Rescuing the Initial Mass Function for Arp 78

TL;DR

This study uses deep imaging to confirm that the outer regions of Arp 78 host young, massive star-forming sites with a normal initial mass function, extending our understanding of star formation in galaxy outskirts.

Contribution

It provides detailed imaging evidence that the outer arms of Arp 78 contain young, massive star-forming regions with a standard IMF, aligning UV and Halpha observations.

Findings

Outer arms host star-forming sites younger than 15 Myr

Star formation rates from Halpha and UV are consistent

Outer arm regions have a normal stellar IMF

Abstract

We present deep R and narrow-band Halpha 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 Halpha-radius. Our new Halpha 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(Halpha) 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.