To See or Not to See a z ∼ 13 Galaxy? That is the Question. Targeting the [C ii ] 158 µm emission line of HD1 with ALMA

Context. “When did the ﬁrst galaxies form?” is still one of the greatest unanswered questions in astronomy. Theory and current stellar population models imply that the ﬁrst galaxies formed at least at z = 14 − 15. Yet, the highest redshift galaxy to have been securely conﬁrmed remains GN-z11, at z ∼ 11. Aims. The galaxy “HD1” was recently proposed to be a z ∼ 13 . 27 galaxy based on its potential Lyman break and tentative [O iii ]88 µ m detection with ALMA. We hereby aim to test this scenario with new ALMA Band 4, DDT observations of what would be the [C ii ]158 µ m emission, if HD1 is at z ∼ 13 . 27. Methods. We carefully analyse the new ALMA Band 4 observations as well as re-analysing the existing ALMA Band 6 data on the source to determine the proposed redshift. Results. We ﬁnd a tentative 4 σ feature in the Band 4 data that is spatially o ﬀ set by 1 (cid:48)(cid:48) . 7 and spectrally o ﬀ set by 190 km s − 1 from the previously-reported 3 . 8 σ “[O iii ]88 µ m” feature. Through various statistical tests, we demonstrate that these tentative features are fully consistent with being random noise features. The chances of ﬁnding a noise peak of the same signiﬁcance as the tentative [C ii ]158 µ m or [O iii ]88 µ m feature are ∼ 50% and 100%, respectively. Given the noise properties of the ALMA data, we recover at least a ∼ 50% chance of ﬁnding two, matched ≥ 3 . 8 σ noise peaks that are spatially and spectrally o ﬀ set by ≤ 10kpc and 1000km s − 1 . Conclusions. We conclude that we are more likely to be recovering noise features than both [O iii ]88 µ m and [C ii ]158 µ m emission from a source at z ∼ 13 . 27. Although we ﬁnd no evidence of a z ∼ 13 . 27 galaxy, we cannot entirely rule out this scenario. Non-detections are also possible for a z ∼ 13 source with a low interstellar gas-phase metallicity and density. Determining where and exactly what type of galaxy HD1 is, will now likely require JWST / NIRSpec spectroscopy.