The term "dark galaxy" may be used to refer two different types of object:
- A dark matter galaxy composed almost exclusively
of dark matter and in which virtually no
star formation has taken place.
Several candidate dark matter galaxies have been identified.
- An object in the early universe composed largely of hydrogen gas that
hasn't yet consolidated into stars. In 2012, astronomers published evidence
gathered using the Very Large Telescope for this type of dark galaxy
in the vicinity of a quasar.
Dark matter galaxies
It's thought that dark matter galaxies should be very common, particularly
since theories of large-scale
structure work much better if the existence of these dark matter galaxies
is assumed. Also, surveys have revealed fewer bright galaxies in the universe
than expected, suggesting that many galaxies, for reasons yet to be understood,
fail to trigger star birth.
|Region of space occupied by suspected dark matter
In 2005 astronomers in the United Kingdom announced the discovery of what
may a dark matter galaxy, VIRGOHI21, lying only about 50 million light-years
away in the Virgo Cluster.
Radio observations of the rotation of hydrogen gas in VIRGOHI21 revealed
there must be at least 100 times as much dark matter as hydrogen in the
galaxy and that its total mass is about one-tenth that of the Milky Way.
According to a counter theory, VIRGOHI21 may simply be the result of a close
encounter between two other galaxies – NGC 4254, which lies nearby,
and NGC 4262, which ago hurtled past NGC 4254 at 900 km/s millions of years.
A prominent arm of stars stretching out from NGC 4254 and bending toward
VIRGOHI21 lends some support to this idea. However, if VIRGOHI21 did originate
in material pulled out of NGC 4254 then it ought to contain stars in addition
to its several hundred million solar masses of hydrogen. In 2007, observations
of a patch of sky 50,000 by 50,000 light-years across, centered on the hydrogen
cloud's position, found only 119 red giant stars, which is only the number
expected in a typical region of the same size in intergalactic space. One
possibility is that NGC 4254's long stellar arm has been drawn out by the
gravity of VIRGOHI21 itself.
|Neutral hydrogen gas streams extends from NGC 4254
(top left) to the dark galaxy VIRGOH121 (center right) in this radio
image. NGC 4262 is at bottom left. Image credit: R. Minchin/Arecibo
Observatory/Cardiff U/Isaac Newton Telescope/WSRT)
If VIRGOHI21 really is a dark galaxy it poses astronomers with a challenge.
Its ratio of dark matter to normal matter appears to be at least 10 times
greater than the ratio found in other galaxies.
dwarf dark matter galaxy
Ten billion light-years away – very much further than VIRGOHI21 –
lies a dwarf galaxy whose presence is only known because of a discrepancy
in a gravitational lens effect caused by
a large, distant elliptical galaxy. The lens created by the big galaxy is
slightly different than what computer modeling predicts, leading to the
conclusion that an invisible companion – a dark galaxy, with a mass
of about 200 million solar masses – lies at the periphery of the larger
|The light bent by a dark matter galaxy can form what
is known as an "Einstein ring"
In the present-day cosmos, dwarf galaxies are often found at the periphery
of larger galaxies. The Milky Way has a couple of dozen such satellites.
But there seem to be many fewer dwarf galaxies around than theories of galaxy
formation predict. One way to explain this is that exist but can't be seen
because they're made of dark matter.
The other type of object called a dark galaxy is a cloud of material, made
mostly of hydrogen gas, in a pre-stellar stellar state in the early universe.
Such dark galaxies are thought to have been very common in the first billion
years or so after the Big Bang. The first direct observational evidence
for their existence came in 2012, when astronomers published data collected
using the Very Large Telescope. It revealed the presence of
a dozen objects fluorescing because of the light falling on them from a
nearby quasar. The astronomers who did this
research estimated that the mass of the gas in each object is about a billion
times that of the Sun, which, theory suggests, would be typical for a gas-rich,
low-mass galaxy in the early universe.
|A region of sky around the quasar HE0109-3518, labeled
with a red circle. The quasars high-energy radiation makes 12 nearby
dark galaxies (blue circles) glow. Credit: ESO, Digitized Sky Survey
2 and S. Cantalupo (UCSC)
- Minchin, R., Davies, J., Disney, M., et al. "A dark hydrogen cloud
in the Virgo Cluster". Astrophysical Journal, 622, (1): L21–L24
- Cantalupo1, S., Lilly, S. J., and Haehnelt, M. G. "Detection of dark
galaxies and circum-galactic filaments fluorescently illuminated by
a quasar at z=2.4." Mon. Not. R. Astron. Soc., accepted June