The relationship between CO emission and visual extinction traced by dust emission in the magellanic clouds

Cheoljong Lee, Adam K. Leroy, Scott Schnee, Tony Wong, Alberto D. Bolatto, Remy Indebetouw, Monica Rubio

Research output: Contribution to journalArticlepeer-review


To test the theoretical understanding that finding bright CO emission depends primarily on dust shielding, we investigate the relationship between CO emission (ICO) and the amount of dust (estimated from infrared emission and expressed as 'AV') across the Large Magellanic Cloud (LMC), the Small Magellanic Cloud, and the MilkyWay.We show that at our common resolution of 10 pc scales, ICO given a fixed line of sight AV is similar across all three systems despite the difference in metallicity. We find some evidence for a secondary dependence of ICO on radiation field; in the LMC, ICO at a given AV is smaller in regions of high Tdust, perhaps because of an increased photodissociating radiation field. We suggest a simple but useful picture inwhich theCO-to-H2 conversion factor (XCO) depends on two separable factors: (1) the distribution of gas column densities, whichmaps to an extinction distribution via a dustto- gas ratio; and (2) the dependence of ICO on AV. Assuming that the probability distribution function (PDF) of local MilkyWay clouds is universal, this approach predicts a dependence of XCO on Z between Z-1 and Z-2 above about a third solarmetallicity. Below this metallicity, CO emerges from only the high column density parts of the cloud and so depends very sensitively on the adopted PDF and the H2/H I prescription. The PDF of low-metallicity clouds is thus of considerable interest and the uncertainty associated with even an ideal prescription for XCO at very low metallicity will be large.

Original languageEnglish (US)
Pages (from-to)2708-2726
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Jul 1 2015


  • Galaxies: ISM-Magellanic Clouds
  • ISM: clouds
  • ISM: molecules

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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