According to our best models, the early Universe was filled with clouds of gas that were the source of its current galaxies. However, not all of these protogalactic nebulas were massive enough to compress their gas sufficiently to make stars. This means a number of "dark galaxies" should exist: galaxy-sized clouds of gas with few or no stars. This lack of stars makes dark galaxies extremely hard to find, but a group of astronomers using the Very Large Telescope (VLT) in Chile have identified 12 candidates from the early days of the Universe. These results have the potential to fill in some of the observational gaps in our understanding of early galaxy formation and evolution.
The trick for spotting these dark galaxies is described in Monthly Notices of the Royal Astronomical Society by Sebastiano Cantalupo, Simon J. Lilly, and Martin G. Haehnelt, who located a quasar, a supermassive black hole pumping out large quantities of ultraviolet (UV) light. Just as UV emission from mercury drives fluorescent lights, the UV light from the quasar caused gas in the dark galaxies to fluoresce, making them faintly visible.
The researchers estimated the candidate clouds' masses would be comparable to dwarf galaxies today, much smaller than star-forming galaxies at the same distance from Earth. They also identified filaments of gas, which might be the signature of material falling in to the dark galaxies.
In the theory of large-scale structure (LSS), early on in the history of the Universe, dark matter collected in clumps known as halos due to gravitational attraction. (Dark matter is the invisible mass that neither absorbs nor emits light; it comprises about 80 percent of the mass in the Universe.) These halos in turn attracted gas; if the halo was of sufficiently high mass, the gas could form a dense rotating disk, where the star formation magic happened. Astronomers have identified many of these early bright galaxies.
But if the halos were not massive enough, the gas wouldn't compress, and star formation would be very slow or nonexistent. These non-star-forming gas clouds are the dark galaxies, and they're predicted by theory to exist in large numbers, since it is easier to make low-mass halos than high-mass halos. However, this gas acted as a reservoir for future star formation, since galaxies grew by mergers between smaller galaxies—including the dark ones. According to the model, dark galaxies consisted mostly of neutral hydrogen, which is very difficult to observe at the cosmological distances at which galaxy formation occurred.
Since 1987, however, astronomers have recognized that strong UV emission could stimulate the neutral hydrogen gas in dark galaxies to fluoresce at 122 nanometers (1.22×10-7 meters). This wavelength is known as Lyman alpha (Lyα) emission. Supermassive black holes (SMBHs), which are found at the centers of most large galaxies, can emit a great deal of radiation at UV wavelengths. Active SMBHs that formed in the early Universe are known as quasars.
The researchers focused their observations on the region of space surrounding quasar HD0109-3518, at a distance appropriate for early galaxy formation. HD0109-3518 is one of the brightest quasars in the sky, which is why the researchers selected it: its radiation would cause fluorescence in the largest number of surrounding dark galaxies, if they existed.
They identified 98 objects with Lyα emission using the VLT's FOcal Reducer and Spectrograph (FORS). However, newborn stars also produce a lot of UV light, so the astronomers eliminated clouds showing significant signs of star formation. (The algorithm they used is drolly known as SExtractor, which any professional astronomers in the crowd will no doubt have used at some point in their lives.) That left them with a sample of 12 dark galaxy candidates that were near the quasar, but sufficiently far that they are not part of the quasar's host galaxy.
Three of the brighter dark galaxy candidates appeared large enough in the images that extended structures could be seen. These are long filamentary structures, which could be tails produced from gravitational tidal forces that pulled the gas clouds into strange shapes. However, the authors argued they are consistent with the infall of gas during the formation of the dark galaxy, another feature predicted by theory but never before seen. (These filaments should not be confused with the dark matter filaments from last week's story, since they are composed of hydrogen gas.)
Additionally, the authors found 13 objects near an even more distant quasar, but the extra distance made clear identification too hard for analysis at this point. Nevertheless, these results together offer hope for identifying more dark galaxies near other quasars. A sufficiently large number of dark galaxy candidates will help constrain models for early galaxy formation, as well as elucidate the structure of protogalaxies.
arstechnica.com
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