The Stellar Populations and Evolution of Lyman Break Galaxies

Using deep near-IR and optical observations of the HDF-N from the HST NICMOSand WFPC2 and from the ground, we examine the spectral energy distributions(SEDs) of Lyman break galaxies (LBGs) at 2.0 < z < 3.5. The UV-to-opticalrest-frame SEDs of the galaxies are much bluer than those of present-day spiraland elliptical galaxies, and are generally similar to those of local starburstgalaxies with modest amounts of reddening. We use stellar population synthesismodels to study the properties of the stars that dominate the light from LBGs.Under the assumption that the star-formation rate is continuous or decreasingwith time, the best-fitting models provide a lower bound on the LBG massestimates. LBGs with ``L*'' UV luminosities are estimated to have minimumstellar masses ~ 10^10 solar masses, or roughly 1/10th that of a present-day L*galaxy. By considering the effects of a second component of maximally-oldstars, we set an upper bound on the stellar masses that is ~ 3-8 times theminimum estimate. We find only loose constraints on the individual galaxy ages,extinction, metallicities, initial mass functions, and prior star-formationhistories. We find no galaxies whose SEDs are consistent with young (< 10^8yr), dust-free objects, which suggests that LBGs are not dominated by ``firstgeneration'' stars, and that such objects are rare at these redshifts. We alsofind that the typical ages for the observed star-formation events aresignificantly younger than the time interval covered by this redshift range (~1.5 Gyr). From this, and from the relative absence of candidates for quiescent,non-star-forming galaxies at these redshifts in the NICMOS data, we suggestthat star formation in LBGs may be recurrent, with short duty cycles and atimescale between star-formation events of < 1 Gyr. [Abridged]Comment: LaTeX, 37 pages, 21 figures. Accepted for publication in the Astrophysical Journa