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.

Carbon monoxide (CO) is the primary tracer for interstellar clouds where stars form, but it has never been detected in galaxies in which the oxygen abundance relative to hydrogen is less than 20 per cent of that of the Sun, even though such 'low-metallicity' galaxies often form stars. This raises the question of whether stars can form in dense gas without molecules, cooling to the required near-zero temperatures by atomic transitions and dust radiation rather than by molecular line emission; and it highlights uncertainties about star formation in the early Universe, when the metallicity was generally low. Here we report the detection of CO in two regions of a local dwarf irregular galaxy, WLM, where the metallicity is 13 per cent of the solar value. We use new submillimetre observations and archival far-infrared observations to estimate the cloud masses, which are both slightly greater than 100,000 solar masses. The clouds have produced stars at a rate per molecule equal to 10 per cent of that in the local Orion nebula cloud. The CO fraction of the molecular gas is also low, about 3 per cent of the Milky Way value. These results suggest that in small galaxies both star-forming cores and CO molecules become increasingly rare in molecular hydrogen clouds as the metallicity decreases.

The Small Magellanic Cloud (SMC) provides the only laboratory to study the structure of molecular gas at high resolution and low metallicity. We present results from the Herschel Spectroscopic Survey of the SMC (HS   ( 3 < pc) scales. We estimate A V using far-IR thermal continuum emission from dust and find that the CO/[C II] ratios reach the Milky Way value at high A V in the centers of the clouds and fall to 1 5 1 10-the Milky Way value in the outskirts, indicating the presence of translucent molecular gas not traced by bright CO 12 emission. We estimate the amount of molecular gas traced by bright [C II] emission at low A V and bright CO 12 emission at high A V . We find that most of the molecular gas is at low A V and traced by bright [C II] emission, but that faint CO 12 emission appears to extend to where we estimate that the H 2 -to-H I transition occurs. By converting our H 2 gas estimates to a CO-to-H 2 conversion factor (X CO ), we show that X CO is primarily a function of A V , consistent with simulations and models of low-metallicity molecular clouds.

Gas can be violently stripped from their galaxy disks in rich clusters, and be dispersed over 100 kpc-scale tails or plumes. Young stars have been observed in these tails, suggesting they are formed in situ. This will contribute to the intracluster light, in addition to tidal stripping of old stars. We want to quantify the efficiency of intracluster star formation. We present CO(1-0) and CO(2-1) observations, made with the IRAM-30 m telescope, towards the ram-pressure stripped tail northeast of NGC 4388 in Virgo. We selected HII regions found all along the tails, together with dust patches, as observing targets. We detect molecular gas in 4 positions along the tail, with masses between 7 × 10 5 to 2 × 10 6 M . Given the large distance from the NGC 4388 galaxy, the molecular clouds must have formed in situ, from the HI gas plume. We compute the relation between surface densities of star formation and molecular gas in these regions, and find that the star formation has very low efficiency. The corresponding depletion time of the molecular gas can be up to 500 Gyr and more. Since this value exceeds a by far Hubble time, this gas will not be converted into stars, and will stay in a gaseous phase to join the intracluster medium.