The Presence of Weak Active Galactic Nuclei in High Redshift Star-forming Galaxies

We present [O III 5007 Å] observations of the star-forming galaxy (SFG) HDF-BMZ1299 (z = 1.598) using Keck Observatory's adaptive optics system with the near-infrared integral field spectrograph OSIRIS. Using previous Hα and [N II] measurements of the same source, we are able for the first time to use spatially resolved observations to place a high-redshift galaxy's substructure on a traditional H II diagnostic diagram. We find that HDF-BMZ1299's spatially concentrated nebular ratios in the central ~1.5 kpc (0farcs2) are best explained by the presence of an active galactic nucleus (AGN): log ([N II]/Hα) = -0.22 ± 0.05 and 2σ limit of log ([O III]/Hβ) gsim0.26. The dominant energy source of this galaxy is star formation, and integrating a single aperture across the galaxy yields nebular ratios that are composite spectra from both AGN and H II regions. The presence of an embedded AGN in HDF-BMZ1299 may suggest a potential contamination in a fraction of other high-redshift SFGs, and we suggest that this may be a source of the "elevated" nebular ratios previously seen in seeing-limited metallicity studies. HDF-BMZ1299's estimated AGN luminosity is L _Hα = (3.7 ± 0.5) × 10⁴¹ erg s^-1 and L_{[O III]} = (5.8 ± 1.9) × 10⁴¹ erg s^-1, making it one of the lowest luminosity AGNs discovered at this early epoch.