→ ALMA observations [22hrs granted in cycle 5] for 4/5 galaxies observed with APEX spatially molecular emission (CO21 and CO10) → J-VLA observations [100hrs] to detect the atomic hydrogen (completed, analysis ongoing)
Context. The assumption of a gas-to-dust mass ratio gamma is a common approach to estimate the basic properties of molecular clouds, such as total mass and column density of molecular hydrogen, from (sub) mm continuum observations of the dust. In the Milky Way a single value is used at all galactocentric radii, independently of the observed metallicity gradients. Both models and extragalactic observations suggest that this quantity increases for decreasing metallicity Z, typical of the outer regions in disks, where fewer heavy elements are available to form dust grains. Aims. We aim to investigate the variation of the gas-to-dust ratio as a function of galactocentric radius and metallicity, to allow a more accurate characterisation of the quantity of molecular gas across the galactic disk, as derived from observations of the dust. Methods. Observations of the optically thin (CO)-O-18 (2-1) transition were obtained with the APEX telescope for a sample of 23 massive and dense star-forming regions in the far outer Galaxy (galactocentric distance greater than 14 kpc). From the modelling of this line and of the spectral energy distribution of the selected clumps we computed the gas-to-dust ratio and compared it to that of well-studied sources from the ATLASGAL TOP100 sample in the inner galactic disk. Results. The gradient in gamma is found to be 0.087(-0.025)(+0.047) dex kpc-1 (or equivalently gamma alpha Z(-1.0)(-14)(+0.3)). The dust-to-metal ratio, decreases with galactocentric radius, which is the most common situation also for external late-type galaxies. This suggests that grain growth dominates over destruction. The predicted gamma is in excellent agreement with the estimates in Magellanic clouds, for the appropriate value of Z.
AbstractWith the advent of ALMA, complete surveys of gas and dust in protoplanetary disks are being carried out in different star forming regions. In particular, continuum emission is used to trace the large (mm-sized) dust grains and CO isotopologues are observed in order to trace the bulk of the gas. The attempt is to simultaneously constrain the gas and dust disk mass as well as the gas/dust mass ratio. In this work the observations from the Lupus disk survey have been analyzed with thermo-chemical disk models, including radiative transfer, CO isotope-selective processes and freeze-out. We find that CO-based gas masses are very low, often smaller than 1MJ. Moreover, gas/dust mass ratios are much lower than value of 100 found in the ISM, being mainly between 1 and 10. This result can be interpreted either as rapid loss of gas, or as a chemical effect removing carbon from CO and locking it into more complex molecules or in larger bodies. Current data cannot distinguish between the two scenarios (except for sources with detected HD lines), but future observations of e.g. [CI] and hydrocarbon lines will help to calibrate CO-based gas masses and to constrain disk gas masses.
