We have compared the metallicity (represented by oxygen abundance), X o , and the dust-to-gas ratio, D, in a sample of dwarf galaxies. For dwarf irregulars (dIrrs) we find a good correlation between the two quantities, with a power-law index 0.52 ± 0.25. Blue Compact Dwarf (BCD) galaxies do not show such a good correlation; in addition both the dust-to-gas ratio and the metallicity tend to be higher than for dIrrs. We have then developed a simple but physical analytical model for the above relation. Comparing the model results with the data, we conclude that: (i) for low values of D, the D − X o relation is quasi-linear, whereas for higher values the curve strongly deviates from the linear behavior, implying that the commonly used power-law approximation is very poor; (ii) the deviation from the linear behavior depends critically on the parameter χ, the "differential" mass outflow rate from the galaxy in units of the star formation rate, ψ; (iii) the shape of the D − X o curve does not depend on ψ, but only on χ; however, the specific location of a given galaxy on the curve does depend on ψ; (iv) the BCD metallicity segregation is due to a higher ψ, together with a significant differential mass outflow. Thus, the lack of correlation can be produced by largely different star formation rates and values of χ in these objects.

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.

Recycled dwarf galaxies can form in the collisional debris of massive galaxies. Theoretical models predict that, contrary to classical galaxies, they should be free of non-baryonic Dark Matter. Analyzing the observed gas kinematics of such recycled galaxies with the help of a numerical model, we demonstrate that they do contain a massive dark component amounting to about twice the visible matter. Staying within the standard cosmological framework, this result most likely indicates the presence of large amounts of unseen, presumably cold, molecular gas. This additional mass should be present in the disks of their progenitor spiral galaxies, accounting for a significant part of the so-called missing baryons.When galaxies collide, gravitational forces cause the expulsion of material from their disks into the intergalactic medium. In this debris, dense self-gravitating structures form. They can reach masses typical of those of dwarf galaxies, show ordered rotation and active star formation(1-8), hence deserve to be considered galaxies in their own right, albeit "recycled" ones. Whether these recycled dwarf galaxies contain dark matter can put strong constraints on the nature and distribution of this enigmatic constituent of the Universe. Indeed, standard theory(9-11) predicts that they differ from classical galaxies by being nearly free of non-baryonic dark matter (5,7,12). According to the widely accepted !CDM (Cold Dark Matter with cosmological constant) model(13), the matter density of the Universe is dominated by non-baryonic dark matter. This matter is expected to surround galaxies in the form of large halos supported by random motions(9). Recycled galaxies are expected to have little, if any, dark matter of this type, because only material from rotating disks is involved in the galactic recycling process. In addition to non-baryonic dark matter, part of the baryonic component is "dark" as well, known to exist in the early Universe(14), but hard or impossible to detect locally today. It has been speculated to be cold gas(15,16) but is most widely thought to reside in a diffuse warm-hot intergalactic medium (WHIM) surrounding galaxies(10,11), which cannot be substantially accumulated in collisional debris. Hence, recycled dwarf galaxies are predicted by conventional views to be mostly free of both baryonic and non-baryonic dark matter. We put these views to the test, measuring the mass of three galaxies formed in the collisional debris around galaxy NGC5291(17,18).The galaxy NGC5291 is surrounded by a large, gasrich ring of collisional debris(17). In several places gas has gathered into self-gravitating, rotating dwarf galaxies where new stars form (Fig. 1). We study the kinematics of atomic hydrogen in the ring via its 21-cm emission line, using the NRAO(19) Very Large Array (VLA) interferometer in a high-resolution configuration. We estimate the mass actually present in the dwarf galaxies and compare this to their visible mass (6,18,20,21). We use N-body simulations that model the gravitational dynamics of sta...