[Paper Review] Probing the Dust Properties of Galaxies at Submillimetre Wavelengths II. Dust-to-gas mass ratio trends with metallicity and the submm excess in dwarf galaxies

M. Galametz, S. C. Madden+4 more|arXiv (Cornell University)|Apr 5, 2011

Galaxies: Formation, Evolution, Phenomena132 references117 citations

TL;DR

This study investigates how submillimeter (submm) observations affect dust mass and dust-to-gas mass ratio (D/G) measurements in galaxies, particularly in low-metallicity dwarfs. By modeling spectral energy distributions (SEDs) with and without submm data (≥160 μm), the authors show that submm constraints are essential for accurate dust mass estimates—reducing errors and revealing a submm excess in 9 low-metallicity galaxies. The key finding is that including submm data tightens the correlation between D/G and metallicity, especially when a cold dust component (T = 10 K) is added to account for the excess.

ABSTRACT

We are studying the effects of submm observations on the total dust mass and thus dust-to-gas mass ratio measurements. We gather a wide sample of galaxies that have been observed at submm wavelengths to model their Spectral Energy Distributions using submm observations and then without submm observational constraints in order to quantify the error on the dust mass when submm data are not available. Our model does not make strong assumptions on the dust temperature distribution to precisely avoid submm biaises in the study. Our sample includes 52 galaxies observed at submm wavelengths. Out of these, 9 galaxies show an excess in submm which is not accounted for in our fiducial model, most of these galaxies being low- metallicity dwarfs. We chose to add an independant very cold dust component (T=10K) to account for this excess. We find that metal-rich galaxies modelled with submm data often show lower dust masses than when modelled without submm data. Indeed, these galaxies usually have dust SEDs that peaks at longer wavelengths and require constraints above 160 um to correctly position the peak and sample the submillimeter slope of their SEDs and thus correctly cover the dust temperature distribution. On the other hand, some metal-poor dwarf galaxies modelled with submm data show higher dust masses than when modelled without submm data. Using submm constraints for the dust mass estimates, we find a tightened correlation of the dust-to-gas mass ratio with the metallicity of the galaxies. We also often find that when there is a submm excess present, it occurs preferentially in low-metallicity galaxies. We analyse the conditions for the presence of this excess and find a relation between the 160/850 um ratio and the submm excess.

Motivation & Objective

• To assess the impact of submillimeter (submm) observations on dust mass and dust-to-gas mass ratio (D/G) measurements in galaxies.
• To investigate the presence and physical origin of a submillimeter excess in low-metallicity dwarf galaxies not accounted for by standard SED models.
• To improve the accuracy of dust mass estimates by comparing models with and without submm data constraints.
• To examine how submm data refine the observed correlation between dust-to-gas mass ratio and galactic metallicity.
• To evaluate the necessity of submm observations for reliable D/G measurements in high-redshift and low-metallicity galaxies.

Proposed method

• A sample of 52 galaxies with submm observations (850/870 μm and sometimes 450 μm) was compiled, covering a range from low-metallicity dwarfs to metal-rich systems.
• Spectral energy distributions (SEDs) were modeled using mid-infrared to far-infrared data from the literature, both with and without submm flux constraints.
• A fiducial model without a cold dust component was first applied; for galaxies showing a submm excess, an additional cold dust component (T = 10 K, β = 1) was added to improve the fit.
• The total dust mass and dust-to-gas mass ratio (D/G) were derived for each galaxy under both modeling conditions to quantify the effect of submm data.
• The metallicity dependence of D/G was re-evaluated using submm-constrained models to assess whether the correlation tightens with improved data.
• The 160/850 μm flux ratio was analyzed as a diagnostic for identifying submm excess and its relation to cold dust emission.

Experimental results

Research questions

• RQ1 How do submillimeter observations influence the accuracy of dust mass estimates in galaxies?
• RQ2 What is the origin and physical nature of the submillimeter excess observed in low-metallicity dwarf galaxies?
• RQ3 Does including submillimeter data improve the correlation between dust-to-gas mass ratio (D/G) and metallicity?
• RQ4 To what extent does the absence of submm data lead to systematic over- or underestimation of dust masses?
• RQ5 Is there a measurable relationship between the 160/850 μm flux ratio and the presence of a submm excess in galaxies?

Key findings

• Submm data are essential for accurate dust mass estimation: without them, dust masses in metal-rich galaxies are systematically overestimated due to incorrect peak positioning and Rayleigh-Jeans slope sampling.
• In low-metallicity dwarf galaxies, the inclusion of submm data leads to higher dust mass estimates due to a previously unaccounted-for submillimeter excess, which is best modeled with a cold dust component at 10 K.
• The submm excess is predominantly found in low-metallicity galaxies, with 9 out of 52 galaxies in the sample requiring an additional cold dust component to fit their SEDs.
• The addition of submm constraints tightens the observed correlation between dust-to-gas mass ratio (D/G) and metallicity, improving the reliability of D/G as a metallicity tracer.
• A strong correlation is found between the 160/850 μm flux ratio and the presence of a submm excess, suggesting this ratio can serve as a diagnostic for cold dust emission.
• The results caution against using dust mass estimates derived without rest-frame submm observations, especially for high-redshift and low-metallicity galaxies, where errors can be substantial.