We measure the COtoH_{2}conversion factor (
We present measurements of the restframe UV spectral slope,
 NSFPAR ID:
 10497024
 Publisher / Repository:
 DOI PREFIX: 10.3847
 Date Published:
 Journal Name:
 The Astrophysical Journal Letters
 Volume:
 964
 Issue:
 2
 ISSN:
 20418205
 Format(s):
 Medium: X Size: Article No. L24
 Size(s):
 ["Article No. L24"]
 Sponsoring Org:
 National Science Foundation
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Abstract α _{CO}) in 37 galaxies at 2 kpc resolution, using the dust surface density inferred from farinfrared emission as a tracer of the gas surface density and assuming a constant dusttometal ratio. In total, we have ∼790 and ∼610 independent measurements ofα _{CO}for CO (2–1) and (1–0), respectively. The mean values forα _{CO (2–1)}andα _{CO (1–0)}are and ${9.3}_{5.4}^{+4.6}$ , respectively. The COintensityweighted mean is 5.69 for ${4.2}_{2.0}^{+1.9}\phantom{\rule{0.25em}{0ex}}{M}_{\odot}\phantom{\rule{0.25em}{0ex}}{\mathrm{pc}}^{2}\phantom{\rule{0.25em}{0ex}}{(\mathrm{K}\phantom{\rule{0.25em}{0ex}}\mathrm{km}\phantom{\rule{0.25em}{0ex}}{\mathrm{s}}^{1})}^{1}$α _{CO (2–1)}and 3.33 forα _{CO (1–0)}. We examine howα _{CO}scales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dustmassweighted average interstellar radiation field strength ( ). Among them, $\overline{U}$ , Σ_{SFR}, and the integrated CO intensity ( $\overline{U}$W _{CO}) have the strongest anticorrelation with spatially resolvedα _{CO}. We provide linear regression results toα _{CO}for all quantities tested. At galaxyintegrated scales, we observe significant correlations betweenα _{CO}andW _{CO}, metallicity, , and Σ_{SFR}. We also find that $\overline{U}$α _{CO}in each galaxy decreases with the stellar mass surface density (Σ_{⋆}) in highsurfacedensity regions (Σ_{⋆}≥ 100M _{⊙}pc^{−2}), following the powerlaw relations and ${\alpha}_{\mathrm{CO}\phantom{\rule{0.25em}{0ex}}(2\u20131)}\propto {\mathrm{\Sigma}}_{\star}^{0.5}$ . The powerlaw index is insensitive to the assumed dusttometal ratio. We interpret the decrease in ${\alpha}_{\mathrm{CO}\phantom{\rule{0.25em}{0ex}}(1\u20130)}\propto {\mathrm{\Sigma}}_{\star}^{0.2}$α _{CO}with increasing Σ_{⋆}as a result of higher velocity dispersion compared to isolated, selfgravitating clouds due to the additional gravitational force from stellar sources, which leads to the reduction inα _{CO}. The decrease inα _{CO}at high Σ_{⋆}is important for accurately assessing molecular gas content and star formation efficiency in the centers of galaxies, which bridge “Milky Way–like” to “starburstlike” conversion factors. 
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