Abstract. We investigate the process of galaxy formation as can be observed in the only currently forming galaxies -the so-called Tidal Dwarf Galaxies, hereafter TDGs -through observations of the molecular gas detected via its CO (Carbon Monoxide) emission. These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between two massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a bona fide galaxy. We have detected CO in 8 TDGs (two of them have already been published in Braine et al. 2000, hereafter Paper I), with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few 10 8 M . The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H2. Although TDGs share many of the properties of small irregulars, their CO luminosity is much greater (factor ∼100) than that of standard dwarf galaxies of comparable luminosity. This is most likely a consequence of the higher metallicity (&1/3 solar) of TDGs which makes CO a good tracer of molecular gas. This allows us to study star formation in environments ordinarily inaccessible due to the extreme difficulty of measuring the molecular gas mass. The star formation efficiency, measured by the CO luminosity per Hα flux, is the same in TDGs and full-sized spirals. CO is likely the best tracer of the dynamics of these objects because some fraction of the HI near the TDGs may be part of the tidal tail and not bound to the TDG. Although uncertainties are large for individual objects, as the geometry is unknown, our sample is now of eight detected objects and we find that the "dynamical" masses of TDGs, estimated from the CO line widths, seem not to be greater than the "visible" masses (HI + H2 + a stellar component). Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties, we find that TDGs require no dark matter, which would make them the only galaxy-sized systems where this is the case. Dark matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implying that they are not of tidal origin. We provide strong evidence that TDGs are self-gravitating entities, implying that we are witnessing the ensemble of processes in galaxy formation: concentration of large amounts of gas in a bound object, condensation of the gas, which is atomic at this point, to form molecular gas and the subsequent star formation from the dense molecular component.

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Do molecular clouds collapse to form stars at the same rate in all environments? In large spiral galaxies, the rate of transformation of H 2 into stars varies little. However, the SFE in distant objects (z ∼ 1) is much higher than in the large spiral disks that dominate the local universe. Some small Local Group galaxies share at least some of the characteristics of intermediate-redshift objects, such as size or color. Recent work has suggested that the star formation efficiency (SFE, defined as the star formation rate per unit H 2 ) in local Dwarf galaxies may be as high as in the distant objects. A fundamental difficulty in these studies is the independent measure of the H 2 mass in metal-deficient environments. At 490 kpc, NGC 6822 is an excellent choice for this study; it has been mapped in the CO(2-1) line using the multibeam receiver HERA on the 30 m IRAM telescope, yielding the largest sample of giant molecular clouds (GMCs) in this galaxy. Despite the much lower metallicity, we find no clear difference in the properties of the GMCs in NGC 6822 and those in the Milky Way except lower CO luminosities for a given mass. Several independent methods indicate that the total H 2 mass in NGC 6822 is about 5 × 10 6 M in the area we mapped and less than 10 7 M in the whole galaxy. This corresponds to a N(H 2 )/I CO ≈ 4 × 10 21 cm −2 /(K km s −1 ) over large scales, such as would be observed in distant objects, and half that in individual GMCs. No evidence was found for H 2 without CO emission. Our simulations of the radiative transfer in clouds are entirely compatible with these N(H 2 )/I CO values. The SFE implied is a factor 5-10 higher than what is observed in large local universe spirals. The CO observations presented here also provide a high-resolution datacube (1500 a.u. for the assumed 100 pc distance, 0.41 km s −1 velocity resolution) of a local molecular cloud along the line of sight.

Aims. We study the association between Giant Molecular Clouds (GMCs) and Young Stellar Cluster Candidates (YSCCs), to shed light on the time evolution of local star formation episodes in the nearby galaxy M33. Methods. The CO (J=2-1) IRAM-all-disk survey was used to identify and classify 566 GMCs with masses between 2×10 4 and 2×10 6 M across the whole star forming disk of M33. In the same area, there are 630 YSCCs, identified using Spitzer-24 µm data. Some YSCCs are embedded star-forming sites while the majority have GALEX-UV and Hα counterparts with estimated cluster masses and ages.Results. The GMC classes correspond to different cloud evolutionary stages: inactive clouds are 32% of the total, classified clouds with embedded and exposed star formation are 16% and 52% of the total respectively. Across the regular southern spiral arm, inactive clouds are preferentially located in the inner part of the arm, possibly suggesting a triggering of star formation as the cloud crosses the arm. The spatial correlation between YSCCs and GMCs is extremely strong, with a typical separation of 17 pc, less than half the CO(2-1) beamsize, illustrating the remarkable physical link between the two populations. GMCs and YSCCs follow the HI filaments, except in the outermost regions where the survey finds fewer GMCs than YSCCs likely due to undetected, low CO-luminosity clouds. The distribution of the non-embedded YSCC ages peaks around 5 Myrs with only a few being as old as 8-10 Myrs. These age estimates together with the number of GMCs in the various evolutionary stages lead us to conclude that 14 Myrs is a typical lifetime of a GMC in M33, prior to cloud dispersal. The inactive and embedded phases are short, lasting about 4 and 2 Myrs respectively. This underlines that embedded YSCCs rapidly break out from the clouds and become partially visible in Hα or UV long before cloud dispersal.