Evidence for large-scale structure at z ≈ 2.4 from Lyα imaging

The history of large-scale structure depends on cosmological parameters and on how merging unfolds among both galaxies and groups. There has been recent evidence for clustering among Lyα emitters, Lyman-break galaxies, and Lyman absorbers. We present deep, wide-field medium-band imaging in redshifted Lyα of fields surrounding regions selected to have HST detections of faint Lyα emitters, over a range of surface densities, to characterize the large-scale environment. The radio galaxy 53W002 was previously found, using HST, to be part of a rich grouping at z = 2.39, including ≈5 spectroscopically confirmed, compact, lower luminosity star-forming objects. Our new data show this to be part of a larger structure traced by bright active nuclei, all contained within a projected span of 6′.8 (3.2 Mpc). Of the 14 candidate emitters, six have been spectroscopically confirmed as active nuclei in the range z = 2.390 ± 0.008. Various statistical tests give a significance of 95%-99% for the reality of this structure on the sky. Our data thus strengthen the evidence for clustering at these redshifts. The grouping around 53W002 is more extended than a relaxed King model, at the 90% confidence level. This may be evidence either for a configuration that has yet to decouple fully from the Hubble expansion or for multiple sub-groupings that will themselves at some point form a more compact, relaxed structure. The redshift range for measured members is comparable to the Hubble flow across the structure, which may imply that the structure is seen near turnaround and suggests that its mass cannot be derived from the velocity dispersion. We surveyed two additional 14′ fields at z = 2.4, each centered on an HST WFPC2 field that has been searched for faint Lyα emitters, as well as on three contiguous fields near 53W002 for objects at z ∼ 2.55. Only a single emitter consistent with showing Lyα at z ≈ 2.4 appears in these fields, to a somewhat brighter flux limit, while a total of six candidate emitters appear in the three fields at z ∼ 2.6. Comparison with the (deeper but narrower) WFPC2 surveys suggests that the surface-density contrast from field to field is larger for brighter objects. From this survey alone, groupings such as the 53W002 "cluster" must have an area-covering fraction less than 0.04 in this redshift range. Three of the active galactic nuclei in the structure at z = 2.4 show extended emission-line structure, with equivalent widths suggesting in situ photoionization as far as 50 kpc from the nuclei. Such objects may popufslate deep surveys as diffuse Lyα-emission clouds when their cores are sufficiently obscured.