Σ_H2 distribution in NGC 551. The white ellipse shows a representative annular region (a radial bin) at a radius of 6 kpc over which Σ_H2 was averaged to obtain a data point in the Σ_H2 profile. The Σ_H2 map was taken from Bolatto et al. (2017). The annular ellipses were demarcated, assuming an inclination of 63° and a position angle of 135°. The dashed red line designates the major axis of the galaxy.