Free Astronomy Magazine November-December 2016

star formation rates. The team then turned to a sophisticated simulation of galaxy formation to demonstrate that the giant glowing cloud of Ly- man-alpha emission can be explain- ed if ultraviolet light produced by star formation in the ALMA sources scatters off the surrounding hydro- gen gas. This would give rise to the Lyman-alpha Blob we see. Jim Geach, lead author of the new study, explains: “Think of a street- light on a foggy night — you see the diffuse glow because light is scatter- ing off the tiny water droplets. A similar thing is happening here, except the streetlight is an intensely star-forming galaxy and the fog is a huge cloud of intergalactic gas. The galaxies are illuminating their sur- roundings.” Understanding how galaxies form and evolve is a massive challenge. As- tronomers think Lyman-alpha Blobs are important because they seem to be the places where the most mas- sive galaxies in the Universe form. In particular, the extended Lyman- alpha glow provides information on what is happening in the primordial gas clouds surrounding young gal- axies, a region that is very difficult to study, but critical to understand. Jim Geach concludes, “What’s excit- ing about these blobs is that we are getting a rare glimpse of what’s hap- pening around these young, grow- ing galaxies. For a long time, the origin of the ex- tended Lyman-alpha light has been controversial. But with the combina- tion of new observations and cut- ting-edge simulations, we think we have solved a 15-year-old mystery: Lyman-alpha Blob-1 is the site of for- mation of a massive elliptical galaxy that will one day be the heart of a giant cluster. We are seeing a snap- shot of the assembly of that galaxy 11.5 billion years ago.” ral bright sources at a distance would seem like a single glowing spot, and only at closer quarters would each source be distinguishable. ALMA’s high resolution has resolved what previously appeared to be a single blob into three separate sources.) Astronomers then combined the AL- MA images with observations from the Multi Unit Spectroscopic Explorer (MUSE) instrument mounted on the VLT, which map the Lyman-alpha light. This showed that the ALMA sources are located in the very heart of the Lyman-alpha Blob, where they are forming stars at a rate over 100 times that of the Milky Way. Deep imaging with the NASA/ESA Hubble Space Telescope and spectro- scopy at the W. M. Keck Observatory showed in addition that the ALMA sources are surrounded by numerous faint companion galaxies that could be bombarding the central ALMA sources, helping to drive their high T his diagram explains how a Lyman-alpha Blob, one of the largest and brightest objects in the Universe, shines. [ESO/J. Geach] n