X-ray spectroscopy of cooling clusters
Introduction
Observations show that the X-ray emission from many clusters of galaxies is sharply peaked around the central brightest galaxy. The inferred radiative cooling time of the gas in that peak, where the temperature drops to the center, is much shorter than the age of the cluster, suggesting the existence of a cooling flow there [99]. X-ray spectroscopy over the past 5 yr shows that the temperature drop toward the center is limited to about a factor of three. Just when the gas should be cooling most rapidly it appears not to be cooling at all. This is sometimes known as the cooling flow problem. Careful observations show that gently distributed heat is required over a radius of up to 100 kpc to balance radiative cooling in these regions.
The issues of cooling and heating of hot gas have broad relevance to the gaseous part of galaxy formation and evolution. Brightest cluster galaxies (BCG) are the most massive galaxies known. Calculations of the clustering behavior of cold dark matter predict a power-law mass distribution for large galaxies whereas the stellar mass observed has an exponential distribution [21]. The truncation of the stellar mass distribution in massive galaxies is likely due to the process which stops cooling flows. Simple cooling flows are an ingredient of semi-analytical models for galaxy formation. The cooling of hot gas to form stars is essential for the growth of massive galaxies and cannot be studied directly for isolated systems due to Galactic absorption. The cores of galaxy clusters offer examples which can be directly observed. However they do not appear to operate in any simple manner. The problem appears to be widespread, from the most massive clusters to the centers of individual elliptical galaxies. Heating and cooling problems of hot gases are common in astronomy, with examples ranging from the interstellar medium of our own Galaxy to the Solar Corona.
The diffuse hot ionized plasma in clusters is magnetized which means that MHD processes may be important [222].
Here we briefly review the main X-ray properties and emission processes of the intracluster medium (ICM) before showing the X-ray spectra of cool cores. We then discuss the main solutions which have been proposed for the cooling flow problem.
Section snippets
Clusters of galaxies
Clusters are the most massive bound and quasi-relaxed objects in the Universe. They have total masses of to above . The total gas fraction is about 16 per cent with about 13 per cent in the hot ICM and the remaining 3 per cent in stars in the cluster galaxies. The remaining 84 per cent of the mass is in dark matter. Gas densities in cluster centers range from as much as in peaked clusters to in the non-peaked ones. This is in stark contrast to the mean cosmic
Physics of the intracluster medium
The ICM is plasma that is nearly fully ionized due to the high temperatures created by the deep dark matter gravitational potential. Hydrogen and helium, for example, are fully stripped of their electrons. Heavier elements have retained only a few of their electrons in this hot medium. In addition to free electrons and ions in the plasma, electromagnetic radiation, which is emitted mostly as X-rays, is created by quantum mechanical interactions in the plasma.
The physics of the ICM can be
X-ray instrumentation and observational techniques
A number of X-ray instruments have been launched above the atmosphere, which is opaque to X-rays, to study X-ray sources in the last 40 years. X-ray telescopes typically reflect X-rays at grazing incidence using mirros coated with a high-Z material. CCDs, proportional counters, and microchannel plates have been used to record the positions of X-ray photons and make low resolution energy measurements. Crystals, reflection and transmission gratings have been used to disperse X-rays and produce
X-ray spectra of cooling clusters
A wealth of information has been collected by a number of X-ray satellites on cooling core clusters. Recently, there have been some strict tests to various aspects of the cooling-flow scenario. We review many of the previous spectroscopic and imaging results that led to the formation of the cooling-flow model for cluster thermal evolution. We then discuss how high resolution X-ray spectroscopic results have questioned a number of assumptions in the interpretation of the previous work. These
Are cooling flows ruled out?
In summary, problems with the simple cooling flow picture emerged from two fronts. The first is the enormous implied mass deposition in some objects where for example (e.g. PKS 0745-191; [98]) which is then not seen in cooled form (the ‘mass sink’ problem). The central galaxy of this, and many other brightest cluster galaxies where the surrounding gas have , has much excess blue light, optical/UV/IR emission-line nebulosity ([73] and refs. therein) and even molecular gas
Heating
Some heating is always expected in the central regions of clusters. Examples are supernovae [224], [78], an active central nucleus [11], [245], [198], [235], [25] and many more recent papers cited in Sections 7.2–7.5), conduction [236], [24], [232], [114], [23], [212] and many more recent papers cited in Section 7.1).
A problem with heating the gas is that the cooling rate is proportional to the density squared whereas most heating processes are proportional to volume. This tends to make the gas
Discussions
The simplest explanation for the common appearance of cold core, X-ray peaked clusters is that, when averaged over tens of Myr, the radiative cooling is balanced in part by distributed heating. Thermal conduction as a means of distributing heat from outer gas is ruled out for low and intermediate temperature clusters. It may however have a role in spreading the energy in the central parts. A plausible mechanism is the dissipation of energy propagating through the ICM from a central radio
Future work
Further detailed deep studies with Chandra and XMM-Newton as well as future studies with Constellation-X and Xeus are needed in order to better understand the heating/cooling balance. There is considerable potential for studies of the cool and warm gas and dust around the BCG using HST, Spitzer and ground-based telescopes. Larger cluster samples are needed, particularly at medium to high redshifts. Determining the extent of a heating-cooling balance (or not) in groups, elliptical galaxies, and
Acknowledgments
We thank Mateusz Ruszkowski, Mitch Begelman, Roger Blandford, and an anonymous referee for a careful reading of this manuscript. A.C.F. thanks the Royal Society for support. J.R.P. was supported by a grant from NASA for scientific and calibration support of the RGS at Stanford. This was also supported by the U.S. Department of Energy under contract number DE-AC02-76SF00515.
References (266)
- et al.
JQSRT
(2001) Astrophys. J.
(2003)Mon. Not. R. Astron. Soc.
(1993)- et al.
Mon. Not. R. Astron. Soc.
(1997) - et al.
Mon. Not. R. Astron. Soc.
(2000) Mon. Not. R. Astron. Soc.
(2001)Mon. Not. R. Astron. Soc.
(2001)Mon. Not. R. Astron. Soc.
(2001)- K.A. Arnaud, in: George H. Jacoby, Jeannette Barnes (Eds.), Astronomical Data Analysis Software and Systems V, ASP...
Astrophys. J.
Astron. Astrophys.
Mon. Not. R. Astron. Soc.
Astrophys. J.
Astrophys. J.
Astrophys. J.
Mon. Not. R. Astron. Soc.
Mon. Not. R. Astron. Soc.
Astronom. J
Mon. Not. R. Astron. Soc.
CoPhC
Astrophys. J.
Astrophys. J.
Mon. Not. R. Astron. Soc.
Mon. Not. R. Astron. Soc.
Astrophys. J.
Astrophys. J.
Astrophys. J.
Astron. Astrophys.
Mon. Not. R. Astron. Soc.
Astron. Astrophys.
Astron. Astrophys.
Astrophys. J.
Astrophys. J.
Astrophys. J.
Astrophys. J.
Astrophys. J.
Mon. Not. R. Astron. Soc.
Nature
Astrophys. J.
Mon. Not. R. Astron. Soc.
Mon. Not. R. Astron. Soc.
Mon. Not. R. Astron. Soc.
Astrophys. J.
Astrophys. J.
Astronom. J.
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