UCLA

We present the results of a narrowband survey of quasi-stellar-object (QSO) fields at redshifts that place the [O III] (5007 Å) emission line in the Δλ/λ ∼ 1% 2.16 μm filter. We have observed 3 arcmin 2 and detected one emission-line candidate object in the field around PC 1109 + 4642. We discuss the possibilities that this object is a star-forming galaxy at the QSO redshift, zem = 3.313, or a Seyfert galaxy. In the former case, we infer a star formation rate of 170 M⊙ yr-1 for this K′ = 21.3 object. The galaxy has a compact but resolved morphology, with an FWHM = 0″.6 or 4.2 kpc at z = 3.313 (H0 = 50 km s-1 Mpc-1 and q0 = 0.5). The comoving density of such objects in QSO environments appears to be 0.0033 Mpc-3, marginally lower (≤ 3 σ) than the density observed for Hoα-emitters in absorption-line fields at z ∼ 2.5 but similar to the density of Lyman-break galaxies at z ∼ 3. If, on the other hand, most of the line emission is [O III] from a Seyfert 2 nucleus at z = 3.31, then the high inferred volume density could imply a large evolution in the Seyfert 2 luminosity function from the current epoch. We find the field containing the object to also contain many faint extended objects in the K′ image but little significant excess over the expected number-magnitude relation. We discuss the implication of the emission line being a longer wavelength line at a lower redshift.