Note: This article is an addendum to James Webb Space Telescope Full Science Operations Begin As 5 ‘First Light’ Images Are Released (1 of 5).
Some important discoveries and confirmations have been realized with the first public release of JWST data and imagery. We can unequivocally state that our evolutionary theories of the early universe have been confirmed! Using NIRSpec data, this article will discuss one such confirmation, one that was not discussed but yet was hiding in plain sight!
We modify published NASA graphics, highlighting spectroscopic features of iron already included but not highlighted or discussed by the authors. We comment on the relevance of these features coupled with the spectral features already highlighted, to unequivocally show how heavy-element nucleosynthesis was shaping the early universe. This is something that has long been postulated but we now have clear and compelling evidence that it did, in fact, occur!
In order to provide background and context, I need to briefly discuss the universe's beginning and early evolution.
A Brief History of the Universe
Ever since Georges Lemaître proposed in his 1927 paper that the expanding universe began as a singularity (the 'Big Bang'), the view of ourselves and our universe has been forever altered. If you're reading this article, then you've probably heard of or read Steven Weinberg's "The First Three Minutes" or Stephen Hawking's "A Brief History of Time".
After the first second, the expanding universe cooled such that protons and neutrons emerged from the expanding super-hot plasma.
After about 2 minutes, conditions allow nucleosynthesis to begin in open space. One quarter of all protons and all neutrons fuse to form heavier nuclei. Initially, the products include deuterium nuclei (1 Proton, 1 Neutron) followed then by mainly helium-4. The plasma had cooled to about 1 billion Kelvin, hot enough for nuclear fusion to occur freely in open space!
The Primordial Abundance
After about 20 minutes, the expanding plasma cooled below the point where nuclear fusion can occur. At this point the primordial abundance was established at 75% hydrogen nuclei and 25% helium with trace amounts of lithium. This value established forever the total aggregate mass the universe will ever have. Since all nuclei heavier than hydrogen form as byproducts of nuclear fusion, all the matter that will ever be was created within that 20 minutes as hydrogen and helium nuclei!
In the 18-minute window between 2 and 20 minutes, all the matter that will ever be was created!
From that point hence, the ratio of hydrogen to helium has slowly and inexorably decreased. As more stars form, free hydrogen, the raw material for new star formation, decreases. The observed aggregate mass of the universe contains ~73% hydrogen, a value consistent with the 'Big Bang' model.
The Recombination Epoch
The Recombination Epoch begins at about 18,000 years as electrons combine with helium nuclei (Alpha particles) to form He+. After 100,000 years, neutral helium forms and after 370,000 years, neutral hydrogen atoms (one proton, one electron) form (recombine).
As this epoch begins, the cooling plasma was still too hot to allow neutral atoms to form and thus, existed as an opaque, ionized gas. The ionized gas was hot -and opaque- and thus, no light could travel through it. This epoch concludes after 370,000 years with the formation of neutral hydrogen. At this point the universe becomes transparent for the first time.
The newly-formed atoms, mostly hydrogen and helium with traces of lithium, quickly reach their ground state energy by releasing photons. These photons are observed today as the cosmic microwave background (CMB).
The Cosmic Dark Ages and The first Stars and Galaxies
This epoch begins after Recombination at 370,000 years and continues until the first stars form at about 400 million years. During this time, neutral hydrogen, the raw material for star formation exists in abundance but gravity is slow to act. Stars form as pockets of gas contract, self-gravitating to form proto-stars, heating up as they contract. Once temperatures reach 10 million Kelvin in the core, nuclear fusion begins and a star is born. This process was slow to start during this epoch but picks up after 400 million years.
The Stelliferous Era
With the formation of the first stars, the Stelliferous Era, the 2nd of 6 ages of the universe, begins. It will continue until all possible stars have formed and the last of the red-dwarfs ceases to shine. This point on the cosmological time line is 10,000x the current age of the universe or 100 trillion years! To make sense of this staggering time scale, time is scaled logarithmically. Thus, each cosmic decade is 10x longer than the previous. Although the sun formed late, its life span is approximately equal to the current age of the universe. It could live and die 10,000 times before the end of the Stelliferous Era! We've only just begun!
As the first stars grow in number, gravity draws them together to form the first galaxies and large-scale structures of the early universe. It continues to expand and evolve over the intervening 13 billion years to the present day.
And this is where our story begins.
July 12, 2022
Of the 5 studies released on that day, the SMACS 0723 Galaxy Cluster study was probably the most content rich of all. Of particular note was the long-distance spectroscopy of 4 high-redshift galaxies:
Spectroscopic studies of 4 high-z (high redshift) galaxies illustrate why Webb is necessary to study the early universe. Four well-known emission lines in the visible spectrum are redshifted well into the infrared. In the first spectrum, from right to left: Hydrogen-alpha (red at 656 nm wavelength), two visible oxygen lines (blue-green at 496 and 500 nm wavelength - this last line is the famous doubly-ionized oxygen line O-III) and Hydrogen-beta at 486 nm. Take particular note of how strong the O-III line is. The strength of this line and others will play a central role in this article. This line is indicative of active stellar evolution in this galaxy, even at this very young age.
The greater the redshift, the further into the infrared the lines are shifted. With the exception of the first two spectra, Hydrogen-alpha, the first in the "Balmer" series, is redshifted off the scale. The redshift values range from 2.81 for the first galaxy at 11.3 billion light-years to 8.50 for the last galaxy at 13.1 billion light-years. With a z (redshift) of 2.81, the first galaxy is receding at 87% the speed of light. The last galaxy, the object of study, with a z of 8.50, receding at 98% the speed of light! For convenience we'll refer to it as z-13.1.
And this brings us to the central point of this article.
Population III Stars
Much conjecture surrounds the makeup and stellar evolution of Population III (the first) stars. Were they ultra-high mass stars over 100 solar masses? How long did they live? What was the stellar diversity of the young universe? With the published data from the SMACS-0723 study, we may now be in a position to answer these questions.
The only relics remaining from the early universe would be low-mass K and M stars with the Pop-III K stars now evolving off the main sequence. Their low relative luminosity makes them difficult to first, detect and then determine metallicity. This has been known for a while and is nothing new.
Stellar Metallicity and Supernovae
Metallicity is the ratio of trans-helium elements (elements heavier than helium) to hydrogen. Presumably, the smaller this ratio, the older the star. How so? Population III stars would necessarily reflect the environment from which they formed. Thus, the stars that formed then would reflect the paucity of heavy elements in that environment.
Elements heavier than hydrogen are produced in the cores of stars through nuclear fusion. With the exception of Type-Ia supernovae, most stars will forever retain the heavier-element byproducts of nuclear fusion. Hence, the only way the Interstellar Medium (ISM) is enriched with heavier elements is through supernovae. And this only happens with core-collapse (Type II) or Type-Ia (white-dwarf and companion) supernovae.
Core-collapse supernovae are the end-state for high-mass stars. The last burning cycle (silicon) of a high-mass star (> 8 solar masses) produces isotopes of iron and nickel. When the star attempts to use these heavy metals as a new source of nuclear fuel, it implodes -and rebounds- violently destroying itself in a supernova. The ISM is where all new stars form, thus the metallicity of the star is directly related to the abundance of heavy elements in the ISM.
Through further analysis of the SMACS-0723 NirSpec data, we present compelling evidence of active stellar evolution and heavy-element production in the early universe.
We analyze the NirSpec spectrum of z-13.1.
NirSpec Data of High-Z Galaxy and Supernovae
Redshift is the shift in the observed wavelength of light emitted by a receding source relative to its rest wavelength. A high-z or high-redshifted galaxy is one whose measured redshift is greater than 6. That is to say, such a galaxy is moving with a velocity such that this ratio is > 6! These high-redshift objects include galaxies known to have formed during the first billion years following the big bang. z-13.1 is 13.1 billion light years distant. It thus formed during the intervening 690-million years between 13.1 billion years and the 13.79 billion-year age of the universe.
Why this Galaxy?
Its redshift is 8.50, not the highest measured by Webb but the one selected as a representative study using NirSpec.
This graphic, modified from the original, shows the strong presence of Iron (neutral and singly ionized)- chart included below. Why is this significant? As we stated previously, iron and nickel are the byproducts of the last burning cycle of a high-mass star.
In addition to the other elements (Oxygen and Neon), there is only one way Iron could appear in z-13.1's spectrum: Type II, Core-collapse Supernovae!
Type Ia supernovae are ruled out as there is insufficient time for the white-dwarf progenitor to form. In the case of solar-mass stars, a white dwarf is the very end-state in the star's evolution, typically 10+ billion years after birth.
The presence of Oxygen, Neon and Iron in this spectrum indicates only one thing: active or recent Type-II supernovae! As stated above, the only place these elements form is in the nuclear cauldrons of high-mass stars. And the only way they make it to the ISM is through supernovae. Full Stop!
Further analysis also shows that some of these spectra originate from ionized species of these elements. In other words, they originated in high-energy environments such as the immediate aftermath of a supernova. These are highlighted in bold text in the chart below.
|Hydrogen, H-beta (Redshifted wavelength: 4.61 microns), rest wavelength: 486 nm, (Neutral Hydrogen) - hydrogen line furthest to the right|
|Hydrogen, H-gamma (Redshifted wavelength: 4.12 microns), rest wavelength: 434 nm, (Neutral Hydrogen)|
|Hydrogen, H-delta (Redshifted wavelength: 3.897 microns), rest wavelength: 410 nm, (Neutral Hydrogen)|
|Iron, (Redshifted wavelength: 3.824 microns), rest wavelength: 404.58 nm, Fe I (Neutral Iron) first Iron line at the left|
|Iron, (Redshifted wavelength: 4.49 microns), rest wavelength: 472.4 nm, Fe II (Singly ionized iron) - part of "Iron Forest"|
|Iron, (Redshifted wavelength: 4.497 microns), rest wavelength: 473.3 nm, Fe II (Singly ionized iron) - part of "Iron Forest"|
|Iron, (Redshifted wavelength: 4.505 microns), rest wavelength: 474.2 nm, Fe II (Singly ionized iron) - part of "Iron Forest"|
|Iron, (Redshifted wavelength: 4.650 microns), rest wavelength: 492.4 nm, Fe II (Singly ionized iron) - part of "Iron Forest"|
|Iron "Forest" centered on the H-beta line at 486 nanometers - see graphic below|
|Oxygen, (Redshifted wavelength: 3.54 Microns), rest wavelength: 372.2 nm, O II (Singly ionized Oxygen) (Typical for Supernovae) - first oxygen line in graphic|
|Oxygen, (Redshifted wavelength: 4.144 microns), rest wavelength: 434.9 nm, O I (Neutral Oxygen)|
|Oxygen, (Redshifted wavelength: 4.71 microns), rest wavelength: 496 nm, O II (Singly ionized Oxygen)|
|Oxygen, (Redshifted wavelength: 4.75 microns), rest wavelength: 500 nm, O II (Singly ionized Oxygen)|
|Oxygen, (Redshifted wavelength: 4.76 microns), rest wavelength: 500.7 nm, O III (Doubly ionized Oxygen and typical for Supernovae) - last oxygen line in graphic|
Note: nm abbreviation for nanometer - 1 billionth of a meter
Recent studies of supernovae illustrate the presence of these lines in corresponding spectra. For example, spectral profile of Type II supernova SN2007W after 72 days is illustrated below. It's important to note that this supernova occurred in a low-luminosity host galaxy, NGC-5105 in Virgo. z-13.1 also appears to be low luminosity. The "Fe line Forest" centered on the H-beta line at 486 nm is illustrated here in the lower-left. This same iron-line forest is clearly exhibited in the NirSpec spectra of z-13.1 and can be seen centered on the 4.6 micron mark in the full image above. An examination of the 12.6 b-ly galaxy's spectra shows the same Iron forest centered on the H-beta line, perhaps even more pronounced. By comparison, the 13.0 b-ly galaxy's spectra reveals no iron forest or iron components at all.
Implications and Conclusions
This analysis also allows us to more accurately constrain the timelines of the early epochs.
Active stellar evolution occurred in the early universe! Remember, it took 400 million years for the first stars to begin forming13.39 billion years ago. This value may now have to be revised based on these findings. At 690 million years after the big bang, there is compelling evidence of active stellar evolution. Are we to believe that the first stars formed, galaxies coalesced from them and the nucleosynthesis of heavier elements ongoing after only 690 million years? That leaves only 290 million years for galaxy formation? If this is accurate, then the universe was evolving and changing rapidly, as has been suggested.
The only way heavy elements are produced is in the cores of high-mass (O, B) stars and Type Ia supernovae. These high-mass stars live no more than a few tens of millions of years. For example, a 25 solar-mass star lives less than 10 million years before ending spectacularly as a Type II Supernova:
It has been speculated that these early stars were massive, between 100 and 300 solar masses. While there is an upper limit to mass according to the Eddington Limit of ~180 solar-masses, there remains considerable uncertainty. So, for now, this will remain an open question.
690 million years may sound like a long time. On cosmological time scales, it is merely the wink of an eye. Our own milky way galaxy rotates once in 230 million years, rotating exactly 3 times during this interval.
Considering the morphological characteristics of z-13.1, i.e. its shape, it appears small and irregular with no apparent structure. As described above, one other galaxy's spectra presents with iron characteristics, much like z-13.1. The other spectra, aside from the strong hydrogen and oxygen lines, are pretty flat. That all 4 galaxies are morphologically irregular with 50% presenting with iron components, 50% not, speaks to a diversity in the early universe.
We see strong heavy-element representation within a few hundred million years of the first stars forming. This result is remarkable and consistent with studies concluding that star formation proceeded rapidly. Some studies also proposed ultra-high mass stars living short, punctuated lives of 1-3 million years that rapidly enriched the ISM. As stated, the prevalence and possible contribution of these ultra-high mass stars in the early universe remains an open question.
Perhaps with JWST, we'll get more new answers than new questions.
National Aeronautics and Space Administration (NASA)
Goddard Space Flight Center (NASA/GSFC)
Jet Propulsion Laboratory (NASA/JPL)
NASA JWST (Webb) https://webb.nasa.gov/
Space Telescope Science Institute (STSci.edu)
The Early Release Observations and associated materials were developed, executed, and compiled by the ERO production team: Hannah Braun, Claire Blome, Matthew Brown, Margaret Carruthers, Dan Coe, Joseph DePasquale, Nestor Espinoza, Macarena Garcia Marin, Karl Gordon, Alaina Henry, Leah Hustak, Andi James, Ann Jenkins, Anton Koekemoer, Stephanie LaMassa, David Law, Alexandra Lockwood, Amaya Moro-Martin, Susan Mullally, Alyssa Pagan, Dani Player, Klaus Pontoppidan, Charles Proffitt, Christine Pulliam, Leah Ramsay, Swara Ravindranath, Neill Reid, Massimo Robberto, Elena Sabbi, Leonardo Ubeda. The EROs were also made possible by the foundational efforts and support from the JWST instruments, STScI planning and scheduling, and Data Management teams.
Astronomy For Change: https://astronomyforchange.org
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