This paper contributes to solving a long-standing mystery related to the origin of Lyman-alpha, Lyman-continuum, and high O III radiation in small size, young objects.
Small-size, young objects were all that existed at the time of Reionisation, so they must be responsible for the process in some way. The photons capable of ionising hydrogen come from 2 sources: Lyman-continuum radiation, or from individual strong X-ray sources (accreting black holes or extreme star binaries). Currently, the total from both these sources appears to be insufficient.
Lyman-continuum is produced whenever stars are present (especially, it seems, young stars) but it struggles to escape from the galaxy. Models of Reionisation only require ~20% of the radiation which we know is being produced to escape (the ‘escape fraction’), but observations of local galaxies in ideal conditions have consistently reported much, much smaller values (Ly-C is extremely rarely seen). Another type of radiation, Lyman-alpha, is (believed to be) produced in nearly identical contexts as Lyman-C, and is extremely common. This has led to the hypothesis that the escape of Lyman-C is not as much rare as extremely inhomogeneous: all of it escapes through a ‘gap’ from the galaxy. Our lack of detection of it is because the gap is tiny and facing away from Earth, but we know Lyman-C must be there because there is so much Lyman-alpha (and Lyman-alpha emission is not nearly as inhomogeneous). Problem: early galaxies don’t even show very much Lyman-alpha.
Various other mechanisms have been proposed. For instance, AGN radiation could be much more efficient at escaping galaxies; unfortunately AGN are extremely rare in the early universe (or maybe they don’t look like what we expect, and we haven’t found them yet). Or, AGN beams could carve the holes through which Ly-C then escapes.
This paper provides a ‘field test’ from some of these radiation processes in a local, well-understood situation. It looks at system Haro 11, the most-easily studied Ly-C. Haro 11 formed very recently (<50 My), and is full of young, low-metalicity (-ish) giant stars. It is also divided quite neatly into 3 separate blobs, all with different properties:
- Blob C is the oldest (the WR stars have started dissapearing) and is a strong Lyman-alpha emitter, AND shows signs of containing an AGN.
- Blob B shows signs of containing a strong AGN, emits X-rays, is young and shows only weak Lyman-alpha.
- Blob A is young but shows (very) weak Lyman-alpha.
Question: which of these 3 blobs is Lyman-C actually ‘leaking’ from?
In a surpise twist, it turns out that Blob A is the most likely source by far. Although it is currently impossible to detect Ly-C with a sufficiently good spatial accuracy, the authors are able to measure which of the blobs are most ‘transparent to us’ using ratios of OIII and OII emission.
If this is true, it flies in the face of most of the theories mentionned above. It has often been assumed that Ly-C sources are a special type of Lyman-alpha sources, but here, it is the weakest Lyman-alpha emitter which is most transparent. And the X-ray sources present in Blobs B and C have not ‘carved escape channels’ as expected.
Link to article: arxiv