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We present new mid-infrared ($5 - 35\mu$m) and ultraviolet (1539 -- 2316 \AA)observations of the interacting galaxy system Arp 143 (NGC 2444/2445) from theSpitzer Space Telescope and GALEX. In this system, the central nucleus of NGC2445 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 expandingshock wave between the nucleus and the western knots. At this ridge, the fluxratio between H$_2$ and PAH emission is nearly ten times higher than in thenucleus. Arp 143 is one of the most extreme cases known in that regard. Fromour multi-wavelength data we derive a narrow age range of the star-formingknots between 2 Myr and 7.5 Myr, suggesting that the ring of knots was formedalmost simultaneously in response to the shock wave traced by the H$_2$emission. However, the knots can be further subdivided in two age groups: thosewith 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 attributethis finding to an ageing effect of the massive clusters which, after about 6Myr, no longer excite the PAHs surrounding the knots.

Powerful H$_2$ Emission and Star Formation on the Interacting Galaxy System Arp 143: Observations with Spitzer and GALEX