ESA Science & Technology - Optical to mid-IR observations of Lyman-alpha galaxies at redshift 5 in the HUDF: a young and low mass population

Publication date: 14 August 2007

Authors: Pirzkal, N. et al.

Journal: arXiv:astro-ph/0612513
Page: 1-28
Year: 2007

To appear in The Astrophysical Journal

High redshift galaxies selected on the basis of their strong Lyman-alpha emission tend to be young ages and small physical sizes. We show this by analyzing the spectral energy distribution (SED) of 9 Lyman-alpha emitting (LAE) galaxies at 4.0 < z < 5.7 in 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 as maximally massive and maximally old, properties of all 9 objects. We show that these faint and distant objects are all very young, being most likely only a few millions years old, and not massive, the mass in stars being ~10⁶-10⁸ M_Sun. Deep Spitzer Infrared Array Camera (IRAC) observations of these objects, even in cases where objects were not detected, were crucial in constraining the masses of these objects. The space density of these objects, ~1.25x10^-4 Mpc^-3 is comparable to previously reported space density of LAEs at moderate to high redshifts. These Lyman-alpha galaxies show modest star formation rates of ~8 M_Sun yr^-1, which is nevertheless strong enough to have allowed these galaxies to assemble their stellar mass in less than a few x10⁶ 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.