Hubble Space Telescope imaging of a radio-quiet galaxy at redshift z = 3.4

We have observed with the Wide Field/Planetary Camera (WF/PC) on the Hubble Space Telescope (HST) a radio-quiet Lya-emitting galaxy at redshift z = 3.428 (G2 below). The images probe the rest-frame UV light around 1250 A with an angular resolution of ≈0″.1, corresponding to 1.4 h50-1 kpc at redshift z = 3.4 (in this Letter we use q0 = O and H0 = 50 h50-1 km s-1 Mpc-1). The light profile of the central ∼ 10 h50-1 kpc region is well fitted by an r1/4 law with re ≈ 1,3 h50-1 kpc, suggesting a dynamically relaxed state. The outer regions are characterized by the presence of substructures, such as an elongated formation and low surface brightness nebulosities. The isophotal analysis shows no evidence of an AGN-like unresolved source in the center. The structural properties of G2 are consistent with a dynamically hot stellar system observed during an early phase of star formation, very likely the progenitor of an elliptical or the bulge of a spiral galaxy.