EMIR confirmation of the high-z Lyα blob SHARDS20018464

A spectroscopic follow-up of the Ly$\alpha$ blob SHARDS20018464 was performed using the infrared spectrograph EMIR (Espectrógrafo Multiobjeto Infra-Rojo) at the 10.4 m GTC telescope, equipped with its newly upgraded H2RG focal plane array detector. We observed the blob in the Ks band, which corresponds, at the photometric redshift of $z=3.49$ presented in the literature, to the rest-frame visible part of the spectrum. Thanks to the sensitivity of EMIR, we were able to identify the [OIII] emission doublet at the position of the blob and the companion galaxy. The flux of the detected lines is of the order of ${10}^{-17}$ erg s${}^{-1}$ cm${}^{-2}$, obtained with 4-hour exposure. The mean redshift derived by analysing the two lines individually is $z=3.59\pm 0.01$, slightly higher than the photometric one, although in agreement within the uncertainties. We also estimated the flux of the two lines and their ratio, which was found to be $R=2.92$, very close to the theoretical prediction of 2.98. This is a strong analytical confirmation that the observed emissions correspond to the [OIII] doublet. Within uncertainties, the redshift of the blob confirms the photometric one reported in the literature, as well as the physical properties inferred for it. Its remarkably small size (5-8 kpc vs. $\sim 100$ kpc for typical samples) and its position aside the companion galaxy also confirm the previously-proposed idea that this object could be the first of a new class of compact Ly$\alpha$ emitters. Moreover, the data support the scenario of photo-ionisation due to a cluster of young and massive stars.