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

TL;DR

This study uses spatially resolved spectroscopy to identify a weak active galactic nucleus in a high-redshift star-forming galaxy, revealing potential contamination in nebular diagnostics.

Contribution

First spatially resolved detection of an embedded AGN in a high-redshift galaxy, highlighting its impact on nebular ratio measurements and galaxy classification.

Findings

Detected an embedded AGN in HDF-BMZ1299 using [OIII] observations.

Found that central nebular ratios indicate AGN activity, while overall ratios are composite.

Estimated the AGN luminosity as one of the lowest at this epoch.

Abstract

We present [OIII 5007A] observations of the star forming galaxy HDF-BMZ1299 (z=1.598) using Keck Observatory's Adaptive Optics system with the near-infrared integral field spectrograph OSIRIS. Using previous Halpha and [NII] 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 HII diagnostic diagram. We find that HDF-BMZ1299's spatially concentrated nebular ratios in the central ~1.5 kiloparsec (0."2) are best explained by the presence of an AGN: log([NII]/Halpha)=-0.22+/-0.05 and 2sigma limit of log([OIII]/Hbeta)>0.26. 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 HII regions. The presence of an embedded AGN in HDF-BMZ1299 may suggest a potential contamination in a fraction of other high-redshift star forming galaxies, 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_Halpha = 3.7e41 erg/s and L_[OIII] = 5.8e41 erg/s, making it one of the lowest luminosity AGN discovered at this early epoch.