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-t o-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 ga laxies, namely NGC4254, NGC4303, NGC4321, and NGC4501. Methods. We fitted PACS and SPIRE observations with a single-temperat ure 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 di fferent dependencies on metallicity of the CO-to-H2 conversion factor (XCO ), used to transform the 12 CO 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 XCO value, since the molecular gas is the dominant component in the inner parts. We found that for three galaxies of our sample, namely NGC4254, NGC4321, and NGC4501, the slopes of the oxygen and of the dust-to-gas radial gradients agree up to∼0.6-0.7R25 using XCO values in the range 1/3-1/2 Galactic XCO . For NGC4303 a lower value of XCO∼0.1× 10 20 is necessary. Conclusions. We suggest that such low XCO values might be due to a metallicity dependence of XCO (from close to linear for NGC4254, NGC4321, and NGC4501 to superlinear for NGC4303),especially in the radial regions RG <0.6-0.7R25 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 XCO , and thus we cannot put constraints on its value.