My first Magazine Sky & Telescope - 04.2019 | Page 17

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The Spiraliferous Period
The modern universe abounds with so many spiral galax-
ies that it’s easy to imagine they will endure forever. About
70% of the bright galaxies within a billion light-years are
spirals. Most galaxies in Charles Messier’s great catalogue are
spirals. And the Local Group’s three most luminous galaxies
— Andromeda, the Milky Way, and M33 — are all spirals. Just
as geologists call the time when much of our planet’s coal
formed the Carboniferous Period, so we might call the pres-
ent time the Spiraliferous Period.
This spiral-spangled period took a long time to start,
however, in part because the universe’s fi rst galaxies were
too small to spin fast. “Spirals tap rotational energy,” says
astronomer Bruce Elmegreen (IBM Research Division).
“Low-mass galaxies can’t form spirals because they rotate too
The great spiral galaxies that adorn the modern
universe are running out of gas and may
someday lose their beautiful shapes.
slowly.” Even after galaxies
grew massive enough to spin
fast, spirals remained rare
for billions of years after the
universe’s birth. “You don’t
get spirals then, because
everything’s very turbulent,”
he says. “The galaxy is accret-
ing gas like crazy, which stirs
it, and it’s also got a high
star-formation rate, which
stirs it.” This vigorous mixing p WAFER This Hubble image of
NGC 5866 in Draco reveals the
acted like a kitchen blender,
galaxy’s edge-on disk. From this
preventing spiral structure.
perspective, the dust lane and
Galaxy collisions in the
blue stellar disk slice through the
crowded early universe added
central bulge.
still more turbulence.
Turbulence is a spiral galaxy’s foe, for a spiral needs tran-
quility. In a tranquil fast-spinning galaxy, stars and gas race
around the galaxy’s center, but they do so slowly relative to
one another. They have what astronomers call a low velocity
dispersion, much lower than the galaxy’s rotation speed. So
the stars and gas don’t drift far above or below the galaxy’s
plane, and the disk stays thin. That keeps the stars and gas
close together, making the disk dense.
“You want high density, so that gravity is important,”
Elmegreen says. “Gravity does all the action to make spi-
rals.” In such a dense environment, gravity has the chance
to enhance concentrations of stars and gas that happen to
develop in the disk. These excesses feed on themselves as
their gravity pulls in additional stars and gas. The galaxy’s
rapid rotation shears this material, whipping it into a spiral
pattern just as stirring cream into coffee does. But too much
turbulence in the coffee — or the galaxy — would blur the
spiral and ruin the beautiful pattern.
The farthest known spiral galaxy, Abell 1689 B11 in the
constellation Virgo, makes the case for tranquility. The galaxy
has a redshift of 2.54, which means its light has traveled for
on years to reach us, so the spiral existed when the
universe was only 2.6 billion years old. “No
one was expecting to see spiral arms at this
redshift,” says Tiantian Yuan (Swinburne Uni-
versity of Technology, Australia), who calls the
spiral a “weirdo.” By good luck, it resides in an
unusually quiet neighborhood, with no other
galaxies to jostle it, so it sports a thin disk and
thus sprouted spiral arms long before its peers.
Elmegreen estimates that spiral galaxies
emerged in earnest 9 to 10 billion years ago,
after the universe had settled down. Today
spirals are flourishing. They now dominate
most galaxy groups and garnish the outskirts
of galaxy clusters.
But spiral galaxies can’t last unless they
find more gas. Gas is not just a galaxy’s means
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HUBBLE
ECTORS
pirals are the most spectacular of galaxies. Like a giant
vortex in space, a spiral galaxy spins through the cos-
mos, its mighty arms squeezing gas and dust into new-
born stars. Blue supergiant stars and pink clouds of ionized
hydrogen gas sparkle in the spiral arms, where the occasional
fl ash of a supernova explosion casts newly minted chemical
elements into the galaxy.
These magnifi cent galaxies lend grace and beauty to the
night. Moreover, they work hard to create new light. Five out
of every six stars born each year are born in spiral galaxies,
some in normal spirals like the Whirlpool, others in barred
spirals like the Milky Way.
Yet these same exquisite galaxies face an immense prob-
lem. As spiral galaxies spawn new suns, they exhaust the very
gas and dust that enable them to be spirals in the first place.
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