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

L. Magrini, S. Bianchi+21 more|UWA Profiles and Research Repository (University of Western Australia)|Jun 3, 2011

Astrophysics and Star Formation Studies1 references48 citations

TL;DR

This study investigates the radial dust-to-gas mass ratio and metallicity gradients in four late-type spiral galaxies (NGC4254, NGC4303, NGC4321, NGC4501) in the Virgo Cluster using Herschel PACS and SPIRE photometry, HI and CO gas maps, and HII region oxygen abundances. It finds that the dust-to-gas ratio gradients closely match oxygen abundance gradients only when using a metallicity-dependent CO-to-H2 conversion factor (X_CO), with values 0.1–0.9× the Galactic X_CO, especially in inner regions dominated by molecular gas, suggesting a superlinear or linear X_CO-metallicity dependence in these cluster galaxies.

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$\sim0.1 imes$ 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

Motivation & Objective

• To investigate the radial correlation between dust-to-gas mass ratio and gas-phase metallicity (oxygen abundance) in Virgo cluster spiral galaxies.
• To assess the impact of the CO-to-H2 conversion factor (X_CO) on derived dust-to-gas ratios and their radial gradients.
• To test whether a metallicity-dependent X_CO is required to reconcile dust-to-gas and oxygen abundance gradients in cluster galaxies.
• To constrain X_CO values in galaxies with disturbed morphologies and varying metallicities, particularly in regions dominated by molecular gas.
• To evaluate the robustness of metallicity gradient measurements by aligning literature oxygen abundances with a common calibration (KK04) and using Galactic X_CO as a lower limit.

Proposed method

• Fitted Herschel PACS and SPIRE photometry with a single-temperature modified blackbody to derive dust mass surface density maps.
• Combined HI moment-1 maps with CO line data to derive total gas mass, using different X_CO values to convert CO emission to H2 mass.
• Extracted radial profiles of dust, atomic gas (HI), and molecular gas (H2) from the derived maps, using geometric parameters from HI kinematics.
• Compared radial gradients of dust-to-gas mass ratio with literature oxygen abundance gradients from HII regions, recalibrated to the KK04 scale for consistency.
• Tested three X_CO dependencies: constant, X_CO ∝ Z⁻¹ (linear), and X_CO ∝ Z⁻².⁵ (superlinear), to match observed dust-to-gas and O/H gradients.
• Used Galactic X_CO as a lower bound to fix the zero-point of oxygen abundance, reducing uncertainty in gradient comparisons.

Experimental results

Research questions

• RQ1 Do the radial gradients of dust-to-gas mass ratio and gas-phase oxygen abundance in Virgo spiral galaxies match when using standard X_CO values?
• RQ2 What X_CO value is required to reproduce the observed dust-to-gas ratio gradient if the oxygen abundance gradient is assumed correct?
• RQ3 Is there evidence for a metallicity dependence of X_CO in these cluster galaxies, particularly in regions dominated by molecular gas?
• RQ4 How do uncertainties in X_CO affect the derived dust-to-gas ratio in the inner versus outer disk regions?
• RQ5 Can the observed agreement between dust-to-gas and oxygen abundance gradients be explained by a non-Galactic X_CO, and if so, what functional form best fits the data?

Key findings

• For NGC4254, NGC4321, and NGC4501, the dust-to-gas mass ratio and oxygen abundance gradients agree within ∼0.6–0.7 R25 when using X_CO values in the range 0.5–0.9× the Galactic value.
• NGC4303 requires an extremely low X_CO value of ∼0.1× the Galactic value to match the observed gradients, suggesting a strong metallicity dependence.
• A linear X_CO-metallicity dependence (X_CO ∝ Z⁻¹) best explains the data for NGC4254, while a superlinear dependence (X_CO ∝ Z⁻².⁵) is needed for NGC4303.
• For NGC4501 and NGC4321, an X_CO intermediate between the Galactic value and the linear dependence is required to match the gradients.
• The inner disk regions (R < 0.6–0.7 R25), dominated by molecular gas, are most sensitive to X_CO choice, while outer regions dominated by atomic gas are less affected and cannot constrain X_CO.
• The study supports a metallicity-dependent X_CO in Virgo cluster spirals, particularly in the radial regions where molecular gas dominates, and highlights the need for a larger sample to confirm this trend.