  Lyman-alpha emitters are thought to be young, low-mass galaxies with ages of
~10^8 yr. An overdensity of them in one region of the sky (the SSA 22 field)
traces out a filamentary structure in the early Universe at a redshift of z =
3.1 (equivalent to 15 per cent of the age of the Universe) and is believed to
mark a forming protocluster. Galaxies that are bright at (sub)millimetre
wavelengths are undergoing violent episodes of star formation, and there is
evidence that they are preferentially associated with high-redshift radio
galaxies, so the question of whether they are also associated with the most
significant large-scale structure growing at high redshift (as outlined by
Lyman-alpha emitters) naturally arises. Here we report an imaging survey of
1,100-um emission in the SSA 22 region. We find an enhancement of submillimetre
galaxies near the core of the protocluster, and a large-scale correlation
between the submillimetre galaxies and the low-mass Lyman-alpha emitters,
suggesting synchronous formation of the two very different types of
star-forming galaxy within the same structure at high redshift. These results
are in general agreement with our understanding of the formation of cosmic
structure.
