Active nuclei and star-forming objects at z > 2: Metallicities, winds, and formation histories

We present near-infrared observations of the active galactic nuclei (AGNs) and star-forming objects in the field of the radio galaxy 53W002 at z = 2.39. The star-forming objects are of special interest as candidate protogalactic objects. The 1.1-2.2 μm passbands sample the emitted-optical range at this redshift, providing new diagnostics of the structure, metal abundance, and age of the members of this grouping originally selected through Lyαa emission. The star-forming objects are uniformly very blue in continuum slope, which fits with the strong Lyα emission in indicating metal abundances that are less than half solar; some are as blue as the most metal-poor local objects. They fall in a range of luminosity and metallicity that is not populated by local objects, indicating a shorter star-forming history at this early epoch. The best local analogs, such as Mrk 66 and 357, either have several times lower luminosity at comparable [O/H] or significantly higher [O/ H] for comparable luminosity. Spectroscopy from the Near-Infrared Camera and Multi-Object Spectrometer yields detections of [O III] emission for two objects and interesting [O III] and Hβ limits for the rest, augmented by Hα limits from Infrared Telescope Facility imaging. These data are satisfied by model stellar populations that have been forming stars for the last 2-5 × 10⁶ yr before z = 2.39. We do not see evidence for older preexisting stellar populations, either in the broadband colors or as redder halos in which the star-forming regions are imbedded. These results suggest that the compact star-forming objects we see at z = 2.0-2.5 are indeed early stages in the building of galaxies rather than transient star-forming events in larger pre-existing dynamical systems. The results also allow an alternative scheme, in which these are low-mass systems that are blowing winds rather than self-enriching, in which case they should fade rapidly with cosmic epoch. For the three prominent AGNs at z = 2.39, Hα and [O III] emission were measured. Unlike the fainter star-forming objects, their line ratios (specifically Lyα/Hα) show metallicities just as high as in nearby systems. If the AGNs occur in those systems that started with the highest density and began active star formation before the less massive surrounding objects, they will have higher metallicity (as we see in their emitted-ultraviolet line ratios). The "ionization cones" seen prominently in Lyα also appear in [O III] and Hα, with a role for continuum reflection in some cases as well. The contrast between the AGNs and fainter star-forming objects can be broadly accommodated in a hierarchical formation picture, although there are still important unknowns as to the fate of the star-forming objects.