Discovery of a faint, star-forming, multiply lensed, Lyman-α blob

¹ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, via Saragat 1, 44122 Ferrara, Italy
e-mail: gbcaminha@fe.infn.it
² Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
e-mail: karman@astro.rug.nl
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
⁴ University Observatory Munich, Scheinerstrasse 1, 81679 Munich, Germany
⁵ INAF−Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
⁶ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁷ INAF−Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
⁸ INAF−Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 17 December 2015
Accepted: 30 August 2016

Abstract

We report the discovery of a multiply lensed Lyman-α blob (LAB) behind the galaxy cluster AS1063 using the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT). The background source is at z = 3.117 and is intrinsically faint compared to almost all previously reported LABs. We used our highly precise strong lensing model to reconstruct the source properties, and we find an intrinsic luminosity of L_Lyα = 1.9 × 10⁴² erg s^-1, extending to 33 kpc. We find that the LAB is associated with a group of galaxies, and possibly a protocluster, in agreement with previous studies that find LABs in overdensities. In addition to Lyman-α (Lyα) emission, we find C iv, He ii, and O iii] ultraviolet (UV) emission lines arising from the centre of the nebula. We used the compactness of these lines in combination with the line ratios to conclude that the Lyα nebula is likely powered by embedded star formation. Resonant scattering of the Lyα photons then produces the extended shape of the emission. Thanks to the combined power of MUSE and strong gravitational lensing, we are now able to probe the circumgalatic medium of sub-L_∗ galaxies at z ≈ 3.