JoXSZ: Joint X-SZ fitting code for galaxy clusters

The thermal Sunyaev-Zeldovich (SZ) effect and the X-ray emission offer separate and highly complementary probes of the thermodynamics of the intracluster medium. We present JoXSZ, the first publicly available code designed to jointly fit SZ and X-ray data coming from various instruments to derive the thermodynamic profiles of galaxy clusters. JoXSZ follows a fully Bayesian forward-modelling approach, accounts for the SZ calibration uncertainty and X-ray background level systematic. It improves upon most state-of-the-art, and not publicly available, analyses because it adopts the correct Poisson-Gauss expression for the joint likelihood, makes full use of the information contained in the observations, even in the case of missing values within the datasets, has a more inclusive error budget, and adopts a consistent temperature across the various parts of the code, allowing for differences between X-ray and SZ gas mass weighted temperatures when required by the user. JoXSZ accounts for beam smearing and data analysis transfer function, accounts for the temperature and metallicity dependencies of the SZ and X-ray conversion factors, adopts flexible parametrization for the thermodynamic profiles, and on user request allows either adopting or relaxing the assumption of hydrostatic equilibrium (HE). When HE holds, JoXSZ uses a physical (positive) prior on the radial derivative of the enclosed mass and derives the mass profile and overdensity radii $r_\Delta$. For these reasons, JoXSZ goes beyond simple SZ and electron density fits. We illustrate the use of JoXSZ by combining Chandra and NIKA data on the high-redshift cluster CL J1226.9+3332. The code is written in Python, it is fully documented and the users are free to customize their analysis in accordance with their needs and requirements. JoXSZ is publicly available on GitHub.Comment: 11 pages, 9 figures, accepted for publication in Astronomy and Astrophysics. Code available on GitHub at https://github.com/fcastagna/JoXSZ. v2 updated with language editin

Richness-based masses of rich and famous galaxy clusters

We present a catalog of galaxy cluster masses derived by exploiting the tight correlation between mass and richness, I.e., a properly computed number of bright cluster galaxies. The richness definition adopted in this work is properly calibrated, shows a small scatter with mass, and has a known evolution, which means that we can estimate accurate (0.16 dex) masses more precisely than by adopting any other richness estimates or X-ray or SZ-based proxies based on survey data. We measured a few hundred galaxy clusters at 0.05 14M☉. Finally, in a technical appendix we illustrate with Planck clusters how to minimize the sensitivity of comparisons between masses listed in different catalogs to the specific overlapping of the considerd subsamples, a problem recognized but not solved in the literature. Full Table 1 is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/587/A158A web front-end is available at the URL http://www.brera.mi.astro.it/~andreon/famous.htm

Two-Color Surface Photometry of Brightest Cluster Members

The Gunn g, r and i CCD images of a representative sample of 17 Brightest Cluster Galaxies (BCM) have been analyzed in order to derive surface brightness and color profiles, together with geometrical parameters like eccentricity and position angle. The sample includes both X-ray and optically selected clusters, ranging in redshift from z=0.049 to z=0.191. We find that BCMs are substantially well described by de Vaucouleurs' law out to radii of $\sim 60-80$ kpc, and that color gradients are generally absent. Only in two cases we find a surface brightness excess with respect to the $r^{1/4}$ law, which for A150 is coupled with a change in the $g-r$ color. The rest frame colors of BCMs do not show any intrinsic dispersion. By parametrizing the environment with the local galaxy number density, we find that it is correlated with the BCM extension, i.e. BCMs with larger effective radii are found in denser environments.Comment: accepted for publication in Aj, May 1997, 25 pages LaTeX format (aas style files), including tables, plus 6 figures (postscript

Simulation view of galaxy clusters with low X-ray surface brightness

X-ray selected samples are known to miss galaxy clusters that are gas poor and have a low surface brightness. This is different for the optically selected samples such as the X-ray Unbiased Selected Sample (XUCS). We characterise the origin of galaxy clusters that are gas poor and have a low surface-brightness by studying covariances between various cluster properties at fixed mass using hydrodynamic cosmological simulations. We extracted approx. 1800 galaxy clusters from a high-resolution Magneticum hydrodynamic cosmological simulation and computed covariances at fixed mass of the following properties: core-excised X-ray luminosity, gas fraction, hot gas temperature, formation redshift, concentration, galaxy richness, fossilness parameter, and stellar mass of the bright central galaxy. We also compared the correlation between concentration and gas fractions in non-radiative simulations, and we followed the trajectories of particles inside galaxy clusters to assess the role of AGN depletion on the gas fraction. In simulations and in observational data, differences in surface brightness are related to differences in gas fraction. Simulations show that the gas fraction strongly correlates with assembly time, in the sense that older clusters are gas poor. Clusters that formed earlier have lower gas fractions because the feedback of the active galactic nucleus ejected a significant amount of gas from the halo. When the X-ray luminosity is corrected for the gas fraction, it shows little or no covariance with other quantities. Older galaxy clusters tend to be gas poor and possess a low X-ray surface brightness because the feedback mechanism removes a significant fraction of gas from these objects. Moreover, we found that most of the $L_X$ covariance with the other quantities is explained by differences in the gas fraction.Comment: 9 pages, 8 figures, accepte

A Serverless Architecture for Efficient and Scalable Monte Carlo Markov Chain Computation

Computer power is a constantly increasing demand in scientific data analyses, in particular when Markov Chain Monte Carlo (MCMC) methods are involved, for example for estimating integral functions or Bayesian posterior probabilities. In this paper, we describe the benefits of a parallel computation of MCMC using a cloud-based, serverless architecture: first, the computation time can be spread over thousands of processes, hence greatly reducing the time the user should wait to have its computation completed. Second, the overhead time required for running in parallel several processes is minor and grows logarithmically with respect to the number of processes. Third, the serverless approach does not require time-consuming efforts for maintaining and updating the computing infrastructure when/if the number of walkers increases or for adapting the code to optimally use the infrastructure. The benefits are illustrated with the computation of the posterior probability distribution of a real astronomical analysis.Comment: 6 pages, 3 figures. Appeared in ICCBDC '23: Proceedings of the 2023 7th International Conference on Cloud and Big Data Computing - August 202

Do gas-poor galaxy clusters have different galaxy populations? The positive covariance of hot and cold baryons

Galaxy clusters show a variety of intra-cluster medium properties at a fixed mass, among which gas fractions, X-ray luminosity and X-ray surface brightness. In this work we investigate whether the yet-undetermined cause producing clusters of X-ray low surface brightness also affects galaxy properties, namely richness, richness concentration, width and location of the red sequence, colour, luminosity, and dominance of the brightest cluster galaxy. We use SDSS-DR12 photometry and our analysis factors out the mass dependency to derive trends at fixed cluster mass. Clusters of low surface brightness for their mass have cluster richness in spite of their group-like luminosity. Gas-poor, low X-ray surface brightness, X-ray faint clusters for their mass, display 25\% lower richness for their mass at $4.4\sigma$ level. Therefore, richness and quantities depending on gas, such as gas fraction, $M_{gas}$, and X-ray surface brightness, are covariant at fixed halo mass. In particular, we do not confirm the hint of an anti-correlation of hot and cold baryons at fixed mass put forth in literature. All the remaining optical properties show no covariance at fixed mass, within the sensitivities allowed by our data and sample size. We conclude that X-ray and optical properties are disjoint, the optical properties not showing signatures of those processes involving gas content, apart from the richness-mass scaling relation. The covariance between X-ray surface brightness and richness is useful for an effective X-ray follow-up of low surface brightness clusters because it allows us to pre-select clusters using optical data of survey quality and prevent expensive X-ray observations