The Herschel Virgo Cluster Survey IX. Dust-to-gas mass ratio and metallicity gradients in four Virgo spiral galaxies

Context. Using Herschel data from the open time key project the Herschel Virgo Cluster Survey (HeViCS), we investigated the relationship between the metallicity gradients expressed by metal abundances in the gas phase as traced by the chemical composition of HII regions, and in the solid phase, as traced by the dust-to-gas mass ratio. Aims. We derived the radial gradient of the dust-to-gas mass ratio for all galaxies observed by HeViCS whose metallicity gradients are available in the literature. They are all late type Sbc galaxies, namely NGC 4254, NGC 4303, NGC 4321, and NGC 4501. Methods. We fitted PACS and SPIRE observations with a single-temperature modified blackbody, inferred the dust mass, and calculated two dimensional maps of the dust-to-gas mass ratio, with the total mass of gas from available HI and CO maps. HI moment-1 maps were used to derive the geometric parameters of the galaxies and extract the radial profiles. We examined different dependencies on metallicity of the CO-to-H-2 conversion factor (X-CO), used to transform the (CO)-C-12 observations into the amount of molecular hydrogen. Results. We found that in these galaxies the dust-to-gas mass ratio radial profile is extremely sensitive to choice of the X-CO value, since the molecular gas is the dominant component in the inner parts. We found that for three galaxies of our sample, namely NGC 4254, NGC 4321, and NGC 4501, the slopes of the oxygen and of the dust-to-gas radial gradients agree up to similar to 0.6-0.7 R-25 using X-CO values in the range 1/3-1/2 Galactic X-CO. For NGC 4303 a lower value of X-CO similar to 0.1 x 1020 is necessary. Conclusions. We suggest that such low X-CO values might be due to a metallicity dependence of X-CO (from close to linear for NGC 4254, NGC 4321, and NGC 4501 to superlinear for NGC 4303), especially in the radial regions R-G < 0.6-0.7 R-25 where the molecular gas dominates. On the other hand, the outer regions, where the atomic gas component is dominant, are less affected by the choice of X-CO, and thus we cannot put constraints on its value there.