$. We also show other applications of $R_1$, including estimating $V_{\rm max}$ and the Einasto concentration $c_{\rm e}\equiv r_{\rm vir}/r_{-2}$. The calculation of $R_1$ is efficient and robust, and we recommend including it as one of the halo properties in halo catalogs of cosmological simulations.

Characterize the assembly of dark matter halos with protohalo size histories: I. Redshift evolution, relation to descendant halos, and halo assembly bias
Kai Wang, et al.
MNRAS, 2024

We propose a novel method to quantify the assembly histories of dark matter halos with the redshift evolution of the mass-weighted spatial variance of their progenitor halos, i.e. the protohalo size history. We find that the protohalo size history for each individual halo at z~0 can be described by a double power-law function. The amplitude of the fitting function strongly correlates to the central-to-total stellar mass ratios of descendant halos. The variation of the amplitude of the protohalo size history can induce a strong halo assembly bias effect for massive halos. This effect is detectable in observation using the central-to-total stellar mass ratio as a proxy of the protohalo size. The correlation to the descendant central-to-total stellar mass ratio and the halo assembly bias effect seen in the protohalo size are much stronger than that seen in the commonly adopted half-mass formation time derived from the mass accretion history. This indicates that the information loss caused by the compression of halo merger trees to mass accretion histories can be captured by the protohalo size history. Protohalo size thus provides a useful quantity to connect protoclusters across cosmic time and to link protoclusters with their descendant clusters in observations.

Massive Dark Matter Halos at High Redshift: Implications for Observations in the JWST Era
Yangyao Chen, et al.
MNRAS, 2023
Recent observations made by the JWST have revealed a number of massive galaxies at high redshift ($z$). The presence of these galaxies appears at odds with the current $\Lambda$CDM cosmology. Here we investigate the possibility of alleviating the tension by incorporating uncertainties from three sources in counting massive galaxies at high $z$: cosmic variance, error in stellar mass estimate, and contribution by backsplash. We find that each of the sources can significantly increase the cumulative stellar mass density $\rho_*(>M_*)$ at the high-mass end, and the combination of them can boost the density by more than one order of magnitude. Assuming a star formation efficiency of $\epsilon_* \sim 0.5$, cosmic variance alone can reduce the tension to $2\sigma$ level, except the most massive galaxy at $z=8$. Including in addition a lognormal dispersion with a width of 0.3 dex in the stellar mass can bring the observed stellar mass density at $z \sim 7 - 10$ to the $2\sigma$ range of the cosmic variance. The tension is completely eliminated when gas stripped from backsplash halos is also taken into account. Our results highlight the importance of fully modeling uncertainties when interpreting observational data of rare objects. We use the constrained simulation, ELUCID, to investigate the descendants of high $z$ massive galaxies. We find that a significant portion of these galaxies end up in massive halos with mass $M_{\rm halo} > 10^{13} h^{-1}M_\odot $ at $z=0$. A large fraction of central galaxies in $M_{\rm halo} \geqslant 10^{14.5} h^{-1}M_\odot$ halos today are predicted to contain significant amounts of ancient stars formed in massive galaxies at $z\sim 8$. This prediction can be tested by studying the structure and stellar population of central galaxies in present-day massive clusters.
A Conditional Abundance Matching Method of Extending Simulated Halo Merger Trees to Resolve Low-Mass Progenitors and Sub-halos
Yangyao Chen, et al.
MNRAS, 2023
We present an algorithm to extend subhalo merger trees in a low-resolution dark-matter-only simulation by conditionally matching them to those in a high-resolution simulation. The algorithm is general and can be applied to simulation data with different resolutions using different target variables. We instantiate the algorithm by a case in which trees from ELUCID, a constrained simulation of $(500h^{-1}{\rm Mpc})^3$ volume of the local universe, are extended by matching trees from TNGDark, a simulation with much higher resolution. Our tests show that the extended trees are statistically equivalent to the high-resolution trees in the joint distribution of subhalo quantities and in important summary statistics relevant to modeling galaxy formation and evolution in halos. The extended trees preserve certain information of individual systems in the target simulation, including properties of resolved satellite subhalos, and shapes and orientations of their host halos. With the extension, subhalo merger trees in a cosmological scale simulation are extrapolated to a mass resolution comparable to that in a higher-resolution simulation carried out in a smaller volume, which can be used as the input for (sub)halo-based models of galaxy formation. The source code of the algorithm, and halo merger trees extended to a mass resolution of $\sim 2 \times 10^8 h^{-1}M_\odot$ in the entire ELUCID simulation, are available.
Environmental dependence of the mass-metallicity relation in cosmological hydrodynamical simulations
Kai Wang, et al.
ApJ, 2023
We investigate the environmental dependence of the gas-phase metallicity for galaxies at $z=0$ to $z\gtrsim 2$ and the underlying physical mechanisms driving this dependence using state-of-the-art cosmological hydrodynamical simulations. We find that, at fixed stellar mass, central galaxies in massive halos have lower gas-phase metallicity than those in low-mass halos. On the contrary, satellite galaxies residing in more massive halos are more metal-rich. The combined effect is that massive galaxies are more metal-poor in massive halos, and low-mass galaxies are more metal-rich in massive halos. By inspecting the environmental dependence of other galaxy properties, we identify that the accretion of low-metallicity gas is responsible for the environmental dependence of central galaxies at high $z$, whereas the AGN feedback processes play a crucial role at low $z$. For satellite galaxies, we find that both the suppression of gas accretion and the stripping of existing gas are responsible for their environmental dependence, with negligible effect from the AGN feedback. Finally, we show that the difference of gas-phase metallicity as a function of stellar mass between protocluster and field galaxies agrees with recent observational results, for example from the MAMMOTH-Grism survey.
Dissect two-halo galactic conformity effect for central galaxies: The dependence of star formation activities on the large-scale environment
Kai Wang, et al.
MNRAS, 2023
We investigate the two-halo galactic conformity effect for central galaxies, which is the spatial correlation of the star formation activities for central galaxies to several Mpcs, by studying the dependence of the star formation activities of central galaxies on their large-scale structure in our local Universe using the SDSS data. Here we adopt a novel environment metric using only central galaxies quantified by the distance to the n-th nearest central galaxy. This metric measures the environment within an aperture from ~1 Mpc to ≳ 10 Mpc, with a median value of ~4 Mpc. We found that two kinds of conformity effects in our local Universe. The first one is that low-mass central galaxies are more quenched in high-density regions, and we found that this effect mainly comes from low-mass centrals that are close to a more massive halo. A similar trend is also found in the IllustrisTNG simulation, which can be entirely explained by backsplash galaxies. The second conformity effect is that massive central galaxies in low-density regions are more star-forming. This population of galaxies also possesses a higher fraction of spiral morphology and lower central stellar velocity dispersion, suggesting that their low quiescent fraction is due to less-frequent major merger events experienced in the low-density regions, and as a consequence, less-massive bulges and central black holes.
Relating galaxies across different redshift to study galaxy evolution
Kai Wang, et al.
MNRAS, 2023
We propose a general framework leveraging the galaxy-halo connection to link galaxies observed at different redshift in a statistical way, and use the link to infer the redshift evolution of the galaxy population. Our tests based on hydrodynamic simulations show that our method can accurately recover the stellar mass assembly histories up to z ~ 3 for present star-forming and quiescent galaxies down to 1010 h-1 M⊙. Applying the method to observational data shows that the stellar mass evolution of the main progenitors of galaxies depends strongly on the properties of descendants, such as stellar mass, halo mass, and star formation states. Galaxies hosted by low-mass groups/haloes at the present time have since z ~ 1.8 grown their stellar mass ~2.5 times as fast as those hosted by massive clusters. This dependence on host halo mass becomes much weaker for descendant galaxies with similar star formation states. Star-forming galaxies grow about 2-4 times faster than their quiescent counterparts since z ~ 1.8. Both TNG and EAGLE simulations overpredict the progenitor stellar mass at z > 1, particularly for low-mass descendants.
Late-formed halos prefer to host quiescent central galaxies. I. Observational results
Kai Wang, et al.
MNRAS, 2023
The star formation and quenching of central galaxies are regulated by the assembly histories of their host halos. In this work, we use the central stellar mass to halo mass ratio as a proxy of halo formation time, and we devise three different models, from the physical hydrodynamical simulation to the empirical statistical model, to demonstrate its robustness. With this proxy, we inferred the dependence of the central galaxy properties on the formation time of their host halos using the SDSS main galaxy sample, where central galaxies are identified with the halo-based group finder. We found that central galaxies living in late-formed halos have higher quiescent fractions and lower spiral fractions than their early-formed counterparts by $\lesssim 8\%$. Finally, we demonstrate that the group finding algorithm has a negligible impact on our results.
Galaxy populations in groups and clusters: evidence for a characteristic stellar mass scale at $M_\ast\sim 10^{9.5}M_\odot$
Jiacheng Meng, et al.
ApJ, 2023

We use the most recent data release (DR9) of the DESI legacy imaging survey and SDSS galaxy groups to measure the conditional luminosity function (CLF) for groups with halo mass $M_{\rm h}\ge 10^{12}M_{\odot}$ and redshift $0.01\le z\le 0.08$, down to a limiting $r$-band magnitude of $M_{\rm r}=-10\sim-12$. For a given halo mass we measure the CLF for the total satellite population, as well as separately for the red and blue populations classified using the $(g-z)$ color. We have the following findings:

  • A clear faint-end upturn is seen in the CLF of red satellites, with a slope $\alpha\approx-1.8$ which is almost independent of halo mass, This faint-end upturn is not seen for blue satellites and for the total population.
  • Our stellar population synthesis modeling shows that the $(g-z)$ color provides a clean red/blue division, and that group galaxies in the red population defined by $(g-z)$ are all dominated by old stellar populations.
  • The fraction of old galaxies as a function of galaxy luminosity shows a minimum at a luminosity $M_{\rm r}\sim-18$, corresponding to a stellar mass $M_\ast\sim10^{9.5}M_\odot$. This mass scale is independent of halo mass and is comparable to the characteristic luminosity at which galaxies show a dichotomy in surface brightness and size, suggesting that the dichotomy in the old fraction and in galaxy structure may have a common origin.
  • The rising of the old fraction at the faint end for Milky Way (MW)-sized halos found here is in good agreement with the quenched fraction measured both for the MW/M31 system and from the ELVES survey.
  • We discuss the implications of our results for the formation and evolution of low-mass galaxies, and for the stellar mass functions of low-mass galaxies to be observed at high redshift.
The Breakdown Scale of HI Bias Linearity
Zhenyuan Wang, et al.
ApJ, 2021

By employing three approaches to generate the mock HI density from an N-body simulation at low z, we check the assumption that HI gas traces the matter density distribution linearly on large scales. Our main findings are:

  • The assumption of HI linearity is valid at the scale corresponding to the first BAO peak, but breaks down at $k \geq 0.1 h {\rm Mpc}^{−1}$.
  • The nonlinear effects of halo clustering and HI content modulation counteract each other at small scales, and their competition results in a model-dependent “sweet-spot” redshift near z=1 where the HI bias is scale-independent down to small scales.
  • The linear HI bias scales approximately linearly with redshift for z ≤ 3.
MAHGIC: A Model Adapter for the Halo-Galaxy Inter-Connection
Yangyao Chen, et al.
MNRAS, 2021

We develop an empirical model pipeline, MAHGIC, to populate dark matter halos with galaxies. The main features of the model and our main results include:

  • PCA and GBDT learners are used to transform halo properties to galaxy properties.
  • Two sets of hydrodynamic simulations, TNG and EAGLE, are used to train the model, which is then applied to other DMO simulations.
  • The model can reproduce a variety of statistical properties of galaxies. It is verified reliable, flexible and accurate.
How to empirically model star formation in dark matter halos: I. Inferences about central galaxies from numerical simulations
Yangyao Chen, et al.
MNRAS, 2021

Our study provides a framework of using hydrodynamic simulations to discover, and to motivate the use of, key ingredients to model galaxy formation using halo properties. Our findings include:

  • The SFH of central galaxies are tightly related to halo MAH.
  • The classification of SF and quenched populations has significant contamination.
  • We propose a multi-stage halo-based empirical model for the star formation in central galaxies, which reproduces many galaxy statistics and galaxy-halo relations including assembly bias.
Measuring galaxy abundance and clustering at high redshift from incomplete spectroscopic data: Tests on mock catalogs and application to zCOSMOS

We build mock galaxy catalogs for high-z galaxy surveys, and we propose methods to measure GLFs, GSMFs and 2PCFs at high-z Universe. Our findings include:

  • Our methods of estimating GLFs, GSMFs and 2PCFs reliably cancel the bias from target selection and sample imcompleteness.
  • Mock catalogs are constructed for zCOSMOS-bright sample and PFS galaxy evolution survey.
  • We quantify the cosmic variance using the mocks, and find the cosmic variance is reduced by a factor of 3-4 in PFS compared with zCOSMOS.
Finding proto-clusters to trace galaxy evolution: I. The finder and its performance
Kai Wang, et al.
MNRAS, 2021

We develop a method to identify proto-clusters based on dark matter halos at high redshift. Our main findings include:

  • The test with N-body simulations shows that our finder has completeness $\sim 85\%$, purity $\geq 90\%$, mass estimates uncertainty $ \leq 0.25 {\rm dex}$.
  • Our method can recover progenitor stellar mass distribution, providing an avenue to link high-z and low-z galaxies in clusters.
An Extended Halo-based Group/Cluster Finder: Application to the DESI Legacy Imaging Surveys DR8
Xiaohu Yang, et al.
ApJ, 2021

We extend the halo-based group finder to use data simultaneously with either photometric or spectroscopic redshifts. The performance is evaluated with a mock from N-body simulation. Our main results include:

  • For magnitude $z \leq 21 $ galaxies in DESI, $\geq 60\%$ members in $\sim 90\%$ halos with $M_{\rm h} \geq 10^{12.5} h^{-1}M_\odot$ can be identified. Detected groups with $M_{\rm h} \geq 10^{12} h^{-1}M_\odot$ has purity $\geq 90\%$.
  • Group mass assignment has uncertainty from 0.2 dex (high mass end) to 0.45 dex (low mass end).
  • Group with 10 members has redshift accuracy $\sim 0.08$.
  • A group catalog is provided for DR8.
Relating the Structure of Dark Matter Halos to Their Assembly and Environment
Yangyao Chen, et al.
ApJ, 2020

We use a large N-body simulation to study the relation of structural properties of dark matter halos to their assembly history and environment. Our main conclusions are:

  • The complexity of individual halo assembly histories can be well described by a small number of principal components, which are preferred over formation times for several reasons.
  • 60%, 10%, 20% of the variances in halo concentration, axis ratio and spin, respectively, can be explained by combining four dominating predictors $\rm PC_{MAH,1}$, $M_{\rm halo}$, $\alpha_\mathcal{T}$, $b$. Degeneracies between predictors are found and analyzed, and are still hold for mass-binned samples.
  • Tidal field provides important environmental information, with $\alpha_\mathcal{T}$ shows strongest assembly bias signal.
Identifying galaxy groups at high redshift from incomplete spectroscopic data - I. The group finder and application to zCOSMOS
Kai Wang, et al.
MNRAS, 2020

High-z spectroscopic surveys, usually incomplete in redshift sampling, present both opportunities and challenges to identifying groups in the high-z Universe. We develop a group finder that is based on incomplete redshift samples combined with photometric data. Our main findings are:

  • Mock test shows that $\geq 90\%$ of groups with $M_{\rm h}\geq 10^{12} h^{-1}{\rm M}_\odot$ are successfully identified.
  • The standard deviation in the halo mass estimation is smaller than 0.25 dex at all masses.
  • We apply our group finder to zCOSMOS-bright and describe basic properties of the group catalog obtained.
ELUCID. VI. Cosmic Variance of the Galaxy Distribution in the Local Universe
Yangyao Chen, et al.
ApJ, 2019

We propose a method based on conditional stellar mass functions to estimate global GSMF. Our findings include:

  • We extend the halo merger trees from N-body simulation to a higher resolution.
  • We use constrained N-body simuation and empirical approach to construct a 'real' mock catalog, which recovers the galaxy distribution in the local Universe (SDSS volume).
  • The low-mass end GSMF estimated from SDSS sample can be significantly affected by the Cosmic Variance (CV).
  • We propose a new method based on CGSMF, provide unbiased estimate of GSMF which show significant upture below $M_* \leq 10^{9.5} h^{-1}{\rm M}_\odot$ and is missed in many earlier works.

Part-Time Works

Major contributor in the development of the following websites:

  • LIG: the data server of the galaxy and cosmology group in Tsinghua DoA.
  • Tsinghua High-z Team: the homepage of the high-z team in Tsinghua DoA.
  • ELUCID-project: the data server of the ELUCID project.

Developer of the following softwares:

  • HIPP: a modern C++ toolkit for HPC.
  • PyHIPP: a modern Python toolkit for HPC.
  • TwoPhaseGalaxyModel: A two-phase model of galaxy formation.
  • HaloFactory: Semi-analytical dark matter halo generators.
  • HaloProps: a calculator/predictor for halo structural properties. Python API and web application are available.
  • AstroHammer: lectures on astronomical techniques for beginners.

Academic Activity

Academic Meetings

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