The stellar population of a $z\sim 3.25$ Ly$\alpha$ emitting group associated with a damped Ly$\alpha$ absorber

Abstract

We present near-infrared observations, acquired with the Wide Field Camera 3 (WFC3) on board of the Hubble Space Telescope (HST), of a Ly$\alpha$ double-clumped emitting nebula at $z\approx 3.25$ associated with a damped Ly$\alpha$ absorber (DLA). With the WFC3/F160W data we observe the stellar continuum around $3600$ $\mathring{\mathrm{A}}$ in the rest frame for a galaxy embedded in the West clump of the nebula, $G_{\mathrm{W}}$, for which we estimate a star formation rate SFR${}_{G_{\mathrm{W}}}=5.0\pm 0.4$ M${}_{\odot }$ yr${}^{-1}$ and maximum stellar mass M${}_{G_{\mathrm{W}}}<9.9\pm 0.7\times 10^9$ M${}_{\odot }$. With the enhanced spatial resolution of HST, we discover the presence of an additional faint source, $G_{\mathrm{E}}$, in the center of the East clump, with a star formation rate of SFR${}_{G_{\mathrm{E}}}=0.70\pm 0.20$ M${}_{\odot }$ yr${}^{-1}$ and maximum stellar mass M${}_{G_{\mathrm{E}}}<1.4\pm 0.4\times 10^9$ M${}_{\odot }$. We show that the Ly$\alpha$ 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 $\lesssim 0.24$ for $G_{\mathrm{W}}$ and $\lesssim 0.34$ for $G_{\mathrm{E}}$. The fact that $G_{\mathrm{W}}$ is offset by $\approx 8$ kpc from the West clump does not fully rule out the presence of additional fainter star-forming sources that would further contribute to the photon budget inside this $\approx 10^{12}$ M${}_{\odot }$ galaxy group that extends over a region encompassing over $30\times 50$ kpc.

9 pages, 7 figures, 1 table. Re-submitted to A&A

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