Galactocentric variation of the gas-to-dust ratio and its relation with metallicity
1 INAF−Istituto di Radioastronomia, and Italian ALMA Regional Centre, via P. Gobetti 101, 40129 Bologna, Italy
2 Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, 53121 Bonn, Germany
3 INAF−Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy
4 School of Physical Sciences, University of Kent, Ingram Building, Canterbury, Kent CT2 7NH, UK
Received: 7 August 2017
Accepted: 9 October 2017
Context. The assumption of a gas-to-dust mass ratio γ 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 C18O (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 γ is found to be 0.087+0.047-0.025 dex kpc-1 (or equivalently γ ∝ Z-1.4+0.3-1.0). 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 γ is in excellent agreement with the estimates in Magellanic clouds, for the appropriate value of Z.
Key words: dust, extinction / ISM: clouds / Galaxy: disk / galaxies: ISM / submillimeter: ISM / stars: formation
© ESO, 2017
In the past decade many surveys of the galactic plane have been carried out in the continuum, covering wavelengths from the millimetre regime to the infrared (IR). They provide a complete picture of the dust emission, tracing both very cold material (at millimetre, sub-mm and far-IR wavelengths; for example ATLASGAL, Schuller et al. 2009; and Hi-GAL, Molinari et al. 2010), and hot dust and PAHs (in the mid- and near-IR; for example MSX, Egan et al. 2003; MIPSGAL, Carey et al. 2009; WISE, Wright et al. 2010). The temperature, mass and column density of the dust can be estimated by constructing and modelling the spectral energy distribution of the thermal dust emission (SED; e.g. König et al. 2017). The dust, however, constitutes only a minor fraction of the total mass of molecular clouds. One has to assume a gas-to-dust mass ratio (γ) to derive the mass and column density of molecular hydrogen. A direct, local determination shows that the hydrogen-to-dust mass ratio is ~100, corresponding to a gas-to-dust mass ratio γ ≈ 136, when accounting for helium (Draine et al. 2007). Current research uses a constant value of the gas-to-dust ratio irrespective of the galactocentric distance of the cloud (typically 100−150, e.g. Elia et al. 2013, 2017; König et al. 2017), and while these values are reasonable within the solar circle they are not likely to be reliable for the outer parts of the disk, where the metallicity and average disk surface density might be substantially lower.
Heavy elements are the main constituents of dust grains, and therefore when their abundance with respect to hydrogen changes, dust may be influenced too. Models combining chemical evolution of the Galaxy with dust evolution indeed suggest that γ increases with decreasing metallicity Z (Dwek 1998; Mattsson & Andersen 2012; Hirashita & Harada 2017). This is also supported by observations in nearby galaxies (e.g. Sandstrom et al. 2013).
Except for a few cases, the data for external galaxies are averaged over the entire Galaxy, and in all cases optically thick CO lines are used to obtain the mass of molecular gas. Moreover, in studies in which the gradient in γ with Z can be spatially resolved, the resolution is of the order of a kpc, introducing large uncertainties, for example, by assuming a uniform single temperature for dust or a specific calibration in deriving the metallicity (e.g. Sandstrom et al. 2013). As Mattsson & Andersen (2012) discuss, this could lead to a dust content which, in the central regions, often is larger than the amount of available metals in the interstellar medium (ISM).
The study of the metallicity-γ relation in the Milky Way not only opens the possibility to have, for the first time, more accurate estimates of the amount of molecular gas in clouds, but also provides the possibility to explore it on spatial scales and sensitivities that are extremely challenging to obtain, if not inaccessible, in galaxies other than our own. Issa et al. (1990) studied how the gradient in gas-to-dust ratio depends on the galactocentric radius, but for a limited range of RGC (9−11 kpc) and using optically thick CO lines to estimate the amount of molecular gas, via the integrated intensity of the CO (1–0) line-to-molecular mass conversion factor XCO.
In this work we use a sample of 23 sources in the far outer Galaxy, complemented by 57 sources from the ATLASGAL TOP100 in the inner Galaxy (Fig. 1) to expand this pioneering work, exploring the variation of γ across the entire disk of the Milky Way. This opens up the possibility of using the appropriate value of the gas-to-dust ratio to obtain more precise estimates of the very basic properties of molecular clouds throughout the Milky Way from publicly available surveys, such as the total mass and H2 column density. From these quantities it is possible to derive molecular abundances and, in combination with complete surveys of the galactic disk, a reliable distribution of mass of molecular gas in the Milky Way.
From the Wouterloot & Brand (1989) IRAS/CO catalogue and that compiled by König et al. (in prep.) using 12CO(2–1) and 13CO (2–1)1, we selected a sample of 23 sources in the far outer Galaxy (RGC > 14 kpc; FOG) with the following criteria: i) the source must be associated with IR emission in WISE images ii) Herschel data must be available to estimate the dust content, and iii) the surface density of dust (Σdust) at the emission peak must exceed 3 × 10-5 g cm-2, or NH2 = 8.75 × 1020 cm-2 (i.e. Σgas ~ 19 M⊙ pc-2), assuming γ = 136. According to the model of Hirashita & Harada (2017), the latter condition is sufficient to ensure that the vast majority of gas is in molecular form for Z ≳ 0.2 Z⊙. In the FOG, in fact, the metallicity ranges from ~0.5 Z⊙ at RGC ~ 14 kpc to ~0.2 Z⊙ at RGC ~ 21 kpc (using the results in L"24">sl4o9/full_e h">~ In the201/aa31ml#R7">ed hey1728signhtmla). Thaff
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