Researchers Measure Size-Luminosity Relation of Galaxies Less Than a Billion Years After Big Bang

Figure 1. Two exceptionally bright galaxies were captured in GLASS-JWST program. These galaxies existed approximately 450 and 350 million years after the Big Bang (with a redshift of approximately 10.5 and 12.5, respectively), and sizes are roughly 500 parsecs and 170 parsecs, respectively. Credit: NASA, ESA, CSA, Tommaso Treu (UCLA)

An international team of researchers, including those from the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), have published a new study in The Astrophysical Journal Letters that examines the relationship between the size and luminosity of some of the earliest galaxies in the universe, as captured by the recently launched James Webb Space Telescope (JWST) – less than a billion years after the Big Bang.

The findings are part of the Grim Lens-Amplified Survey from Space (GLASS) Early-Release Science Program, led by Professor Tommaso Treu from the University of California, Los Angeles. The program’s aim is to study the early universe when the first stars and galaxies ignited, leading to the ionization of neutral gas and the emergence of light, known as the epoch of reionization.

However, details of reionization have remained unknown because telescopes until today have not been capable of observing galaxies in this period of the universe’s history in detail. Finding out more about the epoch of reionization would help researchers understand how stars and galaxies have evolved to create today’s universe as we see it.

Figure 2: Size–luminosity relationships of galaxies observed in five wavelength bands with fixed slope owing to limited data. The black solid and dashed–dotted lines in the F150W panel show the relation derived from HST data by Shibuya et al. (2015; z ∼ 8) and Huang et al. (2013; z ∼ 5) at a similar rest-frame wavelength, respectively. Credit: Yang et al.

One study, led by Kavli IPMU JSPS Fellow Lilan Yang, and including Project Researcher Xuheng Ding, used multiband NIRCAM imaging data from the GLASS-JWST program to measure galaxy size and luminosity to figure out the morphology and the size-luminosity relation from rest-frame optical to UV.

“It’s the first time that we can study the galaxy’s properties in rest-frame optical at a redshift larger than 7 with JWST, and the size-luminosity is important for determining the shape of luminosity function which indicates the primary sources responsible for the cosmic reionization, i.e., numerous faint galaxies or relatively less bright galaxies.

“The original wavelength of light will shift to a longer wavelength when it travels from the early universe to us. Thus, the rest-frame wavelength is used to clarify their intrinsic wavelength, rather than the observed wavelength.

Previously, with Hubble Space Telescope, we know the properties of galaxies only in the rest-frame UV band. Now, with JWST, we can measure longer wavelength than UV,” said first author Yang.

The researchers found the first rest-frame optical size-luminosity relation of galaxies at a redshift larger than 7, or roughly 800 million years after the Big Bang, allowing them to study the size as a function of wavelength. They found the median size at the reference luminosity is roughly 450-600 parsecs and decreased slightly from rest-frame optical to UV. But was this expected?

“The answer is we don’t know what to expect. Previous simulation studies give a range of predictions,” said Yang.

The team also found the slope of the size–luminosity relationship was somewhat steeper in the shortest wavelength band when allowing the slope to vary.

“That would suggest higher surface brightness density at shorter wavelength, hence less observational incompleteness correction when estimating luminosity function, but the result is not conclusive. We don’t want to over-interpret here,” said Yang.

Reference: “Early Results from GLASS-JWST. V: The First Rest-frame Optical Size–Luminosity Relation of Galaxies at z > 7” by L. Yang, T. Morishita, N. Leethochawalit, M. Castellano, A. Calabrò, T. Treu, A. Bonchi, A. Fontana, C. Mason, E. Merlin, D. Paris, M. Trenti, G. Roberts-Borsani, M. Bradac, E. Vanzella, B. Vulcani, D. Marchesini, X. Ding, T. Nanayakkara, S. Birrer, K. Glazebrook, T. Jones, K. Boyett, P. Santini, V. Strait and X. Wang, 18 October 2022, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ac8803