Institute of Astronomy

An emission line study of a unique, strongly lensed galaxy at z ~ 5: connecting Lyman-α and MgII λ 2796 Å emission at high redshift

SpeakerTalk DateTalk Series
Joris Witstok21 October 2020Institute of Astronomy Seminars


We present a detailed analysis of a highly magnified galaxy at z = 4.88, behind the strong lensing cluster RCS 0224–0002. Only 2 sources at z > 5 are known with this lensing magnification, and this particular source has been shown to exhibit widespread, high equivalent width CIV λ 1549 Å emission, suggesting it is a unique example of a metal-poor galaxy, with a hard radiation field and high χ_ion, likely representing the galaxy population that is responsible for Cosmic Reionization. X-shooter observations rule out the presence of an AGN in this source from the UV nebular line ratio diagnostics, suggesting a stellar origin of the hard radiation field in this source. Furthermore, we present a new detection of [NeIII] λ 3870 Å and use the [NeIII]/[OII] line ratio (the highest redshift measurement of this rest-frame optical line diagnostic to date) to put this galaxy in context of the z ~ 0-2 galaxy population and put constraints on the ionisation parameter and gas-phase metallicity. Closely related to the widely used [OIII]/[OII] ratio, our [NeIII]/[OII] measurement suggests this source is similar to extreme emission line galaxies such as "Green Pea" galaxies in the local Universe. Finally, we present the highest redshift detection to date of the MgII line at 2796 Å, observed at high equivalent width in emission, in contrast to more evolved systems where MgII is mostly seen in absorption. Strong MgII emission has been observed in most of the z ~ 0 Lyman-continuum leakers known and has recently been proposed as an indirect tracer of escaping ionising radiation in the Epoch of Reionization, where the widely used Lyman-α line is longer be observable due to the neutral IGM. Our detection of bright MgII at z ~ 5 – just 300 Myr after Reionization ends – demonstrates the feasibility of observing this line in the near future with JWST and ELTs at z > 6. In conclusion, this galaxy illustrates how gravitational lensing can present us invaluable insights regarding our understanding of the early Universe and offer a glimpse into the discovery space of next-generation observatories like JWST.


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