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Many massive galaxies at the centres of relaxed galaxy clusters and groups have vast reservoirs of warm (∼10 000 K) and cold (≲100 K) gas. In many such low-redshift systems this gas is lifted into the hot interstellar medium in filamentary structures, which are long lived and are typically not forming stars. Two important questions are how far do these reservoirs cool and if cold gas is abundant what is the cause of the low star formation efficiency? Heating and excitation of the filaments from collisions and mixing of hot particles in the surrounding X-ray gas describes well the optical and near infrared line ratios observed in the filaments. In this paper we examine the theoretical properties of dense, cold clouds emitting in the far infrared and sub-millimetre through the bright lines of [C II] λ157 μm , [OI] λ63 μm and CO, exposed to such energetic ionizing particles. We find that optical depth effects and thermal pressure support alone cannot account for the line ratios; however, a very modest additional pressure support can fit the observed [O I] λ63 μm/[C II] λ157 μm line ratios by decreasing the density of the gas. Thismay also help stabilize the filaments against collapse leading to the low rates of star formation. We make predictions for the line ratios expected from cold gas under these conditions and present diagnostic diagrams for comparison with further observations. We provide our code as an Appendix. [ABSTRACT FROM AUTHOR]

ABSTRACT Far-ultraviolet emission has been detected from a knot of Hα emission in the Horseshoe filament, far out in the NGC 1275 nebula. The flux detected relative to the brightness of the Hα line in the same spatial region is very close to that expected from hydrogen two-photon continuum emission in the particle heating model of Ferland et al. if reddening internal to the filaments is taken into account. We find no need to invoke other sources of far-ultraviolet emission such as hot stars or emission lines from C iv in intermediate-temperature gas to explain these data. [ABSTRACT FROM AUTHOR]

ABSTRACT We present a 100 ks Chandra observation studying the extended X-ray emission around the powerful z= 1.04 quasar PKS 1229−021. The diffuse cluster X-ray emission can be traced out to ∼15 arcsec (∼120 kpc) radius and there is a drop in the calculated hardness ratio inside the central 5 arcsec consistent with the presence of a cool core. Radio observations of the quasar show a strong core and a bright, one-sided jet leading to the south-west hotspot and a second hotspot visible on the counter-jet side. Although the wings of the quasar point spread function (PSF) provided a significant contribution to the total X-ray flux at all radii where the extended cluster emission was detected, we were able to accurately subtract the PSF emission using Chandra Ray Tracer and marx simulations. The resulting steep cluster surface brightness profile for PKS 1229−021 appears similar to the profile for the FR II (Fanaroff-Riley class II) radio galaxy 3C 444, which has a similarly rapid surface brightness drop caused by a powerful shock surrounding the radio lobes. Using a model surface brightness profile based on 3C 444, we estimated the total cluster luminosity for PKS 1229−021 to be . We discuss the difficulty of detecting cool-core clusters, which host bright X-ray sources, in high redshift surveys. [ABSTRACT FROM AUTHOR]

We present a Chandra observation of the only low redshift, , galaxy cluster to contain a highly luminous radio-quiet quasar, H1821+643. By simulating the quasar point spread function, we subtract the quasar contribution from the cluster core and determine the physical properties of the cluster gas down to ) from the point source. The temperature of the cluster gas decreases from down to in the centre, with a short central radiative cooling time of , typical of a strong cool-core cluster. The X-ray morphology in the central shows extended spurs of emission from the core, a small radio cavity and a weak shock or cold front forming a semicircular edge at radius. The quasar bolometric luminosity was estimated to be , requiring a mass accretion rate of , which corresponds to half the Eddington accretion rate. We explore possible accretion mechanisms for this object and determine that Bondi accretion, when boosted by Compton cooling of the accretion material, could provide a significant source of the fuel for this outburst. We consider H1821+643 in the context of a unified active galactic nucleus (AGN) accretion model and, by comparing H1821+643 with a sample of galaxy clusters, we show that the quasar has not significantly affected the large-scale cluster gas properties. [ABSTRACT FROM AUTHOR]

It has long been known that photoionization, whether by starlight or other sources, has difficulty in accounting for the observed spectra of the optical filaments that often surround central galaxies in large clusters. This paper builds on the first of this series in which we examined whether heating by energetic particles or dissipative magnetohydrodynamic (MHD) wave can account for the observations. The first paper focused on the molecular regions which produce strong and CO lines. Here we extend the calculations to include atomic and low-ionization regions. Two major improvements to the previous calculations have been made. The model of the hydrogen atom, along with all elements of the H-like iso-electronic sequence, is now fully nl-resolved. This allows us to predict the hydrogen emission-line spectrum including excitation by suprathermal secondary electrons and thermal electrons or nuclei. We show how the predicted H i spectrum differs from the pure-recombination case. The second update is to the rates for inelastic collisions. We now use the values computed by Wrathmall et al. The rates are often much larger and allow the ro–vibrational level populations to achieve a thermal distribution at substantially lower densities than previously thought. We calculate the chemistry, ionization, temperature, gas pressure and emission-line spectrum for a wide range of gas densities and collisional heating rates. We assume that the filaments are magnetically confined. The gas is free to move along field lines so that the gas pressure is equal to that of the surrounding hot gas. A mix of clouds, some being dense and cold and others hot and tenuous, can exist. The observed spectrum will be the integrated emission from clouds with different densities and temperatures but the same pressure . We assume that the gas filling factor is given by a power law in density. The power-law index, the only free parameter in this theory, is set by matching the observed intensities of infrared lines relative to optical H i lines. We conclude that the filaments are heated by ionizing particles, either conducted in from surrounding regions or produced in situ by processes related to MHD waves. [ABSTRACT FROM AUTHOR]

The optical filaments found in many cooling flows in galaxy clusters consist of low-density cool gas surrounded by significant amounts of cosmic-ray and magnetic field energy. Their spectra show anomalously strong low-ionization and molecular emission lines when compared with Galactic molecular clouds exposed to ionizing radiation such as the Orion complex. Previous studies have shown that the spectra cannot be produced by O-star photoionization. Here, we calculate the physical conditions in dusty gas that is well shielded from external sources of ionizing photons and is energized either by cosmic rays or dissipative magnetohydrodynamics waves. Strong molecular hydrogen lines, with relative intensities similar to those observed, are produced. Selection effects introduced by the microphysics produce a correlation between the H2 line upper level energy and the population temperature. These selection effects allow a purely collisional gas to produce H2 emission that masquerades as starlight-pumped H2 but with intensities that are far stronger. This physics may find application to any environment where a broad range of gas densities or heating rates occur. [ABSTRACT FROM AUTHOR]

We use a deep XMM–Newton Reflection Grating Spectrometer observation to examine the X-ray emission from the core of the Centaurus cluster of galaxies. We clearly detect Fe xvii emission at four separate wavelengths, indicating the presence of cool X-ray emitting gas in the core of the cluster. Fe ions from Fe xvii to xxiv are observed. The ratio of the Fe xvii 17.1 Å lines to 15.0 Å line and limits on O vii emission indicate a lowest detected temperature in the emitting region of 0.3 to 0.45 keV (3.5 to ). The cluster also exhibits strong N vii emission, making it apparent that the N abundance is supersolar in its very central regions. Comparison of the strength of the Fe xvii lines with a solar metallicity cooling flow model in the inner 17 kpc radius gives mass deposition rates in the absence of heating of . Spectral fitting implies an upper limit of below 0.4 keV, below 0.8 keV and below 1.6 keV. The cluster contains X-ray emitting gas over at least the range of 0.35 to 3.7 keV, a factor of more than 10 in temperature. We find that the best-fitting metallicity of the cooler components is smaller than the hotter ones, confirming that the apparent metallicity does decline within the inner 1 arcmin radius. [ABSTRACT FROM AUTHOR]

We present Spitzer high-resolution spectra of off-nuclear regions in the central cluster galaxies NGC 1275 and NGC 4696 in the Perseus and Centaurus clusters, respectively. Both objects are surrounded by extensive optical emission-line filamentary nebulae, bright outer parts of which are the targets of our observations. The 10–37 μm spectra show strong pure-rotational lines from molecular hydrogen revealing a molecular component to the filaments which has an excitation temperature of ∼300−400 K. The flux in the 0−0 S(1) molecular hydrogen line correlates well with the strength of the optical lines, having about 3 per cent of the Hα+[N ii] emission. The 11.3-μm polycyclic aromatic hydrocarbon feature is seen in some spectra. Emission is also seen from both low- and high-ionization fine-structure lines. Molecular hydrogen cooler than ∼400 K dominates the mass of the outer filaments; the nebulae are predominantly molecular. [ABSTRACT FROM AUTHOR]

We use line-of-sight velocity information on the filamentary emission-line nebula of NGC 1275 to infer a dynamical model of the nebula's flow through the surrounding intracluster gas. We detect outflowing gas and flow patterns that match simulations of buoyantly rising bubbles from which we deduce that some of the nebula filaments have been drawn out of NGC 1275. We find a radial gradient of the ratio [N ii]λ6584/Hα which may be due to a variation in metallicity, interactions with the surrounding intracluster medium or a hardening of the excitation mechanism. We find no preferred spatial correlation of stellar clusters within the filaments and there is a notable lack of [O iii]λ5007 emission, therefore it is unlikely that the filaments are ionized by stellar ultraviolet. [ABSTRACT FROM AUTHOR]

We present the first results from a very deep Chandra X-ray observation of the core of the Perseus cluster of galaxies. A pressure map reveals a clear thick band of high pressure around the inner radio bubbles. The gas in the band must be expanding outwards and the sharp front to it is identified as a shock front, yet we see no temperature jump across it; indeed, there is more soft emission behind the shock than in front of it. We conclude that in this inner region either thermal conduction operates efficiently or the co-existing relativistic plasma seen as the radio mini-halo is mediating the shock. If common, isothermal shocks in cluster cores mean that we cannot diagnose the expansion speed of radio bubbles from temperature measurements alone. They can at times expand more rapidly than currently assumed without producing significant regions of hot gas. Bubbles may also be significantly more energetic. The pressure ripples found in earlier images are identified as isothermal sound waves. A simple estimate based on their amplitude confirms that they can be an effective distributed heat source able to balance radiative cooling. We see multiphase gas with about at a temperature of about 0.5 keV. Much, but not all, of this X-ray emitting cooler gas is spatially associated with the optical filamentary nebula around the central galaxy, NGC 1275. A residual cooling flow of about may be taking place. A channel is found in the pressure map along the path of the bubbles, with indications found of outer bubbles. The channel connects in the south (S) with a curious cold front. [ABSTRACT FROM AUTHOR]

We present clear spectroscopic detections of molecular hydrogen in the outer filaments of the Hα nebula surrounding the central galaxy of the Perseus cluster, NGC 1275. This implies the presence of warm molecular gas clouds at projected distances up to 24 kpc from the nucleus of the host galaxy, embedded in the hot intracluster medium. The emission-line intensity ratios reveal that the H2 is predominately thermally excited with excitation temperatures of 1600–2200 K, suggesting a lack of pressure balance between the molecular component and its surrounding medium. Excitation by stellar ultraviolet or by the central active galactic nucleus is shown to be unlikely, while thermal excitation by X-rays or conduction from the ICM or shocks are all possibilities. Evidence for a non-thermal component is found in the spectra of some regions based on a low ortho-to-para ratio. [ABSTRACT FROM AUTHOR]

We present results from an analysis of two Chandra observations of the rich, nearby galaxy cluster Abell 2199. We find evidence (having corrected for projection effects) for radial gradients in temperature and metallicity in the X-ray emitting gas: the temperature drops from kT ~ 4.2 keV at R = 200 kpc to 1.6 keV within R = 5 kpc of the centre. The metallicity rises from ~0.3 solar at R = 200 kpc to ~0.7 solar at R = 30 kpc before dropping to 0.3 solar within the central 5 kpc. We find evidence for structure in the surface brightness distribution associated with the central radio source 3C 338. No evidence is found for the gas having a large spread in temperature at any particular location despite the cooling time being short (< 10[SUP9] yr) within the central ~15 kpc. Heating and mass cooling rates are calculated for various assumptions about the state of the gas. [ABSTRACT FROM AUTHOR]

This is a progress report on our numerical simulations of conditions in the cold cores of cooling flow condensations. The physical conditions in any non-equilibrium plasma are the result of a host of microphysical processes, many involving reactions that are research areas in themselves. We review the dominant physical processes in our previously published simulations, to clarify those issues that have caused confusion in the literature. We show that conditions in the core of an X-ray-illuminated cloud are very different from those found in molecular clouds, largely because carbon remains substantially atomic and provides powerful cooling through its far infrared lines. We show how the results of the Opacity Project have had a major impact on our predictions, largely because photoionization cross-sections of atoms and first ions are now calculated to be far larger than estimated previously. Finally, we show that the predicted conditions are strongly affected by complexities such as microturbulence or the presence of small amounts of dust. Large masses of cold dense gas, in addition to the warmer molecular gas detected recently, could be present in cooling flows. [ABSTRACT FROM AUTHOR]