CO map and steep Kennicutt-Schmidt relation in the extended UV disk of M63

Abstract

Results from the UV satellite GALEX revealed large extensions of disks in some nearby spiral galaxies, extending out to 3 to 4 times the isophotal radius, r25. M63 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 M63 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 M63 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 M63. The radial profiles of the CO emission and of the Halpha, 24 micron, NUV and FUV star formation tracers and HI 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 HI. 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 HI 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 M63 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, in the external UV region the SFR regime is highly nonlinear and characterized by a steep K-S relation and very low star formation efficiency.

12 pages, 8 figures, A&A accepted

References in corpus (5)

• The GALEX Ultraviolet Atlas of Nearby Galaxies
• Cold gas accretion in galaxies
• An H-alpha Imaging Survey of Galaxies in the Local 11 Mpc Volume
• Spitzer Observations of Star Formation in the Extreme Outer Disk of M83 (NGC5236)
• The Detection of Molecular Gas in the Outskirts of NGC 6946

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• HI Global Scaling Relations in the WISE-WHISP Survey
• The giant low surface brightness galaxy Malin 1: new constraints for its molecular gas mass from GBT/ARGUS observations
• First Detection of the Molecular Cloud Population in the Extended Ultraviolet (XUV) Disk of M83
• Extended far-ultraviolet emission in distant dwarf galaxies
• A Comparison of the UV and HI Properties of the Extended UV (XUV) Disk Galaxies NGC 2541, NGC 5832 and ESO406-042
• Scaling Relations of Mass, Velocity and Radius for Disk Galaxies
• Star Formation Beyond the Optical Disk : The Low-Density Outskirts of NGC2090
• Compact and High Excitation Molecular Clumps in the Extended Ultraviolet Disk of M83