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We present integral field spectroscopy of a Lyman-alpha blob at redshift2.38, with a spectral resolution three times better than previous publishedwork. As with previous observations, the blob has a chaotic velocity structure,much of which breaks up into multiple components. Our spectroscopy shows,however, that some of these multiple components are extremely narrow: they havevelocity widths of less than 100 km/s.  Combining these new data with previous observations, we argue that thisLyman-alpha blob resides in a dark-matter halo of around 10^13 solar masses. Atthe centre of this halo are two compact red massive galaxies. They aresurrounded by hot gas, probably a super-wind from merger-induced nuclearstarbursts. This hot gas has shut down star formation in the non-nuclear regionof these galaxies, leading to their red-and-dead colours.  A filament or lump of infalling cold gas is colliding with the hot gas phaseand being shocked to high temperatures, while still around 30kpc from the redgalaxies. The shock region is self-absorbed in Lyman-alpha but produces C IVemission.  Further out still, the cold gas in a number of sub-halos is being lit up,most likely by a combination of tidally triggered star formation, bow-shocks asthey plough through the hot halo medium, resonant scattering of Lyman-alphafrom the filament collision, and tidal stripping of gas which enhances theLyman-alpha escape fraction. The observed Lyman-alpha emission from the Blob isdominated by the sum of the emission from these sub-halos.  On statistical grounds, we argue that Lyman-alpha blobs are not greatlyelongated in shape, and that most are not powered by ionisation or scatteringfrom a central active galactic nucleus or starburst.

Hot Gas, Cold Gas and Sub-Halos in a Lyman-alpha Blob at Redshift 2.38