Active Nuclei and Star-Forming Objects at z>2: Metallicities, Winds, and Formation Histories

Abstract

We present near-IR observations of AGN and star-forming objects in the field of the radio galaxy 53W002 at z=2.39. The star-forming objects, of special interest as candidate protogalactic objects, are uniformly very blue in continuum slope, which fits with the strong Lyman alpha emission in indicating metallicity less than half solar. They fall in a range of luminosity and metallicity not populated by local objects. NICMOS spectroscopy detects [O III] emission for two objects and sets interesting limits for the rest. These data are satisfied by model stellar populations which have been forming stars for the last 2-5 million years. We see no evidence for older populations, either in broadband colors or as redder halos around the star-forming regions. 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 systems. They also also permit an alternative scheme, in which these low-mass systems are blowing winds rather than self-enriching, and should fade rapidly with cosmic epoch. For the three prominent AGN at z=2.39, emission-line ratios show metallicities just as high as in nearby systems. The "ionization cones" appear in [O III] and H lines, with continuum reflection in some cases as well. The contrast between the AGN and fainter star-forming objects can be broadly accomodated in a hierarchical formation picture, though there are still important unknowns as to the fate of the star-forming objects.