arXiv subjects

Shulei Cao

Publications and source records attributed to Shulei Cao.

At least 19 recordsLinked to original sources

arXiv2026

Calibrating optical galaxy cluster projection effects with sparse spectroscopic samples: A clustering redshift approach

Wide-field optical imaging surveys are efficient at identifying galaxy clusters, but optically identified clusters suffer from projection effects--physically unassociated galaxies along the line of sight can be misidentified as cluster members due to distance uncertainties. Previous studies have used spectroscopic follow-up observations of cluster members to quantify projection effects; however, such follow-up efforts cannot keep pace with the rapidly growing cluster samples. On the other hand, spectroscopic surveys designed for large-scale structure studies collect tens of millions of spectra but tend to have sparse spectra in cluster regions. To bridge this gap, we develop a clustering redshift approach that cross-correlates cluster members with sparse, non-cluster-targeted spectroscopic galaxy samples. We validate this approach using the Cardinal simulation, recovering the correct spectroscopic distribution and projection effect parameters of redMaPPer cluster members. Our approach is insensitive to the selection of the spectroscopic sample and paves the way for calibrating the upcoming LSST clusters using DESI and Roman spectroscopic samples.

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arXiv2026

Unifying cluster and galaxy cosmology analyses using the galaxy-halo connection

Galaxies and galaxy clusters trace the same cosmic density field, but their statistics have been modeled separately in cosmological analyses. We present a unified, simulation-based framework to model them using the galaxy-halo connection. Our analysis includes cluster lensing, galaxy clustering, and galaxy-cluster cross-correlation. We validate our method on the FLAMINGO hydrodynamic simulation. Relative to the cluster-only approach, combining these probes improves the $\sigma_8-\Omega_m$ figure of merit by a factor of 15. Our framework enables stringent tests of cosmological models and exploits small-scale information.

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arXiv2025

Standardizing reverberation-mapped H$\beta$ active galactic nuclei using time-averaged radius$-$luminosity relations with 5100\,\AA\,, broad H$\beta$, and narrow \OIII\ luminosities

Active galactic nuclei (AGN) have been studied as alternate probes in cosmology due to their large and stable luminosities and broad redshift range. Previously it was shown that higher-redshift AGN that were reverberation-mapped (RM) using broad Mg\,\textsc{ii} and C\,\textsc{iv} lines are standardizable and yield weak cosmological constraints that are consistent with those from better-established probes. In contrast, lower-redshift AGN that were reverberation-mapped using the broad H$\beta$ line exhibited tensions with the standard cosmological model, in particular they preferred currently decelerating cosmological expansion. Here we study the standardizability of a homogeneous RM H$\beta$ sample of $\sim 100$ AGN (over redshifts $0.00308 \leq z \leq 0.8429$), whose H$\beta$ time delays and three luminosity tracers (at 5100\,\AA\,, broad H$\beta$, and narrow [O\,\textsc{iii}]) are averaged over several epochs. We find that this averaged sample is standardizable using three $R-L$ relations. While for luminosities corresponding to 5100\,\AA\, and the broad H$\beta$ line the cosmological constraints prefer currently decelerating cosmological expansion, the cosmological parameters for the narrow [O\,\textsc{iii}] luminosity are more consistent with those from better-established probes and they are in agreement with currently accelerating cosmological expansion. This demonstrates for the first time that narrow-line region [O\,\textsc{iii}] can be utilized for AGN standardization and cosmological constraints. Selecting proper photoionizing flux proxies for the broad-line region is thus crucial in studies of RM AGN standardizability.

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arXiv2025

What Galaxy Clusters Have to Say About Dynamical Dark Energy and $H_0$

We show that, in flat $\Lambda$CDM, low-redshift structure probes -- cluster abundances, 3$\times$2-point analyses, and full-shape clustering -- are mutually consistent, jointly delivering precise constraints on $\sigma_8$ and $\Omega_{\rm m}$ that agree with geometrical datasets (CMB+BAO+SN). In $w_0w_a$CDM, adding clusters to the geometry dataset reduces the evidence for evolving dark energy while relaxing the $H_0$ tension, suggesting a $\Lambda$CDM evolution of the late-time Universe and a sound horizon that differs from its standard value.

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arXiv2025

Cosmological Constraints from Dark Energy Survey Year 1 Cluster Lensing and Abundances with Simulation-based Forward-Modeling

We present a simulation-based forward-modeling framework for cosmological inference from optical galaxy-cluster samples, and apply it to the abundance and weak-lensing signals of DES-Y1 redMaPPer clusters. The model embeds cosmology-dependent optical selection using a counts-in-cylinders approach, while also accounting for cluster miscentering and baryonic feedback in lensing. Applied to DES-Y1, and assuming a flat $\Lambda$CDM cosmology, we obtain $\Omega_m=0.254^{+0.026}_{-0.020}$ and $\sigma_8=0.826^{+0.030}_{-0.034}$, consistent with a broad suite of low-redshift structure measurements, including recent full-shape analyses, the DES/KiDS/HSC 3$\times$2 results, and most cluster-abundance studies. Our results are also consistent with \textit{Planck}, with the difference being significant at $2.58\sigma$. These results establish simulation-based forward-modeling of cluster abundances as a promising new tool for precision cosmology with Stage~IV survey data.

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arXiv2025

Impact of projection-induced optical selection bias on the weak lensing mass calibration of galaxy clusters

Weak gravitational lensing signals of optically identified clusters are impacted by a selection bias -- halo triaxiality and large-scale structure along the line of sight simultaneously boost the lensing signal and richness (the inferred number of galaxies associated with a cluster). As a result, a cluster sample selected by richness has a mean lensing signal higher than expected from its mean mass, and the inferred mass will be biased high. This selection bias is currently limiting the accuracy of cosmological parameters derived from optical clusters. In this paper, we quantify the bias in mass calibration due to this selection bias. Using two simulations, MiniUchuu and Cardinal, with different galaxy models and cluster finders, we find that the selection bias leads to an overestimation of lensing mass at the 20-50% level, with a larger bias (20-80%) for large-scale lensing (>3 Mpc). Even with a moderate projection model, this selection bias significantly outweighs other currently known cluster lensing systematics. This work confirms the need to account for this bias in future optical cluster cosmology analyses, and we discuss strategies for mitigating this bias.

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arXiv2025

Association between optically identified galaxy clusters and the underlying dark matter halos

Clusters of galaxies trace massive dark matter halos in the Universe, but they can include multiple halos projected along lines of sight. As a case study, we quantify the properties of halos contributing to clusters identified by the redMaPPer algorithm using the Cardinal simulation, which mimics the Dark Energy Survey data. For each cluster, we identify the halos hosting its member galaxies, and we define the main halo as the one contributing the most to the cluster's richness ($\lambda$, the estimated number of member galaxies). At $z=0.3$, for clusters with $\lambda > 60$, the main halo typically contributes to $92\%$ of the richness, and this fraction drops to $67\%$ for $\lambda \approx 20$. Defining "clean" clusters as those with $\geq50\%$ of the richness contributed by the main halo, we find that $100\%$ of the $\lambda > 60$ clusters are clean, while $73\%$ of the $\lambda \approx 20$ clusters are clean. Three halos can usually account for more than $80\%$ of the richness of a cluster. The main halos associated with redMaPPer clusters have a completeness ranging from $98\%$ at virial mass $10^{14.6}~h^{-1}M_{\odot}$ to $64\%$ at $10^{14}~h^{-1}M_{\odot}$. In addition, we compare the inferred cluster centers with true halo centers, finding that $30\%$ of the clusters are miscentered with a mean offset $40\%$ of the cluster radii, in agreement with recent X-ray studies. These systematics worsen as redshift increases, but we expect that upcoming surveys extending to longer wavelengths will improve the cluster finding at high redshifts. Our results affirm the robustness of the redMaPPer algorithm and provide a framework for benchmarking other cluster-finding strategies.

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arXiv2025

Standardizing a larger, higher-quality, homogeneous sample of reverberation-mapped H$\beta$ active galactic nuclei using the broad-line region radius-luminosity relation

We present a high-quality, homogeneous sample of 157 H$\beta$ reverberation-mapped active galactic nuclei (RM AGNs) spanning redshifts $0.00308 \leq z \leq 0.8429$, which is approximately 3.8 times larger than the previously available high-quality homogeneous sample. Using the broad-line region radius$-$luminosity relation ($R-L$), which involves the broad H$\beta$ line time delay and the monochromatic luminosity at 5100\,\AA\,, we show that the sample is standardizable by using six spatially flat and nonflat cosmological models. The inferred cosmological model parameters are consistent within 2$\sigma$ uncertainties with those from better established baryon acoustic oscillation and Hubble parameter measurements, with the exception of two nonflat models that are ruled out by other data. The $R-L$ relation slope is found to be flatter ($\gamma=0.428 \pm 0.025$ in the flat $\Lambda$CDM model) than the slope expected from a simple photoionization model as well as the slope found previously for the smaller homogeneous sample. In addition, we find a mild dependence of H$\beta$ $R-L$ relation parameters as well as its intrinsic scatter on the Eddington ratio by comparing the $R-L$ relations for low- and high-accreting equal-sized subsamples. A future analysis of a larger homogeneous sample containing a broader range of luminosities and Eddington ratios is necessary to confirm the standardizability of H$\beta$ AGNs.

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arXiv2025

Testing the consistency of new Amati-correlated gamma-ray burst dataset cosmological constraints with those from better-established cosmological data

Gamma-ray bursts (GRBs) are promising cosmological probes for exploring the Universe at intermediate redshifts ($z$). We analyze 151 Fermi-observed long GRBs (datasets A123 and A28) to simultaneously constrain the Amati correlation and cosmological parameters within six spatially flat and nonflat dark energy models. We find that these datasets are standardizable via a single Amati correlation, suggesting their potential for cosmological analyses. However, constraints on the current value of the nonrelativistic matter density parameter from A123 and the combined A123 + A28 data exhibit $>2\sigma$ tension with those derived from a joint analysis of better-established Hubble parameter [$H(z)$] and baryon acoustic oscillation (BAO) data for most considered cosmological models. This tension indicates that these GRB data are unsuitable for jointly constraining cosmological parameters with better-established $H(z)$ + BAO and similar data. Although the A28 data constraints are consistent with the $H(z)$ + BAO data constraints, its limited sample size (28 GRBs) and high intrinsic scatter ($\sim0.7$) diminishes its statistical power compared to existing datasets.

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arXiv2024

Standardizing reverberation-mapped H$\alpha$ and H$\beta$ active galactic nuclei using radius--luminosity relations involving monochromatic and broad H$\alpha$ luminosities

We test the standardizability of a homogeneous sample of 41 lower-redshift ($0.00415\leq z \leq 0.474$) active galactic nuclei (AGNs) reverberation-mapped (RM) using the broad H$\alpha$ and H$\beta$ emission lines. We find that these sources can be standardized using four radius$-$luminosity ($R-L$) relations incorporating H$\alpha$ and H$\beta$ time delays and monochromatic and broad H$\alpha$ luminosities. Although the $R-L$ relation parameters are well constrained and independent of the six cosmological models considered, the resulting cosmological constraints are weak. The measured $R-L$ relations exhibit slightly steeper slopes than predicted by a simple photoionization model and steeper than those from previous higher-redshift H$\beta$ analyses based on larger datasets. These differences likely reflect the absence of high-accreting sources in our smaller, lower-redshift sample, which primarily comprises lower-accreting AGNs. The inferred cosmological parameters are consistent within 2$\sigma$ (or better) with those from better-established cosmological probes. This contrasts with our earlier findings using a larger, heterogeneous sample of 118 H$\beta$ AGNs, which yielded cosmological constraints differing by $\gtrsim 2\sigma$ from better-established cosmological probes. Our analysis demonstrates that sample homogeneity$-$specifically, the use of a consistent time-lag determination method$-$is crucial for developing RM AGNs as a cosmological probe.

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arXiv2024

Optical galaxy cluster mock catalogs with realistic projection effects: Validations with the SDSS clusters

Galaxy clusters identified in optical imaging surveys suffer from projection effects: Physically unassociated galaxies along a cluster's line of sight can be counted as its members and boost the observed richness (the number of cluster members). To model the impact of projection on cluster cosmology analyses, we apply a halo occupation distribution model to N-body simulations to simulate the red galaxies contributing to cluster members, and we use the number of galaxies in a cylinder along the line of sight (counts in cylinders) to model the impact of projection on cluster richness. We compare three projection models: uniform, quadratic, and Gaussian, and we convert between them by matching their effective cylinder volumes. We validate our mock catalogs using SDSS redMaPPer clusters' data vectors, including counts vs. richness, stacked lensing signal, spectroscopic redshift distribution of member galaxies, and richness remeasured on a redshift grid. We find the former two are insensitive to the projection model, while the latter two favor a quadratic projection model with a width of approximately 180 Mpc/h (equivalent to the volume of a uniform model with a width of 100 Mpc/h and a Gaussian model with a width of 110 Mpc/h, or a Gaussian redshift error of 0.04). Our framework provides an efficient and flexible way to model optical cluster data vectors, paving the way for a simulation-based joint analysis for clusters, galaxies, and shear.

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arXiv2024

Testing the standardizability of, and deriving cosmological constraints from, a new Amati-correlated gamma-ray burst data compilation

By using gamma-ray burst (GRB) data to simultaneously constrain Amati correlation parameters and cosmological parameters in six spatially-flat and nonflat dark energy cosmological models, we show that an updated 220 GRB version of the Jia et al. [Mon. Not. R. Astron. Soc. 516, 2575 (2022)] GRB data compilation are standardizable through the Amati correlation and so can be used for cosmological analyses. However, the resulting GRB data constraints on the current value of the nonrelativistic matter density parameter, $\Omega_{m0}$, are in $>2\sigma$ tension with those from a joint analysis of better-established Hubble parameter [$H(z)$] and baryon acoustic oscillation (BAO) data for most of the cosmological models we consider, indicating that these GRB data cannot be jointly used with better-established $H(z)$ + BAO data to constrain cosmological parameters.

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arXiv2023

Low- and high-redshift H II starburst galaxies obey different luminosity-velocity dispersion relations

To determine whether or not H II starburst galaxies (H IIG) are standardizable candles, we study the correlation between the H$\beta$ luminosity ($L$) and the velocity dispersion ($\sigma$) of the ionized gas from H IIG measurements by simultaneously constraining the $L-\sigma$ relation parameters and the cosmological model parameters. We investigate six flat and nonflat relativistic dark energy cosmological models. We find that low-redshift and high-redshift H IIG data subsets are standardizable but obey different $L-\sigma$ relations. Current H IIG data are too sparse and too non-uniformly distributed in redshift to allow for a determination of why the samples follow different relations, but it could be caused by the high-redshift sample containing relatively fewer intrinsically dimmer sources (Malmquist bias) or it could be a consequence of H IIG evolution. Until this issue is better understood, H IIG data cosmological constraints must be treated with caution.

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arXiv2023

Effects of heterogeneous data sets and time-lag measurement techniques on cosmological parameter constraints from MgII and CIV reverberation-mapped quasar data

Previously, we demonstrated that MgII and CIV reverberation-mapped quasars (RM QSOs) are standardizable and that the cosmological parameters inferred using the broad-line region radius-luminosity (R-L) relation are consistent with those determined from better-established cosmological probes. With more data expected from ongoing and future spectroscopic and photometric surveys, it is imperative to examine how new QSO data sets of varied quality, with their own specific luminosity and time-delay distributions, can be best used to determine more restrictive cosmological parameter constraints. In this study, we test the effect of adding 25 OzDES MgII RM QSOs as well as 25 lower-quality SDSS RM CIV QSOs, which increases the previous sample of RM QSOs by $\sim 36\%$. Although cosmological parameter constraints become tighter for some cosmological models after adding these new QSOs, the new combined data sets have increased differences between R-L parameter values obtained in different cosmological models and thus a lower standardizability for the larger MgII + CIV compilation. Different time-delay methodologies, particularly the ICCF and CREAM methods used for inferring time delays of SDSS RM QSOs, slightly affect cosmological and R-L relation parameter values, however, the effect is negligible for (smaller) compilations of robust time-delay detections. Our analysis indicates that increasing the sample size is not sufficient for tightening cosmological constraints and a quality cut is necessary to obtain a standardizable RM QSO sample.

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arXiv2023

$H_0=69.8\pm1.3$ $\rm{km \ s^{-1} \ Mpc^{-1}}$, $Ω_{m0}=0.288\pm0.017$, and other constraints from lower-redshift, non-CMB, expansion-rate data

We use updated Type Ia Pantheon+ supernova, baryon acoustic oscillation, and Hubble parameter (now also accounting for correlations) data, as well as new reverberation-measured C IV quasar data, and quasar angular size, H II starburst galaxy, reverberation-measured Mg II quasar, and Amati-correlated gamma-ray burst data to constrain cosmological parameters. We show that these data sets result in mutually consistent constraints and jointly use them to constrain cosmological parameters in six different spatially-flat and non-flat cosmological models. Our analysis provides summary model-independent determinations of two key cosmological parameters: the Hubble constant, $H_0=69.8\pm1.3$ $\rm{km \ s^{-1} \ Mpc^{-1}}$, and the current non-relativistic matter density parameter, $Ω_{m0}=0.288\pm0.017$. Our summary error bars are 2.4 and 2.3 times those obtained using the flat $Λ$CDM model and Planck TT,TE,EE+lowE+lensing cosmic microwave background (CMB) anisotropy data. Our $H_0$ value is very consistent with that from the local expansion rate based on the Tip of the Red Giant Branch data, is 2$σ$ lower than that from the local expansion rate based on Type Ia supernova and Cepheid data, and is 2$σ$ higher than that in the flat $Λ$CDM model based on Planck TT,TE,EE+lowE+lensing CMB data. Our data compilation shows at most mild evidence for non-flat spatial hypersurfaces, but more significant evidence for dark energy dynamics, 2$σ$ or larger in the spatially-flat dynamical dark energy models we study.

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arXiv2023

Cosmological constraints from standardized non-CMB observations

The current expansion of the Universe has been observed to be accelerating, and the widely accepted spatially-flat concordance model of general relativistic cosmology attributes this phenomenon to a constant dark energy, a cosmological constant, which is measured to comprise about 70% of the total energy budget of the current Universe. However, observational discrepancies and theoretical puzzles have raised questions about this model, suggesting that alternative cosmological models with non-zero spatial curvature and/or dark energy dynamics might provide better explanations. To explore these possibilities, we have conducted a series of studies using standardized, lower-redshift observations to constrain six different cosmological models with varying degrees of flatness and dark energy dynamics. Through comparing these observations with theoretical predictions, we aim to deepen our understanding of the evolution of the Universe and shed new light on its mysteries. Our data provide consistent cosmological constraints across all six models, with some suggesting the possibility of mild dark energy dynamics and slight spatial curvature. However, these joint constraints do not rule out the possibility of dark energy being a cosmological constant and the spatial hypersurfaces being flat. Overall, our findings contribute to the ongoing efforts to refine our understanding of the Universe and its properties, and suggest that multiple cosmological models remain viable.

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arXiv2023

Accretion disks, quasars and cosmology: meandering towards understanding

As Setti & Woltjer noted back in 1973, quasars could be used to construct the Hubble diagram but the actual application was not that straightforward. It took years to implement the idea successfully. Most of the ways to use quasars for cosmology now require an advanced understanding of their structure, step by step. We briefly review this progress, with unavoidable personal bias, and concentrate on bright unobscured sources. We will mention the problem of the gas flow character close to the innermost stable circular orbit close to the black hole, discussed 50 years ago, which later led to the development of the slim disk scenario, but was recently revived in the context of Magnetically Arrested Disks (MAD) and Standard and Normal Evolution (SANE) disk models. We also discuss the hot/warm corona issue, which is still under discussion and complicates the analysis of the X-ray reflection. We present the scenario of the formation of the low ionization part of the Broad Line Region as a mostly failed wind powered by radiation pressure acting on dust (FRADO - Failed Radiatively Driven Dusty Outflow model). Next, we discuss the cosmological constraints that are currently achievable with quasars, mostly concentrating on light echo methods (continuum time delays and spectral line time delays with respect to the continuum) which are (or should be) incorporating the progress mentioned above. Finally, we briefly mention future prospects in this direction.

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arXiv2022

Potentialities of Hubble parameter and expansion rate function data to alleviate Hubble tension

Taking advantage of Gaussian process (GP), we obtain an improved estimate of the Hubble constant, $H_0=70.41\pm1.58$ km s$^{-1}$ Mpc$^{-1}$, using Hubble parameter [$H(z)$] from cosmic chronometers (CCH) and expansion rate function [$E(z)$], extracted from type Ia supernovae, data. We also use CCH data, including the ones with full covariance matrix, and $E(z)$ data to obtain a determination of $H_0=72.34_{-1.92}^{+1.90}$ km s$^{-1}$ Mpc$^{-1}$, which implies that the involvement of full covariance matrix results in higher values and uncertainties of $H_0$. These results are higher than those obtained by directly reconstructing CCH data with GP. In order to estimate the potential of future CCH data, we simulate two sets of $H(z)$ data and use them to constrain $H_0$ by either using GP reconstruction or fitting them with $E(z)$ data. We find that simulated $H(z)$ data alleviate $H_0$ tension by pushing $H_0$ values higher towards $\sim70$ km s$^{-1}$ Mpc$^{-1}$. We also find that joint $H(z)$ + $E(z)$ data favor higher values of $H_0$, which is also confirmed by constraining $H_0$ in the flat concordance model and 2-order Taylor expansion of $H(z)$. In summary, we conclude that more and better-quality CCH data as well as $E(z)$ data can provide a new and useful perspective on resolving $H_0$ tension.

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