Metal dilution in early galaxies as evidence for excessive pristine gas infall?

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Metal dilution in early galaxies as evidence for excessive pristine gas infall?
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The mystery: how did we get from pristine, neutral gas to galaxies as we see them today?

For many years now, astronomers have known that galaxies over the last ~12 billion years have followed rules for evolution of their chemical abundance, or the amount of metals they contain, also called metallicity. We have seen this time and time again, through studies of the Fundamental Metallicity Relation (FMR)-- a tight correlation between star formation, stellar mass, and metallicity. The FMR seems to describe the way that star formation and metallicity regulate growth of mass in galaxies: for a galaxy of a given metallicity and star formation rate, there is a predictable stellar mass. This makes sense with the standard understanding of the Baryon Cycle: gas from between galaxies flows in and supplies gas for stars to form. Those stars  in turn increase the stellar mass of the galaxy while simultaneously producing metals, increasing the metallicity.

But the question for many years leading up to the launch of the James Webb Space Telescope (JWST) has been: When does this relation break down and why? As astronomers, we want to know if very early galaxies follow the same fundamental relation for formation and evolution that we see in galaxies more nearby, or if there is instead something very different about the early universe. In this study, myself and my co-authors, led by Assistant Professor Kasper Heintz, go on a journey to find out.

The challenges of measuring the fundamental metallicity relation: why JWST?

The FMR is important for understanding the evolution of metals in galaxies over time, but is difficult to measure. It requires spectroscopy of emission lines of ideally a large sample of galaxies to probe metallicity and star formation rate, in addition to imaging and/or spectroscopy across a broad range of wavelengths to measure the galaxy's stellar mass. Since those features become redder as galaxies get more distant, they will at some point be redshifted out of the capabilities of telescopes we have at our disposal. Before JWST, it was impossible to measure metallicities for very early galaxies (earlier than ~2 billion years after the Big Bang, or around z~3) for this reason. At that time, astronomers mainly relied on a combination of data from ground based telescopes like Keck and imaging from space telescopes like Hubble and Spitzer to measure metallicity and stellar mass. While powerful, each of these telescopes suffered from various limitations: a wavelength range that does not extend red enough, atmospheric emission if observing from the ground, and limited sensitivity due to mirror size. Now with JWST's greatly improved wavelength range and sensitivity in the near-infrared, we can see more distant galaxies with better detail than before, and we can make the necessary measurements for the FMR. In our paper, we use spectroscopy from JWST to measure all three variables in the FMR: stellar mass, star formation rate, and metallicity. 

Our observations and result

In this paper, my coauthors and I used public JWST spectroscopy of 16 galaxies at z>7, or earlier than 1 billion years after the Big Bang. We measured their metallicities, masses, and star formation rates with JWST spectroscopy using the Prism setting on the instrument NIRSpec, which allowed us to see a very wide wavelength range for our galaxies (see Figure below).

Spectrum of one of the galaxies in our sample. In the bottom panel, the wide wavelength range from ~1-5 microns, or the rest-frame ultraviolet to the rest-frame near-infrared provided by the NIRSpec/Prism can be seen. This allows us to get accurate measurements of stellar mass. Shown in the top right panel is a zoom in of the emission lines used to calculate metallicity and star formation rate. Shown in the top left panel is a color image created from three wavelengths of JWST/NIRCam imaging, showing the shape of the galaxy. This combination of data has a wealth of information allowing us to measure all the necessary quantities described here, which we have for all galaxies in our sample. 

This allowed us to get a more accurate measurement of stellar mass essentially simultaneously with metallicity and star formation rates from emission lines. Our main result is that those early galaxies do not fall on the same FMR as the galaxies we have seen later in the universe; that is, for a given stellar mass those early galaxies have significantly higher star formation rates and significantly lower metallicities (see Figure below).

The Fundamental Metallicity Relation: metallicity vs a combination of stellar mass and star formation rate. In red points are our measurements, and in the black line is the FMR for galaxies from z~0-3 from earlier works. It can be seen that our points do not overlap with the previously found FMR, revealing that something is different about how galaxies form and evolve at early times.

 Something about the way star formation and metals regulate galaxy growth is different in the early universe. Now the question is: what exactly is it? To answer that question, it helps to think about what the universe may have looked like less than a billion years after the Big Bang. There was likely a lot of diffuse, metal-free gas in between overdensities that have begun to look like galaxies, forming stars and accreting gas now for a few hundred million years. Compared to the later universe, those early galaxies would have been embedded in, and interacting with, the gas around them to a greater degree. So maybe what we are seeing when we see heightened star formation rates and lower metallicities at early times is a faster, more efficient inflow of pristine gas into galaxies, diluting their metal content and providing fuel for star formation. 

Caveats

As with most astronomical results, there are assumptions that go into the measurements. For this study there are two places where it is worth keeping that in mind: the metallicity measurements and the stellar mass measurements. The most precise way to measure a galaxy’s metallicity requires a direct measurement of the electron temperature of the gas, which requires very deep data that we did not have for our entire sample. Instead, we used a technique wherein another set of galaxies’ directly measured metallicities were calibrated to the relative strengths of two emission lines which are generally much brighter and more easily detected. We used this calibration to estimate metallicities, which assumes that our galaxy sample is similar enough to the calibration sample to fairly do so. 

The second caveat is that measuring stellar mass is notoriously difficult due to the many assumptions that go into it, including how many of what mass stars are in the galaxy at a given time, or what the past star formation of a given galaxy looked like. A change in either of these assumptions can result in a change in stellar mass measurement of up to an order of magnitude. 

While these are important to keep in mind, we believe that the assumptions we used are appropriate and have accounted for the uncertainties they introduce. And importantly: even given those uncertainties, our result seems to hold.

Why does it matter?

This result could be a sign of one of the earliest stages of galaxy evolution, where there exists a much more intimate connection between galaxies and the intergalactic medium than we have seen before. This is one of the first results showing a hint at a clearer picture of the evolution of the gas in the universe on large scales. The sample size is not huge, and there are caveats to the measurements. But if seen again and again, and we think it likely will be, we could have a much clearer idea of how galaxies began their assembly journey in the first billion years after the Big Bang. 

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Astronomy, Cosmology and Space Sciences
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