Science Assessment

Active Model

The active science workflow is intentionally small:

  • fit z_obs from photometry with no photo-z prior;

  • fit log10_formed_mass_msun as the SED amplitude;

  • fit lognormal SFH shape parameters sfh_t_peak and sfh_tau;

  • fit scalar stellar metallicity log10_metallicity;

  • derive current SFR from fitted mass plus fitted SFH shape;

  • inject COSMOS proxy dust columns when dust_model: cosmos_proxy_fixed is selected;

  • use ssp_flux as the SSP spectral table, with separate SSP emission-line tables used only for diagnostics.

log10_sfr and dust_av are not free parameters in the main science config. log10_sfr is only a fixed internal SFH scale before mass normalization. With dust_model: cosmos_proxy_fixed, COSMOS dust columns drive attenuation and dust_av is marked inactive, not inferred.

Dust catalogue columns are proxy-only. Even if a future config frees dust_av, the FS2/COSMOS dust proxy should not be plotted as ground truth.

Redshift

The science preset does not initialize z_obs from phz_median and does not use a photo-z prior. redshift.initial: fixed is only the MAP starting value; fitted z_obs is free within configured bounds. Redshift multi-start MAP was removed, so posterior runs should be used when redshift degeneracy is scientifically important.

PHZ interval priors were removed. Redshift validation must compare fitted z_obs against z_true_gal.

Photometric Errors

The active 10-band config uses continuum flux columns as fit targets and per-band catalog uncertainties from *_el_model3_ext_odonnell_ext_error for LSST and Euclid. Those uncertainties are converted from flux units into local AB-magnitude errors for reporting, but the default MAP and MCMC likelihood is Gaussian in Fnu cgs flux space. Set fit.likelihood_space: mag only for legacy diagnostics.

Truth And Proxies

Catalog truth columns are diagnostics, not likelihood terms.

log_stellar_mass is converted from catalog h units for mass comparison. The fitted DSPS amplitude is log10_formed_mass_msun. This is formed mass, not necessarily surviving stellar mass, so an offset relative to catalog stellar mass can be semantic rather than a pure conversion bug. log_sfr_true is compared to derived current SFR. metallicity_true is treated as a proxy only: gas-phase oxygen abundance is not the same observable as DSPS stellar metallicity.

Fit-vs-catalog plots only include comparable active/free or derived parameters. Fixed inactive quantities such as dust_av under COSMOS proxy dust are audited but not plotted as inferred parameters. Dust is deliberately excluded from fit-vs-catalog truth metrics because the catalog value is not a DSPS attenuation truth.

SED Diagnostics

COSMOS-template SED reconstruction is pseudo-ground truth. It helps inspect whether fitted DSPS SEDs have plausible broad-band shapes. It is not physical SPS truth.

Photometric markers in SED plots are model-anchored ratios. They are visual residual diagnostics, not an independent rest-frame spectrum. SED diagnostics also include a flux-space residual panel showing (F_obs - F_model) / sigma_F per band.

Batch MAP runs split SED diagnostics between worst-fit and best-fit galaxies. COSMOS proxy SED CSVs and manifests include the scalar normalization factor and normalization-band residuals. Large COSMOS/DSPS height mismatches should be debugged as normalization, filter, dust, redshift, or template-unit issues before being interpreted as physical disagreement.

Non-Detections And Calibration

In flux-space mode, finite non-positive fluxes with valid errors are kept by selection.nondetection_policy: gaussian_flux. upper_limit is reserved but not implemented.

band_calibration.mode: fixed_offsets can test fixed per-band zero-point offsets. Offsets are applied to the model during likelihood evaluation, never to both data and model.

Goodness Of Fit

reduced_chi2 is the DOF-corrected value chi2 / dof. The number of valid bands, effective free parameters, and DOF are written per object. The old chi2 / n_bands diagnostic is still available as chi2_per_band.

Priors

Current named prior sets:

  • weak_physical: broad stabilizing priors used by configs/fs2_phz1_science.yaml;

  • flat_debug: flat priors inside configured fit bounds;

  • popcosmos_like: reserved and intentionally unavailable until exact POP-COSMOS parameter mapping, units, and selection treatment are implemented.

Current priors are not POP-COSMOS priors yet. Do not label results as POP-COSMOS-like until that mapping exists.

Out Of Likelihood

Emission-line catalog columns remain diagnostics only. The active fit uses ssp_flux as the spectral table. The newer SSP asset also provides ssp_emline_luminosity, ssp_emline_wave, and line names, when available. The pipeline reads those datasets and writes diagnostic line/filter crossings near fitted-redshift attractors, but it does not add the line table to the likelihood. Adding it naively could double-count lines already present in ssp_flux.

DSPS Parameter Count

The active science MAP fit has five free parameters:

  • z_obs;

  • log10_formed_mass_msun;

  • sfh_t_peak;

  • sfh_tau;

  • log10_metallicity.

This is not “the DSPS paper parameter count”. DSPS is a differentiable SPS framework. The paper demonstrates differentiability through SFH, metallicity, dust, and nebular choices, but does not define one canonical Euclid+LSST photo-z fit. Any future paper-like preset must state the exact SFH model, metallicity model, dust model, nebular model, active parameter list, units, and priors.