The ramp-up of interstellar medium enrichment at z > 4

The discovery of a hydrogen fluoride absorption line in a galaxy at z = 4.4 provides further insight into the origin of this element. This discovery led to the publication of a letter in the journal Nature Astronomy and was the subject of an ESO press release.

Here are some of the details of this research.

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The galaxy

Our work focused on the observation of a distant galaxy, NGP-190387. This galaxy is very luminous in IR. We wanted to understand how this galaxy, which is located 12 billion light-years away from us, i.e. we see this galaxy as it was 1.5 billion years after the Big Bang, could form so many stars, by studying in detail the diffuse and dense gas reservoirs of this galaxy with ALMA.

A serendipitous discovery

We were a little disappointed when we saw the ALMA images. We realised that this galaxy was so bright not because it formed an astronomic amount of stars but because it formed a lot of stars and its light was amplified by gravitational lensing. Our disappointment was short-lived because when we analysed the spectrum of this galaxy, we noticed a drop in intensity at a very precise wavelength, a so-called absorption line, which corresponded exactly to the wavelength at which the hydrogen fluoride molecule absorbs light.

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HF absorption line

This detection is exciting, as it is the first time that the amount of fluorine in a star-forming galaxy can be measured so far back in the history of the Universe. 

We know that fluorine can be produced by 3 types of stars. Stars that are roughly like our sun, or a little more massive, up to a few times the mass of the sun, AGB stars.  These stars will evolve into red supergiants, these stars live for a very long time. The second type of star is more massive stars, to give you an order of magnitude 10 times the mass of the Sun and these stars will explode into a type II supernova and these stars have a relatively short life time. The last possibility is that the Wolf Rayet stars, which are very rare and very massive, spin around quite rapidly and eject a lot of material in the form of stellar wind, have a cataclysmic end of life and their life span is very short, only a few million years.

The nucluosynthesis of fluorine 

We have run chemical evolution models that predict the amount of fluorine that can be detected in a galaxy, and since our galaxy was seen shortly after the big bang, our models tell us that the relatively low-mass stars have not had time to enrich the interstellar medium sufficiently. Even when we add the contribution of the 10 solar mass stars that will evolve into type II supernovae, the amount of fluorine produced is still not enough and it is likely that most of the fluorine seen in this galaxy is produced by Wolf Rayet stars.

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The ideal tracer of H2

Fluorine is the most electronegative element. It associates very strongly with most other elements and in particular, with H2. This reaction is exothermic and takes place without the need for external energy. For this reason, hydrogen fluoride represents nearly 100% of gas-phase fluoride over a wide range of temperature and density.

A problem we often encounter when studying a high redshift galaxy is that hydrogen in its molecular form is difficult to probe, because of the high excitation temperature of the lowest rotational transition and the lack of permanent dipole moment. The gas mass of a galaxy is however a key element for understanding galaxy evolution. In order to estimate this quantity, we use other tracers (mostly low-J CO transitions) but these proxies have many limitations (they can depend on the metallicity of the galaxy, be destroyed by cosmic rays from star formation or be almost useless in very cold regions or for especially low column densities). Fluorine, thanks to its affinity for H2 and its high stability in the interstellar medium, could be a decisive ally in this respect. Fluorine would not be used to measure H2 directly, but if a simple relation between H2 and HF can be characterised at high redshift (the current lack of detections prevents a clear vision of this for the moment), HF could be used to calibrate existing tracers of H2.