Authors: Souradeep Bhattacharya, Magda Arnaboldi, Orwin Gerhard, Chiaki Kobayashi, and Kanak Saha.
First Author’s Institution: Inter-University Center for Astronomy and Astrophysics, Ganeshkhind
Status: Submitted to ApJ
Often acting as the main characters in many aspects of astronomy, stars contain what seems like infinite information about the universe. Today’s bite focuses on one of those such informational bits, namely how the elements on our periodic table are formed in stars and what that can tell us about the universe when those stars formed.
In the stellar life cycle, baby stars form in pockets of molecular gas inside galaxies. They then grow up, live for tens to hundreds of millions of years as they burn up all their hydrogen, and eventually die in a supernova explosion. When they die, the stars become so energetic that they can fuse “heavy” elements like magnesium, argon, or even iron. All of these elements then get blasted into the gas surrounding the star, enriching the interstellar medium (ISM). New stars then form out of this newly-enriched expelled gas. These elements become part of the star, allowing astronomers to analyze the star’s spectra to understand what the ISM looked like when the star formed!
This comes in handy when astronomers are looking at some of the earliest stars ever formed; We aren’t entirely sure when in the universe’s history stars began to form, and even JWST hasn’t seen that far back (yet), so we have to rely on the aforementioned method to understand what kind of stars existed back then. Stars take millions of years to die-bigger stars die quicker, living around 30 million years, while smaller stars like our Sun can live for billions of years or more. They also die in different ways: larger stars die in what’s called a core-collapse supernova (CCSN), while smaller stars die in what we call a Type Ia supernova (think Ring Nebula). Each type of explosion produces its own unique set of elements which act as signals to astronomers, so that when we see them in a galaxy spectrum we can say that hey, some stars went supernova here!
With all these ingredients in hand, astronomers can test their theories about what kind of stars were living and dying in the first billions of years after the Big Bang. The authors of today’s paper choose to use abundances of oxygen and argon to explore a sample of galaxies that existed in the early universe (redshift ~1.3-7.7).
Chemical tracers of galaxy history
Oxygen and argon are both produced in the core-collapse supernovae of very short-lived massive stars, but argon is also formed in Type Ia supernovae. This means that if we can measure the amount of oxygen in a galaxy relative to the amount of argon, we have a kind of test of how many CCSN have occurred in the galaxy relative to how many Type Ia have occurred.
To measure oxygen and argon abundances, the authors of this paper take spectra of galaxies which include emission lines from the oxygen and argon in the galaxy. The galaxy spectra taken with JWST are shown in Figure 1. The authors find that the oxygen values they find are consistent with previously published values for these galaxies, while argon is measured for the first time in most of these galaxies.
Fitting these galaxies into the universe’s history
The authors also explore how the chemical makeup of these galaxies changes over time (also called chemical enrichment). They look at the ratio of the abundances of oxygen and argon (O/Ar) compared to the ratio of the abundances of argon and hydrogen (Ar/H). If the O/Ar ratio moves away from a value of 1, it lets us know something about how many Type Ia supernovae events are happening. A low O/Ar ratio coupled with a high Ar/H value suggests a higher presence of argon, indicating that Type Ia supernovae events have occurred and have enriched the ISM.
Figure 2 shows the comparison between the O/Ar and Ar/H ratios for sources in intermediate and high redshift categories. The intermediate redshift sample follows the Milky Way-like chemical enrichment pattern (black line in Figure 2) relatively well, but the higher redshift sample is quite clearly offset from the Milky Way line, indicating that these galaxies have an unusually high argon production. This presents a quandary: how did these galaxies get such high argon relative to oxygen at such high redshift?
In need of new stellar evolution models?
With these observations in hand, the authors set out to explore what kinds of models can hope to explain this departure from the Milky Way-type models. One option is that there could have been two bursts of star formation very early in the universe, separated by some pristine gas (i.e. just hydrogen and helium) falling into these galaxies. This model is shown in Figure 3 as the colored line: the first burst of starburst creates Type Ia supernova that increases the Ar/H while keeping minimum O/Ar, then a pristine gas infall reduces the argon abundance while keeping O/Ar constant, and finally another starburst raises both the Ar/H and O/Ar values. In this case, the basic physics and the rates of Sne Ia and CCSNe are as assumed in the Milky Way. This model works well for three of the higher redshift galaxies but falls short to explain two of them.
Since the above option assumes similar supernovae rates to the Milky Way and run-of-the-mill supernova physics, the authors naturally next consider models with different SN rates and some strange physics. They posit that another explanation for this low O/Ar at high redshift is there could just be more Type Ia SNe occurring in these galaxies than in the Milky Way-not a bad guess, considering high redshift galaxies are generally quite different from the Milky Way. Another explanation is that there are slightly different flavors of SNe Ia happening in these galaxies, possibly some with some freaky quantum physics going on.
All options are very difficult to verify, but the authors are hopeful that upcoming surveys and data will provide larger samples of oxygen and argon abundances for high redshift galaxies so that models can be tested more thoroughly. The high-redshift universe remains a major mystery to all, but efforts like this combined with more sophisticated models will hopefully shed light on the early universe.
Astrobite edited by Pranav Satheesh
Featured image credit: NASA
