Scientific Rationale

This is the first in the new series of Breakthrough Workshops that we are starting at ISSI. Breakthrough Workshops are designed to address one key question in science, by gathering the main experts on the topic, invite them to spend a week in a neutral and welcoming environment, discuss the issues, and produce one high visibility peer reviewed paper to state the current understanding on the topic in question, including what there is agreement on, and where there is not, to be submitted within three months from the completion of the Workshop.

The specific aim of this Breakthrough Workshop is to develop a comprehensive and shared understanding of the chronology of the first billion years in the early evolution of the Universe, according to JWST.

The James Webb Space Telescope (JWST) is one of the most ambitious scientific experiments of the 21st century. By far the largest and most sophisticated observatory ever sent into space and over 100 times more powerful in terms of sensitivity and resolution than its predecessors, JWST is poised to revolutionize our understanding of galaxies in the Universe, both near and far. JWST has started science operations in summer 2022 and has already produced unexpected findings that are challenging our understanding of the Universe.

We are bringing together researchers with highly specific expertise from different subdisciplines related to cosmology for a focused workshop in early 2024. This timeframe is ideal, as the first year of JWST data will have been digested by the astronomical community and the second year of data has just started to arrive, especially spectroscopy.

We are going to answer some specific questions:

  • How extreme were the stars in the first generation of galaxies and where did they form?

Early spectra of very high-redshift galaxies with JWST indicate highly ionized low metallicity gas — conditions at least as extreme as can be found as in the most extreme galaxies today, that require extreme ionizing sources (stars). At some still higher redshift, the first stars should have been zero-metallicity “Population III”. These stars may be findable by JWST if gravitationally lensed, or caught in their supernova deaths.

  • When did the first proto-globular clusters form?
  • In what ways are the first galaxies unique?

Since the first generations of stars and galaxies form out of primordial gas, their metallicities are expected to be significantly lower than at later cosmic times. Indeed, the first spectroscopic observations of z>6 galaxies confirm metallicities that are subsolar, but perhaps not as low as expected. Additionally, the excitation and ionization properties of galaxies at z~6-9 appear to be consistent with galaxies at z~2. The question is: how different are galaxies during the reionization epoch from their descendants at the peak of cosmic star-formation, ~2 billion years later? How does star formation proceed in rapidly assembling galaxies? At what redshift are the scaling relationships for galaxies in place?

At lower redshift, star-forming galaxies show tight correlations between the mass in stars and these quantities: oxygen abundance, the star formation rate, and the mass of the supermassive central black hole — which suggests that star formation may be a tightly regulated process. Similarly, at lower redshifts, galaxies dominated by old stars show tight relationships among size, luminosity, and mass. How quickly do galaxies get orderly? How quickly are regulatory processes established, to bring order to the chaos? And what does this tell us about feedback processes?

  • Are AGN important in the first billion years?

Most models find that the ionization budget of the early universe was dominated by galaxies, with black hole accretion a distant second place. It may therefore be safe to assume that black hole accretion is not important to the first billion years. On the other hand, the discovery of billion solar mass black holes powering z=7 quasars indicates that black holes can accrete extremely rapidly and efficiently. How important is black hole accretion to the story of the first billion years?

  • Are JWST discoveries challenging the current cosmological framework?

Perhaps, the most surprising early discovery of JWST was a large number of luminous, and apparently very massive massive, galaxies in the first few JWST datasets. The implied number and mass density of some of these galaxies is so high that it cannot be explained by our standard galaxy formation models, that is galaxies forming inside dark matter halos with a relatively low star-formation efficiency. In fact, some galaxies appear to be too massive too early on that they cannot be explained even if all the baryons in dark matter halos were turned into stars. One extreme possibility would be that our cosmology would need to be changed. Alternatively, other possible astrophysical explanations for these galaxies need to be examined and tested.

This  Breakthrough Workshop will have a format that is slightly different from the typical ISSI Workshops.

We have invited PIs of relevant large and medium JWST Cycle 1 programs and other experts, including theorists. We have encouraged in-person participation for the success of the meeting. The format includes ample discussion time, and time to draft the final written product, hackathon style. We have achieved close to  50% representation by women and non-binary scientists. We have included a large number of Early Career Scientists. Our goal is to provide an environment fostering the free and respectful exchange of ideas, to collectively advance the scientific knowledge on this topic and reach, if possible, a consensus opinion on the questions above.

The written product will be one high visibility refereed paper, co-authored by all participants, to be published soon after the workshop. This paper is intended to capture the current state-of-the-art understanding of the chronology of the first billion years in the formation of the Universe, including what there is agreement, and where there is not. This paper can only be written by the experts who have data and knowledge on the individual questions, and by inviting them to discuss together. As such, the paper is not intended to replace, but rather add to the publications already being produced by the individual teams.