Possible High-Redshift, Low-Luminosity Active Galactic Nuclei in the Hubble Deep Field

In the Hubble Deep Field (HDF), 12 candidate sources of high-redshift (z > 3.5) active galactic nuclei (AGN) have been identified. The color selection criteria were established by passing spectra of selected quasars and Seyfert galaxies (appropriately redshifted and modified for "Lyman forest" absorption), as well as stars, observed normal and starburst galaxies, and galaxy models for various redshifts through the filters used for the HDF observations. The actual identification of AGN candidates also involved convolving a Laplacian-of-Gaussian filter with the HDF images, thereby removing relatively flat galactic backgrounds and leaving only the pointlike components in the centers. Along with positions and colors, estimated redshifts and absolute magnitudes are reported, with the candidates falling toward the faint end of the AGN luminosity function. One candidate has been previously observed spectroscopically, with a measured redshift of 4.02. The number of sources reported here is consistent with a simple extrapolation of the observed quasar luminosity function to magnitude 30 in Johnson B. Implications for ionization of the intergalactic medium and for gravitational lensing are discussed.