  We present new HST ACS medium- and narrow-band images and long-slit, optical
(4000 - 7200A) spectra obtained using the Isaac Newton Telescope (INT) on La
Palma, of the merging system Mrk273. The HST observations sample the
[OIII]4959,5007 emission from the galaxy and the nearby continuum. The images
show that the morphologies of the extended continuum and the ionised gas
emission from the galaxy are decoupled, extending almost perpendicular to each
other. In particular, we detect for the first time a spectacular structure of
ionised gas in the form of filaments extending ~23 kpc to the east of the
nuclear region. The quiescent ionised gas kinematics at these locations
suggests that these filaments are tidal debris left over from a secondary
merger event that are illuminated by an AGN in the nuclear regions. The images
also reveal a complex morphology in the nuclear region of the galaxy for both
the continuum and the [OIII] emission. Kinematic disturbance, in the form of
broad (FWHM > 500 km s-1) and/or strongly shifted (abs(\DeltaV) >150 km s-1)
emission line components, is found at almost all locations within a radius of
~4 kpc to the east and west of the northern nucleus. We fit the profiles of all
the emission lines of different ionisation with a kinematic model using up to 3
Gaussian components. From these fits we derive diagnostic line ratios that are
used to investigate the ionisation mechanisms at the different locations in the
galaxy. We show that, in general, the line ratios are consistent with
photoionization by an AGN as the main ionisation mechanism. Finally, the
highest surface brightness [OIII] emission is found in a compact region that is
coincident with the so-called SE nuclear component. The compactness, kinematics
and emission line ratios of this component suggest that it is a separate
nucleus with its own AGN.
