CW-LRD-z10: Early AGN & SMBH Growth

• The paper demonstrates that CW-LRD-z10 is the first robust LRD at z ≳ 10, revealing key insights into early active galactic nuclei and initial supermassive black hole growth.
• The multi-band imaging and V-shaped SED derived from NIRCam and MIRI data provide tight constraints on its photometric redshift and physical characteristics.
• Joint analysis of morphology and clustering suggests that early SMBH fueling, possibly triggered by mergers, plays a crucial role in early galaxy assembly.

CW-LRD-z10 is the designation for the first robust "Little Red Dot" (LRD) candidate identified at redshift $z \gtrsim 10$ in the COSMOS-Web survey using joint JWST NIRCam and MIRI imaging. LRDs are a recently discovered population of compact, extremely red high-redshift sources with properties interpreted as evidence for early active galactic nuclei (AGN) and nascent supermassive black hole (SMBH) growth. The discovery and analysis of CW-LRD-z10 provide key constraints on the emergence of such sources and their demographic evolution at the earliest cosmic epochs.

1. Identification and Selection of CW-LRD-z10

CW-LRD-z10 was identified through a multi-stage selection protocol integrating deep JWST NIRCam and MIRI imaging. The criteria were as follows:

• Non-detection in F115W, indicating a Lyman break consistent with $z \gtrsim 10$.
• Robust detections in redder bands: F150W, F277W, F444W (NIRCam), and F770W (MIRI).
• Compactness criterion: a comparison of F444W fluxes measured through $0.2''$ and $0.5''$ apertures confirming an unresolved morphology, consistent with a nuclear (AGN-like) or extremely concentrated source.
• Visual inspection to remove spurious sources and artifacts, yielding CW-LRD-z10 as the only robust candidate in the analyzed area.

This search utilized the largest available joint NIRCam–MIRI coverage ($0.20\,{\rm deg^2}$) in the COSMOS-Web field (Tanaka et al., 31 Jul 2025).

2. Spectral Energy Distribution and Physical Properties

CW-LRD-z10 exhibits a characteristic "V-shaped" spectral energy distribution (SED):

• The rest-frame ultraviolet (sampled by F150W–F277W) shows a flat or mildly rising (blue) continuum.
• A sharp inflection at F444W, followed by a steeply rising red continuum through the MIRI/F770W band.
• The color $m_{\rm F444W} - m_{\rm F770W} \approx 1.8-2.0$.

This SED structure is interpreted as a combination of a UV excess and a significant Balmer break or extremely red continuum, potentially associated with AGN activity or reprocessed emission. The analysis excludes alternative explanations such as strong nebular emission line contamination or a purely stellar population, as these would require unphysically high equivalent width lines or a Balmer break much stronger than stellar population synthesis models predict.

Photometric redshift fitting yields $z_{\rm phot} = 10.5^{+0.7}_{-0.6}$ and an absolute UV magnitude $M_{\rm UV} = -19.9^{+0.1}_{-0.2}\,{\rm mag}$. The derived physical properties position CW-LRD-z10 as a luminous, compact source active within the first 500 Myr after the Big Bang (Tanaka et al., 31 Jul 2025).

3. LRDs at High Redshift: Statistical and Demographic Context

Based on the observed detection and completeness simulations, the effective volume density for $M_{\rm UV} \sim -20$ LRDs at $z \sim 10$ is:

$$\Phi_{\rm LRD}(M_{\rm UV} = -20) = 1.2^{+2.7}_{-1.0} \times 10^{-6}\,{\rm Mpc^{-3}\,mag^{-1}}$$

Monte Carlo simulations determine the completeness $C(M_{\rm UV}, z)$, and the survey volume is computed accordingly. Comparison with the evolving galaxy UV luminosity function indicates that while the absolute number density of LRDs falls with redshift, this decline is slower than that of UV-bright galaxies in general. Consequently, the fraction of $M_{\rm UV} = -20$ galaxies classified as LRDs increases with redshift, reaching $\sim3\%$ at $z\sim10$ (Tanaka et al., 31 Jul 2025).

4. Observational Constraints from NIRCam and MIRI

The joint use of NIRCam (up to $\sim5\,\mu$m) and MIRI (particularly F770W, $7.7\,\mu$m) is central for robust LRD identification. At $z \sim 10$, the Balmer break is redshifted beyond the NIRCam range; MIRI/F770W is required to characterize the redder side of the SED. This combination enables a color–color selection in, for example, the $$(m_{\rm F277W} - m_{\rm F444W}, m_{\rm F444W} - m_{\rm F770W})$$ plane, effectively distinguishing $z \sim 10$ LRDs from contaminants at lower redshift (Tanaka et al., 31 Jul 2025).

Detection in F770W rules out extreme emission line galaxies with lower redshifts, and reinforces that the observed SED is due to a genuinely red continuum or Balmer break.

5. Implications for Early Supermassive Black Hole Formation

The properties of CW-LRD-z10 and other LRDs at high redshift have direct implications for models of early SMBH seed formation and growth. The presence of compact, intense (likely AGN-powered) nuclear emission at $z \sim 10$ suggests that SMBHs underwent rapid accretion phases shortly after the onset of galaxy formation. LRDs may represent episodes of super-Eddington or high-efficiency accretion, or phases where nascent SMBHs briefly outshine their host galaxies.

The rising LRD fraction with redshift, as inferred from the data, points to an increasing prevalence of these early AGN as cosmic time moves closer to the epoch of reionization (Tanaka et al., 31 Jul 2025).

6. Limitations and Need for Spectroscopic Confirmation

Although CW-LRD-z10 satisfies all photometric and morphological criteria for a $z\sim10$ LRD, spectroscopic confirmation is necessary for definitive redshift and physical characterization. Current grism spectroscopy with JWST has insufficient sensitivity or wavelength coverage to detect the expected broad Balmer emission lines unambiguously at this redshift and faintness. Deep spectroscopy will provide crucial tests of the LRD hypothesis by confirming redshifts and directly detecting the AGN emission signature.

A plausible implication is that the discovery of CW-LRD-z10 and constraints on LRD demographics set important empirical bounds on the timeline and mechanisms of SMBH formation, provided future spectroscopic campaigns confirm the present photometric interpretations.

7. Broader Context: LRD Clustering and Connection to Galaxy Assembly

The broader LRD population exhibits strong small-scale clustering, as evidenced by the detection of dual LRD systems with sub-arcsecond (1–2 kpc) projected separations (Tanaka et al., 18 Dec 2024). The angular auto-correlation function of LRDs shows an excess clustering factor of $\sim3\times10^2$ compared to the extrapolated power-law ACF of general JWST-selected AGNs at larger separations. This observation supports models in which galaxy mergers and close interactions facilitate early SMBH fueling, suggesting that LRDs likely trace sites of intense baryonic inflow and SMBH assembly during the first billion years.

In summary, CW-LRD-z10 represents a pivotal addition to the empirical inventory of high-redshift AGN-like kernels, offering new observational constraints on the mechanisms underlying the earliest black hole growth episodes and the joint evolution of galaxies and their central black holes.