Jets, Arcs and Shocks: NGC 5195 at radio wavelengths
  We studied the nearby, interacting galaxy NGC 5195 (M51b) in the radio,
optical and X-ray bands. We mapped the extended, low-surface-brightness
features of its radio-continuum emission; determined the energy content of its
complex structure of shock-ionized gas; constrained the current activity level
of its supermassive nuclear black hole. In particular, we combined data from
the European Very Long Baseline Interferometry Network (~1-pc scale), from our
new e-MERLIN observations (~10-pc scale), and from the Very Large Array
(~100-1000-pc scale), to obtain a global picture of energy injection in this
galaxy. We put an upper limit to the luminosity of the (undetected)
flat-spectrum radio core. We find steep-spectrum, extended emission within 10
pc of the nuclear position, consistent with optically-thin synchrotron emission
from nuclear star formation or from an outflow powered by an active galactic
nucleus (AGN). A linear spur of radio emission juts out of the nuclear source
towards the kpc-scale arcs (detected in radio, Halpha and X-ray bands). From
the size, shock velocity, and Balmer line luminosity of the kpc-scale bubble,
we estimate that it was inflated by a long-term-average mechanical power ~3-6 x
10^{41} erg/s over the last 3-6 Myr. This is an order of magnitude more power
than can be provided by the current level of star formation, and by the current
accretion power of the supermassive black hole. We argue that a jet-inflated
bubble scenario associated with previous episodes of AGN activity is the most
likely explanation for the kpc-scale structures.
