Median CO-to-H₂ conversion factor estimates as a function of the total surface density. The grey circles correspond to the α_CO values based on dust emission for nearby disc galaxies (Sandstrom et al. 2013). The ULIRG data correspond to the estimates presented in Downes & Solomon (1998) and Papadopoulos et al. (2012). In the latter case, we show the α_CO values reported from their “one-” and “two-component” multi-transition models. The colour bands indicate the traditional CO-to-H₂ conversion factors and their uncertainties for Milky Way- and ULIRG-like systems (Bolatto et al. 2013). The green line corresponds to the α_CO parametrization suggested by Bolatto et al. (2013) for galaxies with solar metallicity. For HATLAS114625, we find a lower α_CO estimate than the expected value from the Bolatto et al. (2013) parametrization. In the case of HATLAS121446, we find an agreement within 1-σ uncertainty. This figure is adapted from Bolatto et al. (2013).