http://www.skyandtelescope.com.au 9Milky Way mass
still up in the air
Gwendolyn Eadie and William Harris
(McMaster University, Canada) have
observed the motion of distant globular
clusters to measure our galaxy’s mass.
The new estimate, including both
normalanddarkmatterwithinaradius
of 600,000 light-years, lies between
600 and 750 billion Suns’ worth of
material. This value is far below previous
measures, which have put the Milky
Way’s mass at roughly a trillion Suns,
theteamreportedatanAmerican
Astronomical Society meeting and
in the October 1, 2016,Astrophysical
Journal. However, Dennis Zaritsky
(UniversityofArizona)andHelene
Courtois (Lyon University, France) claim
in the March 2017Monthly Notices of
the Royal Astronomical Societythat
mass estimates well below a trillion Suns
implytoolowafractionofdarkmatter.
Since astronomers know dark matter’s
contribution via other methods, that
oughttoruleoutlowerestimatessuch
as the one obtained by Eadie and Harris.
■MONICA YOUNG
Learn more about the study athttps://
is.gd/mwgmassJuno swoops past Jupiter’s south pole
SNASA’s Juno spacecraft took this image of Jupiter’s south pole on February 2 from a
range of 76,600 km. The flyby was the spacecraft’s fourth perijove pass, which brought it
just 4,300 km above the Jovian cloudtops. Despite previous problems, all eight science
instruments operated during this close pass. Juno’s preliminary data suggest that Jupiter’s
extensive magnetic field is larger and more powerful than thought. Also, the zones and
belts seen along the Jovian cloudtops could extend deep into the planet’s interior, though
how deep they go is still unclear.
■DAVID DICKINSONone part of the gravitational landscape
makes the Local Group flow faster
toward the dense concentrations in
the other direction than it would
otherwise.
This discovery might solve a cosmic
conundrum. Astronomers knew that
the Local Group moves with respect
to the cosmic microwave background
(CMB). This motion is called the CMB
dipole. But the velocity (630 km/s,
or 2.2 million kph) is about double
what it should be, if Shapley and the
other clusters were solely responsible.
The void’s effect essentially doubles
Shapley’s pull, explaining why the
Local Group moves as fast as it does.
The team thus labels the region “the
dipole repeller” in their January 30
Nature Astronomy paper.
■ CAMILLE M. CARLISLEWatch a video explaining the result at
https://is.gd/cosmicrepellerOrigin of our galaxy’s most distant stars
THE MOST DISTANT STARS discovered
in the Milky Way’s outer reaches
might have been ripped away from
a companion galaxy. Over the last
decade, multiple groups of astronomers,
including the author, have identified
stars more than 300,000 light-years
from Earth. (Our galaxy’s disk is
roughly 100,000 light-years across.)
Possible explanations for how the stars
got so far away have included that they
were ejected from the Milky Way’s disk,
that they are the brightest members
of a nearly invisible dwarf galaxy, or
that they are the remnants of a galaxy
shredded by the Milky Way’s gravity.
Now, in an upcoming Astrophysical
Journal issue, Marion Dierickx and
Abraham Loeb (Harvard University)
argue that some of these stars are
probably members of what was once the
Sagittarius dwarf galaxy.Sagittarius is the best-studied dwarf
remnant. This object passes close by the
Milky Way in its eccentric orbit and
every time it sweeps by, it sheds stars.
The streams loop around our galaxy in
majestic curves that crisscross the sky.
Starting with maps of these streams,
Dierickx and Loeb created simulations
that wound the clock backward
more than 8 billion years, when the
Sagittarius dwarf would first have
started interacting with the Milky Way.
Then they varied the initial velocity
and direction of the dwarf and let time
run. Five of the 11 most distant stars
astronomers have discovered matched
the positions, velocities, and distances
expected for Sagittarius members at
very large distances. The other six might
be members of another former dwarf
galaxy, but this is less certain.
■ JOHN BOCHANSKIDAMIAN PEACH