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

TL;DR

This study examines how dust-to-gas mass ratio and metallicity gradients in four Virgo spiral galaxies are affected by the CO-to-H2 conversion factor, revealing its significant impact on inner galaxy regions.

Contribution

It provides the first detailed analysis of the relationship between metallicity gradients and dust-to-gas ratios in Virgo spiral galaxies, highlighting the importance of the CO-to-H2 conversion factor.

Findings

Dust-to-gas ratio profiles are highly sensitive to oactors.

Metallicity and dust-to-gas gradients agree within certain radii for specific oactors.

Lower oactors suggest metallicity-dependent oactors in galaxy centers.

Abstract

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. 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 NGC4254, NGC4303, NGC4321, and NGC4501. We examined different dependencies on metallicity of the CO-to-H$_2$ conversion factor (\xco), used to transform the $^{12}$CO observations into the amount of molecular hydrogen. 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 $\sim$0.6-0.7R$_{25}$ using \xco\ values in the range 1/3-1/2 Galactic \xco. For NGC4303 a lower value of \xco$\sim0.1\times$ 10$^{20}$ is necessary. 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 R$_G<$0.6-0.7R$_{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 \xco, and thus we cannot put constraints on its value.