A core in a star-forming disc as evidence of inside-out growth in the early Universe

The morphological evolution and structural diversity of galaxies are key unknowns in extragalactic astrophysics. In the hierarchical Λ cold dark matter cosmological model, galaxies maintain star formation through a quasi-steady state involving gas inflow, outflow, and consumption. Gas cooling at later cosmic epochs possesses higher angular momentum, leading to the expectation of inside-out galaxy growth. However, the actual process is more complex, influenced by stellar feedback, black hole feedback, cosmic rays, galaxy-galaxy interactions, and mergers. The morphological structure and spatially resolved growth rates of galaxies provide a sensitive but complex probe of galaxy formation physics. Galaxies in the local Universe exhibit a range of morphologies, from disc-dominated spirals to bulge-dominated ellipticals, typically classified by the Hubble sequence. Studies of local star-forming galaxies confirm inside-out growth, although some galaxies exhibit outside-in growth, possibly reflecting different growth phases. Most galaxy mass formation occurs during the redshift range 1 ≤ z ≤ 3 ('cosmic noon'), a period of peak cosmic star-formation rate density. Observations at these redshifts reveal galaxies with massive bulges and rotating discs, but probing the build-up of these bulges requires investigation of earlier cosmic times (z ≥ 6), during the epoch of reionization. The theoretical expectation is that the galaxy merger rate increases towards higher redshifts, potentially leading to more pronounced central starbursts. Recent numerical models suggest that early, low-mass galaxies undergo rapid size fluctuations and compaction driven by the interplay of feedback-driven gas outflows and cold inflows, potentially leading to a flat or inverted size-mass relation. Direct observations of early galaxies are crucial to understanding their stellar mass and size growth and the physics regulating star formation. The James Webb Space Telescope (JWST) offers new opportunities to study the formation of the Hubble sequence and bulge-disc formation in the early Universe.

Previous research on galaxy formation has highlighted the role of inside-out growth, primarily based on observations of local galaxies. However, these observations have not fully captured the complexity of galaxy formation processes, especially the influence of mergers and feedback mechanisms at high redshifts. Studies at cosmic noon (z~1-3) have shown galaxies with massive bulges and discs, but the early phases of bulge formation remain poorly understood. Theoretical models predict a higher merger rate at higher redshifts, potentially influencing the formation of central starbursts. Numerical simulations have attempted to model the processes governing galaxy growth, suggesting the importance of feedback and the possibility of rapid size fluctuations and compaction in low-mass galaxies. These simulations, while insightful, require observational confirmation, particularly at high redshifts.

This study utilizes JWST data from the JADES survey, focusing on the galaxy JADES-GS+53.18343-27.79097 at a spectroscopic redshift of 7.43. NIRCam imaging in nine filters and NIRSpec spectroscopy provide extensive coverage of the rest-frame ultraviolet and optical wavelengths. The data were reduced using custom pipelines accounting for noise, scattered light, and background effects, followed by astrometric alignment and mosaic construction. Morphological and photometric analyses were performed using ForcePho, which simultaneously models multiple PSF-convolved Sérsic profiles in all individual exposures and filters. A three-component model (a disc, a compact core, and an off-centred clump) was found to best reproduce the data. Stellar population properties of the three components were determined using the Bayesian SED-fitting tool Prospector, employing a flexible star-formation history and a variable dust attenuation law. Radial profiles of stellar mass and star-formation rate surface density were derived from the unconvolved best-fit Sérsic profiles. Comparisons were made with galaxies at lower redshifts and local stellar systems to understand the spatially resolved growth from stellar mass and SFR distribution. The analysis involved detailed error propagation and testing of different model variations to ensure robustness and validity of the results.

JADES-GS+53.18343-27.79097 exhibits a compact core (half-light radius ~80 pc) surrounded by a more extended star-forming disc (~400 pc) and an off-center clump. The core is significantly more massive than the disc despite its smaller size, indicating a high central stellar mass density comparable to that of the most massive present-day ellipticals, although with a much lower total stellar mass. The star formation history analysis shows that the core experienced earlier star formation with a recent decline, while the disc exhibits a recent burst of star formation. The specific star-formation rate (sSFR) profile increases with radius, strongly supporting inside-out growth. The galaxy's central stellar mass density, although similar to that of massive ellipticals, is contained within a much smaller total mass. Comparison with galaxies at z~2 and local stellar systems (globular clusters, ultracompact dwarfs, compact ellipticals) indicates that JADES-GS+53.18343-27.79097 is a potential progenitor of quiescent galaxies at z~2 and a present-day elliptical galaxy. Two evolutionary scenarios are proposed for the core’s formation: continuous inside-out growth from an extremely compact disc, or initial disc formation followed by gas inflow and compaction leading to core formation, with subsequent disc reformation.

The findings provide strong observational evidence for inside-out galaxy growth during the epoch of reionization. The high central stellar mass density of JADES-GS+53.18343-27.79097 challenges previous assumptions and suggests that bulge formation can commence at very early cosmic times. The observed inside-out growth pattern supports hierarchical galaxy formation models. The evolutionary tracks show that with continued star formation, JADES-GS+53.18343-27.79097 could evolve into a typical quiescent galaxy at redshift 2 and potentially a present-day elliptical galaxy, supporting a two-phase growth process (star-forming phase followed by quiescent phase via mergers). The consistent results from both spectroscopic and photometric analysis further strengthens the conclusions. The study highlights the importance of studying these early galaxies on spatially resolved scales to understand their growth and evolution.

The discovery of JADES-GS+53.18343-27.79097 provides compelling evidence for inside-out growth in a galaxy during the epoch of reionization. This galaxy’s characteristics suggest that it may be a progenitor of present-day elliptical galaxies. Further research should focus on investigating a statistically significant sample of galaxies at similar redshifts to determine if this inside-out growth is common, and to further explore the mechanisms driving bulge formation at early epochs. More sophisticated modeling incorporating detailed kinematics would enhance understanding of the processes involved.

The study focuses on a single galaxy, limiting the generalizability of the findings. The three-component model, while providing the best fit, may not be unique, and there could be other possible interpretations. Uncertainties in the adopted stellar population synthesis models and dust attenuation laws can also affect the interpretation of the results. The analysis relies heavily on the accuracy of the PSF modeling, although the authors have performed extensive validation tests.