Sky and Telescope - September 2018 - 10

NEWS NOTES
G4

MILKY WAY

More Mystery Objects in
the Galactic Center
fornia, Los Angeles), Randy Campbell
(W. M. Keck Observatory), and their
colleagues announced the discovery
of three red, dusty objects near our
galaxy's supermassive black hole, Sgr A*,
at the American Astronomical Society
meeting in Denver. The objects are
bright in hydrogen emission and compact - unlike disrupted gas clouds.
This isn't the first time that astronomers have detected such objects near
the black hole. Two others, called G1
and G2, follow slingshot paths around
Sgr A*. Observers across the world
monitored the G2 object as it plunged
near the black hole, but although its
shroud was disrupted, the object itself
survived the close pass (S&T: Jan. 2015,
p. 16). Many astronomers suspect a star
hides inside the shroud, holding the
object together.

G1
G3

G2

0.2 arcsec

p Preliminary data analysis shows the motion of
G3, G4, and G5 from 2006 through 2017 (green
to yellow). The black arrows show which direction each object is moving. Observations of G1
and G2 are shown in blue and magenta.

Now, as part of a reanalysis of 12
years of Keck data, Ciurlo, Campbell,
and their colleagues have found three
more G objects: G3, G4, and G5.
The team thinks these objects might
be something extraordinary: new stars
made when two stars have merged. If a
binary system were orbiting the supermassive black hole, the black hole would
stretch the stars' orbits around each

t Artist's concept of a starburst galaxy (upper
right) hidden behind a veil of dust (lower left).
ALMA observations peered through the veil to
measure interstellar carbon monoxide.

STAR FORMATION

Too Many Massive Stars
in Early Universe
A NEW METHOD OF MEASURING star

formation in four galaxies in the early
universe finds that they're producing
more massive stars than expected.
Astronomers have long thought that
the same basic processes ought to shape
star formation no matter where it happens. However, the nearby star-forming
region 30 Doradus (S&T: May 2018,
p. 9) recently challenged that assumption and now, in the June 14th Nature,
new work on early galaxies appears to
confirm that the stars that form depend
on their environment.
10

S E P TE M B E R 2 018 * SK Y & TELESCOPE

The galaxies, whose light took more
than 10 billion years to travel to Earth,
are bursting with stars. But they're also
dusty, which makes them immune to
measurements requiring ultraviolet, visible, or infrared light. So Zhi-Yu Zhang
(University of Edinburgh, UK) and
colleagues trained the Atacama Millimeter/submillimeter Array on these
galaxies, searching for carbon monoxide
emission, a signal tied to a galaxy's history of star formation.
The scientists measured two forms of
carbon monoxide: 13CO and C18O. Stars
of all masses release 13C, whereas only
more massive stars produce 18O. Since
massive stars live brief lives, the relative
abundance of 13CO to C18O serves as
a fossil record, showing that too many
massive stars formed relative to lowmass stars in these four galaxies.
If the measurements hold up, then
it might change our understanding of

other, eventually inducing them to join. The
new, more massive star
G5
would be surrounded by
a dusty, red gas cloud
that looks similar to the
G objects. The cloud would then dissipate in about a million years, leaving
behind a young, massive star like those
seen near Sgr A* (see page 22).
But there could be a wee snag in
this explanation. The team doesn't
have enough data yet to conclusively
nail down the new objects' orbits, but
preliminary data show G3's orbit to
be roughly circular, which means the
black hole would have had a harder time
forcing the two stars to combine. That
raises the question of whether all the
G objects really have the same origin.
■ CAMILLE M. CARLISLE

starbirth. For example, the galaxies we
call "starbursts" might not be making
more stars compared to other galaxies, as astronomers assume; rather they
might be pouring more resources into
making massive stars, resulting in a
fewer total stars. This could throw off
our understanding of cosmic star formation, which astronomers currently
think peaked when the universe was
roughly a third its current age.
However, the carbon monoxide
signature measures the interstellar
medium rather than the stars themselves, so it's gauging the galaxy's
entire history of star formation. While
galaxies in the early universe have a
pretty short history, there's still time for
confounding effects to complicate the
measurements, points out Kevin Covey
(Western Washington University).
Zhang's team may not have convinced all their colleagues, but they're
only getting started: Zhang says more
systematic surveys, including of nearby
galaxies, are in the works.
■ MONICA YOUNG

G OB JECT ORBITS: A NN A CIURLO E T A L. / 232ND A AS;
DUST Y STA RBURST: ESO / M. KOR NMESSER

ANNA CIURLO (University of Cali-

Sgr A*

Direction of motion
S stars
New G objects (2006-2017)
Old G objects (2003-2014)



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