2025A&A...693A..34V

Aims. We have devised a model for estimating the ultraviolet (UV) and optical line emission (i.e. CIII] 1909 Å, Hβ, [OIII] 5007 Å, Hα, and [NII] 6583 Å) that traces HII regions in the interstellar medium (ISM) of a subset of galaxies at z ∼ 4-6 from the ALMA large programme ALPINE. The aim is to investigate the combined impact of binary stars in the stellar population and an abrupt quenching in the star formation history (SFH) on the line emission. This is crucial for understanding the ISM's physical properties in the Universe's earliest galaxies and identifying new star formation tracers in high-z galaxies.
Methods. The model simulates HII plus PhotoDissociation Region (PDR) complexes by performing radiative transfer through 1D slabs characterised by gas density (n), ionisation parameter (U), and metallicity (Z). The model also takes into account (a) the heating from star formation, whose spectrum has been simulated with Starburst99 and Binary Population and Spectral Synthesis (BPASS) to quantify the impact of binary stars; and (b) a constant, exponentially declining, and quenched SFH. For each galaxy, we selected from our CLOUDY models the theoretical ratios between the [CII] line emission that trace PDRs and nebular lines from HII regions. These ratios were then used to derive the expected optical/UV lines from the observed [CII].
Results. We find that binary stars have a strong impact on the line emission after quenching, by keeping the UV photon flux higher for a longer time. This is relevant in maintaining the free electron temperature and ionised column density in HII regions unaltered up to 5 Myr after quenching. Furthermore, we constrained the ISM properties of our subsample, finding a low ionisation parameter of log U≈ - 3.8 ± 0.2 and high densities of log(n/cm^–3)≈2.9 ± 0.6. Finally, we derive UV/optical line luminosity-star formation rate relations (log(L_line/erg s^–1) = α log(SFR/M_⊙ yr^–1) + β) for different burstiness parameter (k_s) values. We find that in the fiducial BPASS model, the relations have a negligible SFH dependence but depend strongly on the k_s value, while in the SB99 case, the dominant dependence is on the SFH. We propose their potential use for characterising the burstiness of galaxies at high z.