The accreted core of a cooler sub-cluster

The discovery of a large dust lane in the HST-WFPC2 image of ESO349-G010 suggests the accretion of a dust and gas rich companion galaxy within the past $10^8$yrs. It is possible that this galactic-merger event was actually the late stages of the merger of a smaller galaxy cluster or group with A 4059. In this case, one may attempt to identify the SW ridge with the remnant ICM core of the minor cluster. The well known correlation between the X-ray luminosities and temperatures of galaxy clusters and groups, $L\sim T^3$, means that the accreted minor system is likely to possess an ICM that is significantly cooler than the ambient temperature of A 4059 ($kT\sim 4\,keV$). We note that similar ideas have been proposed to explain the cold fronts observed in other clusters (Markevitch et al. 2000; Bialek, Evrard & Mohr 2002; Nagai & Kratsov 2002).

However, this scenario may be problematic for the case of A 4059 (although it may well explain some of the classical cold fronts seen in other clusters). While it is true that the ICM temperature of the accreted subcluster may initially be cooler than that of A 4059, it will compress and heat as it enters the higher pressure environment of the richer cluster. The relevant thermodynamic quantity to consider is the entropy of the ICM cores of A 4059 and the accreted cluster. In fact, for clusters of the mass of A 4059 and smaller, the entropy of the ICM core is almost constant from one cluster to another (Lloyd-Davies, Ponman & Cannon 2000; Mushotzky et al. 2003). Thus, even if it evolved adiabatically, the ICM-core of the accreted group would be compressionally heated to approximately the ambient temperature of A 4059. Any departure from adiabatic evolution (e.g., the effects of shocks) will only increase the entropy and temperature of the accreting ICM. In order to produce a colder region, radiative cooling needs to dominate the evolution of the accreted core. While this may be true, either fine tuning or significant feedback (either via conduction or radio-galaxy heating) is needed to prevent the core from cooling completely.