March 2018^ DISCOVER^39P. VAN DOKKUM ET AL./ASTROPHYSICAL JOURNAL LETTERS/VOL. 798, NO. 2/JANUARY 7 2015 (2)
Arrested development, like in Coma,
or delayed development à la Malin 1 —
either way, the universe’s faint galaxies
don’t mesh with conventional theory.
galaxies began as accumulations of dark matter in the
early universe that acted as seedbeds for normal matter.
As the universe expanded, those dark matter seeds
spread out into a “cosmic web,” linking galaxy clusters
at the densest points with thin filaments of dark
matter. If we have only seen the brightest galaxies
in the universe, we don’t have the full picture about
how matter and dark matter are truly distributed.
“Low-surface-brightness galaxies are one of the keys
to figuring out what the clumpiness of the cosmic
web looks like,” says NRAO’s O’Neil. “They’ll help
us understand how the universe really did grow to
be what it is.”
Finally, by allowing for a more proper accounting of
the matter content of the universe, dim galaxies could
also help solve the long-standing “missing baryon
problem.” Although few cosmologists seriously doubt
that normal matter, made of particles called baryons,
makes up only about 5 percent of the universe’s totalmass-energy budget, the observable matter we know
of is still only about half what we’d expect. “There
could be a hell of a lot of stuff missing,” says Disney,
“and it could be in the form of hidden galaxies.”A SCANNER DARKLY
Increasingly, astronomers are, in a sense, seeing
the dark. In July 2016, a team reported finding a
humongous low-surface-brightness spiral galaxy, the
first unearthed to rival Malin 1’s size. Van Dokkum,
meanwhile, is expanding the Dragonfly Telephoto
Array to dig up more secretive galaxies. Stony Brook’s
Koda is optimistic. “There will be a lot of discoveries
in the low-surface-brightness universe,” he says,
“because many people are now looking into it and
trying to develop new techniques to find what’s there.”Disney, for one, thinks a huge observing campaign
with the upgraded VLA in New Mexico could finally
give us a handle on the true population of invisible
galaxies. Some dark galaxies, like those in the Coma
cluster but with even less hydrogen, will be tougher
to bring into the fold. While Bothun is skeptical
that human technology could ever detect the darkest
of galaxies, he is confident they are out there. “The
logical extension of a low-surface-brightness galaxy is
a dark galaxy,” he says. “There is no reason to think
they don’t exist.”
Looking back on the failed radio observations at
the turn of the millennium, Disney is eager to make up
for lost time and continue the search for the phantom
universe’s elusive galaxies. “I was the person who did
more to get things wrong than anyone,” says Disney.
“I’ve just literally spent 40 years of my life on this, and
I’d like to know the answer, one way or the other.”
Disney considers it a real gift that we earthlings
might one day be so lucky to behold the cosmos,
flooded as we are in sun- and starlight. “In such a
glare,” says Disney, “it is a wonder we can do any
astronomy at all.”^ DAdam Hadhazy, a freelance science writer based in New Jersey,
also writes for BBC Future and New Scientist, among other
publications.