Shock heating in the nearby radio galaxy NGC 3801

We report the Chandra detection of shock-heated shells of hot gas surrounding the radio lobes of the nearby (D-L similar to 53 Mpc) low-power radio galaxy NGC 3801. The shells have temperatures of 1 and 0.7 keV, compared to an ISM temperature of 0.23 keV. The estimated expansion speed of the shells is similar to 850 km s(-1), corresponding to a Mach number of similar to 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 1.7 x 10(56) ergs, which is equivalent to the thermal energy of the ISM within similar to 11 kpc of the galaxy center, and a factor of similar to 25 larger than the inferred P dV 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 FR I 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 fueling of its AGN and the role of this type of system in feedback models.