Rescuing the Initial Mass Function for Arp 78

We present deep R and narrowband [image] images of Arp 78 obtained with the WIYN 3.5 m telescope on Kitt Peak. GALEX observations had shown a very extended UV structure for this system, reaching beyond the optical radius of Arp 78 and also beyond its previously known H alpha radius. Our new [image] data now show agreement not only with the spatial extent of the near- and far-UV maps, but also in terms of structural details. Star formation rates derived from L(H alpha ) and L(FUV) are in reasonable agreement, indicating that in this case the upper stellar IMF in the UV-bright outer arm is relatively normal. The star-forming sites in the outer arms are younger than [image]15 Myr and massive enough to properly sample the IMF up to high masses; their low optical visibility evidently is a property of their youth.