Distant galaxy A1689-zD1 found to have unusually low dust-to-gas ratio
Tomasz Nowakowski
astronomy writer
Sadie Harley
scientific editor
Robert Egan
associate editor
Using the James Webb Space Telescope (JWST) and the Atacama Large Millimeter/sub-millimeter Array (ALMA), an international team of astronomers has carried out comprehensive multiwavelength observations of a distant massive galaxy known as A1689-zD1.
The new observations, detailed in a paper October 9 on the pre-print server arXiv, yield important insights into the properties of the galaxy, especially regarding dust production in this system.
A1689-zD1 is a bright highly-lensed massive galaxy at a redshift of approximately 7.13. It has a diameter of about 3,000 light years and its stellar mass is estimated to be some 2.6 billion solar masses.
Previous observations of A1689-zD1 have found that it has a metallicity close to the solar value and that it contains a substantial amount of dust—with an estimated mass of 15 million solar masses. Due to this, A1689-zD1 is an excellent place to study the existence of interstellar dust at early cosmic epochs.
That is why a group of astronomers led by Kasper E. Heintz of the University of Copenhagen, Denmark, decided to explore the dust content with JWST and ALMA.
"We revisited this galaxy to gauge the baryonic matter components in the ISM [interstellar medium], with particular focus on constraining the build up of cosmic dust," the researchers explained.
Hintz's team performed the rest-frame ultraviolet to far-infrared modeling of the spectral energy distribution (SED) of A1689-zD1 to determine its stellar mass, dust mass, visual attenuation, and star-formation rate. The ALMA observations were also used to constrain the total dynamical mass of the source, and infer the gas mass using common gas tracers but bounded by the overall dynamics of the system.
The study found that although A1689-zD1 has a substantial dust mass, its dust-to-gas (DTG) and dust-to-metal (DTM) mass ratios are remarkably low—at a level of 0.00051 and 0.061, respectively. The astronomers note that this is due to the high metallicity of A1689-zD1 and its substantial gas mass, which was calculated to be 28 billion solar masses.
Therefore, the DTG and DTM mass ratios for A1689-zD1 are an order of magnitude lower than that found in the Milky Way and the Large Magellanic Cloud (LMC) or the Small Magellanic Cloud (SMC). These ratios also suggest that the bulk neutral atomic hydrogen (HI) gas in the line-of-sight to A1689-zD1 is relatively dust-poor compared to its chemical enrichment.
The authors of the paper conclude that the obtained results point to a potential change in the relative dust abundance or composition of early galaxies.
"We find that this deviation in the DTG and DTM mass ratios appears to be ubiquitous in other metal-rich galaxies at similar redshifts, z ≳ 6. This suggests that the processes that form and destroy dust at later times, or the dust emissivity itself, are drastically different for galaxies in the early universe," the scientists conclude.
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More information: Kasper E. Heintz et al, Inefficient dust production in a massive, metal-rich galaxy at z=7.13 uncovered by JWST and ALMA, arXiv (2025).
Journal information: arXiv
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