Sky and Telescope - September 2015 - 11

DARK MATTER I Mapping the (Not-So-) Invisible

Wolf-Rayet Stars?
Both of these observations hint at a companion star.
Astronomers usually assume that massive stars expel
their outer layers as a stellar wind, an outﬂow of matter
coupled with irregular ejections. But the typical mass
loss from stellar winds might not be enough to produce
as many Wolf-Rayets as we've observed; there are just
too many of them relative to other, less-evolved massive
stars. If a stellar companion is involved, however, it would
pull on the Wolf-Rayet's loosely bound outer envelope,
unraveling the star's outer hydrogen layer more quickly.
The X-ray emission could be from the two stars' colliding
stellar winds, the team suggests in the July 1st Monthly
Notices of the Royal Astronomical Society.
■ ANNE MCGOVERN

-45°

100 million light-years

Declination

-50°

-55°

DARK ENERGY SURVEY

Two projects are mapping the distribution of dark matter in the universe, probing scales both large and small.
One, the Dark Energy Survey, is tackling the largescale universe using the Dark Energy Camera (S&T:
Jan. 2013, p. 15). The instrument is a 570-megapixel
CCD camera that's in the process of surveying a huge,
5,000-square-degree swath of Southern Hemisphere sky.
Using preliminary data that covers just 3% of the full
survey, Vinu Vikram (Argonne National Laboratory) and
colleagues examined the shapes of more than 1 million faraway galaxies, whose light has traveled between
5.8 billion and 8.5 billion years to reach us. The team
was looking for the smearing eﬀect of intervening dark
matter, which acts as a gravitational lens to magnify and
distort distant galaxies' light.
Using the observed smearing, Vikram's team constructed a 2D dark matter map (shown at right) and
plotted out how much dark matter lies along lines of sight
within a 139-square-degree subset of the area. Generally,
the observations support the picture of the universe as
a cosmic spider web, with galaxy clusters strung along
nodes and ﬁlaments of dark matter like so many caught
ﬂies. But clusters don't exactly trace the underlying dark
matter distribution, because normal matter and dark
matter follow diﬀerent physical laws. The team posted the
result May 7th on the open-access preprint site arXiv.org.
Working on a much smaller scale, Mathew Madhavacheril (Stony Brook University) and colleagues
conducted a diﬀerent study using the Atacama Cosmology Telescope to measure dark matter's smearing eﬀect,
not on the light from faraway galaxies, but on the most
well-traveled light in the universe: the cosmic microwave
background (CMB).

- 60°
6h

Right Ascension
By measuring dark matter's smearing eﬀect on galaxy shapes,
the Dark Energy Survey mapped out the mysterious stuﬀ 's
density over a large swath of sky. The color scale reﬂects dark
matter density (red high, blue low); grey circles mark galaxy clusters - bigger circles represent larger clusters.

The team looked for sharp brightness changes in the
CMB on arcminute scales, corresponding in this case to
the few million light-years that an individual galaxy's dark
matter halo spans. The astronomers found about 12,000
blips that matched up with galaxies listed in a Sloan
Digital Sky Survey catalog. Each of these galaxies has a
massive halo roughly 10 times that of the Milky Way, the
team reports April 13th in Physical Review Letters.
Simply measuring the signal from individual galaxies' dark matter halos is an accomplishment - little has
been done on these small scales before. The average dark
halo's mass and concentration, as measured from the
blobby composite image the team created, so far match
what's expected from dark matter simulations. Researchers will apply the same technique to data obtained from
the Advanced ACT polarization survey taking place
between 2016 and 2018, which will cover 10 times the
sky area. Eventually, Madhavacheril hopes to trace the
growth of dark matter halos over cosmic time.
■ MONICA YOUNG

Sk yandTelescope.com September 2015

http://www.arXiv.org http://www.SkyandTelescope.com

Sky and Telescope - September 2015

Table of Contents for the Digital Edition of Sky and Telescope - September 2015