A double-peaked Lyman-α emitter with a stronger blue peak multiply imaged by the galaxy cluster RXC J0018.5+1626

Abstract

We report the discovery of a double-peaked Lyman-α (Ly α) emitter (LAE) at z = 3.2177 ± 0.0001 in VLT/MUSE data. The galaxy is strongly lensed by the galaxy cluster RXC J0018.5+1626 recently observed in the RELICS survey, and the double-peaked Ly α emission is clearly detected in the two counter images in the MUSE field of view. We measure a relatively high Ly α rest-frame equivalent width (EW) of EW_{Ly α, 0} = (63 ± 2) Å. Additional spectroscopy with Gemini/GNIRS in the near-infrared (NIR) allows us to measure the H β, [O III] λ4959 Å, and [O III] λ5007 Å emission lines, which show moderate rest-frame EWs of the order of a few ~10-100 Å, an [O III] λ5007 Å/H β ratio of 4.8 ± 0.7, and a lower limit on the [O III]/[O II] ratio of >9.3. The galaxy has very blue UV-continuum slopes of β_FUV = -2.23 ± 0.06 and β_NUV = -3.0 ± 0.2, and is magnified by factors μ ~ 7-10 in each of the two images, thus enabling a view into a low-mass ($M_{\star }\simeq 10^{7.5}\, \mathrm{M}_{\odot }$) high-redshift galaxy analogue. Notably, the blue peak of the Ly α profile is significantly stronger than the red peak, which suggests an inflow of matter and possibly very low H I column densities in its circumgalactic gas. To the best of our knowledge, this is the first detection of such a Ly α profile. Combined with the high lensing magnification and image multiplicity, these properties make this galaxy a prime candidate for follow-up observations to search for LyC emission and constrain the LyC photon escape fraction.