A double-peaked Lyman- αemitter 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 αemission is clearly detected in the two counter images in the MUSE field of view. We measure a relatively high Ly αrest-frame equivalent width (EW) of EWLyα, 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 Åemission lines, which show moderate rest-frame EWs of the order of a few ~10-100 Å, an [O III ] λ5007 Å/H βratio 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 βFUV= -2.23 ±0.06 and βNUV = -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∗≃ 107.5M⊙) high-redshift galaxy analogue. Notably, the blue peak of the Ly αprofile 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 αprofile. 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.