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The high abundance of heavy elements hidden by dust

21 July 2022

For a long time, ultra-luminous infrared galaxies, resulting from major galaxy mergers, appeared with a relatively low abundance of heavy elements, compared to isolated galaxies of similar mass, also forming stars. This was interpreted by the infall of metal-poor gas from the outer parts of the galaxies, gas which fueled the very active star formation in these merging galaxies. But the measurements came from optical observations. An Anglo-American team led by Nima Chartab observed these ultra-luminous galaxies in infrared spectroscopy, allowing to penetrate the dust, and in fact found an abundance of metals quite comparable to galaxies of the same mass. The low abundance of metals was just a bias due to dust obscuration. Francois Hammer comments on these results in a "News & Views" of Nature Astronomy.

Most large galaxies were formed through merging smaller entities, in a scenario called "hierarchical galaxy formation." It is during these mergers that stars are formed as well as elements heavier than hydrogen and helium. Large galaxies with very luminous infrared emissions are galaxies in the process of merging as illustrated by the sequence shown in the Figure, from left to right.

A big problem with the hierarchical scenario was that these ultra-luminous infrared galaxies seemed to have far fewer heavy elements than other galaxies. So, did those galaxies really form by merger, or did other phenomena explain their formation?

An Anglo-American team led by Nima Chartab (University of California) has lifted the veil, by performing infrared spectroscopy of these galaxies, to deduce the fraction of heavy elements. And there, surprise! Unlike previous measurements all made at visible wavelengths, these galaxies have as many heavy elements as other galaxies of the same mass. The article is published in the journal Nature Astronomy, in May 2022.

Figure: Images of galaxies whose chemistry has been reanalyzed. The images made by the Space Telescope are the result of a composition between the near infrared bands, at 1.2 (blue), at 1.6 (green), and at 2.1micron (red). These images follow a sequence, from left to right, which shows the different stages of the merger between two galaxies. On the left the two galaxies meet (IRAS F08572+3915), then approach and interact forming tidal tails (IRAS 12112+0305), then begin to merge even if the two nuclei can still be distinguished (IRAS 15250+3609), to finally form only one galaxy, whose morphology is strongly affected by the violence of the merger (IRAS 17208-0014).

What happened ? Astronomer François Hammer of the Paris Observatory, Paris Sciences et Lettres, comments these results, in a "News & Views" article published in the journal Nature Astronomy on July 18, 2022. Analyzes made in visible light could only identify a tiny part of the formation of heavy elements in these galaxies, creating an observational bias. The formation of heavy elements in the galaxies shown in the Figure is concentrated in the central regions of these merging galaxies. This is where the dust is formed at the same time, which hides the formation of stars and heavy elements at visible wavelengths.

Hence the conclusion that the ultra-luminous infrared galaxies are indeed part of the same chemical sequence as the other galaxies. In addition, they represent well the different stages of mergers during which all galaxies were formed, whether spiral or elliptical. The hierarchical scenario of the galaxies is well confirmed.

References

–  Chartab, Nima et al., Low gas-phase metallicities of ultraluminous infrared galaxies are a result of dust obscuration, Nature Astronomy 6, 844 (2022)

–  Hammer, François, High abundances hidden in the very dusty medium, Nature Astronomy 6, 776 (2022)

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