Shock heating in the nearby radio galaxy NGC 3801

Abstract

We report the Chandra detection of shock-heated shells of hot gas surrounding the radio lobes of the nearby (D_L ~ 53 Mpc) low-power radio galaxy NGC 3801. The shells have temperatures of 1 keV and 0.7 keV, compared to an ISM temperature of 0.23 keV. The estimated expansion speed of the shells is 850 km/s, corresponding to a Mach number of ~ 4. This is the second X-ray detection of strong shocks produced by a low-power radio galaxy, and allows us to measure directly the contribution of shock heating to the radio galaxy's total energetic input to the ISM. We show that the gas properties of the shells and surrounding ISM are consistent with the Rankine-Hugoniot shock jump conditions. We estimate the energy stored in the hot gas shells (thermal + kinetic energy) to be 1.7x10^56 ergs, which is equivalent to the thermal energy of the ISM within ~11 kpc of the galaxy centre, and a factor of ~25 larger than the inferred PdV work required to inflate the lobe cavities, indicating that energy transfer from the AGN to its environment is dominated by shock heating during this stage of radio-source evolution. Our results provide direct evidence that shock heating in the early supersonic phase of FRI radio-source expansion can have important long-term effects on the properties of the host galaxy ISM. Finally, we discuss the merger history of NGC 3801, the fuelling of its AGN and the role of this type of system in feedback models.