The Far-Infrared Morphology of the Double-ringed Galaxy NGC 4736 (M94): A Ring Surrounding an Extended Nucleus

We have obtained high spatial resolution 100 μm observations of the central region of the double-ringed spiral galaxy NGC 4736 (M94) using the Kuiper Airborne Observatory. Our data show a strong central peak with secondary peaks at the radius of the inner ring (50" = 1.6 kpc). The nuclear emission is extended at 100 μm, with a radius of 15" (500 pc). The far-infrared morphology is similar to that of the molecular gas, while the H I distribution shows a pronounced central depression. Since most of the hydrogen gas in the inner regions of NGC 4736 is in molecular form, we conclude that the far-infrared emission from NGC 4736 arises mainly from dust associated with molecular gas. The Hα distribution differs dramatically from the far-infrared and molecular gas distributions. The ring dominates the Hα emission, while the total 100 μm ring emission is only slightly larger than that of the nucleus, yielding an L(FIR)/L(Hα) for the ~100 times that of the ring. The bolometric luminosity of the stars in the inner 1 kpc of NGC 4736 is sufficient to power the far-infrared from this region, which suggests that a significant fraction of the far-infrared emission in the nuclear region of NGC 4736 is powered by non-OB stars rather than by star formation.