Cold gas precipitation in elliptical galaxy atmospheres heated by AGN cosmic rays

Recent observations have found extended multiphase (MP) gas in a significant fraction of massive elliptical galaxies. The presence of MP gas may be crucial for the co-evolution of the central engine and the gaseous halo in these galaxies, especially in the model of precipitation-driven AGN feedback. We perform 3D hydro simulations of two idealized ellipticals: one representing a typical galaxy characterized by initial conditions conducive to the development of thermal instability (TI) and the other one less likely to develop TI. We find that in one class of ellipticals (hereafter SPG), where the entropy of the hot halo gas rises sharply as a function of radius, the hot halo is thermally stable and runaway cooling only happens in the very centers of galaxies. In the other class of ellipticals (hereafter MPG), characterized by shorter cooling times, non-linear perturbation driven by AGN feedback can cause the hot gas to frequently precipitate and form extended MP filaments. Both MPG and SPG experience cooling-driven AGN feedback cycles. However, long-term evolution of both MPG and SPG leads to the formation of massive cold disks that persist over Gyrs. As such disks are not observed in most ellipticals, we suggest that this aspect of the simulations is not physical. One potential solution is to consider cosmic ray (CR) dominated AGN jet feedback. Here we test a hypothesis that CRs diffusing or streaming out of the AGN-inflated cavities could heat the ISM and potentially effectively offset radiative cooling in the outer parts of the atmospheres in the time-averaged sense thus preventing the formation of the long-lived cold disks. Interestingly, recent simulations of a single short-duration jet in the galaxy cluster show that CR-dominated jets could i) efficiently uplift the hot halo (thus could potentially result in the limiting of the formation of the disk in the long term) and ii) make the ISM more prone to the development of TI in the very centers of the atmospheres on short timescales. We present preliminary results of long-term, self-regulated, and CR-heated atmospheres of MPG and SPG to quantify the net amount of precipitation and assess if CR heating can prevent the formation of cold and long-lived disks.