We present new mid-infrared ( 5 - 35 \mu m ) and ultraviolet ( 1539 – 2316 Å ) observations of the interacting galaxy system Arp 143 ( NGC 2444/2445 ) from the Spitzer Space Telescope and GALEX .
In this system , the central nucleus of NGC 2445 is surrounded by knots of massive star-formation in a ring-like structure .
We find unusually strong emission from warm H _ { 2 } associated with an expanding shock wave between the nucleus and the western knots .
At this ridge , the flux ratio between H _ { 2 } and PAH emission is nearly ten times higher than in the nucleus .
Arp 143 is one of the most extreme cases known in that regard .
From our multi-wavelength data we derive a narrow age range of the star-forming knots between 2 Myr and 7.5 Myr , suggesting that the ring of knots was formed almost simultaneously in response to the shock wave traced by the H _ { 2 } emission .
However , the knots can be further subdivided in two age groups : those with an age of 2–4 Myr ( knots A , C , E , and F ) , which are associated with 8 \mu m emission from PAHs , and those with an age of 7-8 Myr ( knots D and G ) , which show little or no 8 \mu m emission shells surrounding them .
We attribute this finding to an ageing effect of the massive clusters which , after about 6 Myr , no longer excite the PAHs surrounding the knots .