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Register a new userA test of the evolution of gas depletion factor in galaxy clusters using strong gravitational lensing systems
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In this letter, we discuss a new method to probe the redshift evolution of the gas depletion factor, i.e. the ratio by which the gas mass fraction of galaxy clusters is depleted with respect to the universal mean of baryon fraction. The dataset we use for this purpose consists of 40 gas mass fraction measurements measured at $r_{2500}$ using Chandra X-ray observations, strong gravitational lensing sub-samples obtained from SLOAN Lens ACS + BOSS Emission-line Lens Survey (BELLS) + Strong Legacy Survey SL2S + SLACS. For our analysis, the validity of cosmic distance duality relation is assumed. We find a mildly decreasing trend for the gas depletion factor as a function of redshift at about 2.7$sigma$. This is the first result in literature which does not find a constant gas depletion factor as a function of redshift using gas mass fraction measurements at $r_{2500}$.
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The gas mass fraction in galaxy clusters has been widely used to determine cosmological parameters. This method assumes that the ratio of the cluster gas mass fraction to the cosmic baryon fraction ($gamma(z)$) is constant as a function of redshift. In this work, we look for a time evolution of $gamma(z)$ at $R_{500}$ by using both the SPT-SZ and Planck Early SZ (ESZ) cluster data, in a model-independent fashion without any explicit dependence on the underlying cosmology. For this calculation, we use a non-parametric functional form for the Hubble parameter obtained from Gaussian Process regression using cosmic chronometers. We parameterize $gamma(z)$ as: $gamma(z)= gamma_0(1+gamma_1 z)$ to constrain the redshift evolution. We find contradictory results between both the samples. For SPT-SZ, $gamma (z)$ decreases as a function of redshift (at more than 5$sigma$), whereas a positive trend with redshift is found for Planck ESZ data (at more than 4$sigma$). We however find that the $gamma_1$ values for a subset of SPT-SZ and Planck ESZ clusters between the same redshift interval agree to within $1sigma$. When we allow for a dependence on the halo mass in the evolution of the gas depletion factor, the $4-5sigma$ discrepancy reduces to $2sigma$.
Discovery of strongly-lensed gravitational wave (GW) sources will unveil binary compact objects at higher redshifts and lower intrinsic luminosities than is possible without lensing. Such systems will yield unprecedented constraints on the mass distribution in galaxy clusters, measurements of the polarization of GWs, tests of General Relativity, and constraints on the Hubble parameter. Excited by these prospects, and intrigued by the presence of so-called heavy black holes in the early detections by LIGO-Virgo, we commenced a search for strongly-lensed GWs and possible electromagnetic counterparts in the latter stages of the second LIGO observing run (O2). Here, we summarise our calculation of the detection rate of strongly-lensed GWs, describe our review of BBH detections from O1, outline our observing strategy in O2, summarize our follow-up observations of GW170814, and discuss the future prospects of detection.
Although general relativity (GR) has been precisely tested at the solar system scale, precise tests at a galactic or cosmological scale are still relatively insufficient. Here, in order to test GR at the galactic scale, we use the newly compiled galaxy-scale strong gravitational lensing (SGL) sample to constrain the parameter $gamma_{PPN}$ in the parametrized post-Newtonian (PPN) formalism. We employ the Pantheon sample of type Ia supernovae observation to calibrate the distances in the SGL systems using the Gaussian Process method, which avoids the logical problem caused by assuming a cosmological model within GR to determine the distances in the SGL sample. Furthermore, we consider three typical lens models in this work to investigate the influences of the lens mass distributions on the fitting results. We find that the choice of the lens models has a significant impact on the constraints on the PPN parameter $gamma_{PPN}$. We use the Bayesian information criterion as an evaluation tool to make a comparison for the fitting results of the three lens models, and we find that the most reliable lens model gives the result of $gamma_{PPN}=1.065^{+0.064}_{-0.074}$, which is in good agreement with the prediction of $gamma_{PPN}=1$ by GR. As far as we know, our 6.4% constraint result is the best result so far among the recent works using the SGL method.
Strong gravitational lensing along with the distance sum rule method can constrain both cosmological parameters as well as density profiles of galaxies without assuming any fiducial cosmological model. To constrain galaxy parameters and cosmic curvature $(Omega_{k0})$, we use the distance ratio data from a recently compiled database of $161$ galactic scale strong lensing systems. We use databases of supernovae type-Ia (Pantheon) and Gamma Ray Bursts (GRBs) for calculating the luminosity distance. To study the model of the lens galaxy, we consider a general lens model namely, the Extended Power-Law model. Further, we take into account two different parametrisations of the mass density power-law index $(gamma)$ to study the dependence of $gamma$ on redshift. The best value of $Omega_{k0}$ suggests a closed universe, though a flat universe is accommodated at $68%$ confidence level. We find that parametrisations of $gamma$ have a negligible impact on the best fit value of the cosmic curvature parameter. Furthermore, measurement of time delay can be a promising cosmographic probe via time delay distance that includes the ratio of distances between the observer, the lens and the source. We again use the distance sum rule method with time-delay distance dataset of H0LiCOW to put constraints on the Cosmic Distance Duality Relation (CDDR) and the cosmic curvature parameter $(Omega_{k0})$. For this we consider two different redshift-dependent parametrisations of the distance duality parameter $(eta)$. The best fit value of $Omega_{k0}$ clearly indicates an open universe. However, a flat universe can be accommodated at $95%$ confidence level. Further, at $95%$ confidence level, no violation of CDDR is observed. We believe that a larger sample of strong gravitational lensing systems is needed in order to improve the constraints on the cosmic curvature and distance duality parameter.
The gas depletion factor $gamma(z)$, i.e., the average ratio of the gas mass fraction to the cosmic mean baryon fraction of galaxy clusters, plays a very important role in the cosmological application of the gas mass fraction measurements. In this paper, using the newest catalog of 182 galaxy clusters detected by the Atacama Cosmology Telescope (ACT) Polarization experiment, we investigate the possible redshift evolution of $gamma(z)$ through a new cosmology-independent method. The method is based on non-parametric reconstruction using the measurements of Hubble parameters from cosmic chronometers. Unlike hydrodynamical simulations suggesting constant depletion factor, our results reveal the trend of $gamma(z)$ decreasing with redshift. This result is supported by a parametric model fit as well as by calculations on the reduced ACTPol sample and on the alternative sample of 91 SZ clusters reported earlier in ACT compilation. Discussion of possible systematic effects leaves an open question about validity of the empirical relation $M_{tot}$-$f_{gas}$ obtained on very close clusters. These results might pave the way to explore the hot gas fraction within large radii of galaxy clusters as well as its possible evolution with redshift, which should be studied further on larger galaxy cluster samples in the upcoming X-ray/SZ cluster surveys.
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