Sky and Telescope - August 2015 - 11

GALACTIC CENTER
Mysterious X-rays
Spotted in Milky Way
NASA's NuSTAR mission has detected
an unexpected haze of high-energy X-rays
in our galaxy's crowded center. Kerstin
Perez (Columbia University) and colleagues found the X-rays emanating from
within our galaxy's central 10 light-years
or so, they report in the April 30th Nature.
In and of themselves, X-rays from the
galactic center aren't unusual. But the
X-rays NuSTAR detects don't seem to be
associated with structures already known
to exist, such as the supernova remnant
Sgr A East or the dust and gas clouds of
Sgr A West that are falling toward the
supermassive black hole.
Instead, the team proposes that thousands of dead stars could be responsible
for the high-energy X-rays: massive (and
still-growing) white dwarfs, spun-up
pulsars, or black holes and neutrons stars
feeding on low-mass companion stars. But
each of the proposed explanations has its
own set of challenges, and astronomers
don't know which is correct.
Take millisecond pulsars, potentially
the best option. These neutron stars ﬂash
energetic beams in our direction as they
spin like blindingly quick lighthouses.
They began as the cores of massive stars
that went supernova, then spun themselves up with gas stolen from companion stars. Many massive stars die in the
galactic center, so these pulsars' existence
seems inevitable. Such a population might
even help explain a mysterious excess of
gamma rays seen from this region.
But although astronomers expect to
see lots of millisecond pulsars in the
galactic center, so far they've spotted none.
That might be because the gas and dust
between the galactic center and us smears
out the pulsing radio signals. But millisecond pulsars' hot surfaces also emit
low-energy X-rays, so if there were a large
number of these pulsars, then other X-ray
telescopes such as NASA's Chandra X-ray
Observatory should have spotted them.
They haven't.
■ MONICA YOUNG

GALAXIES I Ghosts of Quasars Past
lion galaxy images, only 19 made the
cut. Of these, eight clouds were echoing
light from quasars that had faded from
their former glory, the team reports in
the May Astronomical Journal.
Keel's team found that the host
galaxies of all eight faded quasars show
signs of recent or ongoing mergers,
such as tidal tails or dusty disks warped
by an encounter. The lit-up reservoirs
outside the galaxies are probably the
gaseous remains of these mergers: the
clouds are roughly in the same spot as
the tidal debris and they're also rotating, motion that's likely left over from
the now-cannibalized galaxy.
Surprisingly, none of these mergers
appears to have triggered much, if any,
star formation, despite large reservoirs of
gas. Did the once-bright quasars quench
any existing star formation, as the
authors speculate? Or could we be witnessing these mergers early on, before
activity has had a chance to ignite?
"It is absolutely possible that . . .
either star formation, AGN activity, or
both will be triggered later on, after the
gas settles more," says Freeke van de
Voort (University of California, Berkeley). But, she cautions, it could be that a
gas-rich spiral merging into a gas-poor
elliptical simply doesn't trigger stars.

Astronomers have found eight echoes
from active galactic nuclei (AGN) that
recently went dormant. The echoes
are glowing clouds of gas, ionized by
intense radiation beamed out from the
galaxies' cores. This radiation came
from supermassive black holes that
were voraciously swallowing hot gas,
but the black holes have since gone
quiet. Even so, the echo of light in the
glowing clouds remains, because the
gas atoms' stripped electrons take a
long time to ﬁnd new homes in the
sparse intergalactic environment.
William Keel (University of Alabama) led the charge of nearly 200
Galaxy Zoo volunteers hunting for
these objects in Hubble Space Telescope
images. The search began after a Dutch
schoolteacher discovered a galaxy-size
blob of gas glowing south of the spiral
galaxy IC 2497. The gaseous object,
known as Hanny's Voorwerp (Dutch for
"Hanny's Object"), was ionized roughly
30,000 years ago, when IC 2497's supermassive black hole created a blazing
quasar in the galaxy's core.
The volunteers searched for blobs
with the same distinctive color as
Hanny's Voorwerp (blue or green,
depending on the processing) and oﬀset
by more than 33,000 light-years from
the nearest galaxy. Out of almost a mil-

■ MONICA YOUNG

IN BRIEF
Early Galaxy Caught Merging? A rare
type of gravitational lens has oﬀered
astronomers a close look at a young, dusty
galaxy. Light from the distant galaxy SDP.81
traveled for 12 billion years to arrive at Earth.
The light encountered a massive elliptical
galaxy 8 billion years into its ﬂight, lensing
SDP.81's image into a ring only 3 arcseconds
across. New ALMA observations have taken
advantage of this natural spyglass, revealing
details spanning just 100 light-years, Catherine Vlahakis (ALMA and European Southern
Observatory, Chile) and colleagues report
in Astrophysical Journal Letters. A sepa-

rate analysis by Simon Dye (Nottingham
University, UK) and others reconstructed
SDP.81's shape using both ALMA and
Hubble images. Oddly, while ALMA's radio
image shows a rotating disk of dusty gas,
the visible-light image from Hubble shows
a blob of gas oﬀset from the disk. The disk
and blob might be two distant galaxies only
seemingly close due to our perspective, or
perhaps one galaxy has passed through
another, leaving behind a dusty disk galaxy
that's popping with stars - about 500
Suns' worth a year.
■ MONICA YOUNG

SkyandTelescope.com August 2015

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Sky and Telescope - August 2015

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