Deciphering the Lyman α blob 1 with deep MUSE observations

Context. Lyman alpha blobs (LABs) are large-scale radio-quiet Lyman alpha (Ly alpha) nebula at high-z that occur predominantly in overdense proto-cluster regions. In particular, there is the prototypical SSA22a-LAB1 at z=3.1, which has become an observational reference for LABs across the electromagnetic spectrum.Aims. We want to understand the powering mechanisms that drive the LAB so that we may gain empirical insights into the galaxy-formation processes within a rare dense environment at high-z. Thus, we need to infer the distribution, the dynamics, and the ionisation state of LAB 1's Ly alpha emitting gas.Methods. LAB 1 was observed for 17.2 h with the VLT/MUSE integral-field spectrograph. We produced optimally extracted narrow band images, in Ly alpha lambda 1216, HeII lambda 1640, and we tried to detect CIV lambda 1549 emission. By utilising a moment-based analysis, we mapped the kinematics and the line profile characteristics of the blob. We also linked the inferences from the line profile analysis to previous results from imaging polarimetry.Results. We map Ly alpha emission from the blob down to surface-brightness limits of approximate to 6x10(-19) erg s(-1) cm(-2) arcsec(-2). At this depth, we reveal a bridge between LAB 1 and its northern neighbour LAB 8, as well as a shell-like filament towards the south of LAB 1. The complexity and morphology of the Ly alpha profile vary strongly throughout the blob. Despite the complexity, we find a coherent large-scale east-west velocity gradient of similar to 1000 km s(-1) that is aligned perpendicular to the major axis of the blob. Moreover, we observe a negative correlation of Ly alpha polarisation fraction with Ly alpha line width and a positive correlation with absolute line-of-sight velocity. Finally, we reveal HeII emission in three distinct regions within the blob, however, we can only provide upper limits for CIV.Conclusions. Various gas excitation mechanisms are at play in LAB 1: ionising radiation and feedback effects dominate near the embedded galaxies, while Ly alpha scattering contributes at larger distances. However, HeII/Ly alpha ratios combined with upper limits on CIV/Ly alpha are not able to discriminate between active galactic nucleus ionisation and feedback- driven shocks. The alignment of the angular momentum vector parallel to the morphological principal axis appears to be at odds with the predicted norm for high-mass halos, but this most likely reflects that LAB 1 resides at a node of multiple intersecting filaments of the cosmic web. LAB 1 can thus be thought of as a progenitor of a present-day massive elliptical within a galaxy cluster.