Optical-to-mid-infrared observations of Lya galaxies at z ≈ 5 in the Hubble Ultra Deep Field: A young and low-mass population

High-redshift galaxies selected on the basis of strong Ly a emission tend to be young and have small physical sizes. We show this by analyzing the spectral energy distribution of nine Lyα-emitting galaxies (LAEs) at 4.0 < z < 5.7 in the Hubble Ultra Deep Field. Rest-frame UV-to-optical (700 Å < λ < 7500 Å) luminosities, or upper limits, are used to constrain old stellar populations. We derive best-fit, as well as maximally massive and maximally old, properties of all nine objects. We show that these faint and distant objects are all very young, most likely only a few million years old, and not massive, the mass in stars being ≈10⁶-10 ⁸ M_⊙. Deep Spitzer Infrared Array Camera observations of these objects, even in cases where the object was not detected, proved crucial in constraining the masses of these objects. The space density of these objects, ≈1.25 × 10^-4 Mpc^-3, is comparable to previously reported space densities of LAEs at moderate-to-high redshifts. These Lya galaxies show modest star formation rates of ≈8 M_⊙ yr^_1, which is nevertheless strong enough to have allowed them to assemble their stellar mass in less than a few million years. These sources appear to have small physical sizes, usually smaller than 1 kpc, and are also rather concentrated. They are likely to be some of the least massive and youngest high-redshift galaxies observed to date.