The Hot Gas Environment of the Radio Galaxy 3C 388: Quenching the Accumulation of Cool Gas in a Cluster Core by a Nuclear Outburst

Abstract

We present results from a 35 ks {\em Chandra}/ACIS-I observation of the hot ICM around the FR II radio galaxy 3C 388. 3C 388 resides in a cluster environment with an ICM temperature of $\sim$3.5 keV. We detect cavities in the ICM coincident with the radio lobes. The enthalpy of these cavities is $\sim1.2\times10^{60}$ ergs. The work done on the gas by the inflation of the lobes is $\sim3\times10^{59}$ ergs, or $\sim$0.87 keV per particle out to the radius of the lobes. The radiative timescale for gas at the center of the cluster at the current temperature is a few Gyrs. The gas in the core was probably cooler and denser before the outburst, so the cooling time was considerably shorter. We are therefore likely to be witnessing the quenching of a cluster cooling flow by a radio galaxy outburst. The mechanical power of the lobes is at least 20 times larger than the radiative losses out to the cooling radius. Outbursts of similar power with a $\sim$5% duty cycle would be more than sufficient to continually reheat the cluster core over the Hubble time and prevent the cooling of any significant amount of gas. The mechanical power of the outburst is also roughly two orders of magnitude larger than either the X-ray luminosity of the active nucleus or the radio luminosity of the lobes. The equipartition pressure of the radio lobes is more than an order of magnitude lower than that of the ambient medium, indicating that the pressure of the lobe is dominated by something other than the relativistic electrons radiating at GHz frequencies.