CO map and steep Kennicutt-Schmidt relation in the extended UV disk of M 63

Abstract

Results from the UV satellite GALEX revealed surprisingly large extensions of disks in some nearby spiral galaxies. While the Hα emission, the usual tracer of star formation, drops down at the border of the isophotal radius, r25, the UV emission extends out to 3 to 4 times this radius and often covers a significant fraction of the H I area. M 63 is a remarkable example of a spiral galaxy with one of the most extended UV disks, so it offers the opportunity to search for the molecular gas and characterize the star formation in outer disk regions as revealed by the UV emission. We obtained deep CO(1–0) and CO(2–1) observations on the IRAM 30 m telescope along the major axis of the M 63 disk from the center out to the galactocentric radius rgal = 1.6 r25 and over a bright UV region at rgal = 1.36 r25. CO(1–0) is detected all along the M 63 major axis out to r25, and CO(2–1) is confined to rgal = 0.68 r25, which may betray lower excitation temperatures in the outer disk. CO(1–0) is also detected in the external bright UV region of M 63. This is the fourth molecular gas detection in the outskirts of nearby spirals. The radial profiles of the CO emission and of the Hα, 24 μm, NUV and FUV star formation tracers and H I taken from the literature show a severe drop with the galactocentric radius, such that beyond r25 they are all absent with the exception of a faint UV emission and H I. The CO emission detection in the external UV region, where the UV flux is higher than the UV flux observed beyond r25, highlights a tight correlation between the CO and UV fluxes, namely the amount of molecular gas and the intensity of star formation. This external UV region is dominated by the atomic gas, suggesting that H I is more likely the precursor of H2 rather than the product of UV photodissociation. A broken power law needs to be invoked to describe the Kennicutt-Schmidt (K-S) relation of M 63 from the center of the galaxy out to rgal = 1.36 r25. While all along the major axis out to r25, the K-S relation is almost linear (with a slope of nearly 1 in log space), in the external UV region the SFR regime is highly nonlinear and characterized by a steep K-S relation (with a slope much higher than 1 in log space) and very low star formation efficiency