TXS 2116$-$077: A Gamma-ray Emitting Relativistic Jet Hosted in a Galaxy
  Merger
  What triggers collimated relativistic outflows or jets, from the centers of
galaxies remains a fundamental question in astrophysics. The merging of two
galaxies has been proposed to realize the conditions to successfully launch and
drive such jets into the intergalactic medium. However, evidences for the
operation of this mechanism are scarce. Here we report the first unambiguous
detection of an ongoing merger of a narrow-line Seyfert 1 galaxy, TXS
2116$-$077, hosting a closely aligned, $\gamma$-ray emitting relativistic jet
with a Seyfert 2 galaxy at a separation of $\sim$12 kpc, using the observations
taken with 8.2 m Subaru telescope. Our subsequent followup observations with
10.4 m Gran Telescopio Canarias, 4.2 m William Herschel Telescope, and Chandra
X-ray observatory have provided what is likely to be the first glimpse of the
merging environment hosting a closely aligned relativistic jet. Our finding
that the jet is considerably younger than the merger demonstrates that jet
activity can be triggered by galaxy mergers and that $\gamma$-ray detected
narrow-line Seyfert 1 galaxies represent the beginning phase of that activity.
These results also highlight the crucial role of mergers in shaping the fate of
galaxies in their cosmological evolution and are consistent with recent studies
focused on the host galaxy imaging of this enigmatic class of active galactic
nuclei.
