On the soft X-ray spectrum of cooling flows

Strong evidence for cooling flows has been found in low-resolution X-ray imaging and spectra of many clusters of galaxies. However, high-resolution X-ray spectra of several clusters from the Reflection Grating Spectrometer on XMM-Newton now show a soft X-ray spectrum inconsistent with a simple cooling flow. The main problem is a lack of the emission lines expected from gas cooling below 1–2 keV. Lines from gas at about 2–3 keV are observed, even in a high-temperature cluster such as A1835, indicating that gas is cooling down to about 2–3 keV, but is not found at lower temperatures. Here we discuss several solutions to the problem: heating, mixing, differential absorption and inhomogeneous metallicity. Continuous or sporadic heating creates further problems, including the targeting of the heat at the cooler gas and also the high total energy required. So far there is no clear observational evidence for widespread heating, or shocks, in cluster cores, except in radio lobes which occupy only part of the volume. Alternatively, if the metals in the intracluster medium are not uniformly spread but are clumped, then little line emission is expected from the gas cooling below 1 keV. The low-metallicity part cools without line emission, whereas the strengths of the soft X-ray lines from the metal-rich gas depend on the mass fraction of that gas and not on the abundance, since soft X-ray line emission dominates the cooling function below 2 keV.     

The relationship between cooling flows and metallicity measurements for X-ray-luminous clusters

We explore the relationship between the metallicity of the intracluster gas in clusters of galaxies, determined by X-ray spectroscopy, and the presence of cooling flows. Using ASCA spectra and ROSAT images, we demonstrate a clear segregation between the metallicities of clusters with and without cooling flows. On average, cooling-flow clusters have an emission-weighted metallicity a factor ∼ 1.8 times higher than that of non-cooling-flow systems. We suggest that this is caused by the presence of metallicity gradients in the cooling-flow clusters, coupled with the sharply peaked X-ray surface brightness profiles of these systems. Non-cooling-flow clusters have much flatter X-ray surface brightness distributions and are thought to have undergone recent merger events, which may have mixed the central high-metallicity gas with the surrounding less metal-rich material. We find no evidence for evolution in the emission-weighted metallicities of clusters within z  ∼ 0.3.     

A statistically selected Chandra sample of 20 galaxy clusters – II. Gas properties and cool core/non-cool core bimodality

We investigate the thermodynamic and chemical structure of the intracluster medium (ICM) across a statistical sample of 20 galaxy clusters analysed with the Chandra X-ray satellite. In particular, we focus on the scaling properties of the gas density, metallicity and entropy and the comparison between clusters with and without cool cores (CCs). We find marked differences between the two categories except for the gas metallicity, which declines strongly with radius for all clusters  ( Z ∝ r ^−0.31)  , outside  ∼0.02 r ₅₀₀  . The scaling of gas entropy is non-self-similar and we find clear evidence of bimodality in the distribution of logarithmic slopes of the entropy profiles. With only one exception, the steeper sloped entropy profiles are found in CC clusters whereas the flatter slope population are all non-CC clusters. We explore the role of thermal conduction in stabilizing the ICM and conclude that this mechanism alone is sufficient to balance cooling in non-CC clusters. However, CC clusters appear to form a distinct population in which heating from feedback is required in addition to conduction. Under the assumption that non-CC clusters are thermally stabilized by conduction alone, we find the distribution of Spitzer conduction suppression factors, f _c, to be lognormal, with a log (base 10) mean of  −1.50 ± 0.03  (i.e.   f _c= 0.032  ) and log standard deviation  0.39 ± 0.02  .     

Note on a polytropic β-model to fit the X-ray surface brightness of clusters of galaxies

I suggest that the β -model used to fit the X-ray surface brightness profiles of extended sources, like groups and clusters of galaxies, has to be corrected when the counts are collected in a wide energy band comparable to the mean temperature of the source, and a significant gradient in the gas temperature is observed. I present a revised version of the β -model for the X-ray brightness that applies to an intracluster gas with temperature and density related by a polytropic equation and extends the standard version that is strictly valid for an isothermal gas. Given a temperature gradient observed through an energy window of 1–10 keV typical for the new generation of X-ray observatories, the β parameter can change systematically by up to 20 per cent from the value obtained under isothermal assumption, i.e. by an amount larger than any statistical uncertainty obtained from the present data. Within the virial regions of typical clusters of galaxies, these systematic corrections affect the total gravitating mass estimate by 5–10 per cent, the gas mass by 10–30 per cent and the gas fraction value up to 50 per cent, when compared with the measurements obtained under the isothermal assumption.     

Towards a holistic view of the heating and cooling of the intracluster medium

X-ray clusters are conventionally divided into two classes: 'cool core' (CC) clusters and 'non-cool core' (NCC) clusters. Yet relatively little attention has been given to the origins of this apparent dichotomy and, in particular, to the energetics and thermal histories of the two classes. We develop a model for the entropy profiles of clusters starting from the configuration established by gravitational shock heating and radiative cooling. At large radii, gravitational heating accounts for the observed profiles and their scalings well. However, at small and intermediate radii, radiative cooling and gravitational heating cannot be combined to explain the observed profiles of either CC or NCC clusters. The inferred entropy profiles of NCC clusters require that material is 'pre-heated' prior to cluster collapse in order to explain the absence of low-entropy (cool) material in these systems. We show that a similar modification is also required in CC clusters in order to match their entropy profiles at intermediate radii. In CC clusters, this modification is unstable, and an additional process is required to prevent cooling below a temperature of a few keV. We show that this can be achieved by adding a self-consistent active galactic nuclei (AGN) feedback loop in which the lowest entropy, most rapidly cooling material is heated and rises buoyantly to mix with material at larger radii. The resulting model does not require fine-tuning and is in excellent agreement with a wide variety of observational data from Chandra and XMM–Newton , including entropy and gas density profiles, the luminosity–temperature relation and high-resolution spectra. The spread in cluster core morphologies is seen to arise because of the steep dependence of the central cooling time on the initial level of pre-heating. Some of the other implications of this model are briefly discussed.     

The phase diagram of the intergalactic medium and the entropy floor of groups and clusters: are clusters born warm?

We point out that two problems of observational cosmology, namely the facts (i) that ≳60 per cent of the baryonic content of the Universe is not observed at   z ∼ 0  and (ii) that the properties of small clusters do not agree with simple expectations, could be closely related. As shown by recent studies, the shock heating associated with the formation of large-scale structures heats the intergalactic medium (IGM) and leads to a 'warm IGM' component for the gas. In the same spirit, we suggest the intracluster medium (ICM) to be a mixture of galaxy-recycled, metal-enriched gas and intergalactic gas, shock heated by the collapsing much larger scales. This could be obtained through two processes: (1) the late infalling gas from the external warm IGM is efficiently mixed within the halo and brings some additional entropy, or (2) the shocks generated by larger non-linear scales are also present within clusters and can heat the ICM. We show that, if assumption (1) holds, the entropy brought by the warm IGM is sufficient to explain the observed properties of clusters, in particular the entropy floor and the   L _X– T   relation. On the other hand, we note briefly that scenario (2) would require a stronger shock heating because of the larger density of the ICM as compared with filaments. Although the efficiency of these two processes remains to be checked on a quantitative level, they have the advantage of dispensing with the need to invoke any strong preheating from supernovae or quasars (which has otherwise been introduced for the sole purpose of reproducing the behaviour of clusters). Matter ejection by galaxies is included in the present calculations and, consistently with the metal-enrichment requirements, is indeed shown to yield only a quite moderate entropy increase. Our scenario of clusters being 'born warm' can be checked through the predicted redshift evolution of the entropy floor.     

Observable consequences of kinetic and thermal AGN feedback in elliptical galaxies and galaxy clusters

We have constructed an analytical model of active galactic nuclei (AGN) feedback and studied its implications for elliptical galaxies and galaxy clusters. The results show that momentum injection above a critical value will eject material from low-mass elliptical galaxies, and leads to an X-ray luminosity, L _X, that is  ∝σ⁸⁻¹⁰  , depending on the AGN fuelling mechanism, where σ is the velocity dispersion of the hot gas. This result agrees well with both observations and semi-analytic models. In more massive ellipticals and clusters, AGN outflows quickly become buoyancy dominated. This necessarily means that heating by a central cluster AGN redistributes the intracluster medium (ICM) such that the mass of hot gas, within the cooling radius, should be  ∝ L _X(< r _cool)/[ g ( r _cool)σ]  , where   g ( r _cool)  is the gravitational acceleration at the cooling radius. This prediction is confirmed using observations of seven clusters. The same mechanism also defines a critical ICM cooling time of  ∼0.5 Gyr  , which is in reasonable agreement with recent observations showing that star formation and AGN activity are triggered below a universal cooling time threshold.     

X-ray group and cluster mass profiles in MOND: unexplained mass on the group scale

Although very successful in explaining the observed conspiracy between the baryonic distribution and the gravitational field in spiral galaxies without resorting to dark matter (DM), the modified Newtonian dynamics (MOND) paradigm still requires DM in X-ray bright systems. Here, to get a handle on the distribution and importance of this DM, and thus on its possible form, we deconstruct the mass profiles of 26 X-ray emitting systems in MOND, with temperatures ranging from 0.5 to 9 keV. Initially, we compute the MOND dynamical mass as a function of radius, then subtract the known gas mass along with a component of galaxies which include the cD galaxy with   M / L _K = 1  . Next, we test the compatibility of the required DM with ordinary massive neutrinos at the experimental limit of detection  ( m _ν= 2 eV)  , with density given by the Tremaine–Gunn limit. Even by considering that the neutrino density stays constant and maximal within the central 100 or 150 kpc (which is the absolute upper limit of a possible neutrino contribution there), we show that these neutrinos can never account for the required DM within this region. The natural corollary of this finding is that, whereas clusters  ( T ≳ 3 keV)  might have most of their mass accounted for if ordinary neutrinos have a 2 eV mass, groups  ( T ≲ 2 keV)  cannot be explained by a 2 eV neutrino contribution. This means that, for instance, cluster baryonic dark matter (CBDM, Milgrom) or even sterile neutrinos would present a more satisfactory solution to the problem of missing mass in MOND X-ray emitting systems.     

X-ray observations of the galaxy cluster PKS 0745−191: to the virial radius, and beyond

We measure X-ray emission from the outskirts of the cluster of galaxies PKS 0745−191 with Suzaku , determining radial profiles of density, temperature, entropy, gas fraction and mass. These measurements extend beyond the virial radius for the first time, providing new information about cluster assembly and the diffuse intracluster medium out to  ∼1.5  r ₂₀₀( r ₂₀₀≃ 1.7 Mpc ≃ 15 arcmin  ). The temperature is found to decrease by roughly 70 per cent from 0.3 to  1 r ₂₀₀  . We also see a flattening of the entropy profile near the virial radius and consider the implications this has for the assumption of hydrostatic equilibrium when deriving mass estimates. We place these observations in the context of simulations and analytical models to develop a better understanding of non-gravitational physics in the outskirts of the cluster.     

Statistical properties of metal abundances of the intracluster medium in the central region of clusters

We measured metal abundances of the intracluster medium in the central regions of 34 nearby clusters of galaxies, using ASCA data. Clusters that have a sharp X-ray emission centred on a cD galaxy are commonly found to exhibit a central increment in the Fe abundance, which is more pronounced in lower temperature clusters; +(0.1–0.2) solar at kT >5 keV, compared with +(0.2–0.3) solar at 1.5< kT <4 keV. These central excess metals are thought to be ejected from cD galaxies. Several low-temperature cD type clusters also show significant Si abundance increase by +(0.1–0.2) solar at the central region. Compared with the Si-rich abundances observed in the outer regions of rich clusters, the Si to Fe abundance ratio of central excess metals tends to be near the solar ratio, implying that type Ia products from cD galaxies are dominant for the central excess metals. On the other hand, some other clusters do not show the central Fe abundance increase. As these clusters tend to contain two or three central giant galaxies, it is suggested that galaxy interactions have removed the central abundance increase.     

Coulomb interactions in the intracluster medium

In this paper we discuss the effect of Coulomb collisions on the temperature profiles of the intracluster medium in clusters of galaxies, motivated by recent reports of negative temperature gradients in some clusters by Markevitch et al. The time-scale for electrons and protons to reach temperature equilibrium can exceed a few × 10⁹ years beyond radii of a megaparsec, if the intracluster gas is assumed to be at the usual cluster virial temperature. If a cluster merger has occurred within that time causing the protons, but not the electrons, to be rapidly heated then a small negative temperature gradient can result. This gradient is larger in clusters with higher temperatures and steeper density profiles.   Applying these considerations to the cluster of galaxies A2163, we conclude that, more plausibly, the observed gradient is due to a lack of hydrostatic equilibrium following a merger.     

Supersonic motions of galaxies in clusters

We study motions of galaxies in galaxy clusters formed in the concordance Λ cold dark matter cosmology. We use high-resolution cosmological simulations that follow the dynamics of dark matter and gas and include various physical processes critical for galaxy formation: gas cooling, heating and star formation. Analysing the motions of galaxies and the properties of intracluster gas in a sample of eight simulated clusters at z = 0, we study the velocity dispersion profiles of the dark matter, gas and galaxies. We measure the mean velocity of galaxy motions and gas sound speed as a function of radius and calculate the average Mach number of galaxy motions. The simulations show that galaxies, on average, move supersonically with the average Mach number of ≈1.4, approximately independent of the cluster-centric radius. The supersonic motions of galaxies may potentially provide an important source of heating for the intracluster gas by driving weak shocks and via dynamical friction, although these heating processes appear to be inefficient in our simulations. We also find that galaxies move slightly faster than the dark matter particles. The magnitude of the velocity bias,   b _v ≈ 1.1  , is, however, smaller than the bias estimated for subhaloes in dissipationless simulations. Interestingly, we find velocity bias in the tangential component of the velocity dispersion, but not in the radial component. Finally, we find significant random bulk motions of gas. The typical gas velocities are of order ≈20–30 per cent of the gas sound speed. These random motions provide about 10 per cent of the total pressure support in our simulated clusters. The non-thermal pressure support, if neglected, will bias measurements of the total mass in the hydrostatic analyses of the X-ray cluster observations.     

A statistically selected Chandra sample of 20 galaxy clusters – I. Temperature and cooling time profiles

We present an analysis of 20 galaxy clusters observed with the Chandra X-ray satellite, focusing on the temperature structure of the intracluster medium and the cooling time of the gas. Our sample is drawn from a flux-limited catalogue but excludes the Fornax, Coma and Centaurus clusters, owing to their large angular size compared to the Chandra field of view. We describe a quantitative measure of the impact of central cooling, and find that the sample comprises nine clusters possessing cool cores (CCs) and 11 without. The properties of these two types differ markedly, but there is a high degree of uniformity amongst the CC clusters, which obey a nearly universal radial scaling in temperature of the form   T ∝ r ^∼0.4  , within the core. This uniformity persists in the gas cooling time, which varies more strongly with radius in CC clusters  ( t _cool∝ r ^∼1.3)  , reaching   t _cool < 1 Gyr  in all cases, although surprisingly low central cooling times (<5 Gyr) are found in many of the non-CC systems. The scatter between the cooling time profiles of all the clusters is found to be remarkably small, implying a universal form for the cooling time of gas at a given physical radius in virialized systems, in agreement with recent previous work. Our results favour cluster merging as the primary factor in preventing the formation of CCs.     

Detection of X-ray emission from the host clusters of 3CR quasars

We report the detection of extended X-ray emission around several powerful 3CR quasars with redshifts out to 0.73. The ROSAT HRI images of the quasars have been corrected for spacecraft wobble and compared with an empirical point-spread function. All the quasars examined show excess emission at radii of 15 arcsec and more, the evidence being strong for the more distant objects and weak only for the two nearest ones, which are known from other wavelengths not to lie in strongly clustered environments. The spatial profile of the extended component is consistent with thermal emission from the intracluster medium of moderately rich host clusters to the quasars. The total luminosities of the clusters are in the range ∼4×10⁴⁴–3×10⁴⁵ erg s⁻¹, assuming a temperature of 4 keV. The inner regions of the intracluster medium are, in all cases, dense enough to be part of a cooling flow.     

The intragroup medium in loose groups of galaxies

We have used the ROSAT PSPC to study the properties of a sample of 24 X-ray-bright galaxy groups, representing the largest sample examined in detail to date. Hot plasma models are fitted to the spectral data to derive temperatures, and modified King models are used to characterize the surface brightness profiles.
In agreement with previous work, we find evidence for the presence of two components in the surface brightness profiles. The extended component is generally found to be much flatter than that observed in galaxy clusters, and there is evidence that the profiles follow a trend with system mass. We derive relationships between X-ray luminosity, temperature and optical velocity dispersion. The relation between X-ray luminosity and temperature is found to be L _X∝ T ^4.9, which is significantly steeper than the same relation in galaxy clusters. These results are in good agreement with pre-heating models, in which galaxy winds raise the internal energy of the gas, inhibiting its collapse into the shallow potential wells of poor systems.     

The influence of gas expulsion and initial mass segregation on the stellar mass function of globular star clusters

Recently, De Marchi, Paresce & Pulone studied a sample of 20 globular clusters and found that all clusters with high concentrations have steep stellar mass functions while clusters with low concentration have comparatively shallow mass functions. No globular clusters were found with a flat mass function and high concentration. This seems curious since more concentrated star clusters are believed to be dynamically more evolved and should have lost more low-mass stars via evaporation, which would result in a shallower mass function in the low-mass part.
We show that this effect can be explained by residual-gas expulsion from initially mass segregated star clusters, and is enhanced further through unresolved binaries. If gas expulsion is the correct mechanism to produce the observed trend in the   c –α  -plane, then observation of these parameters would allow to constrain cluster starting conditions such as star formation efficiency and the time-scale of gas expulsion.     

Gamma-ray probe of cosmic ray pressure in galaxy clusters and cosmological implications

Cosmic rays produced in cluster accretion and merger shocks provide pressure to the intracluster medium (ICM) and affect the mass estimates of galaxy clusters. Although direct evidence for cosmic ray ions in the ICM is still lacking, they produce γ-ray emission through the decay of neutral pions produced in their collisions with ICM nucleons. We investigate the capability of the Gamma-ray Large Area Space Telescope ( GLAST ) and imaging atmospheric Čerenkov telescopes (IACTs) for constraining the cosmic ray pressure contribution to the ICM. We show that GLAST can be used to place stringent upper limits, a few per cent for individual nearby rich clusters, on the ratio of pressures of the cosmic rays and thermal gas. We further show that it is possible to place tight (≲10 per cent) constraints for distant  ( z ≲ 0.25)  clusters in the case of hard spectrum, by stacking signals from samples of known clusters. The GLAST limits could be made more precise with the constraint on the cosmic ray spectrum potentially provided by IACTs. Future γ-ray observations of clusters can constrain the evolution of cosmic ray energy density, which would have important implications for cosmological tests with upcoming X-ray and Sunyaev–Zel'dovich effect cluster surveys.     

Heating of the intracluster medium by quasar outflows

We study the possibility of quasar outflows in clusters and groups of galaxies heating the intracluster gas in order to explain the recent observation of excess entropy in this gas. We show that radio galaxies alone cannot provide the energy required to explain the observations but the inclusion of Broad Absorption Line (BAL) outflows can do so, and that in this scenario most of the heating takes place atz ∼ 1–4, the “preheating” epoch being at a lower redshift for lower mass clusters.