Science and techno world topic: Space
Photo: This deep image shows the region of the sky around the
quasar HE0109-3518. The quasar is labelled with a red circle near the centre of
the image. The energetic radiation of the quasar makes dark galaxies glow,
helping astronomers to understand the obscure early stages of galaxy formation.
The faint images of the glow from 12 dark galaxies are labelled with blue
circles. Dark galaxies are essentially devoid of stars, therefore they don’t
emit any light that telescopes can catch. This makes them virtually impossible
to observe unless they are illuminated by an external light source like a
background quasar. This image combines observations from the Very Large
Telescope, tuned to detect the fluorescent emissions produced by the quasar
illuminating the dark galaxies, with colour data from the Digitized Sky Survey
2. Credit: ESO, Digitized Sky Survey 2 and S. Cantalupo (UCSC)
Dark galaxies are small, gas-rich galaxies in the early
Universe that are very inefficient at forming stars. They are predicted by
theories of galaxy formation and are thought to be the building blocks of
today's bright, star-filled galaxies. Astronomers think that they may have fed
large galaxies with much of the gas that later formed into the stars that exist
today.
Because they are essentially devoid of stars, these dark
galaxies don't emit much light, making them very hard to detect. For years
astronomers have been trying to develop new techniques that could confirm the
existence of these galaxies. Small absorption dips in the spectra of background
sources of light have hinted at their existence. However, this new study marks
the first time that such objects have been seen directly.
"Our approach to the problem of detecting a dark galaxy
was simply to shine a bright light on it." explains Simon Lilly (ETH
Zurich, Switzerland), co-author of the paper. "We searched for the
fluorescent glow of the gas in dark galaxies when they are illuminated by the
ultraviolet light from a nearby and very bright quasar. The light from the
quasar makes the dark galaxies light up in a process similar to how white
clothes are illuminated by ultraviolet lamps in a night club."
The team took advantage of the large collecting area and
sensitivity of the Very Large Telescope (VLT), and a series of very long
exposures, to detect the extremely faint fluorescent glow of the dark galaxies.
They used the FORS2 instrument to map a region of the sky around the bright
quasar HE 0109-3518, looking for the ultraviolet light that is emitted by
hydrogen gas when it is subjected to intense radiation. Because of the
expansion of the Universe, this light is actually observed as a shade of violet
by the time it reaches the VLT.
"After several years of attempts to detect fluorescent
emission from dark galaxies, our results demonstrate the potential of our
method to discover and study these fascinating and previously invisible
objects," says Sebastiano Cantalupo (University of California, Santa
Cruz), lead author of the study.
Source: phys.org
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