3.2. Liners and cooling flows

Clusters of galaxies are luminous X-ray sources. The bulk of the X-ray emission arises from Bremsstrahlung and line radiation processes in a hot (T~ 10⁷ K) intracluster medium. In the most luminous systems, the 2-10 keV luminosity exceeds 10⁴⁵ erg s^-1 and the emission extends to radii > 2 Mpc. The total mass of the intracluster medium exceeds that of the galaxies by factors 1-5. In the outer regions of clusters (r > 1 Mpc), the intracluster medium is highly diffuse (Ne ~ 10^-4 cm^-3) and the cooling time far exceeds the Hubble time (t_cool > 10¹¹ yr). In the central regions of most clusters, however, the density of the intracluster medium rises sharply and the cooling time is significantly less than the Hubble time (t_cool < 10⁹ yr). In the absence of forces other than the thermal pressure and gravity, the cooling of the intracluster medium will lead to a slow net inflow of material towards the cluster center, a process known as ``cooling flow'' ([4]). 40% of the clusters from an X-ray flux-limited sample have flows depositing more than 100 M yr^-1 throughout the cooling region ([333]). Large masses of X-ray absorbing material are common in cooling flows ([5]); the mass of the absorbing material is typically ~ 10% of the mass of the X-ray emitting gas in the same region. Theoretical studies of the physical conditions in cooling flows suggest that the X-ray absorbing gas is likely to take the form of small, cold, mainly molecular clouds. Most of the mass deposited by the cooling flows in the central regions of the clusters must reside in some form other than X-ray absorbing gas, perhaps low-mass stars or brown dwarfs ([4]).

Molecular hydrogen has been detected from three central cluster radio galaxies in cooling flows ([208]). CO has been detected in the center of the Perseus cluster indicating a total molecular hydrogen mass of ~ 6 10¹⁰ M ([64]).

The central region of cooling flow clusters often (35%) show optical-line nebulosity which can be extended (up to 20 kpc) and luminous (above 10⁴³ erg s^-1 in H in a few cases). The emission line spectra are characteristic of Liners ([453]; [97]). The emission line luminosity is correlated with the X-ray luminosity of the clusters ([104]; [3]).

Central cluster galaxies with emission lines exhibit strong UV/blue continua in comparison to normal giant elliptical galaxies (gEs); the excess blue continuum has a similar spatial extent (~ 5-10 kpc) as the emission line nebulosities. These continua are better described as due to young stars than by power-law emission models; the use of simple stellar population models shows more than 10⁶ O stars to be present in the most luminous systems; large B, A and F star populations are also inferred. The observed H flux is found to be roughly equal to the predicted flux due to O stars ([2]; [395]; [369]). Filippenko & Terlevich (1992) and [379] have shown that hot, yet normal main sequence O stars (O5 or earlier) irradiating relatively dense clouds of gas can produce line ratios very similar to those observed in the central galaxy of cooling flow clusters.

Large cooling flows tend to have luminous galaxies and powerful radio sources at their center ([333]). Radio sources associated with central galaxies of cooling core clusters are either FR Is or amorphous ([18]). [72] has found that 71% of the cD galaxies with X-ray cooling core are radio loud, whereas a smaller but still significant 23% of cDs without cooling core were detected. This disparity is striking and must point to an intimate connection between the processes giving rise to the radio and the X-ray emissions. A possible explanation is that some of the cooling gas reaches the central engines and powers the radio emission ([18]). Among the radio galaxies in non cooling core clusters, there are luminous and extended wide angle tails, demonstrating that mechanisms other than gas inflow from the cluster can generate relatively powerful extended radio sources in clusters ([149]). Optical nebulosity is common in those cooling flows with a central radio source, but no correlation is found between optical and radio luminosities in objects with both ([113]).

Liners in FR I radio galaxies are generally supposed to be AGNs, while those found in cooling flow clusters are generally extended and probably excited either by shocks or by young stars formed by the cooling flow. But what is then the excitation mechanism of Liners found in FR I radio galaxies which are the central galaxy of cooling flow clusters?