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

TL;DR

This study reveals intense warm H$_2$ emission and recent star formation in the interacting galaxy system Arp 143, highlighting shock-induced star formation and the evolution of star-forming knots over a few million years.

Contribution

First detailed multi-wavelength analysis of Arp 143 showing shock-driven star formation and H$_2$ emission, with insights into the age and evolution of star-forming knots.

Findings

Strong H$_2$ emission associated with shock waves.

Star-forming knots aged 2-7.5 Myr, with evidence of cluster aging effects.

Distinct PAH emission patterns indicating different evolutionary stages.

Abstract

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 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.