Star formation and dust obscuration in the tidally distorted galaxy NGC 2442

Abstract

We present a detailed investigation of the morphological distribution and level of star formation and dust obscuration in the nearby tidally distorted galaxy NGC 2442. Spitzer images in the IR at 3.6, 4.5, 5.8, 8.0, and 24 μm and GALEX images at 1500 Å and 2300 Å allow us to resolve the galaxy on scales between ∼240 and 600 pc. We supplement these with archival data in the B, J, H, and K bands. We use the 8 μm, 24 μm, and FUV (1500 Å) emission to study the star formation rate (SFR). We find that, globally, these tracers of star formation give a range of results of ∼6-11 M⊙ yr-1, with the dust-corrected FUV giving the highest value of SFR. We can reconcile the UV- and IR-based estimates by adopting a steeper UV extinction curve that lies in between the starburst (Calzetti) and Small Magellanic Cloud extinction curves. However, the regions of the highest SFR intensity along the spiral arms are consistent with a starburst-like extinction. Overall, the level of star formation we find is higher than previously published for this galaxy, by about a factor of 2, which, contrary to previous conclusions, implies that the interaction that caused the distorted morphology of NGC 2442 likely also triggered increased levels of star formation activity. We also find marked asymmetry in that the north spiral arm has a noticeably higher SFR than the southern arm. The tip of the southern spiral arm shows a likely tidally distorted peculiar morphology. It is UV bright and shows unusual IRAC colors, consistent with other published tidal features IRAC data. Outside of the spiral arms, we discover what appears to be a superbubble, ∼1.7 kpc across, which is seen most clearly in the IRAC images. Significant Hα, UV, and IR emission in the area also suggest vigorous ongoing star formation. A known, recent supernova (SN 1999ga) is located at the edge of this superbubble. Although speculative at this stage, this area suggests a large star-forming region with a morphology shaped by generations of supernovae. Lastly, we discover an 8 μm (polycyclic aromatic hydrocarbon) circumnuclear ring with an ∼0.8 kpc radius. The Hα emission is largely concentrated inside that ring and shows a vague spiral structure in the rest of the galaxy. The nuclear region shows the highest obscuration levels in the galaxy (A1600 ∼ 3-4) most likely due to the circumnuclear dust ring. © 2010. The American Astronomical Society.