2009ApJ...693.1650B

We present new mid-infrared (IR; 5-35 µm) and ultraviolet (1539-2316 Å) observations of the interacting galaxy system Arp 143 (NGC 2444/2445) from the Spitzer Space Telescope and Galaxy Evolution Explorer. In this system, the central nucleus of NGC 2445 is surrounded by knots of massive star formation in a ringlike structure. We find unusually strong emission from warm H₂ associated with an expanding shock wave between the nucleus and the western knots. At this ridge, the flux ratio between H₂ and polycyclic aromatic hydrocarbon (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 multiwavelength 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 H₂emission. However, the knots can be further subdivided into two age groups: those with an age of 2-4 Myr (knots A, C, E, and F), which are associated with 8 µm emission from PAHs, and those with an age of 7-8 Myr (knots D and G), which show little or no 8 µm emission shells surrounding them. We attribute this finding to an aging effect of the massive clusters which, after about 6 Myr, no longer excite the PAHs surrounding the knots.