This paper reports the discovery of the faintest-yet Lyman-alpha emitter during the Epoch of Reionisation. It is found at redshift 7.64, just 680 million years after the Big Bang.
It has been a bit of a puzzle that the search for Lyman-alpha emitters (i.e., groups of stars) has been quite succesful for bright objects, but very few faint objects have found, even though our models predict that they exist and our instruments are sensitive enough. A theory developed to explain this dubbed ‘inside-out Reionisation’: basically, the ‘ionised bubbles’ around the smallest, faintest objects have smaller diameters. When the light from the galaxies hits the ‘edge’ of their local ionised bubble, it has redshifted due to cosmic expansion, but not enough (it’s still ‘resonantly absorbed’ by the neutral hydrogen). This would mean there is a critical ‘ionising power’ of object below which we can’t find Lyman-alpha emitters.
Fortunately, just finding 1 of those ultra-faint objects doesn’t contradict the theory. This is because the area over which this galaxy was detected is extremely small, smaller than the expected typical size of an ionised bubble. There could easily be a large, more powerful galaxy nearby which has done the heavy lifting of carving a bug enough bubble for this ultra-faint blob to be visible.
There are also signs that the radiation from this galaxy blob is very high energy, as demonstrated by excess light in the red optical which could be due to OIII. Like the previous paper pointed out in a local setting, galaxy blobs can have strong ionising field (Ly-C) without displaying very strong Lyman-alpha.
This system was detected via lensing by a large foreground cluster of galaxies. These authors’ previous paper laid out the candidate selection and narrowed it down to 9, of which 2 were observed, and 1 was not a faint primordial galaxy.
Link to article: arxiv