ALMA observations of a z ≈ 3.1 protocluster: star formation from active galactic nuclei and Lyman-alpha blobs in an overdense environment

We exploit Atacama Large Interferometer Array (ALMA) 870 mu m observations to measure the star formation rates (SFRs) of eight X-ray detected active galactic nuclei (AGNs) in a z approximate to 3.1 protocluster, four of which reside in extended Ly alpha haloes (often termed Lymanalpha blobs: LABs). Three of the AGNs are detected by ALMA and have implied SFRs of approximate to 220-410 M-circle dot yr(-1); the non-detection of the other five AGNs places SFR upper limits of approximate to 210 M-circle dot yr(-1). The mean SFR of the protocluster AGNs (approximate to 110-210 M-circle dot yr(-1)) is consistent (within a factor of approximate to 0.7-2.3) with that found for co-eval AGNs in the field, implying that the galaxy growth is not significantly accelerated in these systems. However, when also considering ALMA data from the literature, we find evidence for elevated mean SFRs (up-to a factor of approximate to 5.9 over the field) for AGNs at the protocluster core, indicating that galaxy growth is significantly accelerated in the central regions of the protocluster. We also show that all of the four protocluster LABs are associated with an ALMA counterpart within the extent of their Ly alpha emission. The SFRs of the ALMA sources within the LABs (approximate to 150-410 M-circle dot yr(-1)) are consistent with those expected for co-eval massive star-forming galaxies in the field. Furthermore, the two giant LABs (with physical extents of greater than or similar to 100 kpc) do not host more luminous star formation than the smaller LABs, despite being an order of magnitude brighter in Ly alpha emission. We use these results to discuss star formation as the power source of LABs.