# High abundances hidden in the dusty medium

François Hammer

► To cite this version:

François Hammer. High abundances hidden in the dusty medium. Nature Astronomy, 2022, 6 (7), pp.776-777. 10.1038/s41550-022-01735-7. hal-04014512

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Submitted on 11 Apr 2023

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# High abundances hidden in the very dusty medium

Author: François Hammer (francois.hammer@obspm.fr)

Affiliation GEPI, Paris Observatory, PSL, CNRS

Certain types of strongly star-forming galaxies have long been thought to lie outside the usual trends in metal abundances followed by other galaxies. A study now reveals the significant effect of dust on observations, bringing these galaxies back into line with models.

One of the most important properties in determining how galaxies evolve is their abundance of heavy metals. It is expected that this metallicity correlates with a galaxy's stellar mass¹, because gas in galaxies is steadily transformed to form both stars and heavy elements, for example through supernovas.

However, the past few decades of observations have suggested that a particular class of galaxies, known as ultra-luminous infrared galaxies (ULIRGs), do not follow this trend, and many galaxy simulations have been run in an attempt to understand why². Writing in *Nature Astronomy*, Nima Chartab and colleagues³ have now revealed that ULIRGs do indeed have the same metal abundances as non-merging galaxies of the same mass, but obscuration by dust plays a much bigger part than previously thought.

ULIRGs are amongst the most massive star-forming galaxies, and they are a result of the later phases of galaxy merging, as Figure 1 shows. It appears at odds for ULIRGs to be lacking in heavy elements because mergers of galaxies are accompanied by gas compression, resulting in large amounts of star formation, and subsequently formation of dust and heavy elements via supernovas.

Although dust can result in the full extinction of visible light, far-infrared light is not affected at all, because its wavelength is larger than the dust grain size (~1µm). To arrive at their result, Chartab and the team used the far-infrared emission lines [OIII] 52µm, [OIII] 88µm, and [NIII] 57µm, which interestingly mostly emit from the galaxy cores; these are indeed the most affected by dust, as attested by their red appearances in Figure 1. In fact, 99% of the visible light emitted by ULIRGs is absorbed by dust and then thermally re-radiated at infrared wavelengths by the heated dust grains.

This result does not mean that former models of merger abundances were wrong, just that they would need different ingredients than previously assumed. For example, they⁴ correctly identified that some low-metallicity gas can be brought from the outskirts to the centre by tidal forces and gravitational torques due to disc-disc interactions. However, a rapid enrichment of heavy elements in the core is also expected, due to supernovas occurring alongside the huge amount of star formation.

Intriguingly, this study of nearby galaxies also affects our understanding of the distant Universe. Although they are very scarce and represent just a small fraction of the total star formation today, luminous infrared galaxies were much more common 6 to 9 billion years ago and account for at least 50% and possibly up to 90% of the star formation in the distant Universe⁵.
In fact, the result from Chartab and colleagues directly affects the long debate about how galactic discs are formed. Today most large galaxies have discs, which are thought to form through mergers of older galaxies such as ULIRGs⁶. Such a hierarchical sequence was problematic when ULIRGs were observed to be chemically different from other galaxies, however this discrepancy now vanishes as ULIRGs are reinserted into the conventional mass-metallicity relation, supporting the hierarchical scenario for all galaxies.

There are several future directions of research to consider after the publication of this result, the first being to try to get additional similar measurements in the nearby Universe. It would be even more interesting to perform them in the distant Universe, where ULIRGs dominate. However distant ULIRGs are much fainter, and their emission lines are redshifted to the sub-millimetre range. This means that with current facilities, we can study them only at high redshifts, z > 4.7, with telescopes such as ALMA, while most of the action is expected from z=1 to z=3, near the peak of star formation. Scientists may have to wait decades before getting a better window onto the peak of star formation in the distant Universe.

The second direction suggests a revisit of how we model star and heavy element formation during galaxy interactions. It has often been assumed that strong star formation is immediately followed by strong feedback, which regulates and even prevents further star formation. This expectation is based on the fact that supernovas expel gas at large distances, creating bubbles within which star formation cannot continue.

However, we are a long way from understanding the efficiency of supernovas during this process. It is likely that in many numerical simulations the feedback has often been made too efficient in regulating the star formation; this can be tested by modifying the simulations to try to reproduce the ULIRGs studied by the authors, and shown in Figure 1. Future observations by the James Webb Space Telescope will also be affected by dust found in highly star-forming galaxies, so this study highlights the need to treat the metal abundance mass relation with caution when trying to understand the earliest galaxies in the Universe.

## References

1. Tremonti, C. A. et al. The Origin of the Mass-Metallicity Relation: Insights from 53,000 Star-forming Galaxies in the Sloan Digital Sky Survey. Astrophys. J. 613, 898–913 (2004).

2. Rupke, D. S. N., Kewley, L. J. & Barnes, J.E., Galaxy Mergers and the Mass-Metallicity Relation: Evidence for Nuclear Metal Dilution and Flattened Gradients from Numerical Simulations. Astrophys. J. 710, L156–L160 (2010).

3. Chartab, N. et al. Low gas-phase metallicities of ultraluminous infrared galaxies are a result of dust obscuration. Nat. Astron. (2022).

4. Rupke, D. S. N., Kewley, L. J. & Galaxy Mergers and the Mass-Metallicity Relation: Evidence for Nuclear Metal Dilution and Flattened Gradients from Numerical Simulations. Astrophys. J. 710, L156–L160 (2010).

5. Flores et al. 15 Micron Infrared Space Observatory Observations of the 1415+52 Canada-France Redshift Survey Field: The Cosmic Star Formation Rate as Derived from Deep Ultraviolet, Optical, Mid-Infrared, and Radio Photometry. Astrophys. J. 517, 148 (1999).

6. Hammer, F., Flores, H., Elbaz, D., Zheng, X. Z., Liang, Y. C., Cesarsky, C., Did most present-day spirals form during the last 8 Gyr? A formation history with violent episodes revealed by panchromatic observations. Astron. & Astrophys., 430, 115 (2005).

7. Scoville, N. Z., et al., NICMOS imaging of infrared luminous galaxies, Astron. J. 119, 991 (2000).
<figure><img src="image_1.png" /><figcaption>**Fig. 1:** Images of the five ULIRGs analysed by Chartab et al. Left: four of the galaxies are shown as a red-green-blue composite image, made using Hubble Space Telescope (HST) observations⁷ in near-infrared bands at 1.2, 1.6, and 2.1 µm. They are arranged to show a merger sequence: from left to right, top to bottom, the first subpanel (IRAS F08572+3915) shows two approaching galaxies, while in the second subpanel (IRAS 12112+0305) two galaxies have interacted and show tidal features. In the third subpanel (IRAS 15250+3609) two galaxies have merged but the two nuclei are still visible, and finally in IRAS 17208-0014 the two galaxies have fully merged and are surrounded by strong tides. Right: a composite (blue and red) image of Mrk 273 from HST observations, showing the gigantic tidal tail that is reminiscent of a toothbrush handle.</figcaption></figure>