The rest-frame optical properties of z similar or equal to 3 galaxies

We present the results of a near-infrared imaging survey of z similar to 3 Lyman break galaxies (LBGs). The survey covers a total of 30 arcmin(2) and includes 118 photometrically selected LBGs with K-s-band measurements, 63 o f which also have J-band measurements, and 81 of which have spectroscopic r edshifts. Using the distribution of optical R magnitudes from previous work and R-K-s colors for this sub sample, we compute the rest-frame optical lu minosity function of LBGs. This luminosity function is described by an anal ytic Schechter fit with a very steep faint-end slope of alpha = -1.85 +/- 0 .15, and it strikingly exceeds locally determined optical luminosity functi ons at brighter magnitudes, where it is fairly well constrained. The V-band luminosity density of only the observed bright end of the z similar to 3 L BG luminosity function already approaches that of all stars in the local un iverse. For the 81 galaxies with measured redshifts, we investigate the ran ge of LBG stellar populations implied by the photometry that generally span s the range 900-5500 Angstrom in the rest frame. The parameters under consi deration are the star AZ formation rate as a function of time, the time sin ce 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 o f the individual galaxies, there are strong trends in the sample as a whole . With a wider wavelength baseline than most previous studies at similar re dshifts, we confirm the trend that intrinsically more luminous galaxies are dustier. We also find that there is a strong correlation between extinctio n and the age of the star formation episode, in the sense that younger gala xies are dustier and have much higher star formation rates. The strong corr elation between extinction and age, which we show is unlikely to be an arti fact of the modeling procedure, has important implications for an evolution ary 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 rap id star formation (hundreds of M. yr(-1)) lasts for roughly 50-100 Myr, aft er whichboth the extinction and star formation rate are considerably reduce d and stars are formed at a more quiescent, but still rapid, rate for at le ast a few hundred megayears. In our sample, a considerable fraction (simila r to 20%) of the LBGs have best-fit star formation ages greater than or sim ilar to1 Gyr, implied stellar masses of greater than or similar to 10(10) M ., and are still forming stars at similar to 30 M. yr(-1).