Quasars Clustering at z ≈ 3 on Scales ≲10 h–1 Mpc

We test the hypothesis of whether high-redshift quasi-stellar objects (QSOs) would preferentially appear in small groups or pairs and whether they are associated with massive young clusters. We carried out a photometric search for Lyα emitters on scales ≲10 h-1 Mpc in the fields of a sample of 47 z ≈ 3 known QSOs. Wide and narrowband filter color-magnitude diagrams were generated for each of the 6.′6 × 6.′6 fields. A total of 13 nonresolved objects with a significant color excess were detected as QSO candidates at a redshift similar to that of the target. All the candidates are significantly fainter than the reference QSOs, with only two of them within 2 mag of the central object. Follow-up spectroscopic observations have shown that five, i.e., about 40% of the candidates, are QSOs at the same redshift of the target; four are QSOs at different z (two of them probably being a lensed pair at z = 1.47); two candidates are unresolved H II galaxies at z ~ 0.3; and one unclassified and one candidate turned out to be a CCD flaw. These data indicate that at least 10% of the QSOs at z ~ 3 do have companions. We have also detected a number of resolved, rather bright Lyα emitter candidates. Most likely, a large fraction of them might be bright galaxies with [O II] emission, at z ≈ 0.3. The fainter population of our candidates corresponds to the current expectations. Thus, there are no strong indications for the existence of an overdensity of Lyα galaxies brighter than m ≈ 25 around QSOs at z ≈ 3.