Sky and Telescope - October 2018 - 18

Our Local Void

then a graduate student at the University of New Mexico, was searching for
21-centimeter radiation from neutral
hydrogen gas in galaxies hiding behind
the galactic plane. He found lots of new
galaxies, but the one in Aquila stood
out by virtue of its small redshift. "It
was special because it was so close to
our own galaxy," McIntyre says. Only
the Great Attractor and the Shapley Conlater did he and his colleagues spot the
centration, which amounts to 455 km/s.
galaxy's stars.
Furthermore, the Local Void's "push"
Because these two void galaxies are
is nearly perpendicular to the pulls from
nearby, the Hubble Space Telescope can
both Virgo and the Great Attractor. To
detect their red giants, aging stars whose
picture the situation, you must know
apparent magnitudes reveal precise
an amazing fact about the eight great
distances to their galactic homes. That's
galaxies nearest our own: Every galactic
important, because by comparing the
goliath within 20 million light-years of
galaxies' distances with their velocities
us resides in nearly the same plane (see
derived via redshifts, astronomers can see
box on page 16).
whether the void is actually expanding.
These great galaxies constitute part
It's not easy, though. "Observing
of what Tully calls the Local Sheet,
empty space is very hard," says Luca
which is a wall of the Local Void. The
Rizzi (W. M. Keck Observatory).
Local Void is above the Local Sheet and
Fortunately, the two void galaxies
causes it to move down. As evidence
nearby are like glowing ﬂotsam tracing
for this movement, he points to the
p KK 246 The gaseous disk of this void galaxy
the currents of a dark sea. "They are in
Leo Spur, a ﬁ lament some 35 milin Sagittarius, marked here as contours, exthe void, but they are closer to us than
lion light-years below the Local Sheet.
tends five times farther out than its stars do, as
Because of the Local Sheet's downward seen in the background black-and-white image. to the center" of the void, Rizzi says.
In a study published last year, his team
movement, the Leo Spur's galaxies
reported that both void galaxies are racing away from the
have smaller redshifts than they would otherwise.
Local Void's center and toward its wall at hundreds of km/s, a
sign that the void is indeed expanding and growing ever empVoid Where Prohibited
tier. "Voids like to get rid of galaxies as fast as they can," he
Despite its name, the Local Void is not completely free of galsays, as galaxies outside the voids tug the void galaxies away.
axies. "The void is partitioned by these wispy ﬁlaments that
Both void galaxies are extremely isolated. Whereas lots of
trace through it," Tully says. The ﬁlaments contain chains of
galaxies dwell close to our own, KK 246 doesn't have a single
galaxies that glow like dimly lit towns on a rural road. Nor is
known neighbor within 10 million light-years.
the Local Void perfectly round. Instead, it's lumpy.
And yet, despite their extreme isolation, the void galaxThe two nearest-known inhabitants of the Local Void are KK
ies seem normal. They have gas, just like most of the Local
246 and ALFAZOA J1952+1428; the two galaxies lie about 25
Group's outlying members, and probably for the same reason:
million light-years away, in Sagittarius and Aquila, respectively.
because there's no giant galaxy nearby to steal it. In fact, KK
Both galaxies are dwarf irregulars, miniature versions of the
246's gas disk extends ﬁve times farther out than its stars
two brightest satellites of the Milky Way, the Large and Small
do. Even that isn't unique, however, as a few dwarf galaxies
Magellanic Clouds. The void galaxies emit only a few percent as
elsewhere have equally impressive gaseous envelopes.
much light as the Large Magellanic Cloud, but like the MagelThus, as long as a dwarf galaxy steers clear of gas-grabbing
lanic Clouds they both possess gas that is spawning new stars.
giants, it can lead a normal life, whether or not it's in a void.
In fact, it was the gas, not the stars, that ﬁrst signaled
Internal processes govern the galaxy's evolution.
the presence of the void galaxy in Aquila. Travis McIntyre,

THE LEO SPUR

The Leo Spur is a Àlament of galaxies beneath the supergalactic plane and thus on the other side of
us from the Local Void. Members include the irregular galaxy NGC 2337, which is 37 million light-years
away in Lynx, and the edge-on spiral NGC 2683, which is 31 million light-years from Earth, also in Lynx.

18

OCTOBE R 2 018 * SK Y & TELESCOPE

DIAG R A M: K RECK EL E T A L., THE ASTR ONOMICAL JOUR NAL, JUNE 2011;
SIDEBA R BACKG ROUND: K RIA N TI / G E T T Y IM AG ES

Patient observers could
help cosmologists
reconcile reality with
theory by turning up new
galaxies in the void.
Sky and Telescope - October 2018

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