The Rest-Frame Optical Properties of z~=3 Galaxies

We present the results of a near-infrared imaging survey of z~3 Lyman break galaxies (LBGs). The survey covers a total of 30 arcmin² and includes 118 photometrically selected LBGs with K_s-band measurements, 63 of which also have J-band measurements, and 81 of which have spectroscopic redshifts. Using the distribution of optical R magnitudes from previous work and R-K_s colors for this subsample, we compute the rest-frame optical luminosity function of LBGs. This luminosity function is described by an analytic Schechter fit with a very steep faint-end slope of α=-1.85+/-0.15, and it strikingly exceeds locally determined optical luminosity functions at brighter magnitudes, where it is fairly well constrained. The V-band luminosity density of only the observed bright end of the z~3 LBG luminosity function already approaches that of all stars in the local universe. For the 81 galaxies with measured redshifts, we investigate the range of LBG stellar populations implied by the photometry that generally spans the range 900-5500 Å in the rest frame. The parameters under consideration are the star formation rate as a function of time, the time since the onset of star formation, and the degree of reddening and extinction by dust. While there are only weak constraints on the parameters for most of the individual galaxies, there are strong trends in the sample as a whole. With a wider wavelength baseline than most previous studies at similar redshifts, we confirm the trend that intrinsically more luminous galaxies are dustier. We also find that there is a strong correlation between extinction and the age of the star formation episode, in the sense that younger galaxies are dustier and have much higher star formation rates. The strong correlation between extinction and age, which we show is unlikely to be an artifact of the modeling procedure, has important implications for an evolutionary sequence among LBGs. A unified scenario that accounts for the observed trends in bright LBGs is one in which a relatively short period of very rapid star formation (hundreds of M_solar yr^-1) lasts for roughly 50-100 Myr, after which both the extinction and star formation rate are considerably reduced and stars are formed at a more quiescent, but still rapid, rate for at least a few hundred megayears. In our sample, a considerable fraction (~20%) of the LBGs have best-fit star formation ages >~1 Gyr, implied stellar masses of >~10¹⁰ M_solar, and are still forming stars at ~30 M_solar yr^-1. Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA and was made possible by the generous financial support of the W. M. Keck Foundation.