F356W-Dropout Sources: High-z vs. Low-z

• F356W-dropout sources are astronomical objects characterized by their missing flux in JWST's F356W filter and strong detections at longer wavelengths, indicating either high-redshift galaxies or unusual low-redshift systems.
• Multi-wavelength SED fitting and NIRSpec spectroscopy are applied to differentiate between genuine high-z Lyman-break galaxies and low-z, dust-obscured or evolved stellar populations.
• Extreme cases like Capotauro highlight classification challenges and the need for refined models and selection criteria to confidently interpret ambiguous spectral signatures.

An F356W-dropout source is an astronomical object identified through the absence (“dropout”) of detectable flux in the JWST/NIRCam F356W filter (centered at 3.56 μm), accompanied by robust detection in longer-wavelength bands such as F410M or F444W. This selection method is rooted in the Lyman-break (or dropout) technique, primarily targeting galaxies at very high redshift or, less commonly, galactic objects with unusual spectral energy distributions (SEDs). The unprecedented imaging depth and wavelength coverage of JWST has enabled the discovery and characterization of diverse F356W-dropout populations, including both rare early-universe systems and exotic galactic sources. Notably, some F356W-dropouts, such as the object labeled “Capotauro,” exhibit photometric and spectroscopic signatures that make their interpretation deeply ambiguous, challenging both observational classification schemes and theoretical models.

1. Selection Criteria and Observational Methodology

The F356W-dropout selection extends the classical Lyman-break method to JWST’s near-infrared parameter space. The key observational criterion is the non-detection (i.e., S/N < 2) in the F356W (3.56 μm) band with significant detection in redder bands (e.g., F410M, F444W), sometimes combined with multiwavelength data from HST and JWST/MIRI.

Principal thresholds defining the selection include:

• For F090W/F115W dropout groups: $m_{356} \leq 25.1\,\mathrm{mag}$
• For F150W/F200W dropout groups: $m_{356} \leq 26.0\,\mathrm{mag}$
• Extremely Red Objects (EROs) criterion: $m_{115} - m_{356} > 2.0\,\mathrm{mag}$

A robust dropout is defined by non-detections in all bands shortward of F356W, a significant color break (typically exceeding 0.8 mag), and strong detections in longer-wavelength filters. The analysis integrates SED fitting employing tools such as Le Phare, EAZY, CIGALE, and, for ambiguous cases, “open-redshift” configurations in codes like BAGPIPES and ZPHOT. Multiwavelength photometry (from 0.6 μm to 21 μm) and, where available, low-resolution JWST/NIRSpec spectra are also used.

This selection process yields a sample that is biased toward sources with extremely red SEDs—either due to cosmological redshift (shifting the Lyman break into the relevant wavelength regime) or due to extreme dust obscuration and/or old stellar populations in low-redshift galaxies.

2. Physical Properties of F356W-Dropout Populations

The F356W-dropout sample encompasses a heterogeneous set of sources. In systematic studies (Sun et al., 9 Feb 2025), 300 very bright dropouts were identified in blank JWST fields. Key characteristics include:

• Red Colors and Extreme Luminosity: Over 80% display $m_{115} - m_{356} > 2.0$ mag, qualifying as EROs. Their F356W magnitudes are exceptionally bright ($m_{356} \leq 25.1$ or $\leq 26.0$ mag), corresponding to absolute UV magnitudes around $M \approx -22$ mag.
• Redshift Distribution: SED fits place ≥67% of these bright dropouts firmly at low redshift ($z_{\mathrm{ph}} \sim 1-4$), typically reflecting dusty or evolved stellar populations. A minority fraction (≥7%) exhibit SEDs consistent with much higher redshift ($z_{\mathrm{ph}} \geq 6$).
• Spectroscopic Confirmation: From a NIRSpec subsample, most secure redshifts confirm the low-z interloper hypothesis, with one notable object (at $z = 8.679$) confirmed as a true high-redshift galaxy with $M_{1500} \approx -22$ and $M_* \approx 10^{9.1} M_\odot$.

A representative case at the faint end is Capotauro (Gandolfi et al., 1 Sep 2025), which is only detected in the two reddest bands (F410M and F444W), entirely undetected shortward of 3.5 μm, with a F444W magnitude of ≈27.68, and a flux drop exceeding 3 mag between 3.5 and 4.5 μm.

3. Interpretation and Redshift Ambiguities

Interpretation of F356W-dropouts often centers on distinguishing between true high-z galaxies and plausible low-z interlopers:

• High-Redshift Scenario: For Capotauro, the sharp flux decrement aligns precisely with the redshifted Lyman break if placed at $z \sim 32$, with the NIRSpec spectrum revealing a rising continuum consistent with rest-frame UV emission beyond the break. Photometric redshift fits (BAGPIPES, CIGALE, ZPHOT) all converge on $z \approx 31.7-31.9$, with only 0.5% of the posterior volume at $z < 25$.
• Low-Redshift Interlopers: Lower-z solutions (i.e., $z < 10$) require a combination of extreme dust extinction (AV up to ~16 mag), strong Balmer breaks, or emission line contamination. Even employing expanded template grids, such models cannot replicate both the amplitude of the flux drop and the absolute non-detections at shorter wavelengths, rendering these solutions physically contrived.
• Galactic Sub-Stellar Hypotheses: For Capotauro, a fit to brown dwarf models (Sonora Cholla, ATMO 2020) yields $T_{\text{eff}} \leq 300\,\mathrm{K}$ and inferred distances $d \gtrsim 130\,\mathrm{pc}$ up to ~2 kpc, parameters that are unprecedented for known Milky Way brown dwarfs. The upper limit on proper motion ($\mu \lesssim 0.137''\,\text{yr}^{-1}$; 2.3-year baseline) is compatible with distant halo or thick disk membership but not typical thin disk nearby objects.

This ambiguity illustrates the inherent challenges of classifying extreme “dropout” sources at the noise and template boundaries of current observations and models.

4. Implications for Galaxy Formation and Stellar/Brown Dwarf Populations

The discovery and characterization of F356W-dropouts bear significant consequences for several research fronts:

• Early Galaxy Formation: The statistical dominance of low-z interlopers underscores the importance of stringent color and non-detection criteria as well as follow-up spectroscopy to validate high-z claims. Presence of extremely luminous galaxies at $z > 8$, such as the confirmed $z = 8.679$ example, may provide critical tests for models of rapid stellar mass and UV luminosity buildup in the early universe. This is particularly salient for exceptionally bright candidates with $M_{\mathrm{UV}} \leq -23$ and $M_* > 10^{10.5} M_\odot$, which, if confirmed, could push galaxy formation models to their limits.
• Exotic Interlopers and Selection Effects: The “Capotauro” case suggests that new classes of dusty, strongly Balmer-break-dominated, or emission-line-dominated galaxies could significantly contaminate dropout selections. The required physical parameters to mimic the observed dropout—and the lack of plausible analogs—imply that many such cases may be genuinely extragalactic or else mark the discovery of previously unclassified astrophysical populations.
• Galactic Sub-Stellar Populations: If Capotauro or similar objects were to be confirmed as Y2–Y3 brown dwarfs or free-floating exoplanets at >130 pc (potentially up to 2 kpc), this would substantially revise current demographic estimates for local sub-stellar populations and the coldest brown dwarfs, far beyond the thin disk neighborhood.

5. Technical Overview: SED Fitting, Spectroscopy, and Future Prospects

The analysis of F356W-dropouts leverages both statistical SED modeling and direct spectroscopy:

• SED Fitting Methodology: Multiple codes are employed with “open-redshift” priors to maximize parameter space exploration across extragalactic and Galactic templates. The best-fit SEDs are judged not only by reduced χ² but by their capacity to reconcile all photometric non-detections and sharp color breaks.
• Spectroscopic Constraints: Low-resolution NIRSpec prism spectra provide the crucial continuum shape and potential emission/absorption diagnostics necessary to disambiguate between Lyman and Balmer breaks, strong emission lines, or molecular features from cold atmospheres.
• Kinematic Discriminants: Proper motion constraints from multi-epoch imaging are vital for separating distant extragalactic sources (expected negligible motion) from galactic interlopers with measurable parallactic or heliocentric shifts.
• Open Issues and Future Work: Definitive classification of ambiguous F356W-dropouts, especially those as extreme as Capotauro, awaits deeper spectroscopy (increased S/N, specific line diagnostics), expanded imaging at intermediate wavelengths, and longer baselines for astrometric motion measurement. A plausible implication is that future studies may require even more stringent multi-band dropout criteria and refined template libraries for both SED and atmospheric modeling.

6. Summary Table: F356W-Dropout Source Classes and Diagnostic Features

Class	Key Observational Signatures	Leading Interpretation(s)
Classic high-z dropout	Strong flux break at F356W, detected only at longer λ	Lyman break at z ≥ 8–10
Low-z interloper	Red SED, possibly with Balmer break, dust, emission lines	Dusty/evolved galaxy z ∼ 1–4
Extreme/unique cases (e.g., Capotauro)	>3 mag break, only detected at >4 μm, ambiguous SED	z ≈ 32 galaxy, peculiar galaxy at z < 10, or cold Y dwarf/free-floating exoplanet

7. Conclusion

F356W-dropout sources, defined via their absence in the 3.56 μm NIRCam filter and exceptionally red colors, represent a powerful yet nuanced probe of the distant universe and rare galactic types. While the majority of very bright F356W-dropouts selected from JWST fields are low-redshift interlopers, a persistently intriguing minority show properties consistent with bona fide high-redshift galaxies and, in extreme cases, challenge both observational classification and theoretical models. The ambiguous nature of objects such as Capotauro illustrates the complexity of interpreting dropout signatures at the sensitivity frontier of contemporary instrumentation. Continued spectroscopic, photometric, and astrometric follow-up will be critical in unraveling the true nature of these sources and refining the methodologies that underpin early-universe studies.