High-Redshift Massive Galaxies

• High-redshift candidate massive galaxies are compact, early-universe systems with stellar masses >10¹⁰ M☉, identified by spectral breaks in broad and medium-band photometry.
• They are selected using dropout techniques, infrared color criteria, and SED fitting, and exhibit high central stellar densities and compact morphologies revealed by JWST imaging.
• Their study informs inside-out galaxy growth models and challenges theoretical predictions, highlighting the need for spectroscopic follow-up to address systematic uncertainties.

High-redshift candidate massive galaxies are systems observed at cosmic epochs corresponding to the first billion years after the Big Bang (z ≳ 3–10) and inferred to possess stellar masses comparable to, or exceeding, $10^{10}$–%%%%1%%%% M$_\odot$. Their identification and characterization have profound implications for theories of cosmic structure formation, galaxy assembly, feedback, and the evolution of the cosmic star formation rate density. Recent advances, particularly with JWST, have revealed both expectations and challenges in their census, physical properties, and the cosmological constraints they enable.

1. Observational Selection and Characterization

High-redshift massive galaxy candidates are typically identified through broad- and medium-band photometry spanning rest-frame UV to near-IR, exploiting spectral discontinuities such as the Lyman and Balmer breaks. Selection techniques include:

• Y-dropout/Lyman-break selection: At $z \sim 7$–8, objects are undetected blueward of the F098M (Y$_{98}$) or F814W filter but detected in redder near-IR bands (e.g., J$_{125}$, H$_{160}$). This isolates galaxies where intergalactic medium absorption leads to strong spectral breaks (Trenti et al., 2011, Repp et al., 2015, Salmon et al., 2017).
• Infrared color-selection: Techniques such as H–[4.5]>2.25 (HIEROs) exploit the redward move of the Balmer/4000 Å break at $z > 3$ and/or heavy dust obscuration missed by standard Lyman-break methods (Wang et al., 2015).
• Rest-frame optical/IR SED fitting: Multi-band photometry enables the identification of massive quiescent and evolved galaxies via features such as the Balmer or D4000 break, using observer-frame color–color diagrams (e.g., J–K$_s$ vs IRAC[3.6]–[4.5]) and SED parameters (age, stellar mass, specific SFR) (Nayyeri et al., 2014, Girelli et al., 2019, Shahidi et al., 2020).
• Spectroscopic confirmation: High S/N near-IR spectra (e.g., from MOSFIRE, NIRSpec) are employed to refine redshift and physical property estimates, detect emission/absorption features, and mitigate contamination by dusty interlopers or AGN (Forrest et al., 29 Apr 2024).

Typical properties of robust massive candidates at z ≳ 7 (as revealed by JWST F200W imaging and SED fitting) include inferred stellar masses $M_* > 10^{10}\,M_\odot$ and red rest-frame optical colors (Labbe et al., 2022, Baggen et al., 2023).

2. Structural and Physical Properties

High-redshift massive galaxy candidates display strikingly compact morphologies. Sérsic profile fitting to deep JWST F200W imaging reveals:

Property	Measured Range	Notes
Effective radius ($r_e$)	$\sim$80–300 pc	Mean $\langle r_e \rangle \approx 150$ pc
Stellar mass ($M_*$)	$>10^{10} M_\odot$	As inferred from SED fits, may reach $10^{11} M_\odot$
Central stellar density ($\rho(r)$)	$\sim10^{12}\,M_\odot\,\textrm{kpc}^{-3}$	Comparable to local massive ellipticals

The light profile is well-modeled by a Sérsic function: $$I(r) = I_e \exp\left[ -b_n \left( \left( \frac{r}{r_e} \right)^{1/n} - 1 \right) \right],$$ where $n$ is the Sérsic index and $b_n$ a function of $n$. The three-dimensional stellar mass density profile, obtained via Abel inversion,

$$\rho(r) = \frac{b_n}{\pi} \frac{I_e}{r_e} \left(\frac{r}{r_e}\right)^{1/n - 1} \int_1^\infty \frac{\exp\left[ -b_n (r/r_e)^{1/n} t \right]}{\sqrt{t^{2n} - 1}} dt$$

shows that these $z\sim7$–9 galaxies, while an order of magnitude smaller in $r_e$ than any other known population at lower redshift, possess inner stellar densities comparable to massive quiescent systems at $z\sim2.3$ and present-day ellipticals (Baggen et al., 2023).

This suggests that the central $\sim$100 pc cores of today’s giant ellipticals—“relic” cores—were assembled within $\sim$600 Myr after the Big Bang.

3. Comparison with Descendant Populations and Implications

While candidate massive galaxies at $z\sim7$–9 display half-light radii up to 20$\times$ smaller than massive quiescent galaxies at $z\sim2$, their central stellar mass densities are remarkably similar. This structural continuity supports a scenario of inside-out galaxy growth, where compact, dense cores form early via highly dissipative processes (e.g., rapid, centrally concentrated star formation or gas infall), and the outer stellar mass builds up at later epochs, likely dominated by minor mergers or extended star formation (Baggen et al., 2023).

This empirical structural sequence may naturally explain the “relic” cores observed in the centers of local elliptical galaxies and the dramatic size growth seen at fixed stellar mass from $z\gtrsim7$ to $z\sim0$.

4. Limitations and Systematic Uncertainties

Several caveats remain in the interpretation of high-redshift candidate massive galaxies:

• The NIRCam point spread function (PSF), especially at the longest wavelengths, is complex and not yet fully characterized; thus, uncertainties in the structural parameters persist (Baggen et al., 2023).
• Stellar masses and redshifts are photometrically inferred; spectroscopic redshifts and emission-line measurements are required to robustly confirm the nature and mass of these objects (Labbe et al., 2022, Forrest et al., 29 Apr 2024).
• Photometric studies are subject to contamination from lower-redshift, dust-obscured galaxies or AGN. Spectroscopic campaigns (e.g., the MAGAZ3NE survey) demonstrate that heavy dust attenuation ($A_V\sim3$ mag) and AGN continuum can systematically bias photometric redshifts and mass estimates high, with emission lines further complicating SEDs. As a result, the number densities of ultra-massive galaxies at $z\gtrsim3$ inferred from photometry alone may be overestimated by factors of $\gtrsim4$ (Forrest et al., 29 Apr 2024).

These uncertainties must be carefully accounted for when comparing observations to predictions from theoretical galaxy formation models.

5. Cosmological and Theoretical Context

The high central densities and rapid emergence of massive compact galaxies at $z>7$ provide unique tests for theoretical models of galaxy assembly and structure growth:

• Implications for galaxy formation: These findings require that the densest cores of today’s massive ellipticals were in place before significant size growth, indicating a need for efficient early star formation and dissipation.
• Hierarchical models: The observed abundance and compactness of massive galaxies at $z\sim7$–9 challenge some hierarchical scenarios, which predict more gradual stellar mass and size assembly (Labbe et al., 2022).
• Spectroscopic and structural confirmation: Resolving outstanding systematic issues, especially through spectroscopy and more advanced PSF characterization, is crucial for validating the mass and redshift estimates and, by extension, for robust constraints on cosmological models.

6. Future Directions

Advancing the paper of high-redshift massive galaxy candidates hinges on:

• Comprehensive spectroscopic follow-up to secure redshifts, measure dynamical properties, and constrain emission line contamination and AGN contributions (Labbe et al., 2022, Forrest et al., 29 Apr 2024).
• High-resolution rest-frame IR imaging to separate structural components, better model the PSF, and mitigate blending/confusion.
• Improved simulations to model mechanisms driving rapid central mass assembly and their subsequent growth to match the observed $z\sim2$ and $z=0$ descendants.
• Quantitative comparison with theoretical predictions of early massive galaxy number densities, core densities, and the prevalence of relic cores in present-day surveys.

The new JWST era thus offers unparalleled potential to elucidate the initial conditions and assembly histories of massive galaxies and their central cores at cosmic dawn, provided systematic biases are addressed and observations are coupled with robust theoretical modeling.