A new nearly dark galaxy has been found by a multinational team of
astronomers by accidently examining deep optical images from the IAC Stripe
82 Legacy Project. The recently discovered galaxy, known as "Nube," is as
huge as the Small Magellanic Cloud (SMC) and has an extremely low surface
brightness. A article detailing the discovery was posted on the pre-print
service arXiv on October 18.
"Almost dark galaxies" are the generic term for galaxies whose center
surface brightness is less than 26 mag/arcsec2. They are typically absent
from optical catalogs of broad field surveys and lack a clear optical
equivalent. On the other hand, further imaging of these small galaxies could
reveal very feeble optical emission.
Now, another galaxy of this uncommon kind has been found by a group of
astronomers headed by Mireia Montes of the University of La Laguna in Spain.
They discovered it while visually inspecting one of the survey areas for the
wide-area weak surface brightness astronomical study known as the IAC Stripe
82 Legacy Project. The study looks at Stripe 82, a 2.5 degree broad stripe
in the Southern Galactic Cap that runs parallel to the Celestial
Equator.
The effective surface brightness of Nube, which is around 350 million light
years distant, is about 26.75 mag/arcsec2. The metallicity of the galaxy was
measured to be -1.1, and its age is estimated to be 10 billion years.
Regarding Nube's other basic characteristics, the research discovered that
it has a half-mass radius of 22,500 light years, making it very extended.
The galaxy is expected to have a total halo mass of 26 billion solar masses,
with a star mass of roughly 390 million solar masses. According to these
findings, the effective surface density is around 0.9 solar masses per
parsec2.
The results led the paper's authors to the conclusion that Nube is the
largest and most extensive galaxy of its sort that has been discovered to
date. The galaxy's radius is three times bigger than that of ordinary
ultra-diffuse galaxies (UDGs) with similar star masses, and it also turned
out to be ten times fainter. Generally speaking, UDGs are very low density
galaxies that are similar in size to the Milky Way but have only 1% more
stars than our own galaxy.
The scientists talk about the nature and formation of this galaxy while
taking into consideration Nube's exceptional characteristics. They looked
into whether these characteristics came about as a result of the galaxy's
initial creation or whether they were the product of a subsequent
evolutionary process brought on by its surroundings.
"To this end, and under the hypothesis that the distribution of stars in
Nube is representative of the distribution of the dark matter halo, we found
that a soliton-shaped profile (typical of fuzzy dark matter) reproduces the
observed distribution of stars very well," the study's authors wrote.
