Deep HST Imaging of a Galaxy Cluster at Z=2.40

Abstract

We present a 67-orbit HST/WFPC2 exposure on the weak radio galaxy 53W002 at z=2.390 and its surrounding cluster. Color Plate 1 shows 12 orbits in I _{F 814} W & V _{F 606} W, and 24 in B _F450W. Potential cluster members were identified through 15 orbits in F410M, optimized for narrowband searches for compact Lyα objects at z≃2.4 (P96), and confirmed through spectroscopy (W91, P96); 16 candidates were found with significant narrow-band emission in F410M: 4 out of 5 had a confirming MMT spectroscopic redshift at z≃.40 (P96). All are located within 60″ from 53W002, or ~ 0.24h ^-1 ₁₀₀ Mpc (q _o=0.5) at z ≃2.4, the physical scale of a group or small galaxy cluster. One object contains a weak (variable) AGN, another is a merger with two companions. Their underlying young stellar population is very compact, with r _h.l.≃0.2″ (≃ 0.8h ^-1 ₁₀₀kpc), and considerably fainter than the L ^*-value at z~2.4, implying sub-galactic sized objects. These results may explain why ground-based Lyα searches for PG’s have been largely unsuccessful. The narrow-line galaxy 53W002 was imaged in the PC at ~0.07″ FWHM (WK95, see also W94). Its AGN component is ≤ 20±4% of the total continuum, surrounded by an extended r ^¼-envelope with r _h.l. ~ 1.1″ (4.3 kpc), and has an SED of ~0.3 Gyr in the center to ~0.5–1.0 Gyr at ~4 kpc. A one-sided cloud is seen kpc West, ~0.3 mag bluer than the SED, aligned with the radio source and its Ly-α cloud, presumably weak scattered AGN light, and/or jet-induced star-formation.