The physical conditions within dense cold clouds in cooling flows - II

Ferland, Gary J.; Fabian, A. C.; Johnstone, R. M.

doi:10.1046/j.1365-8711.2002.05470.x

Cited by 18 publications

(27 citation statements)

References 62 publications

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“…These mixing layers surround cold clouds embedded in the hot X-ray emitting gas which fills the cores of the cooling flows. In parallel, from studies of the physical conditions in the cooling flow centers, Ferland et al (2002) suggest that large masses of cold dense gas, in addition to the warmer molecular gas detected recently, could indeed be present. If those mixing layers are indeed so numerous, and associated with the X-ray emission, as suggested by the combination of (i)−(iii), then charge-exchange between cold gas and relatively moving hot gas could occur in the interfaces and generate X-rays.…”

Section: Galaxy Clusters and Cooling Flowsmentioning

confidence: 81%

On the contribution of charge-exchange induced X-ray emission in the ISM and ICM

Lallement¹

2004

A&A

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Abstract. X-ray emission can be generated by charge-exchange (CXE) between highly charged ions of a hot plasma and neutral species of an interacting cool/warm gas, a phenomenon recently observed in the case of the solar wind interaction with cometary, interstellar, and geocoronal neutrals (Lisse et al. 1996;Cox 1998;Cravens 1997Cravens , 2002. Charge-exchange processes are included in most theoretical models of hot interstellar plasmas interacting with partially neutral gas, resulting in a modification of the physical states of the gases in the interaction region. However, the contribution of the charge-exchange induced X-ray emission produced at these interfaces has hitherto not been considered to be significant. The detailed calculations performed by Wise & Sarazin (1989), motivated by the observations of X-ray emission following chargeexchange in laboratory fusion devices, have shown that the emission is negligible in the case of an SNR fast shock. Our goal here is to investigate its relative importance in different astrophysical cases. We simplistically consider interfaces between partially neutral and hot gas in the following cases: (a) a supernova blast wave propagating in a neutral (or partially ionized) ISM (b) a galactic wind engulfing a halo dense cloud; (c) a high velocity cloud (HVC) moving through the halo, and (c) a dense cloud moving in intra-cluster gas. Although the phenomenon is restricted to the very narrow envelopes defined by the mean free path of the neutrals through the hot plasma, it is easy to show that its efficiency is such that the volume emissivity from these interfaces can be orders of magnitude above the emissivity of the hot gas itself, and, more important, that its relative contribution increases with decreasing hot gas density. As a consequence it should be at maximum in mixing layers in very low density hot gas. Our preliminary results suggest that the charge-exchange X-ray emission from the interface does not contribute significantly in case (a), in agreement with Wise & Sarazin, except marginally for lines of sight tangent to the interfaces, but that it can be a non-negligible fraction of the hot gas emission in cases (b-d). Detailed self-consistent models of plasma and neutral atom distributions in conduction fronts or contact discontinuities are needed for better estimates. In the easiest test case of HVCs, there is a good agreement between our predicted equivalent emission measure EM ≈ 0.01 cm −6 pc for complex C-type clouds and their soft X-ray emission measured by ROSAT (e.g. Kerp et al. 1996). If confirmed by more realistic calculations, a contribution of the CXE emission may play a role in a number of astrophysical cases: (i) tight correlations between Hα and X-ray patterns; (ii) enhanced X-ray limb brightening at interfaces; (iii) spectral and abundance "anomalies", because charge-exchange spectra differ from thermal spectra and contain only emission lines, resulting in some biases when they are interpreted with classical models. Interestingly, interaction between col...

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Section: Galaxy Clusters and Cooling Flowsmentioning

confidence: 81%

On the contribution of charge-exchange induced X-ray emission in the ISM and ICM

Lallement¹

2004

A&A

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“…If the filaments formed from the surrounding hot X‐ray plasma they would likely be dust‐free. Properties of dust‐free clouds within a galaxy cluster were examined by Ferland, Fabian & Johnstone (1994) and Ferland, Fabian & Johnstone (2002). If the gas originated in the ISM of the central galaxy it would most likely contain dust, as we have assumed so far.…”

Section: The Density–heating Planementioning

confidence: 99%

Collisional heating as the origin of filament emission in galaxy clusters

Ferland

Fabian

Hatch

et al. 2009

Monthly Notices of the Royal Astronomical Society

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152

219

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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 H2 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 H0–H2 inelastic collisions. We now use the values computed by Wrathmall et al. The rates are often much larger and allow the ro–vibrational H2 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 P/k=nT. 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 H2 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.

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“…Correlation are also shown between H α emission lines, warm H 2 rovibrationnal lines and cold millimetric emission lines suggesting related exciting mechanisms of these different phases of the gas. The large masses of excited H 2 , around 10 5−6 M could suggest that the cold molecular gas mass could have been underestimated (because of a lower metallicity for example) or is hidden in optically thick dense clouds, see Ferland et al (2002) for a discussion of the physical conditions within dense cold clouds in cooling flows. How much gas is deposited in cooling flows is still an open question.…”

Section: Excited H 2 Detected In Nirmentioning

confidence: 99%

Cold molecular gas in cooling flow clusters of galaxies

Salomé

Combes

2003

A&A

225

290

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Abstract. The results of a CO line survey in central cluster galaxies with cooling flows are presented. Cold molecular gas is detected with the IRAM 30 m telescope, through CO(1-0) and CO(2-1) emission lines in 6-10 among 32 galaxies. The corresponding gas masses are between 3 × 10 8 and 4 × 10 10 M . These results are in agreement with recent CO detections by Edge (2001). A strong correlation between the CO emission and the Hα luminosity is also confirmed. Cold gas exists in the center of cooling flow clusters and these detections may be interpreted as evidence of the long searched for very cold residual of the hot cooling gas.

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The physical conditions within dense cold clouds in cooling flows - II

Cited by 18 publications

References 62 publications

On the contribution of charge-exchange induced X-ray emission in the ISM and ICM

On the contribution of charge-exchange induced X-ray emission in the ISM and ICM

Collisional heating as the origin of filament emission in galaxy clusters

Cold molecular gas in cooling flow clusters of galaxies

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