Flows In Clusters Of Galaxies Research Articles

Hidden cooling flows in clusters of galaxies – III. Accretion on to the central black hole

ABSTRACT Recently, we have uncovered hidden cooling flows (HCFs) in the X-ray spectra of the central brightest galaxies of 11 clusters, 1 group, and 2 elliptical galaxies. Here, we report such flows in a further 15 objects, consisting of 8 clusters, 3 groups, 3 ellipticals, and 1 Red Nugget. The mass cooling rates are about $1\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ in the ellipticals, 2 to $20\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ in the groups, and 20 to $100\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ in regular clusters. The Red Nugget, MRK 1216, has an HCF of $10\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$. We review the fate of the cooled gas and investigate how some of it might accrete on to the central black hole. The gas is likely to be very cold and to have fragmented into low-mass stars and smaller objects before being swallowed whole, with little luminous output. If such a scenario is correct and operates at a few $\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ then such objects may host the fastest growing black holes in the low-redshift Universe. We briefly discuss the relevance of HCF to the growth of early galaxies and black holes.

Hidden Cooling Flows in clusters of Galaxies II: a wider sample

ABSTRACT We have recently uncovered Hidden Cooling Flows (HCFs) in the XMM Reflection Grating Spectrometer spectra of three clusters of galaxies; Centaurus, Perseus, and A1835. Here, we search for them in a wider sample of objects: the X-ray brightest group NGC 5044; four moderate X-ray luminosity clusters Sersic 159, A262, A2052, and RX J0821; and three high X-ray luminosity clusters RX J1532, MACS 1931, and the Phoenix cluster. Finally, we examine two Virgo elliptical galaxies, M49 and M84. All statistically allow the addition of an HCF. We find a significant detection of an HCF in six clusters and two elliptical galaxies. The hidden mass cooling rates are $5{\!-\!}40\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ for the normal clusters, $1000\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ or more for the extreme clusters, and $1{\!-\!}2\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ for the elliptical galaxies. We discuss the implications of the results for the composition of the innermost parts of the massive host galaxies and look forward to future observations.

Cosmic rays and thermal instability in self-regulating cooling flows of massive galaxy clusters

One of the key physical processes that helps prevent strong cooling flows in galaxy clusters is the continued energy input from the central active galactic nucleus (AGN) of the cluster. However, it remains unclear how this energy is thermalised so that it can effectively prevent global thermal instability. One possible option is that a fraction of the AGN energy is converted into cosmic rays (CRs), which provide non-thermal pressure support, and can retain energy even as thermal energy is radiated away. By means of magneto-hydrodynamical simulations, we investigate how CR injected by the AGN jet influence cooling flows of a massive galaxy cluster. We conclude that converting a fraction of the AGN luminosity as low as 10% into CR energy prevents cooling flows on timescales of billion years, without significant changes in the structure of the multi-phase intra-cluster medium. CR-dominated jets, by contrast, lead to the formation of an extended, warm central nebula that is supported by CR pressure. We report that the presence of CRs is not able to suppress the onset of thermal instability in massive galaxy clusters, but CR-dominated jets do significantly change the continued evolution of gas as it continues to cool from isobaric to isochoric. The CR redistribution in the cluster is dominated by advection rather than diffusion or streaming, but the heating by CR streaming helps maintain gas in the hot and warm phase. Observationally, self-regulating, CR-dominated jets produce a γ-ray flux in excess of current observational limits, but low CR fractions in the jet are not ruled out.

Hidden cooling flows in clusters of galaxies

ABSTRACT The radiative cooling time of the hot gas at the centres of cool cores in clusters of galaxies drops down to 10 Myr and below. The observed mass cooling rate of such gas is very low, suggesting that active galactic nucleus feedback is very tightly balanced or that the soft X-ray emission from cooling is somehow hidden from view. We use an intrinsic absorption model in which the cooling and coolest gas are closely interleaved to search for hidden cooling flows in the Centaurus, Perseus, and A1835 clusters of galaxies. We find hidden mass cooling rates of between 10 and $500\,{{\rm M_{\odot }}\,{\rm yr}^{-1}}$ as the cluster mass increases, with the absorbed emission emerging in the far-infrared (FIR) band. Good agreement is found between the hidden cooling rate and observed FIR luminosity in the Centaurus Cluster. The limits on the other two clusters allow for considerable hidden cooling. The implied total mass of cooled gas is much larger than the observed molecular masses. We discuss its fate including possible further cooling and collapse into undetected very cold clouds, low-mass stars, and substellar objects.

Advection by Large-scale Spiral Flows in Galaxy Clusters

Abstract The intracluster medium of a galaxy cluster often shows an extended quasi-spiral structure, accentuated by tangential discontinuities known as cold fronts (CFs). These discontinuities are thought to isolate between low-entropy, high-metallicity gas inside (i.e., below) the CF that was lifted from the center of the cluster by some radial factor f i and high-entropy, low-metallicity gas outside the CF that was pushed inward by a factor f o . We find broad support for such a picture, by comparing the entropy and metallicity discontinuities with the respective azimuthal averages, using newly deprojected thermal profiles in clusters A2029, A2142, A2204, and Centaurus, supplemented by deprojected CFs from the literature. In particular, the mean advection factors f K and f Z , inferred from entropy and metallicity, respectively, strongly correlate ( ) with each other, consistent with large-scale advection. However, unlike sloshing simulations, in which the inside/outside phases are an inflow/outflow settling back to equilibrium after a violent perturbation, our results are more consistent with an outflow/inflow, with the fast, Mach gas inside the CF being a rapidly heated or mixed outflow, probably originating from the cD galaxy, and gas outside the CF being an , slowly cooling inflow. In particular, entropy indicates an outside advection factor f Ko that is approximately constant in all CFs, gauging the distance traversed by inflowing gas within a cooling time. In contrast, and vary considerably among clusters and strongly correlate ( ) with the virial mass, and , suggesting that each cluster sustains a quasi-steady spiral flow.

Cooling+Heating Flows in Galaxy Clusters: Turbulent Heating, Spectral Modeling, and Cooling Efficiency

Abstract The discrepancy between expected and observed cooling rates of X-ray emitting gas has led to the cooling-flow problem at the cores of clusters of galaxies. A variety of models have been proposed to model the observed X-ray spectra and resolve the cooling-flow problem, which involves heating the cold gas through different mechanisms. As a result, realistic models of X-ray spectra of galaxy clusters need to involve both heating and cooling mechanisms. In this paper, we argue that the heating timescale is set by the magnetohydrodynamic (MHD) turbulent viscous heating for the Intracluster plasma, parameterized by the Shakura–Sunyaev viscosity parameter, α. Using a cooling+heating flow model, we show that a value of α ≃ 0.05 (with 10% scatter) provides improved fits to the X-ray spectra of cooling flow, while at the same time, predicting reasonable cooling efficiency, . Our inferred values for α based on X-ray spectra are also in line with direct measurements of turbulent pressure in simulations and observations of galaxy clusters. This simple picture unifies astrophysical accretion, as a balance of MHD turbulent heating and cooling, across more than 16 orders of magnitudes in scale, from neutron stars to galaxy clusters.

Cosmic ray acceleration in accretion flows of galaxy clusters

We investigate acceleration of cosmic rays by shocks and accretion flows in galaxy clusters. Numerical results for spectra of accelerated particles and nonthermal emission are presented. It is shown that the acceleration of protons and nuclei in the nearby galaxy cluster Virgo can explain the observed spectra of ultra high energy cosmic rays.

The High-energy emission of jetted AGN

AbstractQuasars with flat radio spectra and one-sided, arc-second scale, ≈ 100 mJy GHz radio jets are found to have similar scale X-ray jets in about 60% of such objects, even in short 5 to 10 ksChandraobservations. Jets emit in the GHz band via synchrotron radiation, as known from polarization measurements. The X-ray emission is explained most simply, i.e. with the fewest additional parameters, as inverse Compton (iC) scattering of cosmic microwave background (cmb) photons by the relativistic electrons in the jet. With physics based assumptions, one can estimate enthalpy fluxes upwards of 1046erg s−1, sufficient to reverse cooling flows in clusters of galaxies, and play a significant role in the feedback process which correlates the masses of black holes and their host galaxy bulges. On a quasar-by-quasar basis, we can show that the total energy to power these jets can be supplied by the rotational energy of black holes with spin parameters as low asa= 0.3. For a few bright jets at redshifts less than 1, theFermigamma ray observatory shows upper limits at 10 Gev which fall below the fluxes predicted by the iC/cmb mechanism, proving the existence of multiple relativistic particle populations. At large redshifts, the cmb energy density is enhanced by a factor (1+z)4, so that iC/cmb must be the dominant mechanism for relativistic jets unless their rest frame magnetic field strength is hundreds of micro-Gauss.

Precessing AGN jets, bubbles and cooling flows

AbstractSeveral galaxy clusters are known to present multiple and misaligned pairs of cavities seen in X-rays, as well as twisted kiloparsec-scale jets at radio wavelengths. It suggests that the AGN precessing jets play a role in the formation of the misaligned bubbles. Also, X-ray spectra reveal that typically these systems are also able to supress cooling flows, predicted theoretically. The absence of cooling flows in galaxy clusters has been a mistery for many years since numerical simulations and analytical studies suggest that AGN jets are highly energetic, but are unable to redistribute it at all directions. We performed 3D hydrodynamical simulations of the interaction between a precessing AGN jet and the warm intracluster medium plasma, in which dynamics is coupled to a NFW dark matter gravitational potential. Radiative cooling has been taken into account and the cooling flow problem was studied. We found that precession is responsible for multiple pairs of bubbles, as observed. The misaligned bubbles rise up to scales of tens of kiloparsecs, where the thermal energy released by the jets are redistributed. After ~150 Myrs, the temperature of the gas within the cavities is kept of order of ~107 K, while the denser plasma of the intracluster medium at the central regions reaches T ~ 105 K. The existence of multiple bubbles, at diferent directions, results in an integrated temperature along the line of sight much larger than the simulations of non-precessing jets. This result is in agreement with the observations. The simulations reveal that the cooling flows cessed ~50–70 Myr after the AGN jets are started.

Regulation of Thermal Conductivity in Hot Galaxy Clusters by MHD Turbulence

The role of thermal conduction in regulating the thermal behavior of cooling flows in galaxy clusters is reexamined. Recent investigations have shown that the anisotropic Coulomb heat flux caused by a magnetic field in a dilute plasma drives a dynamical instability. A long standing problem of cooling flow theory has been to understand how thermal conduction can offset radiative core losses without completely preventing them. In this Letter we propose that magnetohydrodynamic turbulence driven by the heat flux instability regulates field-line insulation and drives a reverse convective thermal flux, both of which may mediate the stabilization of the cooling cores of hot clusters. This model suggests that turbulent mixing should accompany strong thermal gradients in cooling flows. This prediction seems to be supported by the spatial distribution of metals in the central galaxies of clusters, which shows a much stronger correlation with the ambient hot gas temperature gradient than with the parent stellar population.

Shock heating by Fanaroff–Riley type I radio sources in galaxy clusters

Abstract Feedback by active galactic nuclei (AGN) is frequently invoked to explain the cut-off of the galaxy luminosity function at the bright end and the absence of cooling flows in galaxy clusters. Meanwhile, there are recent observations of shock fronts around radio-loud AGN. Using realistic 3D simulations of jets in a galaxy cluster, we address the question what of fraction of AGN energy is dissipated in shocks. We find that weak shocks that encompass the AGN have Mach numbers of 1.1–1.2 and dissipate at least 2 per cent of the mechanical luminosity of the AGN. In a realistic cluster medium, even a continuous jet can lead to multiple shock structures, which may lead to an overestimate of the AGN duty cycles inferred from the spatial distribution of waves.

Impact of tangled magnetic fields on fossil radio bubbles

There is growing consensus that feedback from active galactic nuclei (AGN) is the main mechanism responsible for stopping cooling flows in clusters of galaxies. AGN are known to inflate buoyant bubbles that supply mechanical power to the intracluster gas [intracluster medium (ICM)]. High Reynolds number hydrodynamical simulations show that such bubbles get entirely disrupted within 100 Myr, as they rise in cluster atmospheres, which is contrary to observations. This artificial mixing has consequences for models trying to quantify the amount of heating and star formation in cool core clusters of galaxies. It has been suggested that magnetic fields can stabilize bubbles against disruption. We perform magnetohydrodynamical simulations of fossil bubbles in the presence of tangled magnetic fields using the high-order pencil code. We focus on the physically motivated case where thermal pressure dominates over magnetic pressure and consider randomly oriented fields with and without maximum helicity and a case where large-scale external fields drape the bubble. We find that helicity has some stabilizing effect. However, unless the coherence length of magnetic fields exceeds the bubble size, the bubbles are quickly shredded. As observations of Hydra A suggest that length-scale of magnetic fields may be smaller than typical bubble size, this may suggest that other mechanisms, such as viscosity, may be responsible for stabilizing the bubbles. However, since Faraday rotation observations of radio lobes do not constrain large-scale ICM fields well if they are aligned with the bubble surface, the draping case may be a viable alternative solution to the problem. A generic feature found in our simulations is the formation of magnetic wakes where fields are ordered and amplified. We suggest that this effect could prevent evaporation by thermal conduction of cold Hα filaments observed in the Perseus cluster.

The effect of heat conduction on the interaction of disk and corona around black holes

Heat conduction plays an important role in the balance between heating and cooling in many astrophysical objects, e.g. cooling flows in clusters of galaxies. Here we investigate the effect of heat conduction on the interaction between a cool disk and a hot corona around black holes. Using the one-radial-zone approximation, we study the vertical structure of the disk corona and derive evaporation and coronal mass flow rates for various reduced thermal conductivities. We find lower evaporation rates and a shift in the evaporation maxima to smaller radii. This implies that the spectral state transition occurs at a lower mass flow rate and a disk truncation closer to the black hole. Reductions of thermal conductivity are thought to be magnetically caused and might vary from object to object by a different configuration of the magnetic fields.

Dynamical Friction and Cooling Flows in Galaxy Clusters

We investigate a model of galaxy clusters in which the hot intracluster gas is efficiently heated by dynamical friction (DF) of galaxies. We allow for both subsonic and supersonic motions of galaxies and use the gravitational drag formula in a gaseous medium presented by Ostriker (1999). The energy lost by the galaxies is either redistributed locally or into a Gaussian centered on the galaxy. We find that the condition of hydrostatic equilibrium and strict energy balance yields a trivial isothermal solution T_iso, independent of radius, or rising temperature distributions provided T_iso/gamma < T < T_iso, where gamma is the adiabatic index of the gas. The isothermal temperature corresponds to the usual scaling relation between the gas temperatures and the velocity dispersions of galaxies. However the minimal temperature associated with the rising solutions is ~ 0.5 T_vir, larger than that inferred from observations, the radial distribution of galaxy masses notwithstanding. Heating by supersonically moving galaxies cannot suppress thermal instability, although it can lengthen the growth time up to the level comparable to the ages of clusters when Mach number of galaxies is less than about two. We show using numerical hydrodynamic simulations that DF-induced heating is generally unable to produce stable equilibrium cores by evolving arbitrary non-equilibrium clusters, although it can lengthen the cooling time. We conclude that DF-induced heating alone is an unlikely solution to the cooling flow problem, although it can still be an important heat supplier, considerably delaying cooling catastrophe. We discuss other potential consequences of DF of galaxies in galaxy clusters.

An Observationally Motivated Framework for AGN Heating of Cluster Cores

The cooling flow problem is a long-standing puzzle that has received considerable recent attention, in part because the mechanism that quenches cooling flows in galaxy clusters is likely to be the same mechanism that sharply truncates the high end of the galaxy luminosity function. Most of the recent models for halting cooling in clusters have focused on AGN heating, but the actual heating mechanism has remained mysterious. Here we present a framework for AGN heating derived from a Chandra survey of gas entropy profiles within cluster cores. This set of observations strongly suggests that the inner parts of cluster cores are shock-heated every ~108 yr by intermittent AGN outbursts, driven by a kinetic power output of ~1045 ergs s-1 and lasting at least 107 yr. Beyond ~30 kpc these shocks decay to sound waves, releasing buoyant bubbles that heat the core's outer parts. Between heating episodes, cooling causes the core to relax toward an asymptotic pure cooling profile. The density distribution in this asymptotic profile is sufficiently peaked that the AGN shock does not cause a core entropy inversion, allowing the cluster core to retain a strong iron abundance gradient, as observed.

Thermal conduction and reduced cooling flows in galaxy clusters

Conduction may play an important role in reducing cooling flows in galaxy clusters. We analyse a sample of sixteen objects using Chandra data and find that a balance between conduction and cooling can exist in the hotter clusters (T > 5 keV), provided the plasma conductivity is close to the unhindered Spitzer value. In the absence of any additional heat sources, a reduced mass inflow must develop in the cooler objects in the sample. We fit cooling flow models to deprojected data and compare the spectral mass deposition rates found to the values required to account for the excess luminosity, assuming Spitzer-rate heat transfer over the observed temperature gradients. The mass inflow rates found are lower than is necessary to maintain energy balance in at least five clusters. However, emission from cooling gas may be partially absorbed. We also compute the flux supplied by turbulent heat transport (Cho et al. 2003) and find conductivity profiles which follow a strikingly similar temperature dependence to the conductivity values required to prevent cooling. Finally, we show that the cluster radio luminosities vary by over five orders of magnitude in objects with X-ray luminosities differing by no more than a factor of a few. This suggests that there is unlikely to be a straightforward correlation between the mechanical power provided by the radio lobes and the rate of energy loss in cooling flow clusters.

Problems in suppressing cooling flows in clusters of galaxies by global heat conduction

I use a simple analytical model to show that simple heat conduction models cannot significantly suppress cluster cooling flows. I build a static medium where heat conduction globally balances radiative cooling, and then perturb it. I show that a perturbation extending over a large fraction of the cooling flow region will grow to the non-linear regime within a Hubble time. Such perturbations are reasonable in clusters which frequently experience mergers and/or AGN activity. This result strengthens previous findings which show that a steady solution does not exist for a constant heat conduction coefficient.

Thermal Conduction in Clusters of Galaxies

We estimate the thermal conductivity of a weakly collisional magnetized plasma with chaotic magnetic field fluctuations. When the fluctuation spectrum extends over two or more decades in wavevector, we find that thermal conduction is very efficient; the conduction coefficient is only a factor of ~5 below the classical Spitzer estimate. We suggest that conduction could play a significant role in cooling flows in clusters of galaxies.

A Moderate Cluster Cooling Flow Model

We propose that the outer portions of cooling flows in clusters of galaxies are frequently disrupted by radio jets and that their effective ages are much shorter than the cluster ages. The inner regions, where the gas density is higher, are more difficult to disrupt and may continue to harbor cooling flows even after disruption events. The main assumption of the proposed scenario is that, on a timescale of ~2-4 × 109 yr, the cD galaxies in cooling flow clusters undergo powerful bursts of active galactic nucleus (AGN) activity that produce strong radio jets. The radio jets excite shocks in the inner regions (r 100 kpc) of cooling flow clusters. A radio burst may result from the accretion of cooling material by the central black hole or from a collision with a subcluster. We assume that the jets remain strong, with kinetic powers of ~1047 ergs s-1 for ~107 yr. The jets excite shock waves moving at several times 103 km s-1 and heat the cooling flow region, hence terminating it in the outer regions. The proposed scenario predicts that the total accreted mass due to the cooling flow is an order of magnitude lower than the mass accreted according to the cooling flow model (which assumes an undisturbed cooling flow for a time equal to the age of the cluster). The scenario, therefore, brings into agreement the observations that (1) a large fraction of clusters harbor cooling flows, (2) strong optical and radio activity are present only in the very inner regions of cooling flows, and (3) there is a lack of a satisfactory reservoir of the expected (in the standard cooling flow model) large mass that has been cooling over the life of the cluster.

On Cooling Flows and the Sunyaev‐Zeldovich Effect

We study the effect of cooling flows in galaxy clusters on the Sunyaev-Zeldovich (SZ) distortion and the possible cosmological implications. The SZ effect, along with X-ray observations of clusters, is used to determine the Hubble constant, H0. Blank-sky surveys of SZ effect are being planned to constrain the geometry of the universe through cluster counts. It is also known that a significant fraction of clusters have cooling flows in them, which change the pressure profile of intracluster gas. Since the SZ decrement depends essentially on the pressure profile, it is important to study possible changes in the determination of cosmological parameters in the presence of a cooling flow. We build several representative models of cooling flows and compare the results with the corresponding case of gas in hydrostatic equilibrium. We find that cooling flows can lead to an overestimation of the Hubble constant. Specifically, we find that for realistic models of cooling flow with mass deposition (varying with radius), there is of the order of ~10% deviation in the estimated value of the Hubble constant (from that for gas without a cooling flow) even after excluding ~80% of the cooling-flow region from the analysis. We also discuss the implications of using cluster counts from SZ observations to constrain other cosmological parameters in the presence of clusters with cooling flows.