Predictions from BPASS synthetic stellar and nebular emission models (Z/Z_⊙ ≃ 0.15, T_e = 1.5 × 10⁴ K, and n_e = 10³ cm⁻³) for the UV continuum slope (β_UV), EW₀ (Hα), and the Balmer break strength ($F_{\nu }^{4200\mathrm{\AA }}/F_{\nu }^{3400\mathrm{\AA }}$) as a function of the age (top) and their combination (bottom). The solid and dashed lines correspond to constant star formation (CSFH) and instantaneous burst models (Burst), respectively. Model sequences are shown for LyC escape fractions f_esc ranging from 0% (red) to 90% (blue); additional models with f_esc = 80% and 95% are shown in the bottom panels only (light and dark blue, respectively). The dot-dashed violet lines (top) and circles (bottom) indicate the observational constraints for U37126, with the shaded regions representing the 1σ uncertainties, except for EW₀ (Hα), which represents the 3σ upper limit.