Sky and Telescope - July 2016 - 14

News Notes

MILKY WAY I Stellar

TECH I Flood Threatens Photographic Plates

Shell in Galaxy's Halo

staﬀ and volunteers packed each box
with plates and ferried it out in "a sort of
human bucket brigade," Grindlay says. By
Thursday afternoon they had evacuated
some 61,000 plates in all, or 12% of the collection, each in its sopping paper envelope.
The roughly 2,000 boxes now sit in
three semi-trailer freezer cars, like an
astronomical cryogenics lab, to keep them
from molding. Over the next year and a
half, disaster recovery contractors will
carefully thaw and photograph each plate
by hand before removing the paper and
gently cleaning the plate itself to preserve
the photographic emulsion.
Grindlay says they're conﬁdent that
they haven't lost the plates; they'll be
able to clean and scan them and ﬁnish
DASCH, which is only about a third done.
The team should be able to complete the
digitization project in early 2018.

The motions of 13 stars in our
galaxy's halo outline a shell-like
structure, Emily Cunningham (University of California, Santa Cruz) and
colleagues report in the March 20th
Astrophysical Journal. The stars might
be the remains of an ancient run-in
with a dwarf galaxy.
The halo is the cloud of stars that
surrounds the Milky Way's disk. These
stars generally follow orbits that take
them in and out of our galaxy's center,
rather than around it in a circular
motion like the stars in the spiral disk.
But Cunningham's team found that
these 13 halo suns seem to be part of a
shell of stars that are piling up as they
turn around in their in-and-out orbits.
If the shell interpretation is correct,
then this turning point in the stars'
orbits could mark what's left of a dwarf
galaxy - or perhaps even a group of
dwarfs - that fell into the Milky Way's
gravitational grasp several billion years
ago. Cosmological models do predict
that shells like this should be out
there, says James Bullock (University
of California, Irvine).
The team is compiling a much
larger sample in a project known as
HALO7D, which will contain information on hundreds of halo stars and
help them conﬁrm whether this shell
and other structures exist.

■ CAMILLE M. CARLISLE

■ MONICA YOUNG

DASCH

On the morning of January 18th, a burst
pipe ﬂooded Harvard College's Observatory Hill, submerging thousands of
historic photographic plates underwater.
Recovery eﬀorts will take about 18 months.
The plates - 500,000 of them -
crowd three stories of shelves in Building
D of the Harvard College Observatory in
Cambridge, Massachusetts. These glass
ﬁlm negatives were taken by astronomers
both at Harvard and at outposts around
the world over the years 1885 to 1992. The
observatory stored them here as a largescale celestial record-keeping project.
At about 5:30 a.m. that morning, the
water main running beneath Observatory
Hill ruptured. With 60 pounds of pressure, the burst 8-inch pipe was like "a ﬁre
hydrant on steroids," laments astronomer
Jonathan Grindlay (Harvard), who directs
the project to digitize the plates, called
Digital Access to a Sky Century at Harvard, or DASCH (S&T: May 2015, p. 20).
The water ﬁlled the basement beneath
the stacks ﬁrst. At 8:30 a.m., when
DASCH staﬀ discovered the ﬂood, there
were 2 feet of standing water in the
archive's bottom story. By the time the
water was turned oﬀ, it had soaked the
four lowest shelves of plates in their paper
envelopes. It also destroyed a half dozen
computers and the commercial scanner
the team was using to digitize the archive.
Harvard's Weissman Preservation
Center arrived with a couple thousand
plastic boxes designed to store valuable
documents under threat. Over three days,

Volunteers and staﬀ evacuated the plates in
"Rescubes," stacked up here ready for loading.

BLACK HOLES I Spin Gauged, Duo Conﬁrmed
Astronomers have measured the spin
of a supermassive black hole and provided evidence that the behemoth has a
companion. Mauri Valtonen (University
of Turku, Finland) and collaborators
studied the quasar OJ 287, which has
double-peaked outbursts roughly every 12
years. Astronomers have suspected that
a smaller black hole orbiting the larger
one could cause the ﬂashes as it punches
through the monster's accretion disk. If
14

July 2016 sky & telescope

so, the smaller monster's orbit should
precess at a rate that depends on the more
massive black hole's spin rate.
Based on what the precession would
have to be to explain the timing of
previous outbursts, the team successfully predicted OJ 287's December 2015
ﬂare. Given this result, the researchers
announce in the March 10th Astrophysical Journal Letters that the heavyweight
rotates at only one-third its maximum

spin. That translates to a rotation speed
of 48,000 km per second (107 million
mph) at the event horizon, which has a
radius of roughly 50 billion km (30 billion
miles). Such a slow spin rate suggests
that random mergers might have played
an important role in the black hole's
growth, or that OJ 287's jets might be
tapping the spin energy and slowing the
black hole's rotation. If the method holds
up, it could be a novel way of calculating
binary black holes' spin. ✦
■ ALLEN ZEYHER
Sky and Telescope - July 2016

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