We present a study of the physical properties of JO201 , a unique disk galaxy with extended tails undergoing extreme ram-pressure stripping as it moves through the massive cluster Abell 85 at supersonic speeds mostly along the line of sight .
JO201 was observed with MUSE as part of the GASP programme .
In a previous paper ( GASP II ) we studied the stellar and gas kinematics .
In this paper we present emission-line ratios , gas-phase metallicities and ages of the stellar populations across the galaxy disk and tails .
We find that while the emission at the core of the galaxy is dominated by an active galactic nucleus ( AGN ) , the disk is composed of star-forming knots surrounded by excited diffuse gas .
The collection of star-forming knots presents a metallicity gradient steadily decreasing from the centre of the galaxy outwards , and the ages of the stars across the galaxy show that the tails formed \lesssim 10 ^ { 9 } yr ago .
This result is consistent with an estimate of the stripping timescale ( \sim 1 Gyr ) , obtained from a toy orbital model .
Overall , our results independently and consistently support a scenario in which a recent or ongoing event of intense ram-pressure stripping acting from the outer disk inwards , causes removal and compression of gas , thus altering the AGN and star-formation activity within and around the galaxy .