NGC3801 caught in the act: A post-merger starforming early-type galaxy
  with AGN-jet feedback
  In the current models of galaxy formation and evolution, AGN feedback is
crucial to reproduce galaxy luminosity function, colour-magnitude relation and
M-sigma relation. However, if AGN-feedback can indeed expel and heat up
significant amount of cool molecular gas and consequently quench star
formation, is yet to be demonstrated observationally. Only in four cases so far
(Cen A, NGC 3801, NGC 6764 and Mrk 6), X-ray observations have found evidences
of jet-driven shocks heating the ISM. We chose the least-explored galaxy, NGC
3801, and present the first ultraviolet imaging and stellar population
analysisis of this galaxy from GALEX data. We find this merger-remnant
early-type galaxy to have an intriguing spiral-wisp of young star forming
regions (age ranging from 100--500 Myr). Taking clues from dust/PAH, HI and CO
emission images we interpret NGC 3801 to have a kinamatically decoupled core or
an extremely warped gas disk. From the HST data we also show evidence of
ionised gas outflow similar to that observed in HI and molecular gas (CO) data,
which may have caused the decline of star formation leading to the red optical
colour of the galaxy. However, from these panchromatic data we interpret that
the expanding shock shells from the young ($\sim$2.4 million years) radio jets
are yet to reach the outer gaseous regions of the galaxy. It seems, we observe
this galaxy at a rare stage of its evolutionary sequence where post-merger star
formation has already declined and new powerful jet feedback is about to affect
the gaseous star forming outer disk within the next 10 Myr, to further
transform it into a red-and-dead early-type galaxy.
