TheHerschelVirgo Cluster Survey

Abstract

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 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 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 ∼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 ∼ 0.1 × 10 20 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.

TheHerschelVirgo Cluster Survey

“…Each pixel was fitted with a single temperature modified black body, with dust emissivity κ ∝ ν β and β = 2; the filter response function was taken into account for the color correction. As shown by Davies et al (2012) for global fluxes, and by Magrini et al (2011) for pixel-by-pixel surface brightness analysis, our choice of the dust emissivity provides a satisfactory fit to the spectral energy distribution. If we exclude the notorious uncertainty of the dust emissivity, the uncertainties due to the fitting procedure amounts to 10-15% for the full depth HeViCS data set.…”

“…The dust surface mass density is derived as in Magrini et al (2011) (see also Smith et al 2010;Bendo et al 2010a). The five images in the Herschel photometric bands were convolved to the poorer resolution of the 500 μm, then registered on the same pixel grid.…”

“…If we exclude the notorious uncertainty of the dust emissivity, the uncertainties due to the fitting procedure amounts to 10-15% for the full depth HeViCS data set. Other sources of uncertainty in the determination of the total mass are discussed in Magrini et al (2011).…”

“…Since we do not have Z c and gradients for all the galaxies in our sample we assume the central metallicity estimated using the mass metallicity relation (Tremonti et al 2004) and reported in Col. 12 of Table 1. We fix the solar value at Z = 8.69, as recently found by Asplund et al (2009), and assume a radial metallicity gradient ∝−0.4 R/R 25 , as found by Magrini et al (2011) for a sample of Virgo spirals. This slope should reproduce the effect of a metallicity gradient.…”

The dust content of galaxies from z = 0 to z = 9

“…These observational constraints include for instance the relation between dust mass and stellar mass (Corbelli et al 2012;Santini et al 2014), the gas fraction of galaxies and dust mass (Cortese et al 2012), dust mass and star-formation rate (SFR; da Cunha et al 2010;Casey 2012;Santini et al 2014), and the dust mass function of galaxies (Dunne, Eales & Edmunds 2003;Vlahakis, Dunne & Eales 2005;Dunne et al 2011;Eales et al 2009;Clemens et al 2013). Two particularly interesting scaling relations are the ratio between dust mass and gas mass in the ISM (dust-togas ratio; DTG), or the ratio between dust mass and the total mass in metals (dust-to-metal ratio; DTM) as a function of metallicity or galaxy stellar mass (Issa, MacLaren & Wolfendale 1990;Lisenfeld & Ferrara 1998;Hirashita, Tajiri & Kamaya 2002;James et al 2002;Hunt, Bianchi & Maiolino 2005;Draine et al 2007;Engelbracht et al 2008;Galametz et al 2011;Magrini et al 2011;Rémy-Ruyer et al 2014a). Rémy-Ruyer et al (2014a) demonstrated that the DTG ratio in galaxies cannot be described by a single powerlaw as a function of metallicity, but is better represented by a double power-law with a break around a metallicity of 0.1 Z (Edmunds 2001).…”