Shocks and Cavities from Multiple Outbursts in the Galaxy Group NGC
  5813: A Window to AGN Feedback
  We present results from new Chandra, GMRT, and SOAR observations of NGC 5813,
the dominant central galaxy in a nearby galaxy group. The system shows three
pairs of collinear cavities at 1 kpc, 8 kpc, and 20 kpc from the central
source, from three distinct outbursts of the central AGN, which occurred
3x10^6, 2x10^7, and 9x10^7 yr ago. The H-alpha and X-ray observations reveal
filaments of cool gas that has been uplifted by the X-ray cavities. The inner
two cavity pairs are filled with radio emitting plasma, and each pair is
associated with an elliptical surface brightness edge, which we unambiguously
identify as shocks (with measured temperature jumps) with Mach numbers of M~1.7
and M~1.5 for the inner and outer shocks, respectively. Such clear signatures
from three distinct AGN outbursts in an otherwise dynamically relaxed system
provide a unique opportunity to study AGN feedback and outburst history. The
mean power of the two most recent outbursts differs by a factor of six, from
1.5--10x10^42 erg/s, indicating that the mean jet power changes significantly
over long (~10^7 yr) timescales. The total energy output of the most recent
outburst is also more than an order of magnitude less than the total energy of
the previous outburst (1.5x10^56 erg versus 4x10^57 erg), which may be a result
of the lower mean power, or may indicate that the most recent outburst is
ongoing. The outburst interval implied by both the shock and cavity ages (~10^7
yr) indicates that, in this system, shock heating alone is sufficient to
balance radiative cooling close to the central AGN, which is the relevant
region for regulating feedback between the ICM and the central SMBH.
