38 DISCOVERMAGAZINE.COM
Dragonfly’s multiple lenses serve as checks on one
another for stray light, and their internal surfaces
are treated with an anti-reflective coating. The lenses
are hooked up to CCDs, which van Dokkum says
are finally getting good enough to distinguish the
universe’s mostly ghostly galaxies. “Even now,” he
says, “we’re only scratching the surface.”
Traditional mirrored telescopes can still help.
Inspired by van Dokkum’s find, Stony Brook
University’s Jin Koda and colleagues looked through
recent Coma Cluster observations from the 8.2-meter
Subaru telescope in Hawaii. The researchers found
854 ultra-diffuse galaxies, with over 300 stretching
to the Milky Way’s size. This boatload had gone
unnoticed because astronomers previously assumed
luminous traces of the galaxies in Coma indicated
small, insignificant bodies, and not just the most
visible central regions of otherwise very dim objects
— the tips of galactic icebergs, as it were.GRASPING IN THE DARK
The newfound dim galaxies in Coma are
strange beasts, and they hark back to
some of the faint galaxies first uncovered
in the late 1980s. Almost entirely gasless,
round and thinly stocked with old red
stars, they have apparently survived
for eons in a dense environment of
visible galaxies. Those comparatively
gas- and mass-rich conspicuous
galaxies and their environs should have
gravitationally pulled the inconspicuous
galaxies to shreds by now — why they
haven’t is a mystery.
Astronomers also are not sure how
the universe made these sorts of objects
in the first place. “That’s why this field
is so exciting at this moment,” says
Koda. “We don’t really know what these
galaxies are.” They might represent
a population of “failed” galaxies.
Although expansive, such failure-to-
launch galaxies might have started out
with insufficient amounts of normal
matter, or somehow lost it, stifling the
formation of new stars.
Assuming that’s the case, the Coma Cluster’s
population of diffuse galaxies might be the opposite
of galaxies like Malin 1. Those in the latter’s class of
low-surface-brightness galaxies are oddly bluish in
color, thanks to the presence of newly made stars that
are azure in color. These galaxies could be cosmic
late bloomers, a kind of slowly evolving galaxy just
now producing plentiful stars, billions of years after
the Milky Way and others went through their peak
periods of star-making. 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.
Dim galaxies also could force some rethinking
about large-scale cosmic structure and its relation to
dark matter. First theorized in the 1930s, dark matter
does not emit any light, betraying its existence solely
through gravity. Its true identity remains a mystery,
but astronomers know it outnumbers regular matter
5 to 1 and acts as gravitational glue, holding galaxies
together. According to recent studies, dim galaxies
like those strangely hardy ones in Coma look to be
almost entirely made of the stuff. Astronomers suspectDragonfly CFHT TelescopeRed squares mark the location of 47
dim galaxies in the Coma Cluster.
The inset shows Dragonfly’s clear
detection of the objects (left column)
and their confirmation as diffuse
objects as imaged by the much larger
Canada-France-Hawaii Telescope.