When I was growing up, ‘blob’ was a word associated with a classic horror movie starring none other than Steve McQueen. Today, blobs are starting to show up in astronomical discussions. Exactly what they are is unknown, but they seem to be as large as galaxies and marked by low luminosity. The latest, an apparently energetic but not very bright object some 11.6 billion light years away, is fully twice the size of our Milky Way and emits the energy of some two billion suns. It is, nonetheless, invisible in images from telescopes looking all the way from the infrared to the x-ray wavebands.
How do you find invisible blobs? Astronomers working with the European Southern Observatory’s Very Large Telescope used a narrow-band filter with the FORS1 spectrograph that allowed them to observe emissions from hydrogen atoms. Applying energy to hydrogen atoms causes their electrons to make a quantum leap to a higher energy level. Upon return to their initial state, the electrons release excess energy in the form of photons, with the various possible transitions resulting in characteristic spectral lines. All of which makes the blob identifiable while leaving its enigmatic nature intact. Says ESO astronomer Kim Nilsson:
“We have tried to explain this blob using the most common explanations, such as the illumination by a galaxy with an active nucleus or a galaxy that produce stars at a frantic rate, but none of them apply. Instead, we are led to the conclusion that the observed hydrogen emission comes from primordial gas falling onto a clump of dark matter. We could thus be literally seeing the building up of a massive galaxy, like our own, the Milky Way.”
Nilsson and team’s work is available as “A Lyman-alpha blob in the GOODS South field: evidence for cold accretion onto a dark matter halo,” a letter to Astronomy and Astrophysics available here (PDF warning). It’s still more evidence of how little we know about one of astronomy’s central mysteries, the nature of dark matter and its role in shaping what we see in the cosmos.