Topic

# True mass

About: True mass is a research topic. Over the lifetime, 155 publications have been published within this topic receiving 5965 citations.

##### Papers published on a yearly basis

##### Papers

More filters

••

University of Padua

^{1}, INAF^{2}, University of Bologna^{3}, University of Rome Tor Vergata^{4}, Harvard University^{5}, University of Trieste^{6}, Max Planck Society^{7}, University of Turin^{8}TL;DR: In this article, the authors examined the systematics affecting the X-ray mass estimators applied to a set of five galaxy clusters resolved at high resolution in hydrodynamic simulations, including cooling, star formation and feedback processes.

Abstract: We examine the systematics affecting the X-ray mass estimators applied to a set of five galaxy clusters resolved at high resolution in hydrodynamic simulations, including cooling, star formation and feedback processes. These simulated objects are processed through the X-ray Map Simulator, X-MAS, to provide Chandra-like long exposures that are analysed to reconstruct the gas temperature, density and mass profiles used as input. These clusters have different dynamic state: we consider a hot cluster with temperature T = 11.4 keV, a perturbed cluster with T = 3.9 keV, a merging object with T = 3.6 keV, and two relaxed systems with T = 3.3 keV and T = 2.7 keV, respectively. These systems are located at z = 0.175 so that their emission fits within the Chandra ACIS-S3 chip between 0.6 and 1.2 R-500. We find that the mass profile obtained via a direct application of the hydrostatic equilibrium ( HE) equation is dependent upon the measured temperature profile. An irregular radial distribution of the temperature values, with associated large errors, induces a significant scatter on the reconstructed mass measurements. At R-2500, the actual mass is recovered within 1 sigma, although we notice this estimator shows high statistical errors due to high level of Chandra background. Instead, the poorness of the beta-model in describing the gas density profile makes the evaluated masses to be underestimated by similar to 40 per cent with respect to the true mass, both with an isothermal and a polytropic temperature profile. We also test ways to recover the mass by adopting an analytic mass model, such as those proposed by Nvarro, Frenk & White and Rasia, Tormen & Moscardini, and fitting the temperature profile expected from the HE equation to the observed one. We conclude that the methods of the HE equation and those of the analytic fits provide a more robust mass estimation than the ones based on the beta-model. In the present work, the main limitation for a precise mass reconstruction is to ascribe to the relatively high level of the background chosen to reproduce the Chandra one. After artificially reducing the total background by a factor of 100, we find that the estimated mass significantly underestimates the true mass profiles. This is manly due (i) to the neglected contribution of the gas bulk motions to the total energy budget and (ii) to the bias towards lower values of the X-ray temperature measurements because of the complex thermal structure of the emitting plasma.

302 citations

••

TL;DR: In this article, the authors study the efficiency and reliability of cluster mass estimators that are based on the projected phase-space distribution of galaxies in a cluster region, and analyze a data-set of 62 clusters extracted from a concordance ACDM cosmological hydrodynamical simulation.

Abstract: Aims. We study the efficiency and reliability of cluster mass estimators that are based on the projected phase-space distribution of galaxies in a cluster region. Methods. We analyse a data-set of 62 clusters extracted from a concordance ACDM cosmological hydrodynamical simulation. We consider both dark matter (DM) particles and simulated galaxies as tracers of the clusters gravitational potential. Two cluster mass estimators are considered: the virial mass estimator, corrected for the surface-pressure term, and a mass estimator (that we call M σ ) based entirely on the velocity dispersion estimate of the cluster. In order to simulate observations, galaxies (or DM particles) are first selected in cylinders of given radius (from 0.5 to 1.5h -1 Mpc) and ≃200h -1 Mpc length. Cluster members are then identified by applying a suitable interloper removal algorithm. Results. The virial mass estimator overestimates the true mass by ≃10% on average, for sample sizes of ≥60 cluster members. For similar sample sizes, M σ underestimates the true mass by ≃15%, on average. For smaller sample sizes, the bias of the virial mass estimator substantially increases, while the M σ estimator becomes essentially unbiased. The dispersion of both mass estimates increases by a factor ∼2 as the number of cluster members decreases from ∼400 to ∼20. It is possible to reduce the bias in the virial mass estimates either by removing clusters with significant evidence for subclustering or by selecting early-type galaxies, which substantially reduces the interloper contamination. Early-type galaxies cannot however be used to improve the M σ estimates since their intrinsic velocity distribution is slightly biased relative to that of the DM particles. Radially-dependent incompleteness can drastically affect the virial mass estimates, but leaves the M σ estimates almost unaffected. Other observational effects, like centering and velocity errors and different observational apertures, have little effect on the mass estimates.

271 citations

••

TL;DR: In this article, the authors used the Millennium Simulation of the concordance Lambda cold dark matter cosmogony to calibrate the bias and error distribution of Timing Argument estimators of the masses of the Local Group and of the Milky Way.

Abstract: We use the very large Millennium Simulation of the concordance Lambda cold dark matter cosmogony to calibrate the bias and error distribution of Timing Argument estimators of the masses of the Local Group and of the Milky Way. From a large number of isolated spiral spiral pairs similar to the Milky Way/Andromeda system, we find the interquartile range of the ratio of timing mass to true mass to be a factor of 1.8, while the 5 and 95 per cent points of the distribution of this ratio are separated by a factor of 5.7. Here, we define true mass as the sum of the `virial' masses, M(200), of the two dominant galaxies. For present best values of the distance and approach velocity of Andromeda, this leads to a median likelihood estimate of the true mass of the Local Group of 5.27 x 10(12) M(circle dot) or M(LG)/M(circle dot) = 12.72, with an interquartile range of [12.58, 12.83] and a 5-95 per cent range of [12.26, 13.01]. Thus, a 95 per cent lower confidence limit on the true mass of the Local Group is 1.81 x 10(12) M(circle dot). A timing estimate of the Milky Way's mass based on the large recession velocity observed for the distant satellite Leo I works equally well, although with larger systematic uncertainties. It gives an estimated virial mass for the Milky Way of 2.43 x 10(12) M(circle dot) with a 95 per cent lower confidence limit of 0.80 x 10(12) M(circle dot).

260 citations

••

TL;DR: In this paper, the authors used N-body/SPH simulation of a large sample of ∼100 galaxy clusters and investigated total mass biases by comparing the mass reconstructed adopting an observational-like approach with the true mass in the simulations.

Abstract: Context. The exploitation of clusters of galaxies as cosmological probes relies on accurate measurements of their total gravitating mass. X-ray observations provide a powerful means of probing the total mass distribution in galaxy clusters, but might be affected by observational biases and rely on simplistic assumptions originating from our limited understanding of the intracluster medium physics. Aims. This paper is aimed at elucidating the reliability of X-ray total mass estimates in clusters of galaxies by properly disentangling various biases of both observational and physical origin. Methods. We use N-body/SPH simulation of a large sample of ∼100 galaxy clusters and investigate total mass biases by comparing the mass reconstructed adopting an observational-like approach with the true mass in the simulations. X-ray surface brightness and temperature profiles extracted from the simulations are fitted with different models and adopting different radial fitting ranges in order to investigate modeling and extrapolation biases. Different theoretical definitions of gas temperature are used to investigate the effect of spectroscopic temperatures and a power ratio analysis of the surface brightness maps allows us to assess the dependence of the mass bias on cluster dynamical state. Moreover, we perform a study on the reliability of hydrostatic and hydrodynamical equilibrium mass estimates using the full three-dimensional information in the simulation. Results. A model with a low degree of sophistication such as the polytropic β-model can introduce, in comparison with a more adequate model, an additional mass underestimate of the order of ∼10% at r500 and ∼15% at r200. Underestimates due to extrapolation alone are at most of the order of ∼10% on average, but can be as large as ∼50% for individual objects. Masses are on average biased lower for disturbed clusters than for relaxed ones and the scatter of the bias rapidly increases with increasingly disturbed dynamical state. The bias originating from spectroscopic temperatures alone is of the order of 10% at all radii for the whole numerical sample, but strongly depends on both dynamical state and cluster mass. From the full three dimensional information in the simulations we find that the hydrostatic equilibrium assumption yields masses underestimated by ∼10–15% and that masses computed by means of the hydrodynamical estimator are unbiased. Finally, we show that there is excellent agreement between our findings, results from similar analyses based on both Eulerian and Lagrangian simulations, and recent observational work based on the comparison between X-ray and gravitational lensing mass estimates.

241 citations

••

TL;DR: In this article, the M 500 − Y X data were used to investigate the power law relation between the X-ray temperature T X and gas mass M g, showing that Y X may be a better mass proxy than T X or M g,500.

Abstract: The quantity Y X , the product of the X-ray temperature T X and gas mass M g , has recently been proposed as a robust low-scatter mass indicator for galaxy clusters. Using precise measurements from XMM-Newton data of a sample of 10 relaxed nearby clusters, spanning a Y X range of 10 13 –10 15 $M_\odot$ keV, we investigate the M 500 – Y X relation. The M 500 – Y X data exhibit a power law relation with slope α = 0.548 ± 0.027, close to the self-similar value (3/5) and independent of the mass range considered. However, the normalisation is ~20% below the prediction from numerical simulations including cooling and galaxy feedback. We discuss two effects that could contribute to the normalisation offset: an underestimate of the true mass due to the hydrostatic equilibrium assumption used in X-ray mass estimates, and an underestimate of the hot gas mass fraction in the simulations. A comparison of the functional form and scatter of the relations between various observables and the mass suggest that Y X may indeed be a better mass proxy than T X or M g,500 .

208 citations