2008ApJ...688L..65K

We present deep R and narrowband Hα 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 Hα radius. Our new Hα 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(Hα) 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 ∼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.