Sky and Telescope - October 2018 - 14

Our Local Void

alaxies are gregarious, most living near others. For
example, our galaxy and its great spiral neighbor 2.5
million light-years away, Andromeda, rule the Local
Group, which boasts more than 100 galaxies.
But on the far side of our galaxy, starting less than 4 million light-years from Earth, lurks a dark domain that bears
almost no galaxies at all. Moreover, this void is gargantuan,
spanning a quarter billion light-years. Its center hides behind
the galactic plane in Aquila or Sagittarius.
At ﬁrst glance, voids might seem as uninviting to astronomers as treeless meadows are to arborists. After all, you're not
likely to dazzle your friends by showing them a void through
your telescope.
But voids are important, because most of the space of the
universe is in one void or another. Voids help sculpt the large-

Sagittarius Dwarf Irregular Galaxy

HOME
This view shows the
Milky Way, as well as
several of the dwarf
galaxies around it, relative to the supergalactic
plane (circular white
lines). This plane contains the Local Sheet of
galaxies as well as the
Virgo Cluster.

The Local Void stretches out ahead of
you for another 250 million light-years.
It's the intergalactic equivalent of
driving through western Kansas.

DIAG R A M: DA NIEL PO M A RÈDE (CE A UNIV ERSITÉ PA RIS-SACL AY ); BA R N A RD'S G A L A X Y:
ESO; SAGIT TA RIUS DWA RF IRREGUL A R: N ASA / ESA / HUBBLE HERITAG E TE A M (STSCI /
AUR A), ACK NOWLEDG MEN T: Y. M O M A N Y (UNIV. OF PA DUA)

scale structure of the universe, as galaxies ﬂow out of them
and into the glowing sheets and ﬁlaments that crisscross the
cosmos. In addition, the few galaxies that still inhabit voids
may evolve differently from galaxies elsewhere.
Because of its proximity to us, the Local Void provides
our best chance to study all of these effects in action. Here,
astronomers can search for galaxies so feeble they would
be undetectable in more distant voids. And because the few
galaxies in the Local Void are nearby, we can scrutinize them
to see whether they differ from galaxies that grew up in more
urban locations.

Discovering the Local Void
Two decades before R. Brent Tully (University of Hawai'i)
and J. Richard Fisher (National Radio Astronomy Observatory) discovered the Local Void, the pair had met in graduate
school. "I was the data guy," Fisher says, "and Brent was the
interpreter." Following the completion of their PhD theses,
Fisher set out to observe more than 1,000 galaxies and measure their redshifts, a sign of their distances from the Milky
Way. Tully then plotted the three-dimensional positions of
these and other galaxies.
"There was a whole sector of sky that was really, really
empty of galaxies," Tully says. Unfortunately, the dusty
galactic plane cuts right through this empty sector, obscuring
any galaxies it might possess. "But it pokes out on each side,
well above and below the plane of the galaxy," he adds. In
their 1987 Nearby Galaxies Atlas, which contains information
on 2,367 nearby galaxies, Tully and Fisher called this barren
region the Local Void.
OCTOBE R 2 018 * SK Y & TELESCOPE

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Sky and Telescope - October 2018

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