JWST provides a new view of cosmic dawn: latest developments in studies of early galaxies

TL;DR

JWST pushes the redshift frontier and reshapes our understanding of the earliest galaxies by providing detailed rest-frame UV–optical spectra and unprecedented sensitivity across a broad wavelength range. The paper surveys how stellar, nebular, and AGN components shape galaxy spectra, how JWST enables new selection techniques and rest-frame optical measurements at z>6, and how these data inform star-formation histories, metal enrichment, and black-hole growth. It highlights major open questions on the timeline and drivers of cosmic reionization, the prevalence of bursty star formation, and the origins of supermassive black holes, as well as surprising findings such as an excess of luminous galaxies at z>10 and the discovery of faint, gas-enshrouded AGN. Together, these results have profound implications for models of early galaxy formation, feedback, and the co-evolution of galaxies and black holes, and set the stage for next-generation facilities and multi-wavelength campaigns.

Abstract

Studies of the distant Universe are providing key insights into our understanding of the formation of galaxies. The advent of the James Webb Space Telescope (JWST) has significantly enhanced our observational capabilities, leading to an expanded redshift frontier, providing unprecedented detail in the characterization of early galaxies and enabling the discovery of new populations of accreting black holes. This review aims to provide an introduction to the basic processes and components that shape the observed spectra of galaxies, with a focus on their relevance to techniques with which high-redshift galaxies are selected. The review further introduces specific topics that have attracted significant attention in recent literature, including the discovery of highly efficient galaxy formation in the early Universe, the relation between galaxies and the process of reionization, new insights into the formation of the first stars and the enrichment of interstellar gas with heavy elements, and breakthroughs in our understanding of the origins of supermassive black holes.

Figures (19)

• Figure 1: Images that showcase JWST's abilities to push the frontiers of extra-galactic astrophysics. On the left is a false-color image constructed from JWST/NIRCam images in the F070W/F200W/F356W filters, $5\times5$" on the side. The central object is one of the most luminous examples of the newly discovered population of 'little red dots' that are accreting supermassive black holes whose red colors are attributed to coverage by dense gas. In the middle is a $3\times3$" JWST/NIRSpec pseudo-narrow band image of the Balmer emission lines of a highly magnified galaxy that is constructed from 3D spectroscopic data (image adapted from Vanzella et al. Vanzella23). The galaxy is at a redshift of $z=6.6$ and has a mass comparable to a globular cluster and a metallicity less than 0.4 % solar -- one of the most metal poor galaxies known. On the right is a zoomed in $1\times1"$ JWST/NIRCam image in the F090W/F115W/F277W filters of the most distant galaxy currently known at $z=14.4$ (image adapted from Naidu et al. Naidu25), whose light has traveled 13.3 billion years to reach us.
• Figure 2: Example spectra of galaxies and quasars in the distant Universe. The two galaxy spectra in red and green are based on models Williams18, while the quasar spectrum (blue) is an empirical template Selsing16. The star-forming galaxy (green) has young stars and little dust attenuation and therefore a blue continuum with strong narrow emission-lines as [O ii], H$\beta$, [O iii] and H$\alpha$. The Balmer break galaxy (red) has an older stellar population and is not actively forming stars. Absorption line features from stellar atmospheres can be identified, as well as a break at the Balmer wavelength of $3.64$$\mu$m. Note that attenuation due to neutral gas in the intergalactic medium is only applied at $\lambda<0.912 \mu$m. The quasar shows a power-law like blue continuum with various strong broad emission-lines as Lyman-$\alpha$, C iv, Mg ii, H$\beta$ and H$\alpha$.
• Figure 3: Model spectra of simple stellar populations with different ages in the rest-frame UV to optical. The models are constructed using the Binary Population and Spectral Synthesis code, version 2.3, and describe stellar populations with a mass $10^6$ M$_{\odot}$, a 20 % solar metallicity and a broken power-law IMF (with slopes of $\alpha=-1.3, -2.35$ at masses below and above 0.5 M$_{\odot}$, respectively). Dotted vertical lines highlight key wavelengths of interest: the Lyman edge at 912 Å, the Ly$\alpha$ transition at 1216 Å, the Balmer edge at 3645 Å and the H$\alpha$ transition at 6564 Å. The models clearly illustrate the strong dependence of the mass-to-light ratio on age, the slight reddening of the SEDs with increasing age, the Lyman break and the development of the Balmer break around $10^8$ yr.
• Figure 4: The ionizing spectrum of various models of young stellar populations (blue, purple) and AGN (red) normalised to the flux density at 13.6 eV (912 Å), inspired by similar figures in the literature Feltre16Gotberg19. In both panels, the filled purple and blue spectrum are for a reference model (Stellar population with binaries as in Fig. \ref{['fig:starsed']}, with metallicity Z=0.004 and $\alpha$/Fe=+0.6) with an age of 1 and 10 Myr. The red shaded regions shows a power-law spectrum with slope $f_{\nu}\propto\nu^\alpha$, where $\alpha=-1.2$ to $-2$ characteric for accretion disks. In the left panel, dashed lines show the spectrum without binary interactions. In the right panel, dashed lines shows a model with a higher metallicity while dashed-dotted lines show a lower metallicity model.
• Figure 5: Rest-frame UV to optical model spectra of a young high-redshift galaxy with stellar and nebular emission. The models are constructed using BAGPIPES Carnall18 and normalised to the flux density at 3000 Å. The fiducial model (black) has no dust attenuation and all ionizing photons are converted into nebular emission ($f_{\rm esc}=0$). The blue model has no dust, but $f_{\rm esc}=0.5$, whereas the red model has a dust attenuation of $A_V=1$ and a zero escape fraction. The differences in the spectra show how dust leads to reddening, whereas the effects of a high escape fraction are subtle but discernable in the continuum around the Balmer limit.