Cooling Flow Research Articles

Solving the cooling flow problem of galaxy clusters by dark matter neutralino annihilation.

Recent x-ray observations revealed that strong cooling flow of intracluster gas is not present in galaxy clusters, even though it is predicted theoretically if there is no additional heating source. I show that relativistic particles produced by dark matter neutralino annihilation in cluster cores provide a sufficient heating source to suppress the cooling flow, under reasonable astrophysical circumstances including adiabatic growth of central density profile, with appropriate particle physics parameters for dark matter neutralinos. In contrast to other astrophysical heat sources, such as active galactic nuclei, this process is a steady and stable feedback over cosmological time scales after turned on.

Cooling by heat conduction inside magnetic flux loops and the moderate cluster cooling-flow model

I study non-radiative cooling of X-ray emitting gas via heat conduction along magnetic field lines inside magnetic flux loops in cooling flow clusters of galaxies. I find that such heat conduction can reduce the fraction of energy radiated in the X-ray band by a factor of 1.5-2. This non-radiative cooling joins two other proposed non-radiative cooling processes, which can be more efficient. These are mixing of cold and hot gas, and heat conduction initiated by magnetic fields reconnection between hot and cold gas. These processes when incorporated into the moderate cooling flow model lead to a general cooling flow model with the following ingredients. (1) Cooling flow does occur, but with a mass cooling rate about 10 times lower than in old versions of the cooling flow model. Namely, heating occurs such that the effective age of the cooling flow is much below the cluster age, but the heating can't prevent cooling altogether. (2) The cooling flow region is in a non-steady state evolution. (3) Non-radiative cooling of X-ray emitting gas can bring the model to a much better agreement with observations. (4) The general behavior of the cooling flow gas, and in particular the role played by magnetic fields, make the intracluster medium in cooling flow clusters similar in some aspects to the active solar corona.

Subclustering in Cooling and Non-cooling Flow Clusters

For 20 hot luminous Beppo-SAX galaxy clusters the occurrence of substrucures has been investigated in 2-D optical data, mainly through the application of the wavelet analysis. Our results are compared with others, coming from 3-D optical data, as well as X-rays. In this sample of galaxy clusters a higher percentage of substructure is observed among non-cooling flow clusters than cooling flow clusters.

The Interaction of AGN Jets with the Intracluster Medium

Chandra revealed that many galaxy clusters exhibit an interaction of AGN jets with intraclaster medium (ICM). For this reason, they are thought to supply a large amount of heat energy to the ICM so that the cooling flow (CF) is restricted. We analyzed 2A 0335+096 and Abell 2199 in detail. As a result, we found that radio jets do not have enough energy to reduce the overall radiative cooling. We also studied the hot gas cavities of NGC 4636 and found a significant increase in temperature and absorption column density around cavities. However, the energy of a gas disturbance is not large in comparison with the clusters. These results indicate that the activity of most of the radio galaxies has faded, and thus they cannot supply a significant amount of heat energy to the ICM.

Development, assessment and application of TRAC-BF1/v2001.2 for beyond design basis BWR LOCA transients

In preparation for the possible transition to risk based licensing, it is increasingly important to demonstrate the applicability of Best Estimate codes to more extreme conditions corresponding for example to limited equipment availability. With this idea in mind, we have reviewed the application of TRAC-BF1 to Large and Small Break LOCAs. In this context this work describes the assessment and applications of the Penn State University (PSU) version 2001.2 of TRAC-BF1 with all the PSI updates, to the analysis of hypothetical Large Break (LB) LOCAs in a BWR/4 with postulated limited ECC availability. Since in contrast to a LB-LOCA in a BWR with full ECC availability, the rod surface temperatures reach relatively high values, additional assessment of the code under such conditions is required. Hence, after analyzing bottom flooding separate-effect experiments in two different heater rod bundles and a TLTA LB-LOCA test, we shall present and discuss the results of the analysis of a LB-LOCA with highly restricted Emergency Core Cooling flow in a BWR/4. In this context, we shall also assess different descretization of some terms of the 3D momentum equations, which was found to be important in the analysis of BWR Small Break LOCAs.

Resolving Molecular Gas in the Central Galaxies of Cooling Flow Clusters

We present aperture synthesis images of the CO (1-0) line emission in five central galaxies in cooling flow clusters using the Owens Valley Millimeter Array. Three of the five sources are significantly resolved, but the majority of the emission is from a compact ( 1022 cm-2) column density. We review the implications of these results and the prospects for observations in the near future.

Observed Non–Steady State Cooling and the Moderate Cluster Cooling Flow Model

We examine recent developments in the cluster cooling flow scenario following recent observations by Chandra and XMM-Newton. We show that the distribution of gas emissivity verses temperature determined by XMM-Newton gratings observations demonstrates that the central gas in cooling flow clusters cannot be in simple steady-state. Combining this result with the lack of spectroscopic evidence for gas below one-third of the ambient cluster temperature is strong evidence that the gas is heated intermittently. While the old steady-state isobaric cooling flow model is incompatible with recent observations, a cooling flow in which the gas undergoes intermittent heating that effectively reduces the age of a cooling flow is consistent with observations. Most of the gas within cooling flows resides in the hottest gas, which is prevented from cooling continuously and attaining a steady-state configuration. This results in a mass cooling rate that decreases with decreasing temperature, with a much lower mass cooling rate at the lowest temperatures. The present paper strengthens the moderate cooling flow model, which can accommodate the unique activities observed in cooling flow clusters.

On the Relationship between Cooling Flows and Bubbles

A common feature of the X-ray bubbles observed in Chandra images of some clusters is that they appear to be surrounded by bright, cool shells. Temperature maps of a few nearby luminous clusters reveal that the shells consist of the coolest gas in the clusters -- much cooler than the surrounding medium. Using simple models, we study the effects of this cool emission on the inferred cooling flow properties of clusters. We find that the introduction of bubbles into model clusters that do NOT have cooling flows results in temperature and surface brightness profiles that resemble those seen in nearby clusters. They also approximately reproduce the recent XMM-Newton and Chandra observations of a high minimum temperature of 1-3 keV. Hence, bubbles, if present, must be taken into account when inferring the physical properties of the ICM. In the case of some clusters, bubbles may account entirely for these observed features, calling into question their designation as clusters with cooling flows. However, since not all nearby clusters show bubble-like features, we suggest that there may be a diverse range of physical phenomena that give rise to the same observed features.

Feedback Heating in Cluster and Galactic Cooling Flows

Cluster cooling flow models that include both active galactic nuclei (AGN) heating and thermal conduction can reduce the overall mass cooling rate and simultaneously sustain density and temperature profiles similar to those observed. These computed flows have no ad hoc mass dropout. To achieve this agreement, the thermal conductivity must be about 0.35 ± 0.10 times the Spitzer value, similar to that advocated by Narayan & Medvedev. However, when applied to galaxy/group scales, the synergistic combination of AGN heating and conduction is less satisfactory. When the computed density profile and the global cooling rate are lowered by AGN heating to match observations of these smaller scale flows, the gas temperatures within ~10 kpc are too large. In addition, best-fitting flows in galaxy/groups with AGN heating and thermal conduction require conductivities much closer to the Spitzer value, ~0.5-1. Another difficulty with galaxy/group flows that combine AGN heating and conduction is that the iron enrichment by Type Ia supernovae is more effective when the gas density is lowered by heating to match the observations. The hot-gas iron abundance in galactic flows with heating and conduction greatly exceeds observed values throughout most of the galaxy. Galactic/group flows with central heating and conduction, therefore, require an additional process that removes the iron: failure of Type Ia supernovae ejecta to go into the hot phase, selective cooling, etc.

ITAL]Chandra[/ITAL] Observations of Abell 2029: No Cooling Flow and a Steep Abundance Gradient

We have obtained high spatial resolution temperature and abundance profiles for the galaxy cluster Abell 2029 with the Chandra ACIS-S instrument. Our observations reveal that the spectra are well fitted by a single-phase gas in each annulus. While the temperature of the intracluster medium drops from ~9 keV at 3' (260 h kpc) to 3 keV in the central 5'' (5 h kpc) of the cluster, there is no evidence of gas emitting at temperatures below 3 keV. The addition of a cooling flow component does not improve the fits, despite previous claims for a massive cooling flow. There is also no evidence of excess absorption above the Galactic NH-value. We also observe a steep Fe abundance gradient, such that ZFe 2 Z☉ (assuming meteoritic solar Fe abundance) in the core, consistent with significant enrichment from Fe-rich Type Ia supernovae in the cD galaxy. The Fe abundance drops to ≈0.5 Z☉ at 3' (260 h kpc), in good agreement with previous BeppoSAX measurements. The high-resolution image reveals neither a strong central point source nor any filamentary structure related to a cooling flow or a merger. The absence of a strong merger signature argues against the creation of the wide-angle-tailed radio source morphology in a merger event.

Do Cooling Flows Survive Cluster Mergers?

We report the results of recent numerical simulations of the head-on merger of a cooling flow cluster with an infalling subcluster of galaxies. The objective of these simulations was to examine the effects of different types of cluster mergers (with 16 : 1 and 4 : 1 mass ratios) on the evolution of cluster cooling flows (with mass accretion rates of 100 and 400 M☉ yr-1). The two-dimensional simulations were performed with a combined hydrodynamics/N-body code on a uniform grid with a resolution of 20 kpc (~12 zones per core radius). In our simulations, cooling flow disruption is indicated by a dramatic increase (by a factor of 10-40) in the central cooling time of the primary cluster. We find that the ram pressure of the infalling gas is crucial in determining the fate of the cooling flow, because disruption occurs when a substantial amount of subcluster gas reaches the primary's core. In such cases, the subcluster gas can increase the central cooling time by displacing the high-density cooling gas and by heating it via shocks and turbulent gas motions. However, the fate of a merging cooling flow is also dependent on its initial cooling time. In cases where the initial cooling time is very short (i.e., 10-40 times shorter than the Hubble time), then even if the flow is disrupted, the central cooling time will remain less than a Hubble time, and the flow will likely reestablish itself. This has an important observational consequence, because such clusters will be classified as cooling flows on the basis of their cooling times, even though they have undergone a significant merger. In addition, we find that there is a time delay between core crossing and the point at which the central cooling time of a disrupted flow becomes of order a Hubble time. Thus, even in the case of disruption, a cluster can be classified as a cooling flow and exhibit substructure (indicative of a merger) for 1-2 Gyr after merging with a subcluster. We argue that our results make it possible to reconcile the high cooling flow frequency inferred by some observations with both high merger rates and a high frequency of substructure.

Heated Cooling Flows

In conventional models of galactic and cluster cooling flows widespread cooling (mass dropout) is assumed to avoid accumulation of unacceptably large central masses. This assumption is often criticized because the observed mass of cooled gas and newly formed stars is much less than the cooling rates imply. Moreover, recent XMM observations have failed to find spectral evidence for locally cooling gas. This remarkable discovery has revived the notion that cooling flows do not cool in the manner previously assumed but are instead heated by some process such as an intermittent or low-level active galactic nucleus in the central galaxy. To explore this hypothesis, we consider the gasdynamical consequences of heated galactic cooling flows under many different heating scenarios without specifying the detailed physics of the heating process. We have been unable to find a single acceptable heated flow in reasonable agreement with well-observed hot gas temperature and density profiles. Even for poor fits to the observations, these models require finely tuned heating scenarios. Idealized flows in which radiative cooling is perfectly balanced by global heating are grossly incompatible with observations. Flows heated by central feedback often result in episodic cooling that is associated with quasi-cyclic changes in the hot gas density profile; at present there is no observational evidence for this. Paradoxically, when cooling flows are centrally heated (or pressurized), they experience spontaneous nonlinear compressions that result in spatially widespread cooling instabilities. Therefore, spectral evidence for cooling gas is expected in either traditional cooling flows with ad hoc mass dropout or heated cooling flows without the dropout assumption.

Photospheric Abundances of the Hot Stars in NGC 1399 and Limits on the Fornax Cluster Cooling Flow

We present far-UV spectroscopy of the giant elliptical galaxy NGC 1399, obtained with the Far Ultraviolet Spectroscopic Explorer. Of all quiescent ellipticals, NGC 1399 has the strongest known upturn—a sharp spectral rise shortward of 2500 A. It is now well established that this emission comes from hot horizontal branch stars and their progeny; however, the chemical composition of these stars has been the subject of a longstanding debate. For the first time in observations of any elliptical galaxy, our spectra clearly show photospheric metallic absorption lines within the UV upturn. The abundance of N is at 45% solar, Si is at 13% solar, and C is at 2% solar. Such abundance anomalies are a natural consequence of gravitational diffusion. These photospheric abundances fall in the range observed for subdwarf B stars of the Galactic field. Although NGC 1399 is at the center of the Fornax Cluster, we find no evidence for O VI cooling flow emission. The upper limit to λλ1032, 1038 emission is 3.9 × 10-15 ergs s-1 cm-2, equivalent to 0.14 M☉ yr-1 and less than the value predicted by simple cooling flow models of the NGC 1399 X-ray luminosity.

Nonthermal X‐Ray Emission: An Alternative to Cluster Cooling Flows?

We report the results of experiments aimed at reducing the major problem with cooling flow models of rich cluster X-ray sources, i.e., the fact that most of the cooled gas or its products have not been found. Here we show that much of the X-ray emission usually attributed to cooling flows can, in fact, be modeled by a power-law component which is indicative of a source(s) other than thermal bremsstrahlung from the intracluster medium. We find that adequate simultaneous fits to ROSAT PSPC and ASCA GIS/SIS spectra of the central regions of 10 clusters are obtained for two-component models that include a thermal plasma component that is attributable to hot intracluster gas and a power-law component that is likely generated by compact sources and/or extended nonthermal emission. For five of the clusters that purportedly have massive cooling flows, the best-fit models have power-law components that contribute ~30% of the total flux (0.14-10.0 keV) within the central 3'. Because cooling flow mass deposition rates are inferred from X-ray fluxes, our finding opens the possibility of significantly reducing cooling rates.

Temperature Profiles of Nearby Clusters of Galaxies

We report results from the analysis of 21 nearby galaxy clusters, 11 with cooling flow (CF) and 10 without cooling flow, observed with BeppoSAX. The temperature profiles of both CF and non-CF systems are characterized by an isothermal core extending out to ~0.2r180; beyond this radius both CF and non-CF cluster profiles rapidly decline. Our results differ from those derived by other authors, who found either continuously declining profiles or substantially flat profiles. Neither the CF nor the non-CF profiles can be modeled by a polytropic temperature profile, the reason being that the radius at which the profiles break is much larger than the core radius characterizing the gas density profiles. For r > 0.2r180, where the gas can be treated as a polytrope, the polytropic indexes derived for CF and non-CF systems are respectively 1.20 ± 0.06 and 1.46 ± 0.06. The former index is closer to the isothermal value, 1, and the latter to the adiabatic value, . Published hydrodynamic simulations do not reproduce the peculiar shape of the observed temperature profile, probably suggesting that a fundamental ingredient is missing.

Evidence for a Heated Gas Bubble inside the "Cooling Flow" Region of MKW 3s

We report on the deep Chandra observation of the central r = 200 kpc region of the cluster of galaxies MKW 3s, which was previously identified as a moderate cooling flow cluster. The Chandra image reveals two striking features—a 100 kpc long and 21 kpc wide filament, extending from the center to the southwest, and a nearly circular, 50 kpc diameter depression 90 kpc south of the X-ray peak. The temperature map shows that the filamentary structure is colder while the surface brightness depression is hotter than the average cluster temperature at any radius. The hot and the cold regions indicate that both cooling and heating processes are taking place in the center of MKW 3s. We argue that the surface brightness depression is produced by a heated, low-density gas bubble along the line of sight. We suggest that the heated bubble is produced by short-lived nuclear outbursts from the central galaxy.

Is the Gas in Cooling Flows Multiphase?

Employing XMM-Newton EPIC data we perform a detailed comparison between different spectral models to test whether the gas in cooling flows is multiphase or not. Our findings all point in the same direction, namely, that gas in cooling flows does not show the wide distribution of temperatures expected from standard multiphase models. This result has profound implications for models. First, the large absorption column depths inferred by previous analysis of spectra are most likely an artifact following from the application of an incorrect spectral model to the data. Second, the mass deposition and mass flow are likely to be much smaller than previously thought. Last, but perhaps not least, the term cooling-flow cluster is probably no longer appropriate, as it describes a phenomenon of smaller entity and impact than previously thought. We propose to substitute it with that of cool-core cluster. The latter definition is less ambitious than the first, as it reflects only an observational fact rather than an inferred physical property, the flow, but has the undeniable advantage of being firmer.

A High‐Resolution Study of the Hydra A Cluster withChandra: Comparison of the Core Mass Distribution with Theoretical Predictions and Evidence for Feedback in the Cooling Flow

The cooling flow cluster Hydra A was observed during the orbital activation and calibration phase of the Chandra Observatory. While the X-ray image of the cluster exhibits complex structure in the central region as reported in McNamara et al., the large-scale X-ray morphology of the cluster is fairly smooth. A spectroscopic analysis of the ACIS data shows that the gas temperature in Hydra A increases outward, reaches a maximum temperature of 4 keV at 200 kpc, and then decreases slightly at larger radii. The distribution of heavy elements is nonuniform, with a factor of 2 increase in the Fe and Si abundances within the central 100 kpc. Beyond the central 100 kpc the Si-to-Fe abundance ratio is twice solar, while the Si-to-Fe ratio of the central excess is consistent with the solar value. One of the more surprising results is the lack of spectroscopic evidence for multiphase gas within the bulk of the cooling flow. Beyond the central 30 kpc, the ACIS spectra are adequately fitted with a single-temperature model. The addition of a cooling flow component does not significantly improve the fit. Only within the central 30 kpc (where the cooling time is less than 1 Gyr) is there spectroscopic evidence for multiphase gas. However, the spectroscopic mass deposition rate is more than a factor of 10 less than the morphologically derived mass accretion rate at 30 kpc. We propose that the cooling flow region is convectively unstable owing to heating by the central radio source, which significantly reduces the net accretion rate. In addition, we show that the mass distribution within the central 30-200 kpc region scales as ρd ∝ r-1.3, intermediate between an NFW and a Moore profile, but with a best-fit NFW concentration parameter (cNFW = 12) approximately 3 times greater than that found in numerical simulations. However, given the limited photon statistics, we cannot rule out the presence of a flat-density core with a core radius less than 30 kpc.

The Detection of a Cooling Flow Elliptical Galaxy from O [CSC]vi[/CSC] Emission

Cooling flow models for the hot gas in elliptical galaxies predict that gas is cooling at a rate of ~1 M☉ yr-1, yet there is little evidence for this phenomenon beyond the X-ray wave band. If hot gas is cooling, it will pass through the 3 × 105 K regime and radiate in the O VI λλ1032, 1038 ultraviolet lines, which can be detected with the Far-Ultraviolet Spectroscopic Explorer (FUSE), and here we report on FUSE observations of the X-ray bright early-type galaxies NGC 1404 and NGC 4636. In NGC 1404, the O VI doublet is not detected, implying a cooling rate less than 0.3 M☉ yr-1, which is below the predicted values from the cooling flow model of 0.4-0.9 M☉ yr-1. In NGC 4636, both O VI lines are clearly detected, indicating a cooling rate of 0.43 ± 0.06 M☉ yr-1, which falls within the range of values from the cooling flow prediction, 0.36-2.3 M☉ yr-1, and is closest to the model where the production of the cooled gas is distributed through the galaxy. The emission-line widths, 44 ± 15 km s-1, are close to the Doppler broadening value (30 km s-1), indicating that the flow is quiescent rather than turbulent and that the flow velocity is less than 30 km s-1.

Numerical investigation of film cooling flow induced by cylindrical and shaped holes.

The present study is the second half of a two part work carried out in collaboration with SNECMA which tends to investigate a shaped hole film cooling experimentally and numerically. The aim of this paper is the numerical study of 3D phenomena induced by cylindrical and shaped hole film cooling on a flat wall. The two calculations show up classical structures such as horseshoe or kidney vortices and their differences according to the shape configuration. A detailed study demonstrates their influence on the jet behaviour. Comparing both cases reveals the impact of shaping on the velocity field and vortex motions. The calculations were performed by resolving the 3D Navier-Stokes equations associated with a k-epsilon turbulence model. The solver is the CANARI code developed by ONERA.