An multinational research team directed by the Instituto de AstrofÃsica de
Canarias (IAC) in partnership with the University of La Laguna (ULL) and
other institutions identified Nube, a virtually undetectable dwarf
galaxy.
The term comes from the object's diffuse look and was proposed by the
5-year-old daughter of one of the group's researchers. The object seems
hazy, almost like a ghost, and its surface brightness is so low that it was
missed in the numerous prior scans of this region of the sky. This is
because "Nube" (Spanish for "Cloud") was nearly invisible due to its stars'
extreme dispersion across such a vast volume.
A number of unique characteristics make this recently found galaxy apart
from other known objects. According to the study team, Nube is a dwarf
galaxy that is both 10 times more extensive and 10 times fainter than other
objects of its sort that have a similar amount of stars.
For those with a basic understanding of astronomy, this galaxy is
equivalent in size to one-third of the Milky Way and has a mass comparable
to the Small Magellanic Cloud.
"We do not know how a galaxy with such extreme characteristics can exist
with our current knowledge," says Mireia Montes, the article's primary
author and a researcher at the ULL and IAC.
As part of the Legado del IAC Stripe 82 project, Ignacio Trujillo, the
article's second author, has been investigating a particular strip of sky
using photos from the Sloan Digital Sky Survey (SDSS) for a number of years.
They discovered a faint region in one of the data revisions that seemed
intriguing enough to launch a study.
The next step was to verify that this patch in the survey was an
exceedingly diffuse item rather than some kind of mistake using ultra-deep
multicolor photos from the Gran Telescopio Canarias (GTC). The Nube is
faint, making it difficult to measure its precise distance.
The authors estimated Nube's distance from Earth to be 300 million light
years using an observation made with the Green Bank Telescope (GBT) in the
United States. However, future observations with the Very Large Array (VLA)
radiotelescope and the optical William Herschel Telescope (WHT) at the Roque
de los Muchachos Observatory, La Palma, should help them demonstrate whether
this distance is accurate.
"If the galaxy turns out to be nearer, it will still be a very strange
object and offer major challenges to astrophysics," says Ignacio
Trujillo.
An further obstacle to the current dark matter model?
As a general rule, the number of stars in galaxies decreases quickly as one
moves out from the center, with a considerably higher density seen in the
inner regions. However, Montes states that in Nube, "the density of stars
varies very little throughout the object, which is why it is so faint, and
we have not been able to observe it well until we had the ultra-deep images
from the GTC."
The astronomers are baffled by Nube. According to the scientists, there
doesn't seem to be any interaction or other sign of its peculiar qualities.
Its "extreme" features cannot be replicated by cosmological models, even
when using diverse circumstances. "We are left without a viable explanation
within the currently accepted cosmological model, that of cold dark matter,"
Montes says.
The cold dark matter hypothesis is able to replicate the universe's
large-scale structures, but it is unable to provide a satisfactory
explanation for certain small-scale events, as the Nube example. It is one
of the most severe occurrences that has been observed to far, as we have
demonstrated that it cannot be produced by any of the theoretical
models.
"It is possible that with this galaxy, and similar ones which we might
find, we can find additional clues which will open a new window on the
understanding of the universe," Montes said.
"One possibility, which is attractive, is that the unusual properties of
Nube are showing us that the particles that make up dark matter have an
extremely small mass," claims Ignacio Trujillo. If this is the case, then
the peculiarities of this galaxy would serve as a galactic example of
quantum physics. "If this hypothesis is confirmed, it would be one of the
most beautiful demonstrations of nature, unifying the world of the smallest
with that of the largest," he says.
The research is
published
in the journal Astronomy & Astrophysics.
