Powerful H2 Emission and Star Formation on the Interacting Galaxy System Arp 143: Observations with Spitzer and GALEX
Abstract
We present new mid-infrared (5 - 35μm) and ultraviolet (1539 -- 2316 ) 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 H2 associated with an expanding shock wave between the nucleus and the western knots. At this ridge, the flux ratio between H2 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 H2 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 8μm emission from PAHs, and those with an age of 7-8 Myr (knots D and G), which show little or no 8μm 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.