Differences and similarities of stellar populations in LAEs and LBGs at $z\sim$ 3.4 - 6.8

Abstract

The differences between the inherent stellar populations (SPs) of LAEs and LBGs are a key factor in understanding early galaxy formation and evolution. We have run a set of SP burst-like models for a sample of 1,558 sources at $3.4<z<6.8$ from the Survey for High-$z$ Absorption Red and Dead Sources (SHARDS) over the GOODS-N field. This work focuses on the differences between the three different observational subfamilies of our sample: LAE-LBGs, no-Ly$\alpha$ LBGs and pure LAEs. Single and double SP synthetic spectra were used to model the SEDs, adopting a Bayesian information criterion to analyse under which situations a second SP is required. We find that the sources are well modelled using a single SP in $\sim 79\%$ of the cases. The best models suggest that pure LAEs are typically young low mass galaxies ($t\sim 26_{-25}^{+41}$ Myr; $M_{\mathrm{star}}\sim 5.6_{-5.5}^{+12.0}\times 10^8{M}_{\odot }$), undergoing one of their first bursts of star formation. On the other hand, no-Ly$\alpha$ LBGs require older SPs ($t\sim 71\pm 12$ Myr), and they are substantially more massive ($M_{\mathrm{star}}\sim 3.5\pm 1.1\times 10^9{M}_{\odot }$). LAE-LBGs appear as the subgroup that more frequently needs the addition of a second SP, representing an old and massive galaxy caught in a strong recent star-forming episode. The relative number of sources found from each subfamily at each $z$ supports an evolutionary scenario from pure LAEs and single SP LAE-LBGs to more massive LBGs. Stellar Mass Functions are also derived, finding an increase of $M^{\ast }$ with cosmic time and a possible steepening of the low mass slope from $z\sim 6$ to $z\sim 5$ with no significant change to $z\sim 4$. Additionally, we have derived the SFR-$M_{\mathrm{star}}$ relation, finding a $\mathrm{SFR}\propto M_{\mathrm{star}}^{\beta }$ behaviour with negligible evolution from $z\sim 4$ to $z\sim 6$.

Accepted for publication in MNRAS. 18 pages, 23 figures

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