Discover 3

34 DISCOVERMAGAZINE.COM

O

ne winter night in 1969, at an

observatory atop Kitt Peak in

Arizona, Michael Disney had

a funny thought. As he peered

into a huge, superluminous

galaxy, he wondered: What

if an alien astronomer there

were staring right back? At the

eyepiece of its own telescope,

the intelligent extraterrestrial might likewise be

ogling Disney’s smaller, fainter home galaxy, the

Milky Way.

Then another thought cut through the whimsy.

The young Welsh astronomer realized the alien had

no chance of seeing the Milky Way, let alone the

universe’s oodles of dimmer galaxies. Overwhelmed

by the glare of all the stars stuffed into its resident

galaxy, the alien would unknowingly be blinded to

most of the cosmos.

Disney wondered if we might be similarly deceived,

awash in the inescapable glow of our own surroundings.

“It occurred to me there could be a whole universe up

there of hidden galaxies, just a little dimmer than those

we can detect from Earth,” says Disney, an emeritus

professor at Cardiff University in Wales.

Since that revelation in the desert nearly a half-

century ago, Disney, now 80, has searched for a

shadow galactic realm. His hunch gained momentum

in the 1980s and 1990s, but at the turn of the century,

the trail ran cold. Disheartened and defeated, Disney

relinquished the hunt.

But recently, serendipitous sightings and new

technology have reinvigorated the concept of a

hidden cosmos. “Most of the universe is likely

undiscovered,” says Greg Bothun, an astrophysicist at

the University of Oregon who has long studied faint

galaxies. The emerging population of dim galaxies

likely outnumbers, and is strikingly different from, the

typical bright galaxies we know and love, challenging

our conventional theories of galaxy formation and

evolution. Dim galaxies may also solve an old mystery

about missing matter in the universe.

By these reckonings, hidden galaxies are the cosmic

norm, not our garish Milky Way and its ilk. Long

overlooked, the dominion of dim galaxies may finally

be getting its due.

BLINDED BY THE LIGHT

Our universe is suffused with luminous galaxies. We

can see the nearest few of these great collections of

stars, gas (mostly hydrogen) and dust with our eyes

from Earth. Telescope surveys suggest as many as

2 trillion are out there, albeit mostly of a small, faint,

“dwarf ” variety. Tidily, these galactic hordes come in

stereotypical shapes and sizes, such as large spirals like

the Milky Way, even bigger football-shaped ellipticals

and those dime-a-dozen dwarf galaxies. They follow

typical life cycles, making abundant stars in their

youths and slowing down as they age.

For all we’ve learned about galaxies and the

wider cosmos, though, astronomers have struggled

with human limitations as heavenly observers. Our

instruments can only readily perceive objects whose

brightness contrasts enough with the glow of the night

sky. For sure, night looks dark — around 50 million

times darker than day — but that’s still just relatively

dark. “We live right next to this bloody luminous star

called the sun,” says Disney. “That’s always going to

make it difficult for us to find this hidden universe.”

The sun’s brilliance affects astronomical viewing

in two roughly equal ways. At night, an “airglow”

lingers in our atmosphere as molecules radiate away

the heat they soaked up during the day. To avoid

airglow, we can send instruments into orbit, like the

Hubble Space Telescope (for which Disney designed

instruments). But these spacecraft still must squint

through the second of the sun’s impacts, its bright

illumination of icy and dusty particles around

it, known as the zodiacal light. Add this to the

copious light generated by all the other stars in our

galaxy, and you get quite a glare. This natural “light

pollution” extends to the entire electromagnetic

spectrum, beyond visible light.

“We truly are imprisoned in our lighted cell,” says

Disney. “It’s like you’re in the middle of a lighted room

at night and you look out the window.” Your room’s

light drowns out anything less bright. In 1976, seven

years after his experience in Arizona, Disney wrote

in a paper in Nature that our catalogs of galaxies

are probably an unrepresentative subset of the true

galactic population. A great number of dimmer and

potentially sizable galaxies likely awaited discovery,

OOOOOOOOOOO

“We truly are imprisoned in our lighted

cell,” says Michael Disney. “It’s like you’re

in the middle of a lighted room at night

and you look out the window.” Your room’s

light drowns out anything less bright.