Emission-weighted Temperature Research Articles

Halo scaling relations and hydrostatic mass bias in the simba simulation from realistic mock X-ray catalogues

ABSTRACT We present a new end-to-end pipeline for Mock Observations of X-ray Haloes and Analysis (moxha) for hydrodynamic simulations of massive haloes, and use it to investigate X-ray scaling relations and hydrostatic mass bias in the simba cosmological hydrodynamic simulation for haloes with M500 ∼ 1013−15M⊙. moxha ties together existing yT-based software packages and adds new functionality to provide an end-to-end pipeline for generating mock X-ray halo data from large-scale or zoom simulation boxes. We compare moxha-derived halo properties in simba to their emission-weighted counterparts, and forecast the systematic mass bias in mock Athena observations. Overall, we find inferred hydrostatic masses are biased low compared to true simba values. For simple mass-weighting, we find $b_\text{MW} = 0.15^{+0.15}_{-0.14}$ (16–84 per cent range), while emission-weighting increases this to $b_\text{LW}=0.30^{+0.19}_{-0.10}$. The larger bias versus mass-weighted values we attribute to the spectroscopic and emission-weighted temperatures being biased systematically lower than mass-weighted temperatures. The full moxha pipeline recovers the emission-weighted hydrostatic masses at R500 reasonably well, yielding $b_\text{X}=0.33^{+0.28}_{-0.34}$. moxha-derived halo X-ray scalings are in very good agreement with observed scaling relations, with the inclusion of lower mass groups significantly steepening the LX − M500, M500 − TX, and LX − TX relations. This indicates the strong effect the simba feedback model has on low-mass haloes, which strongly evacuates poor groups but still retains enough gas to reproduce observations. We find similar trends for analogous scaling relations measured at R500, as expected for halo-wide gas evacuation.

Effects of the K-value solution schemes on radiation heat transfer modelling in oxy-fuel flames using the full-spectrum correlated K-distribution method

Abstract Radiation heat transfer in oxy-fuel flames is more important than in conventional fuel-air flames. The Full-Spectrum Correlated K-distribution methods (FSCK) with the original correlated-K solution scheme (Modest and Zhang, 2002) and a newly proposed one (Cai and Modest, 2014), and the Rank Correlated Full-Spectrum K-distribution method (RC-FSK) are used in radiative calculations of oxy-fuel flames. Twelve one-dimensional flames, including fuel-air, dry and wet oxy-fuel flames with various length scales, as well as a two-dimensional dry oxy-fuel flame are studied. The results show that the reference temperature has a non-negligible impact on the accuracy of original scheme and the emission-weighted temperature leads to a good accuracy. The accuracy of the new scheme is almost unaffected by the reference temperature, except for small-scale dry oxy-fuel flames. The error of the new scheme is mainly in the low-temperature region, and its accuracy depends on the length scale of computational domain. A hybrid correlated-K scheme using either the original or the new scheme according to local temperature is proposed. It combines the advantages of two schemes in different temperature regions, and is independent of the reference temperature. In addition, the RC-FSK demonstrates almost the same accuracy as FSCK with the hybrid scheme when the Planck temperature is set equal to the reference temperature.

SOURCE IDENTIFICATION IN THE IGR J17448-3232 FIELD: DISCOVERY OF THE SCORPIUS GALAXY CLUSTER

We use a 43-ks XMM-Newton observation to investigate the nature of sources first distinguished by a follow-up Chandra observation of the field surrounding INTEGRAL source IGR J17448-3232, which includes extended emission and a bright point source previously classified as a blazar. We establish that the extended emission is a heretofore unknown massive galaxy cluster hidden behind the Galactic bulge. The emission-weighted temperature of the cluster within the field of view is 8.8 keV, with parts of the cluster reaching temperatures of up to 12 keV; no cool core is evident. At a redshift of 0.055, the cluster is somewhat under-luminous relative to the X-ray luminosity-temperature relation, which may be attributable to its dynamical state. We present a preliminary analysis of its properties in this paper. We also confirm that the bright point source is a blazar, and we propose that it is either a flat spectrum radio quasar or a low-frequency peaked BL Lac object. We find four other fainter sources in the field, which we study and tentatively identify. Only one, which we propose is a foreground Galactic X-ray binary, is hard enough to contribute to IGR J17448-3232, but it is too faint to be significant. We thus determine that IGR J17448-3232 is in fact the galaxy cluster up to $\approx$45 keV and the blazar beyond.

The effects of stellar dynamics on the X-ray emission of flat early-type galaxies

Past observational and numerical studies indicated that the hot gaseous haloes of early-type galaxies may be sensitive to the stellar kinematics. With high-resolution zeus 2D hydrosimulations, we study the hot gas evolution in flat early-type galaxies of fixed (stellar plus dark) mass distribution, but with variable amounts of azimuthal velocity dispersion and rotational support, including the possibility of a (counter) rotating inner disc. The hot gas is fed by stellar mass-losses, and heated by supernova explosions and thermalization of stellar motions. The simulations provide γth, the ratio between the heating due to the relative velocity between the stellar streaming and the interstellar medium bulk flow, and the heating attainable by complete thermalization of the stellar streaming. We find that (1) X-ray emission-weighted temperatures and luminosities match observed values and are larger in fully velocity dispersion supported systems; X-ray isophotes are boxy where rotation is significant; (2) γth ≃ 0.1–0.2 for isotropic rotators and (3) γth ≃ 1 for systems with an inner (counter) rotating disc. The lower X-ray luminosities of isotropic rotators are not explained just by their low γth but by a complicated flow structure and evolution, consequence of the angular momentum stored at large radii. Rotation is therefore important to explain the lower average X-ray emission and temperature observed in flat and more rotationally supported galaxies.

The influence of mergers on the cluster temperature function and cosmological parameters derived from it

We develop a parameter-free analytic model to include the effects of mergers into the theoretical modelling of the X-ray temperature function of galaxy clusters. We include this description into our model for the cluster population based on fluctuations of the gravitational potential, which avoids any reference to mass. Comparisons with a numerical simulation reveal that the theoretical model is in good agreement with the simulation results. We show that building the model on the dynamics of spherical rather than ellipsoidal collapse yields better results if emission-weighted temperatures are used, while ellipsoidal collapse yields good agreement between model and simulation for mass-weighted temperatures. Analysing two different samples of X-ray clusters, we quantify the influence of mergers and a conversion between different temperature definitions on the joint determination of Omega_m0 and sigma_8. If effects of mergers are included, temperature functions based on cluster masses and on the gravitational potential built on spherical collapse are in good agreement with other cosmological probes without any conversion of temperatures.

Spherically averaging ellipsoidal galaxy clusters in X-ray and Sunyaev-Zel’dovich studies - II. Biases

This is the second of two papers investigating the spherical averaging of ellipsoidal galaxy clusters in the context of X-ray and Sunyaev-Zel'dovich (SZ) observations. In the present study we quantify the orientation-average bias and scatter in observables that result from spherically averaging clusters described by ellipsoidal generalizations of the NFW profile or a nearly scale-free logarithmic potential. Although the mean biases are small and mostly <1%, the flattest cluster models generally have a significant mean bias; i.e., averaging over all orientations does not always eliminate projection biases. Substantial biases can result from different viewing orientations, where the integrated Compton-y parameter (Y_SZ) and the concentration have the largest scatter (as large as sigma ~10% for Y_SZ), and the emission-weighted temperature (T_X) has the smallest (sigma < ~0.5%). The very small scatter for T_X leads to Y_X and M_gas having virtually the same orientation biases. Substantial scatter is expected for individual clusters (up to sigma ~8%) in the correlation between Y_SZ and Y_X in comparison to the small mean bias (sigma < ~1%) applicable to a random sample of clusters of sufficient size. For ellipsoidal NFW models we show that the orientation bias for the total cluster mass attains a minimum near the radius r_2500 so that the spherically averaged mass computed at this radius is always within ~0.5% of the true value for any orientation. Finally, to facilitate the accounting for orientation bias in X-ray and SZ cluster studies, we provide cubic polynomial approximations to the mean orientation bias and 1-sigma scatter for each cluster observable as a function of axial ratio for the ellipsoidal NFW models.

Substructure and Scatter in the Mass‐Temperature Relations of Simulated Clusters

Galaxy clusters exhibit regular scaling relations among their bulk properties. These relations establish vital links between halo mass and cluster observables. Precision cosmology studies that depend on these links benefit from a better understanding of scatter in the mass-observable scaling relations. Here, we study the role of merger processes in introducing scatter into the M − TX relation, using a sample of 121 galaxy clusters simulated with radiative cooling and supernova feedback, along with three statistics previously proposed to measure X-ray surface brightness substructure. These are the centroid variation (w), the axial ratio (η), and the power ratios (P20 and P30). We find that in this set of simulated clusters, each substructure measure is correlated with a cluster's departures δ ln TX and δ ln M from the mean M − TX relation, both for emission-weighted temperatures TEW and for spectroscopic-like temperatures TSL, in the sense that clusters with more substructure tend to be cooler at a given halo mass. In all cases, a three-parameter fit to the M − TX relation that includes substructure information has less scatter than a two-parameter fit to the basic M − TX relation.

On the evolution of cooling cores in X-ray galaxy clusters

(Abridged) To define a framework for the formation and evolution of the cooling cores in X-ray galaxy clusters, we study how the physical properties change as function of the cosmic time in the inner regions of a 4 keV and 8 keV galaxy cluster under the action of radiative cooling and gravity only. The cooling radius, R_cool, defined as the radius at which the cooling time equals the Universe age at given redshift, evolves from ~0.01 R200 at z>2, where the structures begin their evolution, to ~0.05 R200 at z=0. The values measured at 0.01 R200 show an increase of about 15-20 per cent per Gyr in the gas density and surface brightness and a decrease with a mean rate of 10 per cent per Gyr in the gas temperature. The emission-weighted temperature diminishes by about 25 per cent and the bolometric X-ray luminosity rises by a factor ~2 after 10 Gyrs when all the cluster emission is considered in the computation. On the contrary, when the core region within 0.15 R500 is excluded, the gas temperature value does not change and the X-ray luminosity varies by 10-20 per cent only. The cooling time and gas entropy radial profiles are well represented by power-law functions. The behaviour of the inner slopes of the gas temperature and density profiles are the most sensitive and unambiguous tracers of an evolving cooling core. Their values after 10 Gyrs of radiative losses, T_gas ~ r^0.4 and n_gas ~ r^(-1.2) for the hot (cool) object, are remarkably in agreement with the observational constraints available for nearby X-ray luminous cooling core clusters. Because our simulations do not consider any AGN heating, they imply that the feedback process does not greatly alter the gas density and temperature profiles as generated by radiative cooling alone.

Mismatch between X-Ray and Emission-weighted Temperatures in Galaxy Clusters: Cosmological Implications

The thermal properties of hydrodynamical simulations of galaxy clusters are usually compared to observations by relying on the emission-weighted temperature T_ew, instead of on the spectroscopic X-ray temperature T_spec, which is obtained by actual observational data. In a recent paper Mazzotta et al. show that, if the cluster is thermally complex, T_ew fails at reproducing T_spec, and propose a new formula, the spectroscopic-like temperature, T_sl, which approximates T_spec better than a few per cent. By analyzing a set of hydrodynamical simulations of galaxy clusters, we find that T_sl is lower than T_ew by 20-30 per cent. As a consequence, the normalization of the M-T_sl relation from the simulations is larger than the observed one by about 50 per cent. If masses in simulated clusters are estimated by following the same assumptions of hydrostatic equilibrium and \beta--model gas density profile, as often done for observed clusters, then the M-T relation decreases by about 40 per cent, and significantly reduces its scatter. Based on this result, we conclude that using the observed M-T relation to infer the amplitude of the power spectrum from the X-ray temperature function could bias low \sigma_8 by 10-20 per cent. This may alleviate the tension between the value of \sigma_8 inferred from the cluster number density and those from cosmic microwave background and large scale structure.

Comparing the temperatures of galaxy clusters from hydrodynamicalN-body simulations toChandraandXMM-Newtonobservations

Theoretical studies of the physical processes guiding the formation and evolution of galaxies and galaxy clusters in the X-ray are mainly based on the results of numerical hydrodynamical N-body simulations, which in turn are often directly compared to X-ray observations. Although trivial in principle, these comparisons are not always simple. We demonstrate that the projected spectroscopic temperature of thermally complex clusters obtained from X-ray observations is always lower than the emission-weighed temperature, which is widely used in the analysis of numerical simulations. We show that this temperature bias is mainly related to the fact that the emission-weighted temperature does not reflect the actual spectral properties of the observed source. This has important implications for the study of thermal structures in clusters, especially when strong temperature gradients, like shock fronts, are present. Because of this bias, in real observations shock fronts appear much weaker than what is predicted by emission-weighted temperature maps, and may even not be detected. This may explain why, although numerical simulations predict that shock fronts are a quite common feature in clusters of galaxies, to date there are very few observations of objects in which they are clearly seen. To fix this problem we propose a new formula, the spectroscopic-like temperature function, and show that, for temperature larger than 3 keV, it approximates the spectroscopic temperature better than few per cent, making simulations more directly comparable to observations.

Non-isothermality on the Sunyaev–Zel’dovich effect

Abstract We have proposed a method to calculate the real effect of non-isothermality on the Sunyaev–Zel’dovich effect (SZE). The non-isothermal effect is conventionally approximated by an emission-weighted temperature with the isothermal assumption or only considered the influence of the non-isothermal Compton y -parameter. We have compared the real SZE with those estimated by these two approximative methods for non-isothermal clusters with and without cooling flows. Two temperature profiles, the hybrid model and polytropic model, are considered for the clusters without cooling flows. For investigating the effect of cooling flows on the SZE, the A1835 cluster is taken for example. Temperature profiles in galaxy clusters strongly affect the SZE and consequently the estimated values of the Hubble constant. Different profiles result in very different error ranges for estimating the Hubble constant, ∼−3% to +10% for the hybrid model and ∼−10% to +40% for the polytropic model. Specially, the effect of cooling flows on determining the value of the Hubble constant is dramatic, ∼ + 45% for A1835, when the isothermal emission-weighted temperature is adopted.

Mass-Temperature Relation of X-ray Clusters in Triaxial Halo

We investigate the mass-temperature relation of clusters for both the spherical NFW halo model and a concentric triaxial halo model. We study the temperature and density distributions of both an isothermal and a polytropic intra-cluster gas in hydrostatic equilibrium. We find that both the uncertainties in the concentration parameter and in the eccentricities (in case of the triaxial halo) lead to a greater scatter in the emission-weighted temperature at a given halo mass for less massive clusters. This may be helpful when determining the intrinsic statistical error of the σ8 normalization of the linear power spectrum from cluster abundance.

Scaling laws in X-ray galaxy clusters at redshift between 0.4 and 1.3

(Abriged) We present a study of the integrated physical properties of a sample of 28 X-ray galaxy clusters observed with Chandra at redshift between 0.4 and 1.3. In particular, we have twelve objects in the redshift range 0.4-0.6, five between 0.6 and 0.8, seven between 0.8 and 1 and four at z>1, compounding the largest sample available for such a study. We focus particularly on the properties and evolution of the X-ray scaling laws. We fit both a single and a double beta-model with the former which provides a good representation of the observed surface brightness profiles, indicating that these clusters do not show any significant excess in their central brightness. By using the best-fit parameters of the beta-model together with the measured emission-weighted temperature (in the range 3-11 keV), we recover gas luminosity, gas mass and total gravitating mass out to R500. We observe scaling relations steeper than expected from self-similar model by a significant (>3 sigma) amount in the L-T and Mgas-T relations. We detect hints of negative evolution in the L-T, Mgas-T and L-Mtot relations, thus suggesting that systems at higher redshift have lower X-ray luminosity and gas mass for fixed temperature. In particular, when the 16 clusters at z>0.6 are considered, the evolution becomes more evident and its power-law modelization is a statistically good description of the data.

Mapping small-scale temperature and abundance structures in the core of the Perseus cluster

We report further results from a 191 ks Chandra observation of the core of the Perseus cluster, Abell 426. The emission-weighted temperature and abundance structure is mapped detail. There are temperature variations down to ~1 kpc in the brightest regions. Globally, the strongest X-ray surface brightness features appear to be caused by temperature changes. Density and temperature changes conspire to give approximate azimuthal balance in pressure showing that the gas is in hydrostatic equilibrium. Si, S, Ar, Ca, Fe and Ni abundance profiles rise inward from about 100 kpc, peaking at about 30-40 kpc. Most of these abundances drop inwards of the peak, but Ne shows a central peak, all of which may be explained by resonance scattering. There is no evidence for a widespread additional cooler temperature component in the cluster with a temperature greater than a factor of two from the local temperature. There is however evidence for a widespread hard component which may be nonthermal. The temperature and abundance of gas in the cluster is observed to be correlated in a manner similar to that found between clusters.

Implications of the universal temperature profile for galaxy clusters

We study the X-ray cluster gas density distribution in hydrostatic equilibrium using the universal temperature profile obtained from recent simulations involving only gravitational processes. If this temperature profile is an indicator of the influence of gravitational processes alone on the intracluster medium, then the comparison of various X-ray parameters expected from this profile and the observed data would point towards any additional physics that may be required. We compare the entropy at 0.1 R_{200} and R_{500}, the scaled entropy profile, the gas fraction at 0.3 R_{200} and the gas fraction profile with recent observations and discuss the implications of this temperature profile in light of these data. We find that the entropy imparted to the gas from gravitational processes alone is larger than previously thought. The entropy at R_{500} for rich clusters is consistent with data, whereas the entropy at 0.1R_{200} is still less than the observed values. We also find that the gas fraction in the inner region of clusters, expected from gravitational processes alone, is smaller than previously thought but larger than the observed data. It does show a trend with the emission-weighted temperature ( ) as shown by data. We therefore find that the role of any additional non-gravitational process influencing the physical state of ICM would have to be revised in light of these findings.

Circulation Flows: Cooling Flows with Bubble Return

The failure of the XMM-Newton and Chandra X-ray telescopes to detect cooling gas in elliptical galaxies and clusters of galaxies has led many to adopt the position that the gas is not cooling at all and that heating by an active nucleus in the central E or cD galaxy is sufficient to offset radiative cooling. In this paper we explore an idealized limiting example of this point of view in which hot, buoyant bubbles formed near the center return the inflowing, radiatively cooling gas to distant regions in the flow. We show that idealized steady state, centrally heated non-cooling flows can indeed be constructed. In addition, the emission-weighted temperature profiles in these circulating flows resemble those of normal cooling flows. However, these solutions are valid only (1) for a range of bubble parameters for which there is no independent justification, (2) for a limited spatial region in the cooling flow, and (3) for a limited period of time after which cooling seems inevitable. Our exploration of non-cooling flows is set in the context of galaxy/group flows.

Simulated X-ray cluster temperature maps

Temperature maps are presented of the nine largest clusters in the mock catalogues of Muanwong et al. for both the Preheating and Radiative models. The maps show that clusters are not smooth, featureless systems, but contain a variety of substructure which should be observable. The surface brightness contours are generally elliptical and features that are seen include cold clumps, hot spiral features and cold fronts. Profiles of emission-weighted temperature, surface brightness, density and pressure across the surface brightness discontinuities seen in one of the bimodal clusters are consistent with the cold front in Abell 2142 observed by Markevitch et al.

Hydrodynamic simulations of merging clusters of galaxies

We present the results of high-resolution AP3M+SPH simulations of merging clusters of galaxies. We find that the compression and shocking of the core gas during a merger can lead to large increases in bolometric X-ray luminosities and emission-weighted temperatures of clusters. Cooling flows are completely disrupted during equal-mass mergers, with the mass deposition rate dropping to zero as the cores of the clusters collide. The large increase in the cooling time of the core gas strongly suggests that cooling flows will not recover from such a merger within a Hubble time. Mergers with subclumps having one eighth of the mass of the main cluster are also found to disrupt a cooling flow if the merger is head-on. However, in this case the entropy injected into the core gas is rapidly radiated away and the cooling flow restarts within a few Gyr of the merger. Mergers in which the subcluster has an impact parameter of 500 kpc do not disrupt the cooling flow, although the mass deposition rate is reduced by ∼30 per cent. Finally, we find that equal mass, off-centre mergers can effectively mix gas in the cores of clusters, while head on mergers lead to very little mixing. Gas stripped from the outer layers of subclumps results in parts of the outer layers of the main cluster being well mixed, although they have little effect on the gas in the core of the cluster. None of the mergers examined here resulted in the intracluster medium being well mixed globally.

Preheating the Intracluster Medium in High-Resolution Simulations: The Effect on the Gas Entropy

We present first results from high-resolution tree + smoothed particle hydrodynamics simulations of galaxy clusters and groups that are aimed at studying the effect of nongravitational heating on the entropy of the intracluster medium (ICM). We simulate three systems having emission-weighted temperatures Tew 0.6, 1, and 3 keV with spatial resolutions better than 1% of the virial radius. We consider the effect of different prescriptions for nongravitational ICM heating, such as supernova energy feedback, as predicted by semianalytical models of galaxy formation, and two different minimum entropy floors, Sfl = 50 and 100 keV cm2, imposed at z = 3. Simulations with only gravitational heating nicely reproduce predictions from self-similar ICM models, while extra heating is shown to break the self-similarity, by a degree that depends on the total injected energy and on the cluster mass. We use the observational results on the excess entropy in central regions of galaxy systems to constrain the amount of extra heating required. We find that by setting the entropy floor Sfl = 50 keV cm2, which corresponds to an extra heating energy of about 1 keV per particle, we are able to reproduce the observed excess of ICM entropy.

ChandraX-ray observations of the 3C 295 cluster core

We examine the properties of the X-ray gas in the central regions of the distant (z=0.46), X-ray luminous cluster of galaxies surrounding the powerful radio source 3C 295, using observations made with the Chandra Observatory. Between radii of 50 and 500 kpc, the cluster gas is approximately isothermal with an emission-weighted temperature, kT∼5 keV. Within the central 50-kpc radius this value drops to kT∼3.7 keV. The spectral and imaging Chandra data indicate the presence of a cooling flow within the central 50-kpc radius of the cluster, with a mass deposition rate of approximately 280 M⊙ yr−1. We estimate an age for the cooling flow of 1–2 Gyr, which is approximately 1000 times older than the central radio source. We find no evidence in the X-ray spectra or images for significant heating of the X-ray gas by the radio source. We report the detection of an edge-like absorption feature in the spectrum for the central 50-kpc region, which may be caused by oxygen-enriched dust grains. The implied mass in metals seen in absorption could have been accumulated by the cooling flow over its lifetime. Combining the results on the X-ray gas density profile with radio measurements of the Faraday rotation measure in 3C 295, we estimate the magnetic field strength in the region of the cluster core to be B∼12 μG.