EDP Sciences
Free Access
Issue
A&A
Volume 606, October 2017
Article Number L12
Number of page(s) 14
Section Letters
DOI https://doi.org/10.1051/0004-6361/201731728
Published online 27 October 2017

© ESO, 2017

1. Introduction

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.

2. Observations and sample selection

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">~ ~ 21 kpc (using the results in ) a317 gas-t> ailowing cri> k muSme gr728-nles/aa/full_h(LMC, SMC) hreficity ranges from ~14 the gradient in gas-to-dust ratio depends on the galactocentric radius, bu4 thRuskpclesults Dopeculang2uter paen sl4o9and usin criteria: i) the sof ot e kpc<1ul>From the γ), and hotThis is also supported by observations in nearby galaxies (e.g. ) at ⊙ecular abundancme gcaps">1728&sample ecular abundances and, in combination with comp far outer oatiashref="/articles/aa/full_html/28-17/aaGC ~ 21 kpc (using t30>. According to the model of 1<8-17/aaGC ~ 21 kpc (using t80>. According to the model of L"24">sl4oe Milky Way from s 13les" savai name="I28-nles/aa/full_,>Nγ = 136. According to the model of ~1<>GC ~ 21 kpc (using t230>. According to the model of 0.5 only opens tR46">Wouimpl clouIR-b modele follo,ynticleiii) the surfaceundance wit use a samples/aa/fub>om "> a the am nlyeform> d usin criss="s

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mtance of thMCWee> (makeacugesibipondPyMC/>,ykagem(8">the grRffeooass=terloo >"mlass="sim<_hi/articlrffeooass="mlass="sim sass="sim s/limlass="sim sass="sfull_htmlR the grAsal pypCollabor> valuesives stui.adsabs.hah">rd.edu/#absll_h3A&A...558A..33A/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmfule/componeink_edps"> valuesives stdoig mol>[C-dusRff]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmfule/componeink_edps"> valuesive stwww.ahicag moleref="/articabsll_h3ml/20122068-h3m0122068-h3G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho1"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#abslang[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho3"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_07A&A...464..909B/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesive stwww.ahicag moleref="/articabsll_0tml2rti54tn-06rti54tn-06G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho4"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_09PASP..121...76C/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#absll_05ApJ...623..213C/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>ala, B. T.ubDa) ubD. A.ubBendo, G.ubsen (201207, ApJ, 66<, 866lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stui.adsabs.hah">rd.edu/#absll_07ApJ...663..866D/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#abslang8ApJ...501..643D/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>[C-dusRff]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho8"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h3ApJ...772...45E/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#absll_h7MNRAS.471..100E/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>ubD. A.ubsen (2012)<, N0/aa3, 495, 487lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stui.adsabs.hah">rd.edu/#absl12)[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>< thSt>). S class (TaylorRar forFrugcis)lass="sim sass="slass="sim sass="sim sass="sim saslass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho13"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h4A&A...570A..65G/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesive stwww.ahicag moleref="/articabsll_14ml/20123692-14m0123692-14G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho14"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h7A&A...60[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>[C-dusRff]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmfule/componeink_edps"> valuesive stwww.ahicag moleref="/articabsll_h7/031728"ht8-161728"ht8-16G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho15"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h4ApJ...797...85G/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#absll_h7ApJ...837...98G/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#absll_h0A&A...518L...3G/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>[C-dusRff]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesive stwww.ahicag moleref="/articabsll_h0ml/201h4519-l/201h4519-l/G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho18"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h7MNRAS.467..699H/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho19"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmmubJ. D.01207, Compu). S c. Engin.ub9, 90lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stui.adsabs.hah">rd.edu/#absll_07CSE.....9...90H/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#abslang0A&A...236..237I/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho21"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm,2II, N20J.uber thaLeethC. Wthl_08, Aer thA, 487, 993lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stui.adsabs.hah">rd.edu/#absll_08A&A...487..993K/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesive stwww.ahicag moleref="/articabsll_08/33m0109481-08/0109481-08G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho22"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h4ITTST...4..588K/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>[C-dusRff]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho23"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h7A&A...599A.139K/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesive stwww.ahicag moleref="/articabsll_h7/03m0126841-15m0126841-15G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho24"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h2ApJ...750...33L/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>ala, B. T.hl_01, ApJ, 554, 778lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stui.adsabs.hah">rd.edu/#absll_01ApJ...554..778L/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#absll_11AJ....142..136L/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#absll_h2MNRAS.423...38M/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#absll_h4MNRAS.444..797M/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>[C-dusRff]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho29"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmmrd, B., BallythJ.ubrticles/aa0, PASP, 122, 314lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stui.adsabs.hah">rd.edu/#absll_h0PASP..122..314M/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#abslang4A&A...291..943O/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho31"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmmthi, V.nS., Sofia, U20J.ubMurthythJ.uber thaBabu, B. R.nS.01212, ApJ, 760, 36lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stui.adsabs.hah">rd.edu/#absll_12ApJ...760...36P/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>rd, D.uber thaFonnesbe r, Ces/aa0, J. St>). Soft, 35, 1lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stdoig mol> valuesive stepjh.epjg moleref="/arepjhcabsll_h0m01/h1="h19/h1="h19G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho33"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm gSg. f., 60, 883lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stui.adsabs.hah">rd.edu/#abslang8JQSRT..60..883P/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>LDsstribue="ISt>) st lal Compu)). lass="sim sass="slass="sim sass="sim sass="sim saslass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho35"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h0A&A...518L...2P/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesive stwww.ahicag moleref="/articabsll_h0ml/201h4535-l/201h4535-l/G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho36"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_05A&A...437..957P/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesive stwww.ahicag moleref="/articabsll_05/2317227tn-0517227tn-05G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho37"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h4ApJ...783..130R/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#absll_14ApJ...797...86R/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>ubD. A.ubsen (2012)5, N0/aa3, 525, 218lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stui.adsabs.hah">rd.edu/#absll_h5N0/aa.525..218R/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesive stwww.amsg molN2">scinet-ompitem?mr=337707n e[M2">S cNet]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesive stwww.ncbi.nlm.nih.gov/growez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abs7; valuesives stui.adsabs.hah">rd.edu/#absl1992ApJ...384..508R/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>ubF.ubsen (20121<, ApJ, 777, 5lass="sim sass="slass="sim sass="sim sass="sim sass="sfule/componeink_edps"> valuesives stui.adsabs.hah">rd.edu/#absll_h3ApJ...777....5S/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>ubF.ubMeilh"0 valuesives stui.adsabs.hah">rd.edu/#absll_09A&A...504..415S/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>[C-dusRff]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmfule/componeink_edps"> valuesive stwww.ahicag moleref="/articabsll_09/3517211568-08/0111568-08G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho43"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmmarXiv:1709.00392e gr]lass="sim sass="slass="sim sass="sim sass="sim saslass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho44"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#abslang4ARA&A..32..191W/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol> valuesives stui.adsabs.hah">rd.edu/#absl1989A&AS...80..149W/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho46"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_08A&A...487..237W/abs7;[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>[C-dusRff]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmfule/componeink_edps"> valuesive stwww.ahicag moleref="/articabsll_08/31rti8156-031728156-03G" tit>[EDP S class=]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho47"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm valuesives stui.adsabs.hah">rd.edu/#absll_h0AJ....14[NASA ADS]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmms="sfule/componeink_edps"> valuesives stdoig mol>[C-dusRff]e grlass="simmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmlass="simmmmmmmmmm17/aa31728-17Z398.orgitltml"mlass="sim sass="sim sss="simmmmmmmmmm1i>^lass="sim sass="slass="sim sass="sim sass="sim sass="sful value# ho48"mlass="sim sass="sim sss="( critext)lass="sim sass="sim s/ grlass="simmmmmmmmmmmmmmmmmmmmm17/aa31728-17bold">Tab"/^>Properefes of> crisourss= hichresults of> criSED fit procedure for> crifar class Galaxy sr tle.

sass="s17/aa31728-17bold">Tab"/^>Measurements of> criC17/aa31728-17si tle-N2">">17up>181i>up>1i>^O rot>
sass="s17/aa31728-17bold">Tab"/^>Priors used in MCWeeds.
sass="s17/aa31728-17bold">Tab"/^>Column densiefesubsxg. ">17up>181i>up>1i>^O abuicaass=, gas-to-dust r> crifar class Galaxy sr tle.
sass="s17/aa31728-17bold">Tab"/^>Column densiefesubsxg. ">17up>181i>up>1i>^O abuicaass= hichgas-to-dust r> crisubsr tle of> criTOP100 sel. cis work. sass="s1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig5_small.jpg1">17/aa31728-17bold">Fig. B211i>^>SED fit (top penel) hich criresiduals 17/aa31728-17si tle-N2">">(S17ub>obs1i>ub>−S17ub>mod1i>ub>) /S17ub>obs1i>ub>1i>^ (bottom penel for>eachisourss). critop penel,> criext;rdsbhichupw>rds,iresg. crigrey- hichblack-body is shown byh crivalig dashed eines. cribottom penel exceed |2|,> cripoin) is indicaied byhah riang"/.ftd 1ol> ="2"mful valuesive stdexter.edps class=g moleptlet.php?pdf_id=5er thDOI=>Open with DEXTER> sass="s1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig6_small.jpg1">17/aa31728-17bold">Fig. B211i>^>coroinued.ftd 1ol> ="2"mful valuesive stdexter.edps class=g moleptlet.php?pdf_id=6er thDOI=>Open with DEXTER> sass="ssourss= ii> crifar class Galaxy1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig7_small.jpg1">17/aa31728-17bold">Fig. C211i>^>C17/aa31728-17si tle-N2">">17up>181i>up>1i>^O sg. crisourss= ii> criFOG. We show ii>red tcribest fit from MCWeeds.ftd 1ol> ="2"mful valuesive stdexter.edps class=g moleptlet.php?pdf_id=7er thDOI=>Open with DEXTER> sass="s1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig8_small.jpg1">17/aa31728-17bold">Fig. C211i>^>coroinued.ftd 1ol> ="2"mful valuesive stdexter.edps class=g moleptlet.php?pdf_id=8er thDOI=>Open with DEXTER> sass="s17/aa31728-17bold">Tab"/^>Properefes of> crisourss= hichresults of> criSED fit procedure for> crifar class Galaxy sr tle. critext> 17/aa31728-17bold">Tab"/^>Measurements of> criC17/aa31728-17si tle-N2">">17up>181i>up>1i>^O rot> critext> 17/aa31728-17bold">Tab"/^>Priors used in MCWeeds. critext> 17/aa31728-17bold">Tab"/^>Column densiefesubsxg. ">17up>181i>up>1i>^O abuicaass=, gas-to-dust r> crifar class Galaxy sr tle. critext> 17/aa31728-17bold">Tab"/^>Column densiefesubsxg. ">17up>181i>up>1i>^O abuicaass= hichgas-to-dust r> crisubsr tle of> criTOP100 sel. cis work. critext> 1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig1_small.jpg1">17/aa31728-17bold">Fig. 11i>^>Dsstribuesog of> crisourss= corsidered in cis work. white we show crisourss= ii> criFOG, hichii>red we show clumps from criTOP100 sr tle. Tcribackgrouichimage is aa3erefst i treassog of> criMilky Way as seen from crinor crrnhgalacNASA/JPL-Calat (0, 8.34) kpc.ftd 1ol> ="2"mful valuesive stdexter.edps class=g moleptlet.php?pdf_id=1er thDOI=>Open with DEXTER>ftd 1ol> ="2"31728-17in-txt"mful valueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7G" ti#F11" critext> fdiv31728-17insei">1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig2_small.jpg1">17/aa31728-17bold">Fig. 21i>^>Top:bsxr tle of> criSED fit for>WB17/aa31728-17si tle-N2">">_1i>𗉅, hos)). one of> cri crifurthest clusters from crigalacrdsbhichupw>rds,iresg. crigrey hichblack bodis= hre shown byh crivalig dashed eines. Bottom:bresiduals calculaied as 17/aa31728-17si tle-N2">">(S17ub>obs1i>ub>−S17ub>mod1i>ub>) /S17ub>obs1i>ub>1i>^. TcriSED hich crirbresiduals for>allh crio crrisourss= hre included in Fig. ful valueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7G" ti#F51"B211ia>.ftd 1ol> ="2"mful valuesive stdexter.edps class=g moleptlet.php?pdf_id=2er thDOI=>Open with DEXTER>ftd 1ol> ="2"31728-17in-txt"mful valueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7G" ti#F21" critext> fdiv31728-17insei">1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig3_small.jpg1">17/aa31728-17bold">Fig. 31i>^>Exr tle of> criC17/aa31728-17si tle-N2">">17up>181i>up>1i>^O(2–1) obseh">tsogs for>WB17/aa31728-17si tle-N2">">_1i>𗉅. We indicaie ii>red tcribest fit from MCWeeds. Tcrisg. crigroire sr tle hre shown in Fig. ful valueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7G" ti#F7">C211ia>.ftd 1ol> ="2"mful valuesive stdexter.edps class=g moleptlet.php?pdf_id=3er thDOI=>Open with DEXTER>ftd 1ol> ="2"31728-17in-txt"mful valueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7G" ti#F31" critext> fdiv31728-17insei">1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig4_small.jpg1">17/aa31728-17bold">Fig. 41i>^>Variaesog of> crigas-to-dust r>ourbfiducial case (penel a), corsider). a CO/C17/aa31728-17si tle-N2">">17up>181i>up>1i>^O galacpenel b), hich28-um). that criabuicaass of>COifollowsh criradial oxygen gradigro, r> criC/H (penel c). Tcri cick blue eines indicaie tcribest fit,irepor ed in cribottom right corner;> cri<7/aa31728-17si tle-N2">">68%1i>^ hich<7/aa31728-17si tle-N2">">95%1i>^ highest probability densiey inteh">ls of> crifit perameters hre indicaied byh crilight hichdark yellow-shaded regsogs,iresg. comperison with extergal galaxis= <7/aa31728-17si tle-N2">">log 1i>^ (O/H) + 12 is shown oi> critop axis.ftd 1ol> ="2"mful valuesive stdexter.edps class=g moleptlet.php?pdf_id=4er thDOI=>Open with DEXTER>ftd 1ol> ="2"31728-17in-txt"mful valueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7G" ti#F41" critext> fdiv31728-17insei">1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig5_small.jpg1">17/aa31728-17bold">Fig. B211i>^>SED fit (top penel) hich criresiduals 17/aa31728-17si tle-N2">">(S17ub>obs1i>ub>−S17ub>mod1i>ub>) /S17ub>obs1i>ub>1i>^ (bottom penel for>eachisourss). critop penel,> criext;rdsbhichupw>rds,iresg. crigrey- hichblack-body is shown byh crivalig dashed eines. cribottom penel exceed |2|,> cripoin) is indicaied byhah riang"/.ftd 1ol> ="2"mful valuesive stdexter.edps class=g moleptlet.php?pdf_id=5er thDOI=>Open with DEXTER>ftd 1ol> ="2"31728-17in-txt"mful valueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7G" ti#F51" critext> fdiv31728-17insei">1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig6_small.jpg1">17/aa31728-17bold">Fig. B211i>^>coroinued.ftd 1ol> ="2"mful valuesive stdexter.edps class=g moleptlet.php?pdf_id=6er thDOI=>Open with DEXTER>ftd 1ol> ="2"31728-17in-txt"mful valueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7G" ti#F61" critext> fdiv31728-17insei">1imgn (ponthumbnaiitlsrcueleref="/articfull_" till_h7/l/20131728-h7/0131728-h7-fig7_small.jpg1">17/aa31728-17bold">Fig. C211i>^>C17/aa31728-17si tle-N2">">17up>181i>up>1i>^O sg. crisourss= ii> criFOG. 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