Frequently Asked Questions on the Luminosity--Temperature relation in Groups and Clusters

Efforts to understand the deviation of the L--T relation from a simple scaling law valid for clusters and groups have triggered a number of interesting studies on the subject. Techniques and approaches differ widely but most works agree on the important role played by gas cooling and heating sources like AGNs and SNe. Observations set useful constraints on the evolution of the intracluster medium (ICM): a 100KeV/cm^2 entropy floor in the core of groups and about 5--15% of baryons being converted into stars. However, essential details like the nature of the dominant heating mechanism and the quantitative importance of cooling still need to be addressed. I suggest that a new generation of high resolution N-body simulations and a quantitative comparison of results between different approaches is required to improve results and increase our understanding of the problem.