The stellar population of a z ∼ 3.25 Lyα-emitting group associated with a damped Lyα absorber

Abstract

We present near-infrared observations, acquired with the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST), of a Lyα-emitting double-clumped nebula at z ≈ 3.25 associated with a damped Lyα absorber (DLA). With the WFC3/F160W data we observe the stellar continuum around 3600 Å in the rest frame of a galaxy embedded in the west clump of the nebula, G_W, for which we estimate a star formation rate (SFR) of SFR_GW = 5.0 ± 0.4 M_⊙ yr^‑1 and a maximum stellar mass M_GW < 9.9 ± 0.7 × 10⁹ M_⊙. With the enhanced spatial resolution of HST, we discover the presence of an additional faint source, G_E, in the center of the east clump, with SFR_GE = 0.70 ± 0.20 M_⊙ yr^‑1 and a maximum stellar mass M_GE < 1.4 ± 0.4 × 10⁹ M_⊙. We show that the Lyα emission in the two clumps can be explained by recombination following in situ photoionization by the two galaxies, assuming escape fractions of ionizing photons of ≲0.24 for G_W and ≲0.34 for G_E. The fact that G_W is offset by ≈8 kpc from the west clump means we cannot fully rule out the presence of additional fainter star-forming sources, which would further contribute to the photon budget inside this ≈10¹² M_⊙ galaxy group that extends over a region of 30 × 50 kpc.