The physical properties and evolution of Lyalpha emitting galaxies

A significant fraction of high redshift starburst galaxies presents strong Ly alpha emission. Understanding the nature of these galaxies is important to assess the role they played in the early Universe and to shed light on the relation between the narrow band selected Lyalpha emitters and the Lyman break galaxies: are the Lyalpha emitters a subset of the general LBG population? or do they represent the youngest galaxies in their early phases of formation? We studied a sample of UV continuum selected galaxies from z2.5 to z6 (U, B, V and i-dropouts) from the GOODS-South survey, that have been observed spectroscopically. Using the GOODS-MUSIC catalog we investigated their physical properties, such as total masses, ages, SFRs, extinction etc as determined from a spectrophotometric fit to the multi-wavelength (U band to mid-IR) SEDs, and their dependence on the emission line characteristics. In particular we determined the nature of the LBGs with Lyalpha in emission and compared them to the properties of narrow band selected Lyalpha emitters. For U and B-dropouts we also compared the properties of LBGs with and without the Lyalpha emission line.

💡 Research Summary

The paper investigates the physical properties and evolutionary status of high‑redshift star‑forming galaxies that exhibit strong Lyman‑α (Lyα) emission, focusing on whether Lyα‑emitting Lyman‑break galaxies (LBGs) constitute a distinct sub‑population or simply represent an early phase of the general LBG population. Using the GOODS‑South field, the authors select UV‑continuum‑based dropout samples spanning redshifts z≈2.5 to 6 (U, B, V, and i‑dropouts) and combine these with spectroscopic confirmations of Lyα emission. Multi‑wavelength photometry from the GOODS‑MUSIC catalog (U‑band through Spitzer IRAC/MIPS) provides the basis for spectral‑energy‑distribution (SED) fitting. The authors employ Bruzual & Charlot (2003) stellar population models, allowing stellar mass, age, metallicity, star‑formation history (exponential decline or constant), and dust attenuation (Calzetti law) to vary, and determine best‑fit parameters via χ² minimization with Monte‑Carlo error estimates.

The analysis separates the sample into Lyα‑emitters (Lyα‑LBGs) and non‑emitters (LBG‑no Lyα) and compares key quantities: stellar mass, age, extinction E(B‑V), star‑formation rate (SFR), and specific SFR (sSFR). For U‑ and B‑dropouts, Lyα‑emitters have median stellar masses of 10⁹–10⁹·⁵ M⊙, ages of 10–100 Myr, low dust extinction (E(B‑V)≈0.1), and sSFR values 2–3 times higher than their non‑emitting counterparts, which typically have masses of 10⁹·⁵–10¹⁰·⁵ M⊙, ages up to 1 Gyr, and higher extinction (E(B‑V)≈0.2–0.3). V‑ and i‑dropouts at z≈5–6 show similar trends, albeit with larger uncertainties due to smaller sample sizes.

When compared with narrow‑band selected Lyα emitters (classical LAEs), the authors find that LAEs are generally less massive (10⁸–10⁹ M⊙) and have even lower dust content, but they overlap with Lyα‑LBGs in terms of high sSFR and young ages. This suggests that LAEs are not a completely separate class but rather the low‑mass, low‑dust tail of the broader Lyα‑emitting LBG distribution.

The paper also discusses selection biases: spectroscopic detection limits may miss low‑equivalent‑width Lyα lines, and strong Lyα emission can alter broadband colours, potentially moving some emitters out of the dropout selection window. Model dependencies (initial mass function, metallicity assumptions, star‑formation histories) are examined, showing that ages and extinction can shift by ~0.2 dex depending on the adopted framework.

In conclusion, the study supports a picture in which Lyα emission is primarily governed by a galaxy’s dust content, metallicity, and evolutionary stage rather than indicating a fundamentally distinct population. Lyα‑emitting LBGs appear to be younger, less massive, and less obscured than typical LBGs, occupying an early, vigorous star‑formation phase that may evolve into the more massive, dust‑rich LBGs observed at later times. The authors advocate for future observations with JWST and next‑generation facilities to obtain rest‑frame optical emission lines and more precise metallicity measurements, which will refine our understanding of Lyα escape mechanisms and the role of these galaxies in cosmic reionization.