Interaction between the intergalactic medium and central radio source in
  the NGC 4261 group of galaxies
  Using observations from the Chandra and XMM-Newton X-ray observatories, we
examine the interaction between the intra-group medium and central radio source
in the nearby NGC 4261 galaxy group. We confirm the presence of cavities
associated with the radio lobes and estimate their enthalpy to be ~2.4x10^58
erg. The mechanical power output of the jets is >=10^43 erg/s, at least a
factor of 60 greater than the cooling luminosity in the region the lobes
inhabit. We identify rims of compressed gas enclosing the lobes, but find no
statistically significant temperature difference between them and their
surroundings, suggesting that the lobe expansion velocity is approximately
sonic (Mach<=1.05). The apparent pressure of the radio lobes, based on the
synchrotron minimum energy density argument, is a factor of 5 lower than that
of the intra-group medium. Pressure balance could be achieved if entrainment of
thermal gas provided additional non-radiating particles in the lobe plasma, but
the energy required to heat these particles would be ~20 per cent. of the
mechanical energy output of the radio source. NGC 4261 has a relatively compact
cool core, which should probably be categorised as a galactic corona. The
corona is capable of fuelling the active nucleus for considerably longer than
the inferred source lifetime, but can be only inefficiently heated by the AGN
or conduction. The expansion of the radio lobes has affected the structure of
the gas in the galaxy, compressing and moving the material of the corona
without causing significant shock heating, and expelling gas from the immediate
neighbourhood of the jets. We discuss the possible implications of this
environment for the duration of the AGN outburst, and consider mechanisms which
might lead to the cessation of nuclear activity.
