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Does the CO-to-H2 conversion factor depend on the star formation rate?

Clark, Paul ORCID: and Glover, Simon C. O. 2015. Does the CO-to-H2 conversion factor depend on the star formation rate? Monthly Notices of the Royal Astronomical Society 452 (2) 10.1093/mnras/stv1369

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We present a series of numerical simulations that explore how the ‘X-factor’, XCO – the conversion factor between the observed integrated CO emission and the column density of molecular hydrogen – varies with the environmental conditions in which a molecular cloud is placed. Our investigation is centred around two environmental conditions in particular: the cosmic ray ionization rate (CRIR) and the strength of the interstellar radiation field (ISRF). Since both these properties of the interstellar medium (ISM) have their origins in massive stars, we make the assumption in this paper that both the strength of the ISRF and the CRIR scale linearly with the local star formation rate (SFR). The cloud modelling in this study first involves running numerical simulations that capture the cloud dynamics, as well as the time-dependent chemistry, and ISM heating and cooling. These simulations are then postprocessed with a line radiative transfer code to create synthetic 12CO (1–0) emission maps from which XCO can be calculated. We find that for 104 M virialized clouds with mean density 100 cm−3, XCO is only weakly dependent on the local SFR, varying by a factor of a few over 2 orders of magnitude in SFR. In contrast, we find that for similar clouds but with masses of 105 M, the X-factor will vary by an order of magnitude over the same range in SFR, implying that extragalactic star formation laws should be viewed with caution. However, for denser (104 cm−3), supervirial clouds such as those found at the centre of the Milky Way, the X-factor is once again independent of the local SFR.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Publisher: Oxford University Press
ISSN: 0035-8711
Date of First Compliant Deposit: 11 September 2017
Date of Acceptance: 16 June 2015
Last Modified: 04 May 2023 18:34

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