A population of red candidate massive galaxies ~600 Myr after the Big Bang

Abstract: Galaxies with stellar masses as high as $\sim 10^{11}$ solar masses have been identified out to redshifts $z \sim 6$, approximately one billion years after the Big Bang. It has been difficult to find massive galaxies at even earlier times, as the Balmer break region, which is needed for accurate mass estimates, is redshifted to wavelengths beyond $2.5\mum$. Here we make use of the $1-5\mum$ coverage of the JWST early release observations to search for intrinsically red galaxies in the first ~750 million years of cosmic history. In the survey area, we find six candidate massive galaxies (stellar mass $>10^{10}$ solar masses) at $7.4 < z < 9.1$, 500 - 700 Myr after the Big Bang, including one galaxy with a possible stellar mass of $\sim 10^{11}$ solar masses. If verified with spectroscopy, the stellar mass density in massive galaxies would be much higher than anticipated from previous studies based on rest-frame ultraviolet-selected samples.

• The paper identifies six candidate massive galaxies at redshifts 7.4–9.1 using JWST NIRCam observations and double-break SED features.
• The paper reveals that these galaxies exhibit fiducial masses above 10¹⁰ M☉, potentially exceeding expected early mass densities by factors up to 20–1000.
• The paper employs a robust methodology combining JWST data with advanced SED modeling techniques to differentiate high-redshift candidates from interlopers.

Massive Galaxy Candidates Identified Shortly After the Big Bang

The paper "A population of red candidate massive galaxies ~600 Myr after the Big Bang" presents a significant advancement in the study of galaxy formation by identifying six candidate massive galaxies at high redshifts (7.4 ≤ z ≤ 9.1), merely 500–700 Myr post-Big Bang. These candidates were discovered through early James Webb Space Telescope (JWST) observations, leveraging the Near Infrared Camera's (NIRCam) 1–5 μm coverage, allowing the study of intrinsically red galaxies in the earliest epochs of cosmic history.

Methodology and Data Collection

The analysis is based on data from the JWST's Cosmic Evolution Early Release Science (CEERS) program, which focused on a 'blank' field overlapping with Hubble Space Telescope (HST) imagery. Out of an initial catalog comprising 42,729 objects, 13 galaxies exhibiting a distinct "double-break" spectral energy distribution (SED)—specifically, the Lyman and Balmer breaks—were selected as potential high-redshift candidates. The key selection criteria for these candidates included a lack of detection at optical wavelengths, a blue color in near-infrared, and a distinctive red color at longer infrared wavelengths, to distinguish them from lower redshift interlopers. The evaluation process accounted for modeling uncertainties using three techniques: EAZY, Prospector-α, and Bagpipes.

Results and Interpretation

The analysis yielded six candidate massive galaxies with fiducial masses exceeding 10^10 M☉, suggesting a far higher stellar mass density in early massive galaxies than previously deduced from ultraviolet-selected samples. Notably, one galaxy's mass is hypothesized to surpass even the present-day Milky Way. These galaxies are characterized by exceptionally red SEDs, markedly distinct from those found in previous HST-selected samples at analogous redshifts. The presence of strong emission lines, especially beyond the Balmer break, significantly influenced the continuum emissions in broadband photometry.

This study challenges the extant understanding of early galaxy formation, proposing that these massive galaxies may exceed the anticipated mass density by a factor of ~20 at z ~ 8 and potentially ~1000 at z ~ 9. This presents a juxtaposition against the traditional Lambda Cold Dark Matter (LCDM) cosmology, potentially stretching the constraints posed by the baryon content in dark matter halos.

Implications and Future Prospects

The implications of these findings are profound, questioning our current models of cosmic structure formation and the initial mass functions. If these candidate galaxies are spectroscopically confirmed, it could indicate a reevaluation of the redshift-driven galaxy evolution models and a need to consider previously unaccounted mechanisms, such as unique dust attenuation laws or early assembly histories.

However, the authors remain cautious, acknowledging possible explanations involving lower true masses than estimated or contributions from phenomena such as exotic emission lines or faint AGNs. Future work, particularly with JWST's NIRSpec spectroscopy, could provide the independent verification needed to ascertain accurate redshifts and comprehensively assess emission line contributions.

These pioneering detections by JWST inaugurate a new era in observing early universe structures, laying the groundwork for further explorations into the onset of galaxy formation and contributing to a recalibration of theoretical models of early universal dynamics. The ability to directly observe such high redshift galaxies opens expansive lanes for astrophysical enquiry, potentially recalibrating models of massive galaxy formation and the subsequent hierarchical assembly processes observed in the universe today.