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High redshift galaxies selected on the basis of their strong Lyman-alphaemission tend to be young ages and small physical sizes. We show this byanalyzing the spectral energy distribution (SED) of 9 Lyman-alpha emitting(LAE) galaxies at 4.0 < z < 5.7 n the Hubble Ultra Deep Field (HUDF).Rest-frame UV to optical 700A < lambda < 7500A luminosities, or upper limits,are used to constrain old stellar populations. We derive best fit, as well asmaximally massive and maximally old, properties of all 9 objects. We show thatthese faint and distant objects are all very young, being most likely only afew millions years old, and not massive, the mass in stars being ~10^6-10^8M_sun. Deep Spitzer Infrared Array Camera (IRAC) observations of these objects,even in cases where objects were not detected, were crucial in constraining themasses of these objects. The space density of these objects, ~1.25x10^-4 Mpc^-3is comparable to previously reported space density of LAEs at moderate to highredshifts. These Lyman-alpha galaxies show modest star formation rates of ~8M_sun yr^-1, which is nevertheless strong enough to have allowed these galaxiesto assemble their stellar mass in less than a few x10^6 years. These sourcesappear to have small physical sizes, usually smaller than 1 Kpc, and are alsorather concentrated. They are likely to be some of the least massive andyoungest high redshift galaxies observed to date.Comment: 27 pages. Accepted. To appear in The Astrophysical Journa

Optical–to–Mid‐Infrared Observations of Lyα Galaxies atz ≈ 5 in the Hubble Ultra Deep Field: A Young and Low‐Mass Population