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, r 25 , 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 r gal = 1.6 r 25 and over a bright UV region at r gal = 1.36 r 25 . CO(1-0) is detected all along the M 63 major axis out to r 25 , and CO(2-1) is confined to r gal = 0.68 r 25 , 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 r 25 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 r 25 , 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 H 2 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 r gal = 1.36 r 25 . While all along the major axis out to r 25 , 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.

The properties of the Malin 1 galaxy giant disk

“…Malin 1 was never detected in molecular gas tracers (Braine et al 2000;Lee et al 2014). We thus assume the molecular fraction is low in this very extended low density disk, as may be expected from the extrapolation of the trends found in nearby galaxies (Leroy et al 2008) or in other low density regions (Dessauges-Zavadsky et al 2014). For these reasons, we do not correct the gas profile for inclusion of molecular gas, bearing in mind this introduces a large uncertainty.…”

X-ray spectral variability of Seyfert 2 galaxies

“…This galaxy, also called the Phoenix galaxy or UGC 4203, was optically classified as a Seyfert 2 by Dessauges-Zavadsky et al (2000). Broad lines have been observed in polarized light using spectropolarimetric data Tran 1995).…”

Extended HI disks in nearby spiral galaxies

“…4.5.1). This galaxy became much later a prototype extended UV disk of type 1 (Thilker et al 2007), and CO emission in the patch has been reported by Dessauges-Zavadsky et al (2014). These data, combined with the images in Gonzalez-Delgado et al (2010), make capture of a dwarf galaxy a likely scenario.…”

“…Several facts were discussed in the late 1950s concerning the HI emission in the outskirts of the Milky Way: the HI disk is warped, and also flares as discussed in Gum et al (1960). Since M31 approaches the Milky Way, Kahn & Woltjer (1959) argued that dark matter has to be present in the Local Group.…”